A Closer Look at Global Icing, Turbulence, and Surface Analysis Map Layers

Available with ForeFlight Mobile 9.4, five new weather layers bring global Icing, Turbulence and surface pressure data to the map, with detailed Surface Analysis available to much of North America. These weather products significantly enhance local and global flight planning and your weather decision-making abilities. The new layers are available for customers with Pro Plus, Performance Plus, Business Performance, and MFB Performance subscription plans, on mobile and on the web. In addition, the recently introduced time slider provides frame-by-frame control over animation of the new weather layers. Let’s dive into the details!

Layer Animation Time Slider

The animation Time Slider tool provides better control over weather layer animation, as well as more clarity on the age of the weather product you are viewing. The slider automatically displays when you select any Radar, Satellite, Icing, or Turbulence layer.

Weather Layer Time Slider

In addition to the familiar play/pause button, the Time Slider allows frame-by-frame control over the animation. The slider’s position on the time scale indicates the valid time for the currently displayed weather graphic. Tapping to the right of the slider head advances the layer by one time step, while tapping to the left of the slider head retreats the layer by one time step. Alternatively, tap-hold and move the slider head left and right to manually control animation speed and enable back and forth (rocking) animation, a useful capability when analyzing local storm cell changes on radar.

The time scale changes as appropriate for each weather product displayed. Forecast-based weather layers, such as the Icing and Turbulence layers, use a white vertical bar to indicate the present time, splitting the time slider into two parts: forecasts that are valid for the past (gray line) and for the future (white line).

Products that indicate past information only, such as the radar and satellite layers, are presented with a gray time scale with the product age, relative to the current time, displayed with each frame. The absolute time is displayed in a callout with each frame as you manually scrub left or right, as well as to the left of the time scale.

Providing full manual control over the weather layer animation and easily reading relative age and absolute time for each weather product frame provides better insight to how the weather is trending.

Surface Analysis

The Surface Analysis layer adds a global surface pressure overlay, displaying isobar lines and associated pressure values in millibars. For much of North America, the isobar lines are complemented with depictions of surface fronts, troughs, and high/low pressure center markers. Besides displaying the current surface analysis, the forecasted surface analysis can be viewed for up to two days into the future using the Time Slider.

The Surface Analysis product and its forecast is a collaboration between multiple weather centers and is primarily based on the National Weather Service Global Forecast System (GFS) and the North American Mesoscale (NAM) models, with additional guidance from the European ECMWF and the United Kingdom’s UKMET models. Surface features (fronts, troughs, pressure centers) are analyzed manually by a NWS meteorologist, and are therefore only available for the North America region.

Surface Analysis features follow the standard depiction convention as outlined in the table below:

View global isobars and more detailed weather features for the U.S.

Icing

Icing severity data for the United States has been available in the ForeFlight Mobile Imagery view since 2015. Now, icing severity is available as a dynamic weather layer in the Maps view in ForeFlight Mobile and on the web. And not just for the US, but for the entire world.

You will now see two icing products in the Maps view layer selector: Icing (US) and Icing (Global). Both serve the same purpose, but are based on different weather models.

For the US coverage, the Forecast Icing Product (FIP) is a Numerical Weather Prediction model that employs a 20km grid in the horizontal and a 1000 ft grid in the vertical (from 3000’ to FL450) to calculate icing severity and the potential for SLD (supercooled large droplets). The FIP model is run hourly and forecasts are available out to 18 hours.

For a fast analysis of conditions at each altitude, you can quickly scrub between altitudes using the Altitude Slider in the lower right corner of the Maps view. The Altitude Slider is also available on the Turbulence layer.

Altitude Slider

Altitude Slider

Please note that SLD threat is currently only available to customers flying with the SiriusXM SXAR1 aviation receiver and who are subscribed to the SiriusXM Pilot for ForeFlight plan. SLD threats are depicted with red squares.

The Global Icing product is based on the Global Forecast System (GFS) weather model with a coarser horizontal and vertical grid and is run four times per day (every 6 hours). Global Icing forecasts in ForeFlight are available out to 24 hours.

Because of the coarser grid, as well as the less-frequent model runs of the GFS, it’s worth comparing the two icing depictions side-by-side. The difference in horizontal resolution and model update frequency between the US icing layer (left) and the global icing layer (right) is evident. When operating within the limits of the US icing model coverage area, the US icing layer should be referenced instead of the global layer to take advantage of its frequent updates and finer horizontal and vertical grid resolution.

Regardless of whether you have US or global icing selected, each layer uses the same color scale to depict conditions of no icing, light, moderate and heavy icing, as shown in the legend below:

Icing intensity is based on how long it would take for ice to build up on an airfoil

It’s important to note that the icing severity is roughly based on the accretion rate of ice on an airplane. The severity levels are defined by how long it would take for ¼ inch (65mm) of ice to build up on an airfoil. Time ranges are given for each level because the build-up rate depends on variables like airfoil properties, airspeed, and atmospheric conditions.

Since the icing forecasts are produced with no human modifications, they are intended for flight planning purposes only and should always be used in combination with AIRMETs, SIGMETs and PIREPs.

Turbulence

Similar to the Icing map layer, there are two new Turbulence map layers, one for the United States and the other for Global coverage.

The US turbulence product is based on the Graphical Turbulence Guidance (GTG). The GTG and the associated Eddy Dissipation Rate (EDR) data scale used by GTG were both covered in detail in a previous ForeFlight article. It’s worth re-reading that post to brush up on the turbulence product, especially how the relationship between EDR and in-flight turbulence intensity changes based on aircraft weight. The information in that article applies equally to the US and global turbulence layers.

The US turbulence layer data are still based on GTG-3 with an available lead time of up to 18 hours. This layer displays the maximum EDR of clear air turbulence (CAT) and mountain wave turbulence (MWT). It is not intended to predict convection and thunderstorm turbulence sources, but may provide some guidance if the storm event is widespread. Furthermore, the graphics represent a snapshot at that time and not a forecast for a time range. Finally, it is important to realize that turbulence is a dynamic event and rapidly changing conditions may not be accurately reflected.

The global turbulence layer displays EDR derived from the GFS forecast model using a proprietary algorithm. This layer differs from the US GTG-3 derived layer in that the global turbulence data only forecasts CAT and not MWT. The forecast lead time can be up to 24 hours. The same reduced horizontal and vertical resolutions discussed previously in the global icing section apply to the global turbulence layer. Also, since the global turbulence layer uses GFS model data, the same reduced model run rate limitation applies (every 6 hours for GFS versus hourly for GTG-3), resulting in the US turbulence layer being updated more frequently. For all of these reasons, it’s important to use the US turbulence layer when operating within its coverage area.

Since the layer’s EDR scale is aircraft dependent, it is important to review the scale applicable to your aircraft category (the article linked above contains scales for light, medium, and heavy aircraft classes). The following color scheme is used in the turbulence map layer:

These new weather layers are an exciting and useful feature addition for our US and worldwide customers. Keep in mind that there is no human involvement in creating the turbulence and icing products and the information should be supplemented as much as possible with SIGMETs, AIRMETs, and PIREPs to understand the full weather picture.

Graphical Forecasts for Aviation (GFA) will become operational in April

Effective April 13, 2017, the experimental Graphical Forecasts for Aviation (GFA) produced by the NWS Aviation Weather Center (AWC) will transition to operational status. As you may have heard, the GFA was created in response to a formal request by the FAA to discontinue production of the textual Area Forecasts (FA). According to the NWS headquarters in Silver Spring, Maryland, “the requirements for the underlying meteorological information in the FA have not changed. The FAA recognizes that, given modern advances within the NWS, the legacy text FA is no longer the best source of en route flight planning weather information.”

The new graphical forecasts are designed to provide meteorological information equivalent to the textual FA. The GFA product includes observations and forecasts for the continental United States that provide data critical for aviation safety. The data is overlaid on high-resolution base maps that you can test drive here. Given this will be the replacement for the FA, it means that all of the forecasts will terminate at the U.S. borders. FAs for Hawaii, Alaska, the Caribbean, and the Gulf of Mexico will not be affected at this point in time.

For the time being, the legacy FA will continue to be generated in parallel with the GFA. The GFA is automated whereas the legacy FA is issued by forecasters at the AWC. At some point in the future, forecasters at the AWC will discontinue issuing this textual forecast. And don’t be surprised if the two forecasts contradict one another – let’s look at an example:

Below is the GFA valid at 23Z (issued at 2102Z) for cloud coverage along with tops and bases for the Northeast and Great Lakes. Notice that it forecasts just high cirrus clouds over a majority of Maine.

The GFA cloud forecast shows cloud coverage (color contours) as well as bases and tops. (click for larger image)

However, the legacy FA for this area shown below suggests a totally different forecast. This area forecast was amended by the FA forecaster for the eastern region at 1935Z. This forecast (highlighted below) suggests that after 21Z NW Maine is expected to have overcast clouds with bases at 2,000 – 3000 feet MSL. And NERN Maine is expected to have overcast cloud bases of 1,500 feet MSL. The forecaster also issued an AIRMET for IFR conditions covering most of the northeastern U.S.

000
FAUS41 KKCI 141935 AAA
FA1W  
BOSC FA 141935 AMD
SYNOPSIS AND VFR CLDS/WX
SYNOPSIS VALID UNTIL 151200
CLDS/WX VALID UNTIL 150600...OTLK VALID 150600-151200
ME NH VT MA RI CT NY LO NJ PA OH LE WV MD DC DE VA AND CSTL WTRS
.
SEE AIRMET SIERRA FOR IFR CONDS AND MTN OBSCN.
TS IMPLY SEV OR GTR TURB SEV ICE LLWS AND IFR CONDS.
NON MSL HGTS DENOTED BY AGL OR CIG.
.
SYNOPSIS...SEE MIA FA FOR SYNOPSIS.
.
ME NH VT
NW ME/NRN-SW NH/VT...OVC020-030 TOP FL250. VIS 3SM -SN BR. 21Z
OVC020-030. VIS 3SM -SN BLSN. WND N 20G30KT. OTLK...IFR CIG SN
BLSN WND.
NERN ME...OVC030 TOP FL250. VIS 3-5SM -SN. 21Z OVC015. VIS 3SM
-SN BR. 03Z OVC015. VIS 3SM -SN BLSN. WND NELY G25KT. OTLK...IFR
CIG SN BLSN WND.

Notice the Synopsis section simply says “SEE MIA FA FOR SYNOPSIS.” Most pilots were probably not taught that the FA has a 3,000 character limit. So, with a raging Nor’easter occurring in the Northeast, they didn’t have enough characters available for the Boston FA to provide a complete synopsis. In that case, the forecaster opted to place the Boston synopsis in the Miami FA.

For the potential of clouds in Maine, the legacy FA proved to be much more accurate than the new GFA. Most of Maine was experiencing IFR conditions as denoted by AIRMET Sierra shown here.

At this point in time, the AWC is not providing public access to some of the underlying data you may see on the webpage mentioned above. We are busy at ForeFlight trying to determine how to best incorporate these forecasts from the GFA once they become available. So stay tuned.

True Colors of IR Satellite

Now in ForeFlight Mobile 8.3, you have a choice between one of two satellite layers on the ForeFlight Map view. The legacy satellite layer was renamed to Enhanced Satellite and the new layer is appropriately named Color IR Satellite. For many, the new satellite layer will look quite familiar. That’s because it was created to generally match the infrared (IR) satellite images located within the ForeFlight Imagery view. Or you may have seen similar color images on aviationweather.gov. While there are some differences, this color IR satellite layer has a rather high glance value to depict the locations of significant adverse weather and help to locate the height of the cloud tops.

satellite-selection

The older satellite layer was renamed to Enhanced Satellite with the new layer now called Color IR Satellite.

Why another satellite layer?

Back in November 2014, you may recall that we added color to the global satellite layer. Color was added to enhance or highlight the highest cloud tops that are typically associated with significant large synoptic-scale weather systems and deep, moist convection or thunderstorms. This is especially critical when flying in regions where ground-based radar data is sparse or nonexistent. The new satellite layer takes this a step further by colorizing the entire satellite layer based on a discrete cloud top temperature (in degrees Celsius).

color-satellite-layer

The Color IR Satellite layer should be viewed along with the sky coverage markers. You will notice that many pilot weather reports of icing tend to occur in regions of yellow, green and very light blue.

As I discussed in this earlier blog post high clouds are very cold and emit less infrared radiation than warmer clouds near Earth’s surface. Satellite sensors measure this radiation and meteorologists calibrate this to appropriate temperatures. Knowing the cloud top temperature can help us determine the relative height of the cloud tops and more importantly it can help us understand when supercooled liquid water may dominate the clouds creating a nasty icing threat.

Cloud tops and icing

In this new color satellite image, purple and darker shades of blue are indicative of tops at high altitudes. At the other end of the spectrum, shades of red and orange are indicative of shallow clouds with tops near the earth’s surface.

color-scale-ir

Colors such as dark blue and purple on the left side of this scale (in degrees Celsius) represent the coldest (highest) cloud tops whereas colors on the right side of the scale represent the warmest (lowest) cloud tops.

To use the layer to determine the cloud top height over a particular region, zoom in on the area of concern in the Map view and note the temperature using the color scale above. Next, find the MSL altitude that corresponds to that temperature by referencing the local temperature aloft in that region. That gives you the cloud top height. For example, assume you were departing out of Garden City Regional Airport (KGCK) and wanted to know the height of the tops. Zooming in as shown below provides an orange color representing a temperature of approximately 0 degrees Celsius.

color-height-example

The color IR satellite when zoomed in over Garden City shows mostly orange in this area. This corresponds to a temperature of roughly 0 degrees Celsius.

Using the winds/temperatures aloft provided in the Garden City popover, find the altitude that corresponds to that temperature. Perhaps a more accurate approach is to use a tool called a Skew-T log (p) diagram like the one pictured below. Starting from the surface, work your way up the red environmental temperature line and find the first altitude that corresponds to a temperature of 0 degrees Celsius. In this case, that corresponds to an altitude of 4,285 feet as shown on the left. Additionally, the diagram confirms that saturated conditions occur below this altitude representing the presence of clouds with unsaturated conditions above. This kind of analysis will provide the necessary confidence that a climb to 5,000 feet MSL will get you on top of this cloud deck.

skew-t

A Skew-T log (p) diagram like the one shown here for the Garden City Municipal Airport is an excellent tool to help locate the cloud top height. This depicts a forecast model representation of temperature (red line) and dewpoint temperature (blue line) as a function of height.

The more important colors are perhaps shades of yellow and green and maybe even very light blue. Using the color scale below, clouds with fairly warm subfreezing cloud top temperatures are likely to be dominated by supercooled liquid water and represent a airframe icing threat.

icing-potential

The pale green, yellow and very light blue indicate regions where cloud top temperatures are in the  regime where the clouds below are dominated by supercooled liquid water representing an airframe icing hazard.

Don’t become complacent; clouds with colder (higher) tops can and do contain supercooled liquid water and may pack the threat of supercooled large drop (SLD) icing especially within deep, moist convection. However, these colder-topped clouds of darker shades of blue will normally be dominated by ice crystals or more likely be a mixed phase cloud (containing both ice crystals and supercooled liquid water). However, once ice nuclei begin to activate and ice crystals start to form in the cloud, the cloud tends to grow bigger ice crystals at the expense of supercooled liquid water which lessens the icing threat.

Masking out clear skies

As mentioned above, this layer is a close cousin of the static color IR satellite images found in the ForeFlight Imagery view. The static images show not only the temperature of the cloud tops using the same colors, but also the temperature of the surface of the earth. This can make it difficult to know where clouds exist and where the sky is clear. The main improvement is that the new satellite layer attempts to mask out regions where the sky is clear showing the map background in those regions instead of the surface temperature.

clear-vs-cloudy

Clear regions are masked out to show the underlying map below.

While this masking algorithm works a majority of the time, it can be difficult to get it right every single time simply using temperature alone. For example, anytime there’s a shallow low-topped stratus deck like the one shown below, the tops of the clouds may actually be slightly warmer than the surface of the earth courtesy of a surface-based temperature inversion. So the algorithm may have a difficult time discerning where it is cloudy or clear. So it’s important to always overlay the sky coverage markers to pick up on these issues when they occur.

bad-mask

For some low-topped stratus events, it’s not unusual for the masking algorithm to show clear skies as it did here in the Midwest. The best way to detect this condition is to overlay the cloud coverage markers or during daylight hours check the Enhanced Satellite which operates in the visible spectrum during this time.

So during the late fall, winter and early spring, give this new satellite layer a quick glance. It’ll provide you with a method to determine the tops of most clouds and to reveal where there’s a definite risk of airframe ice.

‘Tis the season for airframe ice

Now that cold air has infiltrated a good portion of North America, it’s time to review one important aspect of airframe icing, namely, precipitation type. The three basic wintry precipitation types include snow, ice pellets (colloquially known as sleet) and freezing rain (also freezing drizzle). Surface observations (METARs) and forecasts such as TAFs typically report these precipitation types based on what’s reaching or expected to reach the surface. That’s a critical element to understand. If the surface temperature is expected to be even a degree or two above freezing, you may see a forecast for rain (RA) or drizzle (DZ) in the TAF instead of freezing rain (FZRA) or freezing drizzle (FZDZ). However, just 500 feet above the ground a serious icing hazard may be lurking. So let’s take a look at the three primary precipitation types and examine the temperature profile aloft that’s common for each.

Snow

Snowflakes are just collections of ice crystals that coalesce as they fall toward the Earth’s surface. For snow (SN) to reach the surface, there needs to be a deep moist layer that is, for the most part, entirely below freezing. More importantly, the key to getting snow is that the top of this moist layer must be sufficiently cold to produce those ice crystals. While there is no definitive temperature, ice crystals begin to dominate when the top of this moist layer is -12 degrees Celsius or colder. Precipitation continues to fall as snow when the temperature remains at or below 0 degrees Celsius from the cloud base to the ground. Wet snow is the result of temperatures slightly above freezing near the surface.

snow

A typical environmental temperature profile that produces snow. Image courtesy of NOAA National Severe Storms Laboratory.

Freezing rain

There are two processes in the atmosphere that can produce freezing rain (FZRA), namely, classical and nonclassical. The classic situation is what most pilots are taught during their primary training. That is, the precipitation starts out high in the cloud as snowflakes. These snowflakes fall through a melting layer that’s warmer than 0 degrees Celsius. If the melting layer is sufficiently warm and/or deep enough, it will melt those snowflakes turning them entirely into raindrops. That rain falls into a subfreezing layer and becomes freezing rain creating a significant airframe icing hazard.

freezingrain

A typical temperature profile that produces classical freezing rain. Image courtesy of NOAA National Severe Storms Laboratory.

The nonclassical case is a bit more complex to explain, but essentially the entire process remains liquid. In other words, the precipitation high in the cloud doesn’t involve snow. This occurs when the weather system isn’t terribly deep and the top of the moist layer is at a temperature warmer than -12 degrees Celsius. Warmer subfreezing temperatures at the tops tend to prefer a liquid process over the production of ice crystals. In the non-classical case, the entire temperature profile aloft may be below freezing or may also have a melting layer. Regardless of the actual profile, the non-classical case is strictly an all-liquid process. In most situations, you’ll see a lot of tiny drops that produce a nasty freezing drizzle environment. Surprisingly, 92 percent of the cases are nonclassical based on a study done by the National Center for Atmospheric Research (NCAR).

Ice pellets

Ice pellets (PL) are similar to the classical freezing rain case mentioned above, except that the melting layer is very shallow. This doesn’t entirely melt the snowflake, and the drop retains a slushy inner core. These slushy drops refreeze as they fall through a deep layer of subfreezing air near the surface, and eventually reach the ground as hard little nuggets that bounce on impact.

sleet

A typical temperature profile that produces ice pellets. Image courtesy of NOAA National Severe Storms Laboratory.

Keep in mind that ice pellets often indicate the presence of supercooled large drop (SLD) icing aloft. While the frozen pellets will bounce right off of your aircraft while in flight (taking a bit of paint with it), they are often mixed with other forms of freezing precipitation including freezing rain especially at altitudes right below the shallow melting layer.

Here’s a little bit of ice pellet trivia. The abbreviation for ice pellets used to be PE. However, when rain and ice pellets occurred together with rain being the dominant precipitation type, the surface observation includes the term RAPE. This was deemed to be politically incorrect in English speaking countries and the abbreviation for ice pellets was then modified to PL.

So the next time you venture out this cold season, pay attention not only to the precipitation types that are being reported or forecast but also get a sense of the temperature profile aloft.

Aging Surface Observations

One of the more common concerns raised by ForeFlight customers is the age of surface observations or METARs shown within the app. They often wonder why the age of a METAR can be 60 or more minutes old in some cases. To understand why this occurs, let’s discuss how routine surface observations are taken throughout the world.

metar-age

The age shown here in the airport popover is based solely on the difference between the current time and the time the METAR was issued.

If you visit most any airport in the U.S., you’ll likely see one of two weather observing systems installed: the Automated Surface Observing System (ASOS) or the Automated Weather Observing System (AWOS). Both of these are capable of generating one or more weather reports each hour. Although these systems observe the weather nearly continuously in time, they will only generate official reports known as an aviation routine weather report or METAR when certain conditions apply.

Routine observations

For an ASOS, only one routine report is issued every hour, which is a key reason for the seemingly excessive age of these observations. If you pay close attention to the issuance time on METARs, you will notice that many routine observations are issued a few minutes before the top of each hour. Starting at 47:20 past the hour, the ASOS begins to make its routine observation. By 53:20, the hourly observation has been prepared and edited and should be ready for transmission. This routine report becomes the official hourly observation for the NWS. That’s the METAR you will see in the ForeFlight Mobile app.

It’s important to understand that the age presented in ForeFlight is based on the issuance time in the METAR regardless of when it was disseminated by the ASOS or AWOS station. Once each minute we pull down those latest observations directly from our interface with NOAA, parse them and add them directly into our database. After the METAR was issued, it is not unusual for several minutes to pass before it becomes available to ForeFlight. ForeFlight doesn’t typically receive and ingest the data until 4 or 5 minutes after this issuance time. Therefore, it’s very common that the routine observations will have an age of 4 or 5 minutes when updated. That means it’s quite normal to see an age of 64 or 65 minutes just before it gets refreshed by the latest hourly observation.

metar-refreshed

When a METAR is refreshed in ForeFlight Mobile an age of 4, 5 or 6 minutes is very common. For example, this METAR for Ellington, the METAR was updated 6 minutes ago.

An AWOS, on the other hand, typically issues three routine observations each hour or every 20 minutes. The typical interval is at 15, 35 and 55 minutes past each hour. However, you will find that these times will vary depending on the location. You may even run across some AWOS stations that operate similar to an ASOS, that is, one routine observation an hour.

SPECIs

If the weather is changing rapidly for the better or worse, special observations (SPECIs) are issued in addition to the routine hourly observations and include operationally significant changes to elements like wind direction, wind speed, ceiling height and visibility just to name a few. Given that the ASOS relentlessly measures the weather and could inundate pilots with more frequent special observations than a human observer, the system is purposely throttled to provide SPECIs only at 5-minute intervals. This is to limit the number of observations that can be transmitted during the hour when the weather is changing rapidly. Like the routine observations, SPECIs will also take several minutes to appear in ForeFlight after it is issued.

1-minute weather

Before you depart or when you approach an airport, it’s common to listen to the local weather broadcast over the dedicated ground-to-air frequency. This broadcast is referred to as the 1-minute weather. You can also get the latest weather by calling the stations dedicated telephone number. In either case, this automated weather is often more up to date than what you’d get over ATIS or via ForeFlight. At the moment, ForeFlight only provides the latest official observations that are disseminated in the form of a METAR or SPECI. In other words, we don’t currently provide the 1-minute weather you’d get over the phone or on the radio broadcast.

airport-wx-freq

You can find the frequency and phone number for the local ASOS or AWOS on the Airports view under Weather and Advisory tab.

Of course, all pilots want the latest and greatest information. However, that does not necessarily mean an hourly observation that’s 30 or more minutes old should be considered stale. In fact, if the weather hasn’t undergone an operationally significant change, the latest observation is likely still very representative of the weather at the airport.

Range of usefulness

You can’t talk about age unless you also wrap in a discussion about the range of usefulness of an observation. It’s not unusual for many pilots to assume that a particular observation is useful as far as 20 or more miles from the airport. That may be the case when the weather is fairly homogeneous across a large region. But in most situations, making that assumption can get you into trouble.

These official surface observations are taken to be representative of the weather within the terminal area. The terminal area is defined as the circular region within 5 statute miles from the center of the airport’s runway complex. In other words, they are point observations. Notice in the table below that many of the parameters reported in a METAR are valid only within 1 to 3 miles of the airport. So there are no guarantees that the weather is similar to what’s shown in the observation as you get outside of the terminal area.

table-validity-asos

This table defines the representative range from the airport of the various weather elements provided by the observing system.

So the next time you look at the age of latest surface observation don’t discount its operational value. When the weather isn’t changing all that rapidly, a single update each hour will be the normal case for many reporting stations throughout the world.

Why Use Convective Outlooks?

Perhaps one of the most underutilized weather products shown on the ForeFlight Map view are the yellow-shaded polygons called convective outlooks. On any given eight-hour shift, they are issued hourly by a highly trained meteorologist at the Aviation Weather Center (AWC) in Kansas City. In fact, convective SIGMETs shown by a red-shaded polygon are also issued by this same forecaster.

wst-outlooks

Convective outlooks, shown in yellow, can be displayed by picking the AIR/SIGMET/CWAs menu selection. Tapping on the TS button will display all convective SIGMETs as well as any convective outlooks.

Let’s start with convective SIGMETs

Convective SIGMETs (WSTs) define regions of airspace with active areas of thunderstorms that meet specific criteria. The important word here is active. In other words, convective SIGMETs represent more of a NOWcast for thunderstorms than a forecast. Here’s the way it works. Each and every hour the convective SIGMET forecaster at the AWC looks for thunderstorms throughout the lower 48 United States and coastal waters that meet specific criteria. A single cell pulse thunderstorm isn’t necessarily hazardous as long as you don’t fly through the same airspace that it occupies. However, when thunderstorms form long lines, are clustered close together in widespread areas, are embedded or severe, they become more of a threat to aviation and the forecaster will issue a convective SIGMET for those areas of thunderstorms at 55 minutes past each hour.

wst-siriusxm

A convective SIGMET outlined in red for a line of embedded thunderstorms as depicted from the SiriusXM satellite weather broadcast.

Despite the fact that convective SIGMETs are valid for two hours when issued, the following hour the forecaster will once again evaluate the convective threat and issue a new round of convective SIGMETs. Each new issuance at 55 minutes past the hour will supersede the previous set of convective SIGMETs. Effectively, no convective SIGMET will ever exist for two hours.

This is not to say that you must fly around convective SIGMET areas. For a convective SIGMET to be issued, the area of convection must contain significant radar echoes that fill a minimum of 40% of the area at least 3,000 square miles or 40% of a line of at least 60 miles in length. This leaves a fair amount of airspace to navigate through some convective SIGMET areas.

What about convective outlooks?

First, they are not “outlook SIGMETs” as I’ve seen them called. In fact, they are not SIGMETs at all. Unlike convective SIGMETs, convective outlooks are truly forecasts; there isn’t a requirement that active thunderstorms exist when they are issued. Instead, they define larger regions of airspace that are expected to contain thunderstorms that meet convective SIGMET criteria in the next two to six hours after the outlook was issued. These may include ongoing areas or lines of convection covered by a convective SIGMET or they may include new areas or lines of thunderstorms that are expected to develop and reach convective SIGMET criteria in the two to six hours valid period.

wst-outlook

A convective outlook is outlined in yellow. This shows the region where convective SIGMETs are likely to be issued within the next two to six hours. The text of the outlook provides the effective time.

That two to six hour window is a perfect “sweet spot” for many of us making flights. There may not be any thunderstorms when you go to depart, but if your proposed route takes you through one of these convective outlook areas in the valid time specified you may see one or more convective SIGMETs issued within this outlook area during your flight.

outlook-with-cwa

When convection doesn’t quite meet convective SIGMET criteria you may still see a Center Weather Advisory (CWA) issued for thunderstorms as shown in this image. CWAs are issued by meteorologists at the Center Weather Service Units and coordinated with forecasters at the Aviation Weather Center.

What about ADS-B or SiriusXM?

At the moment, convective outlooks are not broadcast over the ADS-B ground stations and are not part of the SiriusXM satellite weather broadcast. In ForeFlight, we attempt to preserve the latest convective outlooks until they expire six hours later. So be sure to use the Pack feature of ForeFlight prior to departure.

Tips On Using SiriusXM Satellite Weather In ForeFlight

With the release of ForeFlight Mobile 8.1 you now have the opportunity to use the best portable en route weather system available courtesy of our partnership with SiriusXM Satellite Radio. The SiriusXM Pilot for ForeFlight subscription tier has been uniquely designed to provide all of the essential weather data during every phase of flight. In fact, within about 15 minutes of turning on the SXAR1 and connecting to the ForeFlight Mobile app, you’ll have seamless access to a comprehensive set of weather products well before you close the door on the cockpit and depart. Here are some of my tips to safely use this unique collection of weather data.

Hurricane Hermine

SiriusXM radar depiction of Hurricane Hermine as it approached the Florida coast in early September.

The SiriusXM source label

Knowing the source of the data you are using is paramount since weather data ages quickly. When connected to the SXAR1, you’ll see a SiriusXM label under the tappable timestamp button in the upper left of the Map view. Moreover, every weather product provided through the SiriusXM broadcast includes a source label in parentheses along with its relative age like the one depicted in the image below. This is similar to the ADS-B label shown when connected to Stratus. While connected to the SXAR1 in flight, always be sure to check for the presence of the SiriusXM label. Seeing this label will confirm that you are using the most current weather available.

siriusxm-tag-taf

Products received from the SiriusXM broadcast and displayed in ForeFlight will be labeled with a SiriusXM tag along side the product’s age as shown here for a terminal aerodrome forecast (TAF) for the Cape Girardeau Regional Airport.

Lightning

During the warm season, lightning from ground-based sensors is perhaps one of the most critical weather elements to have available in the cockpit. Any area of weather that includes lightning means there’s a darn good chance you will encounter severe or extreme convective turbulence in and around that weather. While most of the serious thunderstorms will be included within the boundary of a convective SIGMET, not all thunderstorms will meet convective SIGMET criteria. Moreover, thunderstorms often occur outside of these areas, especially during a rapidly developing convective event.

Lightning is broadcast over SiriusXM every five minutes and provides pilots with a birds-eye view of where the truly nasty convective weather is located. Moreover, both cloud-to-ground (CG) and intracloud (IC) lightning are part of this broadcast. It’s quite important that both types are included since many severe storms are often dominated by IC lightning.

With SiriusXM not every lightning strike is broadcast. Instead, a single lightning symbol is shown anytime one or more strikes have occurred within a generous 0.5 nautical mile grid. So when you pinch-and-zoom way in on the ForeFlight map as shown below, you’ll notice the lightning bolt symbols are aligned in this 0.5 nautical mile gridded pattern. ForeFlight retains the most recent 10 minutes of lightning data which tends to align with the most recent radar depiction very well.

SiriusXM Lightning grid

A zoomed-in view of SiriusXM lightning reveals it’s gridded nature.

Lightning is detected even in regions where radar coverage is not present. This can be extremely useful when flying outside of the NEXRAD radar coverage area. You’ll see lightning depicted in regions over the Gulf of Mexico and Caribbean as well as the coastal waters of the U.S. in the western Atlantic and eastern Pacific Oceans. It will also include lightning in Canada, Mexico, Central America and the northern-most regions of South America. Although there is SiriusXM NEXRAD coverage provided around Puerto Rico and the U.S. Virgin Islands (using the base reflectivity from the lowest tilt), having lightning shown in other locations in the Caribbean will help pilots avoid the nasty tropical convection that occurs in these highly traveled areas where there isn’t NEXRAD coverage.

lightning-south-america

SiriusXM radar coverage is available using the base reflectivity layer from the lowest tilt around Puerto Rico and U.S. Virgin Islands. You will also see lightning depicted outside of the standard NEXRAD coverage area as far south as the northern portions of South America.

Storm attribute markers

Pilots have become accustomed to seeing echo top heights and storm track identification markers in ForeFlight. With SiriusXM you’ll get those same NEXRAD storm attributes. This includes a generic storm marker with an echo top height shown in 100s of feet in addition to cells that have signatures of hail, mesocyclone and tornadoes using the symbols shown below. Echo top heights are only shown for tops 20,000 feet and higher.

markers

Storm attribute markers include hail, mesocyclone and tornadic vortex signature. Under the settings, these SiriusXM Storm Markers can be switched on and off as desired.

In most cases these storm attribute markers will also contain a direction and speed of the cell being tracked. Similar to the other storm tracks you will see depicted on the radar mosaic in ForeFlight, SiriusXM tracks will contain an arrow showing the direction of movement as well as the speed. If the cell is moving at a speed of more than 10 knots, you’ll also see two black dots depicted on the arrow that loosely estimates the position of that storm cell in 20 and 40 minutes based on the cell’s current speed and direction provided. The arrowhead represents the estimated location of the cell in 60 minutes.

Confusing Storm Attribute Markers

During a rapidly developing convective event or when thunderstorms are dissipating, it’s quite common to see the storm tracks for adjacent cells point in opposite direction.

While these markers provide additional information about a storm cell, keep in mind that there will be times when the storm tracks for adjacent cells may provide conflicting information as you can see in the example shown above. It’s unlikely these cells are actually moving toward each other. This typically occurs during the initial stage of thunderstorm evolution especially when there’s an area of rapidly developing convection. Animating the radar is perhaps the best way to note the direction of movement of an area of weather.

tvs-hermine

Shown here are several storm attribute markers to include mesocyclone circulation and tornadic vortex signatures from Tropical Storm Hermine as it passed off the coast of South Carolina.

Radar layers

The SiriusXM composite reflectivity and base reflectivity from the lowest tilt have the same 2 km horizontal resolution as you may have experienced with the regional radar broadcast provided by ADS-B. On the left is the regional composite reflectivity mosaic broadcast by ADS-B using the Stratus 2 receiver. On the other hand, the right side is the SiriusXM mosaic just a minute earlier. While the mapping of dBZ levels to color may be a little different for the two composite reflectivity sources, the overall spatial resolution is the same.

ads-b-vs-siriusxm

Regional composite reflectivity from ADS-B shown on the left and composite reflectivity from SiriusXM shown on the right. Both have a similar resolution.

There’s no doubt that the overall qualitative glance value is practically the same between the two radar depictions above. You’ll find, however, that the latest SiriusXM broadcast will be about 5 minutes fresher on average than what you get through ADS-B.

Partial radar refresh

You may occasionally notice that both of the radar mosaics may take a short period of time to completely refresh the Map view for the entire radar coverage area when a new NEXRAD broadcast is being processed. During the refresh, it will be common to see “Radar not available” briefly depicted over regions where coverage is normally provided as shown below for the base reflectivity mosaic from the lowest tilt.

Partial refresh

Partial updates to both the composite reflectivity and base reflectivity from the lowest tilt should be expected when the newest radar broadcast is being processed.

This is because radar data received by the SXAR1 rarely comes as a continuous frame of data. Often this data is broadcast in blocks over a short period of time. This is especially true for the base reflectivity mosaic from the lowest tilt. To avoid holding back the entire radar mosaic until every single byte is received, we decided to provide the newest radar in pieces as it arrives. Whether or not this occurs and how long it takes to provide a complete picture, depends on the amount of radar echoes throughout the entire coverage area. During times of high convective activity or large-scale precipitation, expect the refresh to be a bit slower, typically 20 to 30 seconds.

If you believe in Murphy’s Law, this refresh delay will rear its ugly head at the most inopportune time. If the refresh takes uncomfortably too long while in flight, you can always switch to the other radar depiction in the short term.

Also includes Canada

Unlike ADS-B, the SiriusXM radar depiction from the lowest tilt does include Canadian Doppler radar information as well (Canadian radar is not included in the composite reflectivity mosaic). You won’t see any storm tracks or echo tops depicted by Canadian radar data, but this does extend the radar coverage to the southern most part of Canada for those pilots that fly to this area frequently. In addition to radar, you will see winds and temperatures aloft depicted in Canada as well as METARs, TAFs and PIREPs.

Winds and temperatures aloft

The winds aloft layer is populated by model-based winds (not observations) from the SiriusXM broadcast. These are an accurate representation of the current winds at 3,000 ft MSL up to FL480 at 3,000 ft intervals. This is a similar presentation to what you will find with the winds aloft layer when connected to the Internet. Tapping on any wind barb will provide the wind direction, wind speed and temperature at the altitude selected.

winds-aloft

While in flight, you will see updates to the current winds once each hour. At this time there are no forecasts of winds aloft provided through SiriusXM valid beyond the current time. Consequently, the SiriusXM winds are not used in performance calculations, so you should anticipate using the pack feature to have an estimation of winds aloft along your route while in flight.

Getting The Lowdown On ForeFlight Radar

In the pilot world there is a ubiquitous debate that continues to thrive over what ground-based radar product is better to use – NEXRAD composite reflectivity or NEXRAD base reflectivity from the lowest elevation angle. Without question, both of these radar mosaics provide a high glance value to the pilot to highlight the location and movement of the truly nasty adverse weather along your proposed route assuming you understand each of their inherent limitations. Now in ForeFlight Mobile 7.7, you’ll have the opportunity to wrangle over which is best since we’ve added a high resolution base reflectivity layer from the lowest elevation angle to complement the current composite reflectivity layer within the app.

But wait…there’s more! In addition to this new layer, we now offer two new low resolution NEXRAD mosaics, namely, a composite reflectivity and lowest elevation angle base reflectivity layer. These two four-color ground-based radar mosaics comply with the dBZ-to-color mapping standards defined by the Radio Technical Commission for Aeronautics (RTCA) documented in Table 3.2 of DO-267A. More on these later.

Radar-Selection

You can now select from one of two radar mosaic depictions in ForeFlight Mobile. The selections include Composite reflectivity and reflectivity from the lowest elevation angle or Lowest Tilt.

Base does NOT equal lowest

First, let’s squash a misnomer about base reflectivity. Many pilots (and even weather professionals) may use the term “base” in base reflectivity to imply lowest. That’s not what it means. In fact, every elevation angle generated by the WSR-88D NEXRAD Doppler radars has a base reflectivity product. The amount of energy directed back to the radar is measured and recorded in a logarithmic scale called decibels of Z (abbreviated dBZ), where Z is the reflectivity parameter. Next, these base data returns are processed by a radar product generator (RPG) to produce hundreds of meteorological and hydrological products including a few near and dear to pilots such as reflectivity.

A more accurate description would be to prefix the product with the elevation angle such as “0.5 degree base reflectivity.” Nevertheless, you may see labels like “Composite Reflectivity” and “Base Reflectivity” on various public and subscription-based websites including those from NOAA. It’s likely that the base reflectivity is from the lowest elevation angle (or lowest tilt) of NEXRAD radar. That’s because the lowest elevation sweep is most representative of precipitation that is reaching the surface which is helpful to the average person on the street including hikers, golfers, boaters and anyone else who wants to know if they need to take the umbrella to work. Unfortunately, the elevation angle is usually dropped (likely due to ignorance or brevity) from these labels.

CompositevsBase-Animation

This is an animated comparison of the composite reflectivity and lowest elevation angle for convection in Florida. Notice the composite reflectivity provides a larger footprint since it picks up on the ice crystals that make up the cirrus anvil.

You might be surprised to learn that in many locations across the U.S., the composite reflectivity image you study before or during a flight is largely made up of only three or four of the lowest 14 elevation scans of the radar.  So in these areas the composite reflectivity and base reflectivity from the lowest elevation angle are not all that different. These areas include regions where the NEXRAD coverage is sparse. Which surprisingly doesn’t only occur in the western U.S. Places such as my home town of Charlotte, North Carolina have distinct gaps in radar coverage.

Radar to the max

Each NEXRAD radar makes multiple 360° azimuthal sweeps at increasing elevation angles from 0.5° to 19.5° depending on the current mode of operation. The number of elevation angles (or tilts) depends on the scanning strategy or Volume Coverage Pattern (VCP) of the individual radar which is set by the radar operator that is located at the local weather forecast office that monitors and manages that particular radar site. A composite reflectivity image considers the base reflectivity from all of the most recent sweeps at each elevation angle and shows only the maximum reflected energy in the vertical column above each location within the radar’s effective coverage area.

It’s all about range

With respect to ground-based radar, range or distance is the key.  Even though the lowest elevation angle is only 0.5°, at 124 nautical miles away the center of the radar beam is already nearly 17,000 feet above the surface due to the curvature of the earth.  So it is easy to see how the higher elevation angles may easily overshoot precipitation that is not in the immediate vicinity of a radar site. Moreover, even if the beam is low enough to see the storm, it may still overshoot the precipitation core.  Let’s take a look at an example.

Below is a two-image animation from the NEXRAD located at the Greenville-Spartanburg Weather Forecast Office in Greer, South Carolina. This shows the returns received from the lowest elevation angle or lowest tilt of the radar which is 0.5° and the fourth elevation angle which is only 1.7° (remember that 19.5° is the maximum elevation). Notice the radar at the lowest elevation has identified an area of weather over Fayetteville, North Carolina (seen on the far right). This cell is approximately 150 miles away from the radar site in Greer (on the far left). However, given it’s distance from the radar, the 1.7° elevation scan completely overshoots this area of precipitation. That means the composite reflectivity image in the Fayetteville area is likely made up of only the lowest three elevation angles of the radar. The remaining higher 11 elevation angles overshoot the precipitation in this region.

Angles

This two-image animation from WDT’s RadarScope app shows the base reflectivity from the 0.5 degree and 1.7 degree elevations. The NEXRAD radar producing this image is located in Greer, SC on the far left. Notice that some returns farther from the radar completely disappear as the radar beam overshoots the weather entirely.

Now it’s true that other adjacent radars such as the one from Raleigh Durham, North Carolina might be able to see this area of weather at higher elevation angles. However, due to the curvature of the earth, the radar beam from the highest elevation angles often overshoots much of the precipitation out there unless it is close to the radar site. This means that locations where there is little overlap between adjacent radars, expect the composite reflectivity image to be very similar to the base reflectivity image for the lowest elevation angle in these gaps.

The four-color radar

If you are flying with airborne radar, you may want to look at the new low resolution four-color NEXRAD mosaic now available in ForeFlight Mobile. The colors depicted in this radar mosaic match the standard color-to-dBZ mapping defined by the RTCA as documented in Section 3.8.2 (Table 3-2) of RTCA DO-267A (shown below). This standard is also used for airborne radar displays.

RTCA radar

This is Table 3.2 of DO-267A that defines the color-to-dBZ mapping for airborne radar.

To see the four-color radar depiction, simply select one of the two radar layers on the Map view. Then tap the gear button next to the Map mode button and scroll down the Settings window until you see the setting switch labeled Four-color Radar just above the Radar Opacity slider. Tapping on the right side of this switch will change the radar depiction from the high resolution radar mosaic to the four-color mosaic. You can also find this four-color switch in the general Map View settings.

4-color setting

The four-color radar switch is located in the general Map View settings or can be found under the gear button at the top of the Map view.

If you use the Stratus (FIS-B) to receive weather while in flight, you won’t find the capability to select the lowest tilt, but you will find the four-color radar will also be available for the composite reflectivity mosaic. As you can see below, the four-color radar mosaic (second image) provides a much more ominous depiction of the weather as compared to its higher resolution counterpart (first image).

Stratus-High-Res

Normal resolution radar mosaic from FIS-B (Stratus).

Stratus-4-color

Four-color radar mosaic from FIS-B (Stratus).

The reason for this may not be obvious. The data broadcast for FIS-B radar does not specifically include the raw dBZ values. Instead it uses intensity encoded values or “bins” that map to dBZ ranges as shown in the table below. Notice the wide 10 dBZ ranges for intensity encoded values of 2 and 3. Based on the RTCA standard defined in the table above, these are mapped in the ForeFlight four-color radar to green and yellow, respectively. Red is mapped to intensity encoded values of 4 and 5 with magenta mapped to 6 and 7. Because of the wide ranges as they map to the RTCA standards, the four-color radar depiction from FIS-B will use much “warmer” colors than the standard depiction.

Intensity-To-dBZ-Mapping

This table from RTCA DO-358 defines the intensity-to-dBZ mapping for FIS-B radar broadcasts. The intensity encoded values of 0 and 1 are considered background and are not displayed as a color. ForeFlight chose to use magenta for intensity encoded values of 7.

Keep in mind that the four-color radar mosaic is a low resolution depiction and will not emphasize storm characteristics like you may see with the Internet radar. This is especially true for the initial evolution of convective cells.

New International Forecast Weather Imagery Available in ForeFlight

Good news for our international customers and flight planners – ForeFlight’s Imagery view now provides a greatly expanded collection of international forecast and weather products, covering South America, Europe, and the Atlantic and Pacific oceans, in addition to the Canadian and Mexican imagery that has been available. Products include precipitation and cloud cover, temperature and wind for FL050 and FL180, prog charts, and SIGWX charts.

Tap the Global tab at the bottom of the menu to access the new imagery

The new imagery can be found in the Imagery view by tapping the Global tab at the bottom of the left-hand menu. Don’t forget that you can save a given product as a favorite by tapping the star in the top-right while viewing it, and tap the Send To button in the bottom-right to save it directly to your device, share it via email, or copy it to your device’s clipboard.

Tap the Send To button to save the image to your device, email, or copy it

ForeFlight Goes Vertical at Heli-Expo

2016 Heli Expo hero rev2

Before the horses come to town, there will be helicopters! For the first time HAI Heli-Expo is coming to Louisville, Kentucky and Team ForeFlight will be on hand in Booth 6736 to demonstrate all the latest features that help your pilots fly safer and your flight department stay in sync.

ForeFlight’s hazard awareness features make it easy to comply with Part 135 obstacle height regulations. Profile view, hazard advisors, and Synthetic Vision allow you to quickly evaluate and plan your route, without any surprises. Make informed decisions with better weather, distribute company documents, manage iPad deployments, and make sure you’re flying with the most up-to-date charts.

Still haven’t taken the plunge to attain EFB approval or need help managing your current program? We are here to help—schedule a time to chat or drop by the booth to learn about our comprehensive EFB solution. We can also provide a majority of the paperwork you will need for your FAA approval package in our EFB Documentation Kit. FAA 8900.1 guidance has come a long way and we have several customers who have shared their experience with EFB deployments. You can check out this valuable insight here.

Also, don’t miss our resident Weather Scientist, Scott Dennstaedt, who will be discussing low-level wind shear and how to identify and avoid this dangerous hazard. This seminar will earn you FAA Wings credit and is part of the Rotor Safety Challenge. Attend six of the scheduled safety sessions and receive a certificate of recognition as well as valuable knowledge.

Minimizing Exposure to Low-Level Wind Shear – Presented by Scott Dennstaedt
Mar. 1, 2016 | 2:15 pm – 3:15 pm | Room C201 | pre-register for FAA WINGS credit
Mar. 2, 2016 | 3:30 pm – 4:30 pm | Room C104 | pre-register for FAA WINGS credit

See you there!