About Scott Dennstaedt

ForeFlight Weather Scientist, CFI and former NWS meteorologist

Surface Wind Analysis Now Available with SiriusXM

In ForeFlight Mobile 9.0 we’ve added a high resolution surface wind analysis to the list of map layers you can display through the SiriusXM satellite weather broadcast. This new product includes both windspeed and direction presented as wind barbs similar to the winds aloft layer. Tapping on any wind barb will show the specific details.

The surface wind analysis layer broadcast by SiriusXM will provide an overview of the general circulation of the prevailing wind about 10 meters above the surface. Tapping on any wind barb will display the valid time as well as the windspeed and direction at that location.

Two surface wind layers?

Yes, there will be two surface wind layers when connected to the SXAR1. The layer you have been using in prior releases and the one you can view when connected to the Internet is strictly based on surface observations from the various weather reporting stations around the world (typically airports). This depicts the actual wind reported in the routine observation (METAR) or special observation (SPECI). The surface wind layer is depicted at weather stations as colored wind barbs; at this point in time the wind markers shown include the gust factor.

The surface wind layer that is based on observations is shown as wind barbs color-coded based on the observed wind speed at weather stations.

The new surface wind analysis layer is not observed data from weather stations, but instead is generated by a forecast model, and therefore, completely automated. It’s only available when connected to the SXAR1 and shows an analysis of the prevailing wind at 10 meters above the surface; it does not include the gust factor. Unlike the observed data that is updated when new observations are taken, the surface wind analysis is updated once every hour.  When refreshed, this will provide wind data that will be valid at the top of the previous hour.

The surface wind analysis broadcast by SiriusXM shows low level atmospheric circulations very well as seen here as a Nor’easter deepens over the Delmarva Peninsula.

The primary value of this new layer is to show low level circulations at the synoptic scale level. This will point out high (clockwise) and low pressure (counter-clockwise) circulations as well as lines of convergence in the vicinity of strong frontal boundaries. This is difficult to see with the coarse network of observing sites throughout the U.S. But with the high resolution surface wind barb analysis, these circulations and convergence zones show up nicely.

 

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.

ForeFlight Tops it Off with Two New SiriusXM Weather Layers

If you surveyed a group of IFR pilots, tops are likely one of the most requested features. Now, the wait is over. With ForeFlight Mobile 8.3, you can view both echo tops and cloud tops when connected to the SXAR1 SiriusXM satellite receiver. These two sought-after weather products are now included with the current ForeFlight Mobile SiriusXM pricing tier at no extra cost.

tops-selections

You can find the echo tops and cloud tops selections in between the radar and satellite layers in the ForeFlight Mobile app.

Cloud top height

First and foremost, the cloud tops depiction from SiriusXM is not a satellite image per se. Instead it depicts the height of the cloud tops in reference to mean sea level (MSL). Second, the cloud tops overlay does not infer the depth of the cloud layer. Consequently, a high overcast cirrus deck at 30,000 feet may mask one or more cloud layers below. Third, not all cloud layers may be shown, especially when there are regions of low-topped stratus or scattered to broken fields of fair weather cumulus clouds. So it’s important to always overlay the sky coverage markers to augment the cloud tops layer.

lowstratus-tops

Here’s a common limitation during a low-topped stratus event. Notice that the sky coverage markers around Houston, Texas indicate the presence of overcast skies, however,  the cloud tops layer shows the sky as clear. 

The cloud tops layer is always valid in the recent past since it’s based on observed data. It is typically updated with a new image once or twice an hour. Tops above 25,000 ft MSL are color-coded using blue, orange and red to visually enhance the highest tops. Tops below 25,000 ft are shown as simple shades of gray.

echo-tops-layer

The echo tops layer (left) may appear to look like a radar depiction (right) from a color perspective, However, it has a much lower spatial resolution than the composite or lowest tilt radar mosaic.

Echo top height

Like cloud tops, echo tops depict a height above mean sea level so it’s not a radar depiction per se. Simply put, echo top height is based on the highest elevation angles at which greater than 18 dBZ reflectivities are detected. Keep in mind that echo tops are primarily used by meteorologists to identify more significant storms by locating the highest tops. So it’s important know that echo tops are not the same as cloud tops. The actual top of the cloud is always higher than the echo top.

echotops

In this vertical cross-section of a thunderstorm, reflectivity is shown using colors similar to what you would see on a NEXRAD mosaic. Dark blue represents a reflectivity of 15-20 dBZ. So, the echo tops are likely found near the top of the dark blue regions on this image.

Filtering by altitude

On the ForeFlight Map view, both the echo tops and cloud tops can be filtered by altitude. When selecting either one of these layers, an altitude selector similar to the one that appears with the winds aloft layer is shown. This provides a quick way to determine tops that are above a selected MSL altitude. Initially, the altitude selector will be positioned at the lowest setting, namely, 0 feet MSL. This is the selection that will show all cloud or echo tops. Setting the cloud tops altitude selector to 10,000 feet, for example, will remove any clouds with tops below this altitude leaving only clouds with tops above 10,000 feet. Therefore regions without tops data are regions without clouds or tops that are below the selected altitude.

cloud-tops-filter

The altitude selector allows you to filter all of the cloud tops (or echo tops) below a specific altitude. In this example, all cloud tops below FL300 are removed leaving only those tops above that altitude. For convective tops, it’s also a good idea to overlay the lightning layer.

Echo top clutter

Echo tops received through SiriusXM do not go through a rigorous filter like you may see with the two radar layers. Therefore, it is normal to see echo top clutter around and near the various NWS radar sites as shown below. Typically these are not associated with real areas of precipitation and often occur during fair weather. Simply moving the altitude selector up to the next rung at 5,000 feet will remove many of these annoying areas of clutter.

clutter

Echo tops clutter showing tops below 5,000 feet will often occur around the various NWS radar sites. Here you can see clutter around the NEXRAD sites at Charleston, W. Va., Sterling, Va., Dover, De., and Mount Holly, N.J.

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.

Don’t Let Satellite Weather Get You Down

The radar depictions you see from the SiriusXM broadcast are highly filtered to provide only real precipitation areas. Ground clutter, anomalous propagation, birds, insects and such are carefully removed to provide the cleanest and most representative image. But like any process, there will be times where non-precipitation returns do not get filtered out. More importantly, you may see real areas of precipitation filtered out as well.

While rare, the latter usually occurs in regions where WSI (the weather provider for SiriusXM) implements what is called a manual gross filter. This kind of filter is the most efficient way to eliminate any clutter in large areas that are not expected to see precipitation. But when that filter is left on too long, it’ll be just as efficient at removing real precipitation from the broadcast.

zoomed-in-gross-filter

Lightning and a single hail storm attribute marker with no radar depicted.

Here’s one such example depicted above. While connected to the SXAR1 I panned the map over Texas and I saw some lightning and a lone hail attribute marker showing echo tops at 45,000 feet in north-central Texas, but no radar returns. Hmmm?

I verified that I had the Radar Composite turned on (I did) and zoomed the display out as shown below to see that there are plenty of other precipitation areas shown to the northeast and southeast of this area. Given that the area wasn’t cross-hatched with “Radar not available” why wasn’t there any precipitation shown?

texas-gross-filter

Zoomed out to show the presence of other precipitation on the radar composite.

About 15 minutes later I came back to the map to see if there was any change. Notice below that plenty of lightning and storm attributes are being depicted here in north-central Texas; however, there are still no radar returns being rendered. Given this activity, you’d expect there to be some precipitation shown when both lightning and storm tracks are present. This is a classic indication that the real precipitation in this region was being erroneously filtered.

gross-filter

This is a classic signature for a gross filter being left on too long. With the radar composite on, no precipitation is being shown despite the presence of lightning and storm tracks.

Just five minutes later, the gross filter was removed by WSI and the returns suddenly popped into existence as you can see below.

gross-filter-pulled

Once the gross filter was removed, the NEXRAD returns associated with these thunderstorms were rendered.

I took a look at the NEXRAD archives and discovered that the first precipitation developed in this region around 12:05 p.m. CDT. The gross filter wasn’t removed until 12:50 p.m. CDT. That’s 45 minutes with no radar for this area of rapidly developing and potentially severe thunderstorms. Moral of the story is to always have lightning ON and be sure the SiriusXM Storm Markers are also set to ON in the Maps Settings menu (the gear button on the Maps view). Having both of these layers on will likely expose these kinds of uncommon events.

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 most from ForeFlight radar layers

Now that ForeFlight Mobile 7.7 introduced a second radar layer to the app, what are the practical advantages of each? As I mentioned in my earlier blog post, the composite reflectivity and lowest tilt radar layers both provide a high glance value to the pilot to highlight the location and movement of the truly nasty adverse weather. But I think you’ll find that these two layers are more often similar than they are different.

Go to any pilot gathering discussing weather and you’ll likely discover a majority of pilots genuinely swear by the composite reflectivity mosaic. In fact, you may even hear a few so-called “experts” stand up in front of an audience and attempt to convince them that you should only ever use composite reflectivity. Depending on your particular flying habits and aircraft capabilities, you may find that the base reflectivity from the lowest tilt is actually more useful and accurate. However, before we get into the pertinent differences, let’s examine how each mosaic is built.

The nuts and bolts of NEXRAD

Every NEXRAD radar site throughout the U.S. scans the sky with multiple 360-degree sweeps at increasing elevation angles. It starts the process (called a volume coverage pattern) at 0.5 degrees and finishes at 19.5 degrees assuming the radar is in precipitation mode. The base reflectivity from the lowest elevation angle (called the lowest tilt) is most representative of precipitation, if any, that is falling out of the base of the cloud and reaching the surface. So the lowest tilt is what interests most of the general public so that’s what you are likely to see on various websites that depict weather radar.

The composite reflectivity, on the other hand, includes the base reflectivity from every elevation scan. Depending on the scanning strategy of the particular radar site, this could be up to 14 different elevations. The highest base reflectivity value from each of these elevations is what’s included in the composite reflectivity mosaic. Consequently, you don’t know if the reflectivity depicted is near the base of the cloud, somewhere in the middle or near the top simply by looking at the mosaic.

Cross-section

A cross section of this mesoscale convective system (MCS) provides a better indication of the altitude of the highest reflectivity in the storm. In this case the precipitation core is below 6 km or 20,000 feet.

More is not always better

One of the chief issues with the composite reflectivity mosaic is that it often has a very large footprint when compared to the lowest tilt. It tends to exaggerate the areal impact of the precipitation event making it challenging to determine where it’s safe to fly. Shown below is a two image animation over the southeastern Florida peninsula that toggles between the composite reflectivity and lowest tilt. Notice on the composite reflectivity mosaic at least one-half of the area depicts returns that are not likely to be actual precipitation falling from the sky. Most of the green contours to the northeast of Lake Okeechobee are low dBZ returns from ice crystals in the thunderstorm’s anvil and are not likely a threat to pilots flying at lower altitudes 10 or more miles from the storm, but below the anvil.

CompositevsBase-Animation

An animation comparing the composite reflectivity and base reflectivity from the lowest elevation angle (lowest tilt).

High ice water content

If you fly a turbojet aircraft in the upper flight levels, the composite reflectivity mosaic can be quite important to examine. The thunderstorm anvil like the one shown above can contain a high enough concentration of ice crystals (called high ice water content) to be a problem. These ice crystals can be ingested into jet engines causing power-loss or damage within the engine core. Engine instability such as surge, stall, flameout, rollback and damage of compressor blades due to ice shedding have been reported in these conditions. So if you are a pilot circumnavigating deep, moist convection in a turbojet aircraft, the composite reflectivity mosaic provides some indication of where the high ice water content may be located.

Down low and below

During the warm season when thunderstorms are the most common, the lowest tilt depiction is one that is useful to pilots that like to fly down in the bumpy air below the cloud deck. Typically the footprint of the areas of precipitation will be less giving pilots a cleaner image leaving behind just the cellular structure that’s most important when flying within a convective environment. Even so, it’s still important to keep your distance. Bear in mind that nasty convective wind shear often occurs below building convection or when flying near mature thunderstorms. Gust fronts from thunderstorm outflow as well as microbursts are the biggest threats especially with high-base convection.

What about the radar from my Stratus?

At the moment, the base reflectivity from the lowest elevation angle isn’t part of the ADS-B broadcast. So while en route you will only have the regional and national composite reflectivity mosaic available. The current provider of ADS-B radar does a good job removing most non-precipitation returns, however, they don’t broadcast any returns below 20 dBwhich is typically what you’d see in areas with a thunderstorm anvil.

dBZ

Here is the ForeFlight mapping of colors to dBZ levels found in the Pilot’s Guide. Notice that the first shade of green under ADS-B doesn’t start until 20 dBZ whereas the Internet scheme starts as low as 5 dBZ.

In the end, when both depictions are available as they are in ForeFlight Mobile, each radar should be given its due time during your preflight analysis.