Our Weather Imagery Has Blossomed

Browse the Imagery view in ForeFlight Mobile and you will notice big changes have taken place! In addition to some basic spring cleaning, we nearly doubled the number of collections in the USA Ensembles.

What are all these new charts?

Like anything else that’s new, it’ll take some time for you to fully benefit from all of the imagery we’ve added to the ForeFlight Mobile app. We understand that some of these new charts may be unfamiliar to many customers. Therefore, in the weeks and months to come you can expect to see us offer more insight on how to effectively use this guidance in your day-to-day preflight planning regiment. So stay tuned to Twitter, Facebook, and the ForeFlight blog for more details.

12-hour Probability of Precipitation

Shown here is one of the new forecasts included in the USA imagery ensembles called the 12 HR Probability of Precipitation or 12 HR PoP for short.

What’s with the order of the collections?

Previously in ForeFlight Mobile, the USA collections were roughly ordered alphabetically with Alaska being first and Winds Aloft positioned last. With such a large number of new collections, we want to do a little better than to simply alphabetize the weather guidance. While there is no perfect way to order these collections to meet every pilot’s needs, we implemented an order that we think you will find useful. Here’s what we were thinking…

Picture a preflight weather briefing as a funnel with large-scale features at the top of the funnel and route-specific details at the bottom.  At the top of the funnel you start out with the synoptic overview (big picture) such as the location and movement of high and low pressure systems, fronts and associated areas of precipitation and clouds. The timing of your proposed round-robin flight is often critical, so we’ve placed outlooks and long-range forecasts near the top as well to help you decide which day may provide the best opportunity to minimize your exposure to adverse weather. As you work your way to the bottom of the funnel, this will include finer route-specific details such as en route advisories to include G-AIRMETs and SIGMETs, icing, turbulence, regional satellite, ground-based radar and last, but not least, pilot weather reports.

What happened to my favorite and recent images?

We made an honest attempt to preserve all of your favorite and recent images with this update. A careful mapping was done to point to the right image even for those that were moved from one collection to another. There were also a dozen or so images that existed in more than one collection; so we removed those duplicates. If your favorite image was the one we removed, it was mapped to the other location. Nevertheless, there may be a few images that were deleted and a few favorites or recents that were not preserved. If you are having trouble finding one of those images, please e-mail us at team@foreflight.com and we are happy to track those down.

Speaking of recents and favorites, there is now a recents button for imagery on the iPad version of the app as shown below. Now you can view and swipe through all of your favorites and recents on your iPad or iPhone. These settings sync across your mobile devices. As always, don’t forget to check out the Pilot’s Guide to ForeFlight Mobile to learn more about the new imagery.

Double the Weather Imagery, FreeFlight ADS-B Connectivity, More Flexible Download Manager in ForeFlight Mobile 7.1

ForeFlight Mobile version 7.1 equips you with more weather briefing and analysis tools in the Imagery view to make better go/no-go decisions, delivers subscription-free weather and ADS-B traffic via FreeFlight Systems’ certified RANGR-series ADS-B products, and gives you more options to manage chart downloads.

Enhanced Weather Image Library

We are a company of pilots and so we understand that weather is a top priority when it comes to flight planning and decision-making. Having our own in-house Weather Scientist (Scott Dennstaedt) allows us to focus on advancing the capabilities of the preflight weather briefing tools within the ForeFlight platform. Scott and the development team have been busy thoughtfully organizing ForeFlight Mobile’s Imagery and nearly doubled the number of collections in the library. Eight new weather image sets are available and include forecasts for ceiling and visibility, convection, and precipitation. Additionally, products like icing, turbulence, and AIRMETs are enhanced with greater resolution of altitude and/or time.

This article from Scott provides an introduction to the new weather imagery collections.

Over the next few weeks we will provide more guidance and insight on how to effectively use these weather products in your day-to-day preflight planning regiment. So stay tuned to Twitter, Facebook, your email inbox, or right here on the ForeFlight blog for more details. The Pilot’s Guide to ForeFlight Mobile is also a great resource to learn more about the new weather imagery collections.

Recents button in Imagery viewWe have also added a Recents button to Imagery view on the iPad version of the app. Now you can view and swipe through all of your favorites and recents on your iPad or iPhone. For those of you that move between devices while planning, these settings sync so that you can easily pick up on your iPad where you leave off on your iPhone (or vice versa).

The new forecast weather products are available to all subscribers.

Need a refresher on those weather chart symbols? Grab the current Aviation Weather Services Advisory Circular from the Documents catalog:

Aviation Weather Services

Refresh your weather symbology knowledge with the Aviation Weather Advisory Circular in ForeFlight Documents.

Test Drive the New Prog Charts

New Prog Charts

Test drive the new NDFD Progs.

The Prognostic Charts that pilots have known and used for years are undergoing a facelift this Fall. As part of the Imagery enhancements, we are including these new charts giving you a chance to become familiar with them before they are officially released by the National Weather Service. Read this article from Scott where he walks through the new features of this helpful weather prediction chart.

FreeFlight RANGR-series ADS-B Solutions Deliver Weather and Traffic to ForeFlight Mobile

Our ForeFlight Connect program now includes FreeFlight Systems’ RANGR-series of certified ADS-B products. The FreeFlight Wi-Fi connectivity solution enables you to affordably equip your aircraft well in advance of the FAA’s 2020 mandate and immediately realize the benefits of inflight subscription-free FIS-B weather and TIS-B traffic on your iPad or iPhone. The RANGR GPS receiver also provides position source and data for the ForeFlight Mobile moving map view and instrument panel. ForeFlight Mobile is compatible with any certified FreeFlight Systems product that has the capability to receive data. This includes the FDL-978-RX, FDL-978-XVR, and FDL-978-XVR systems.

Delta Downloads and Download Manager Enhancements

We have received lots of positive feedback on Delta Downloads and the 70%-plus increase in download speed that the system delivers. With 7.1, Delta Downloads is activated on all customer accounts. In addition, the Delta Downloads infrastructure enables some helpful changes to the way US VFR charts and IFR Enroute charts can be managed in the Downloads view.

New Downloads grouping by State.

IFR High and Low Enroutes and US VFR charts are grouped by State or Province.

We have made it easier to manage these charts both as individual downloads or grouped by State or Province. Tap the rotating caret to reveal individual charts in a State or Province group.

New section header showing Packed data.

New section header showing Packed data.

A new heading—“Packed and Unselected Regions”—contains charts you have Packed, as well as those you regions you may have de-selected. The re-organization makes it easier to see charts you may want to delete or unpack.

Apple iOS 8.4 Expected To Resolve GPS Accessory Compatibility Issue

As noted in a previous blog post and via a customer notice email, iOS 8.3 introduced an incompatibility with previous generation GPS accessories like the DUAL XGPS 150 and some Bad Elf devices. The issue was escalated to Apple by ForeFlight and Bad Elf and, based on PIREPs we have received, is expected to be resolved in the forthcoming iOS 8.4 update. This issue has frustrated many pilots who own Wi-Fi only iPads or GPS accessories, and we are glad relief is on the way.

Related Articles

Our Weather Imagery Has Blossomed

Prog Charts Are Changing

Prog Charts Are Changing

The Prog Charts that pilots have been using for the last decade or two (pictured below) will be undergoing a facelift sometime in September 2015.

Old-Progs

So at ForeFlight we’re giving you the opportunity to test drive the new charts before they become operational and are officially released by the National Weather Service (NWS). We’ve added these forecasts to our USA Ensemble Imagery and you can find them under the NDFD Progs collection as shown below.

NDFD-Progs

So What’s Changing?

The current Prog Charts are issued by highly experienced meteorologists at the Weather Prediction Center (WPC) in College Park, Maryland; that won’t change. The new implementation will still use the fronts and sea level pressure (SLP) forecast issued by those same meteorologists at the WPC, however, the precipitation forecast represented by those pale green lines is being replaced. The new instantaneous precipitation forecast is now being extracted from the National Digital Forecast Database (NDFD). Instead of the green contours, you’ll see the new precipitation forecast as shaded and outlined regions like the ones shown below.

New Prog chart

Example of the new NDFD Progs.

The new NDFD Prog Charts contain a mosaic of digital precipitation forecasts issued from all of the local NWS weather forecast offices (WFOs) throughout the United States working in collaboration with the National Center for Environmental Prediction (NCEP) and WPC. The forecasts depicted combine the familiar WPC forecasts of fronts, isobars and high and low pressure centers with the NDFD depiction of expected weather type and likelihood.

The precipitation presented on the new NDFD Progs is forecast coverage just like its legacy counterpart. So it is valid at the time posted on the chart and not over a period of time. Using a color-coding, the legend in the lower left corner of the image describes the precipitation type or weather expected (rain, snow, mixed, ice and thunderstorm) as well as the likelihood (chance versus likely) that the precipitation will occur.

Precipitation type legend

Definitions for the various weather types depicted on the NDFD Progs.

We know that it’ll take some time to become completely comfortable with the new forecast depiction of precipitation, but give them a try now so you’ll be way ahead of other pilots come September.

Flooding Rains In Texas Courtesy Of An MCS

Over the last five or more years a drought of historic proportion has plagued much of Texas. In fact, the National Weather Service reported that 2011 was Texas’ driest year on record. Fast forward to 2015 and that’s hardly been the case over the last few weeks as a good portion of Texas has received more rain in the month of May than they usually receive throughout the entire year. Rainfall totals reported to exceed 20 inches have been pretty common. And to cap it all off, this past Monday a very significant rainfall event occurred throughout central and eastern Texas with more than 10 inches falling in Houston Monday night causing widespread flash flooding in the city. So what caused this extreme rainfall event?

Texas Rainfall

Rainfall totals in May 2015 for the Southern Plains and lower Mississippi Valley. Image courtesy of The Weather Channel.

The phenomenon that was responsible for this deluge of rain on Monday is called a Mesoscale Convective System or MCS. Similar to hurricanes, they are very seasonal. Occurring mostly east of the Continental Divide, they start out in the Southern Plains and Deep South during the month of May. As the jet stream moves north through the summer months of June and July, they tend to occur in the Central Plains, Middle Mississippi Valley as well as the Tennessee and Ohio Valleys. Finally, into July and August, they are seen more in the Northern Plains, Upper Mississippi Valley and Upper Great Lakes regions.

These systems are usually severe and can often produce a few tornadoes, dangerous lightning, large and damaging hail and strong straight-line winds. But perhaps the most devastating feature is the torrential rains that can fall from some of these storms since they are often long-lived weather systems. Nevertheless, these convective systems are absolutely necessary since they provide much of the needed rain for agriculture in the Midwest during the summer months.

MCS on infrared satellite

Many Mesoscale Convective Systems (MCSs) have a signature oval or circular cloud shield as seen on the color-enhanced infrared satellite image. This is the one that provided Houston with over 10 inches of rainfall in just a few hours.

Mesoscale Convective Systems are easy to spot on the color-enhanced infrared satellite found in the ForeFlight Imagery as shown above. When mature, they usually appear as a large circular or oval cloud shield that can cover one or more Midwest states with very cold cloud tops that show up on this image as purple and white. Under this cloud shield is usually a bow-shaped line of strong thunderstorms at the leading edge of the MCS as seen on this NEXRAD mosaic below.

Bow echo associated with the MCS

Often an MCS will have a bow- or crescent-shaped line of echoes which is a good sign of very intense straight-line winds.

You were probably taught that the early morning hours are the best time to fly to avoid thunderstorms. That’s usually sound advice unless you are dealing with an MCS that will often develop and mature in the overnight hours and persist into the next day. So they are often nocturnal beasts that almost seem to create their own environment to feed on.

MCS Pair

The weather system that dumped a copious amount of rainfall on Houston Monday night developed from a pair of thunderstorm complexes in western Texas early that morning. It’s unusual to see a pair of Mesoscale Convective Systems tracking along together.

In fact, the MCS that flooded Houston Monday night was born early that morning in western Texas and began as a pair of MCSs as shown above. Throughout the morning the two systems tracked east and eventually merged (below) into a single complex of storms setting the stage for a very wet evening in Houston.

MCS combined

Just after 12 p.m., the pair of Mesoscale Convective Systems joined up in central Texas to produce one massive convective complex.

This is a very common setting in the Plains where the unique geography of the region favors nocturnal and early morning thunderstorms. During the warm season, this setting promotes a strong flow of low-level moisture northward from the Gulf of Mexico, often referred to by meteorologists as a low-level jet stream. Moisture carried by the low-level jet helps to maintain these systems that often begin during daytime hours on the higher terrain in western Texas and Colorado. Because of the low-level supply of moisture, the MCS can mature and persist well into the nighttime hours.

The Skew-T Log (p) diagram for Houston Monday evening shows the low-level jet as a maximum wind speed at 6,000 feet. This moist, southerly flow keeps the surface dewpoint temperature in the low 70s to offer a good source of moisture for the MCS to ingest.

Skew T Diagram

The Skew-T Log (p) Diagram is an excellent source to visualize the moisture, winds and the instability for a particular location.

Last but not least, the Skew-T diagram shows the atmosphere was very unstable Monday evening with a lifted index of -6, Convective Available Potential Energy (CAPE) approaching 3,000 Joules/kg and a K-Index of 42. A K-Index this high is a good sign of high convective rainfall rates that can produce local flash flooding.

Webinar: Weather Flying and the iPad

Recently ForeFlight’s own Weather Scientist, Scott Dennstaedt, and Sporty’s John Zimmerman hosted a webinar devoted to Weather Flying and the iPad. In this hour long session, learn about the basics of weather, discover how to utilize ForeFlight and the Stratus ADS-B receiver for the most informed and effective weather decision-making, and see ForeFlight and Stratus in action with real-world scenarios. This webinar is geared towards making you a safer, more strategic, and informed pilot in any weather situation.

A Layered Approach In ForeFlight

In addition to all of the other great features introduced in ForeFlight Mobile 6.8, we also enhanced the app to allow more layers to be displayed simultaneously on the Map view. Many of these layers are mutually exclusive of one another. That is, when you select a new layer, it will replace the current layer (if any) that was previously selected.  For instance, the app will not allow the Radar and Satellite layer to be displayed at the same time. Therefore, with the Radar layer on, selecting the Satellite layer will deselect and hide Radar layer.

Having this dependency certainly makes sense when selecting between many of the airport-specific layers such as Flight Category, Temperature and Ceiling just to name a few. That dependency hasn’t changed. However, the Pilot Weather Reports (PIREPs) and Lightning layers are not tied to an airport and can now be overlaid individually or together with any of the other Map layers.

Multiple Overlays on Map View

When viewing the Satellite layer, be sure to select Sky Coverage, PIREPs and Lightning for the most complete picture.

This was primarily done to allow users to select the satellite layer while also displaying Sky Coverage along with Lightning as discussed in this earlier blog post. Adding on the PIREPs layer will provide an even greater situational awareness of the weather occurring at any particular location as shown above. If you were to visit the Aviation Weather Center in Kansas City, Missouri, you will notice on the image below that they also make heavy use of overlays.

AWC Overlay Screen

Forecasters often overlay sky coverage and AIRMETs over the visible satellite image.

But if you really want to go crazy, you can select up to seven layers to be displayed at the same time as shown below.  But let’s not go crazy.

Everything Selected

Adding too many layers can render the Map view unreadable.

 

Automated Ceiling Reports

Every day pilots use surface observations to make many routine operational decisions before or during any particular flight. For example, as you approach an airport you have been trained to listen to the ground-to-air radio broadcast to determine the preferred landing runway based on the current wind direction. Or even before you are in radio coverage you may have been following the latest FIS-B METAR broadcast received by your Stratus and displayed in the ForeFlight Mobile app. Many of these observations come from automated systems. So it is critical that pilots at all experience levels understand how these systems collect and process the weather data especially those automated ceiling reports.

Approach-LNS

Listen to the ground-to-air radio broadcast when approaching an airport.

The two primary automated observing systems deployed at many airports throughout the United States includes the Automated Surface Observing System (ASOS) and the Automated Weather Observation System (AWOS). These automated systems like the one shown here consist of a collection of electronic sensors that measure the environment, and then process the data to create an observation once every minute. Even though these automated systems create a completely new observation every minute, they must have adequate sensor samples to develop an accurate observation. In order to provide a representative observation at an airport, the automated hardware must continuously collect the sensor’s real-time data over a period of time. The automated system applies an algorithm to the collected data to extrapolate the weather to cover a wider area.

METAR-ForeFlight

Get the latest observation from ForeFlight Mobile before departing.

This is especially important when considering the observation for sky cover and cloud base height. When approaching an airport, for example, pilots don’t necessarily want to know what’s happening instantly over the sensor since it may not always be representative of the sky condition throughout the airport’s terminal area. Most importantly, it might vary quite a bit between reports.

Automated systems employ an upward-pointing laser beam ceilometer to determine sky cover and cloud height such as the sensor shown here. The cloud height indicator, for instance, transmits a little over 9,000 pulses in 12 seconds, but it’s not these individual samples that are used for the observation. Instead, this data is collected over a period of 30 minutes before a ceiling or sky cover observation is considered acceptable and broadcast to the pilot. So the ceiling report your see on ForeFlight or the one you hear from the ground-to-air radio broadcast is based on a sampling of 30 minutes of data and not the most recent sensor sample taken.

Airports-View-METAR

Automated report in the Airports View in ForeFlight Mobile.

Based on studies, 30 minutes of data provides a fairly reasonable description of sky conditions. This means that the system will detect and process all the clouds (if any) passing over the sensor in the past 30 minutes. To account for the latest sky conditions, the result is biased by counting the last 10 minutes of data twice, a technique referred to as double weighting. Using the last 30 minutes of data in this way will allow the system to determine the height and sky cover included in the surface observation and this becomes a reasonable estimate of the sky conditions that is valid over a three to five statute mile radius around the airport.

Keep in mind that these automated systems have a few important limitations. For instance, automated systems can only report clouds that are below 12,000 feet AGL. This means that an overcast cloud deck at 15,000 feet AGL will be reported as clear. Effectively, a clear sky report from an automated station means the sky is free of clouds below 12,000 feet AGL even though it still may be an overcast day.

Tower-LNS

Towered airport with a trained observer present.

While many high-impact airports throughout the U.S. still rely on a trained weather observer to construct the routine or special observation (SPECI), automated systems supply them with uniform and objective data for the observation. However, automated systems measure only the weather that passes directly through the sensor array so it is not able to report what’s happening outside the airport’s runway complex. Trained weather observers can certainly augment the observation to add these details such as clouds with bases above 12,000 AGL. By the way, the NWS is looking to extend the capabilities of the ceilometer to automatically report clouds above 12,000 feet, but don’t hold your breath; it may take several years before such a solution becomes available.

Fooling Around With Convective Wind Shear

Most pilots equate wind shear to turbulence or convection. Certainly some forms of wind shear are indeed turbulent and convection can also induce dangerous wind shear. If the wind shear occurs close to the surface when the aircraft is landing or departing, it may be in excess of the inertial capabilities of the aircraft making it difficult for the pilot to recover resulting in an accident that is often fatal. But don’t be fooled; some of the most dangerous wind shear situations come out of rather benign-looking convective environments.

Moist low-level outflow from a thunderstorm.

Moist low-level outflow from a thunderstorm.

Perhaps the most dangerous wind shear occurs in a convective downburst where downdrafts in thunderstorms have been estimated to be greater than 100 miles per hour. These high-speed winds strike the surface and spread out creating a gust front that can also be very dangerous especially if it occurs when landing or departing. When that downburst occurs in a very small area spatially (about 2 miles) it is referred to as a microburst similar to the one in this image. Most microbursts are ephemeral lasting no more than five minutes in most cases. If you are not convinced of the power of a microburst, check out this video. This demonstrates just how much energy can be driven out of a convective outflow.

At this point in time, several large turbojet aircraft have succumb to the forces associated with a microburst. The one that is especially memorable is Delta Airlines Flight 191 that encountered a microburst on approach to the Dallas Fort Worth International Airport (KDFW) in 1985. Delta Flight 191 didn’t fly through or under an intense supercell thunderstorm as you might imagine. In fact, a thunderstorm with a cloud base of nearly 10,000 feet was the culprit. High-base convection with a heavy rain signature should be of particular concern to pilots since they signal a deep mixed layer with a high lapse rate and plenty of precipitation to fuel a strong downdraft. Pilots can be fooled because high-based convection does not seem particularly threatening (especially to pilots flying large turbojet aircraft) which makes them even more likely to stumble into the path of a downburst or microburst.

Here’s a quote from a paper written by Captain William W. Melvin entitled Wind Shear Revisited, Air Line Pilot Magazine, Nov 1994.

Many pilots have been trained to avoid large supercell-type thunderstorms in the belief that this will prevent encounters with microbursts. Yet no evidence exists that any of the known microburst encounters have occurred in supercell storms. Dr. Ted Fujita and Dr. Fernando Caracena recognized authorities in this field have repeatedly emphasized that microbursts are frequently generated from benign-appearing cells. Many “experts” who disagree with Drs. Fujita and Caracena have emphasized the supercell storms with warnings of dangers of gust fronts. These so-called experts are leading pilots down the primrose path for microburst encounters.

In a high-based thunderstorm there’s typically an extremely dry environment between the cloud base and the surface. Initially, little or no rain may reach the surface. As the rain falls out of the cloud into this very dry atmosphere it evaporates quickly which causes a cooling effect relative to the air around the precipitation. Such cooling makes the air much denser, and therefore negatively buoyant, effectively creating a very intense downburst with winds that may exceed hurricane force, even approaching the speeds found in a weak to moderate tornado.

The next time you see what appears to be a very inviting situation in and around active convection, think twice about departing or making that approach to land. Microbursts can happen quickly before any visual cue may exist. Especially avoid areas where you see a circular ring of dust at the surface that may be curled around the edges or any kind of bulge (piling up) of precipitation at the bottom of an evolving rain shaft. These are all characteristics of a intense downdraft, microburst or gust front.

Density Altitude: The Secret Killer

Now that the warm season is approaching, pilots need to start planning for the secret killer, namely, density altitude. In fact, density altitude (DA) is perhaps just as hazardous as airframe icing. In an NTSB study, density altitude contributed to just as many accidents as icing shown in the pie chart below. To be fair, some of the accidents in the NTSB study that were attributed to density altitude were caused by pilots departing in an over-gross weight aircraft or using improper procedures (e.g., improper flap usage). However, every pilot needs to be aware that gross mistakes such as this are not forgiving when the density altitude is high.

NTSB-Wx-Accident-Study

What is Density Altitude?

In simple terms, density altitude is pressure altitude corrected for non-standard temperature. Therefore, if the pressure and temperature throughout the atmosphere matches the standard, then pressure altitude and density altitude are equal. Of course, during the warm season, the temperature is generally above standard in most locations throughout the U.S. creating an invisible hazard if ignored.

If you’ve ever flown out of a high-elevation airport such as Santa Fe, New Mexico (6,348 feet MSL), you have experienced the effect of density altitude. At these airports, the performance of the aircraft decreases. For fixed-wing aircraft you will experience a longer takeoff distance, longer landing rollout and reduced rate of climb. The higher you are above sea level, the lower the pressure and that means the air is less dense. This reduction in air density reduces the wing’s lift and also lowers the efficiency of the propeller or rotor.

What About Temperature?

The temperature of the air can have the same effect even at sea-level airports. As daytime temperatures begin to creep up into the upper 80s and 90s, the air becomes much less dense similar to being at that high-elevation airport. That means there are fewer air molecules in a given volume of airspace. Less air, means poor aircraft performance similar to what was described above. And of course, the combination of high temperatures at high-elevation airports can make for a serious hazard waiting to happen if unchecked.

While a pilot can determine pressure altitude in the cockpit by setting the altimeter to 29.92″, there’s no handy-dandy instrument that you can use to directly measure the density altitude. It must be calculated based on the station’s pressure (not the altimeter setting), temperature and dewpoint temperature. Not to worry, the ForeFlight Mobile app does all of the work for you.

Finding Density Altitude in ForeFlight

To find the density altitude simply select an airport from the Maps view. In the pop-over window, tap the METAR tab at the bottom and scroll down a bit to see the current density altitude for the chosen airport.

Density-AltitudeJust like any other aspect of weather, pilots need to prepare in advance. Before you close the door to the cockpit, double-check the density altitude in ForeFlight to be sure it won’t adversely affect your flight. To learn more about density altitude, please refer to this FAA safety publication.

Forecasting for the Terminal Area is Incredibly Difficult

Perhaps the most difficult forecast any meteorologist has to issue is a Terminal Aerodrome Forecast or TAF. They are the most detailed aviation forecast made available to pilots and they will be around for a long time to come. The terminal area is quite small; it is officially defined as “the area within five statute miles of the center of an airport’s runway complex.” Do you remember the world globe you used in grade school?  Well, imagine placing a single dot on that globe with a sharp pencil – that’s about the size of the terminal area. Consequently, forecasters consider a TAF a point forecast.

Let’s take a look at a specific example. Below is a TAF for the Fort Smith Regional Airport (KFSM) issued at 1736 UTC – well before any thunderstorms had formed. Notice the forecaster believes that moderate rain and thunderstorms will temporarily impact the Fort Smith terminal area between 0000 and 0300 UTC (8 p.m. to 11 p.m. EDT).

FSM-TAFIf you had also looked at the automated ForeFlight MOS forecast below within a similar timeframe, you would have seen a much different forecast. In fact, MOS did not forecast any thunderstorms or precipitation from 1900 to 0200 UTC (3 p.m. to 10 p.m. EDT). Instead, MOS predicted some gusty southwest winds with good visibility and a high scattered cloud deck. Which one provides the best guidance?

FSM-MOS

Actually, both! As it turned out thunderstorms did roll through the terminal area as predicted by the TAF a little bit after 0000 UTC as shown in the METARs below. However, the total amount of precipitation measured in the Fort Smith rain bucket for the event was a meager 3/100 of an inch.  So the primary thunderstorm cell passed through the northern edge of the terminal area with the sun low in the horizon beaming in from the west (notice CLR skies were also reported).

KFSM 250353Z 28005KT 10SM CLR 18/14 A2992 RMK AO2 SLP127  

KFSM 250253Z 27007KT 10SM CLR 19/15 A2990 RMK AO2 SLP122

KFSM 250153Z 26010KT 10SM SCT050 22/15 A2986 RMK AO2 RAE19 TSE33 P0003

KFSM 250139Z 28008KT 10SM FEW038 BKN050 21/16 A2986 RMK AO2 RAE19 TSE33 P0003

KFSM 250053Z 21012KT 10SM -TSRA CLR 24/14 A2982 RMK AO2 RAB49 TSB02 SLP093 LTG ICCC P0000 

KFSM 250008Z 21013KT 10SM TS CLR 26/13 A2980 RMK AO2 TSB02 LTG ICCC 

KFSM 242353Z 21016G21KT 10SM CLR 26/13 A2979 RMK AO2 

KFSM 242253Z 22019G26KT 10SM BKN065 27/12 A2979 RMK AO2

Fort Smith was on the southern-most edge of a fairly broken line of thunderstorms as shown on the ForeFlight Map view below.  This line of storms quickly moved in from the west along and ahead of a surface cold front. As you can see below, there was one small cell that moved through the Fort Smith terminal area approximated by the small red circle. It’s this cell that triggered the thunderstorm observation at 0008 UTC.

FSM-Radar

So it’s easy to see that Fort Smith could have been in that large gap to the northeast creating a situation more representative of the automated MOS forecast. The forecaster that issued the TAF took a little meteorological risk and felt there was a fairly reasonable chance this line of thunder would evolve and impact the Fort Smith terminal area; a gutsy move given how this broken line of storms ultimately evolved.

MOS, on the other hand, wasn’t as certain about the possibility of thunderstorms passing through the terminal area; it was leaning towards a forecast more representative of the weather within the gap. This is a good example of the “edge effect” that happens quite often when issuing such a point forecast whether it is issued by a meteorologist or automated tool. Ten miles can make a huge difference in making a good forecast or getting it wrong.