Weather Without Borders

With ForeFlight Mobile 7.4, SIGMETs issued beyond the U.S. border can now be displayed. These International SIGMETs are advisories that cover a wide range of hazards including convection (thunderstorms), severe turbulence, severe icing, tropical cyclone and volcanic ash just to name a few. In most cases these are displayed on the ForeFlight Map view as polygons similar to the way domestic AIRMETs, SIGMETs and convective SIGMETs are depicted. To help with all of these new advisories, we’ve also added the ability to filter this layer by the type of hazard.

The whole FIR and nothing but the FIR

Unlike advisories issued by forecasters in the U.S., International SIGMETs are not always well defined by the source. Occasionally the origin country may not provide the points that define the advisory area. For those situations, the entire Flight Information Region (FIR) is displayed on the Map as is shown below for a hazard within the Mexican FIR.

Entire FIR

When the source of the SIGMET isn’t specific about the exact location of the hazard, the entire FIR may be outlined in red.

Unspecified conditions

Similarly, when tapping on a SIGMET polygon, you may see “Unspecified Conditions” displayed in the title of the popover as shown below. This means the source of the advisory did not specify the details of the type of hazard. While ForeFlight will make an attempt to determine the hazard by parsing the raw text, there’s no guarantee we will be able to make that determination in every case. In these situations it’s strongly encouraged to review the raw text of the SIGMET for the details.

Unspecified Conditions

In some cases the type of adverse conditions are not specifically provided by the source government. For those situations, Unspecified Conditions will be shown. You are encouraged to read the raw text for those details.

No more clutter

Another feature added to ForeFlight Mobile 7.4 is the ability to filter the AIR/SIGMET/CWAs layer by hazard type. When this layer is displayed, you’ll notice four buttons at the bottom of the Map view labeled Ice, Turb, IFR and TS representing hazards associated with airframe icing, turbulence, IFR conditions and convection, respectively. Tapping on any of these buttons will add or remove advisories for that hazard type from the Map. For example, the Turb, IFR and TS hazards have been filtered with only the Ice hazard displayed as shown below. Please note that these selections are preserved. Therefore, if you’ve removed the layer from the Map or closed the app, the next time you view the AIR/SIGMET/CWAs layer on your device, the hazard selections you made earlier will be restored.


When the AIR/SIGMET/CWA layer is active, use the buttons at the bottom to hide or display the advisories by hazard type.

The only hazards that are never filtered are those SIGMETs issued for tropical cyclones, radioactive cloud or volcanic ash like the one shown below. These SIGMETs often persist for days or even weeks at a time once they are issued.


Not all hazards can be filtered. These include volcanic ash, radioactive cloud and tropical cyclone advisories.

More Turbulence Is Better

No, we’re not gluttons for punishment; however, the turbulence Imagery in ForeFlight Mobile has just gotten way better! Forecasts now go out beyond 12 hours to include lead times of 15 and 18 hours. This is a significant improvement to NOAA’s Graphical Turbulence Guidance (GTG-3) product that now includes an analysis and forecast for clear air turbulence as well as turbulence from mountain wave activity with a new forecast updated every hour. Whereas the lowest altitude in the earlier version of GTG originated at 10,000 feet, the new GTG product includes low-level turbulence beginning at 1,000 feet MSL with a vertical resolution of 2,000 feet that extends to FL450. If that isn’t enough, forecasts now have a higher resolution of turbulence intensities that includes the full range of classifications from light to extreme as shown below making the product even more useful to evaluate the risk of dangerous turbulence along your proposed route.

Eddy Dissipation Rate (EDR)

Turbulence intensities now include light (blue to green), moderate (green to orange), severe (orange to red) and extreme (red to dark red).

Eddy Dissipation Rate (EDR)

On the older GTG version, the legacy terminology such as light or moderate turbulence is somewhat arbitrary and based typically on the response of the aircraft to the turbulence, not the atmospheric conditions themselves. Shown in the scale above, Eddy Dissipation Rate, or EDR, is an objective, aircraft-independent, universal measure of turbulence based on the rate at which energy dissipates in the atmosphere. In other words, it is a measure of the turbulent state of the atmosphere. According to turbulence researcher, Dr. Robert Sharman, “When the atmosphere is dissipating energy quickly (i.e the EDR is large), atmospheric turbulence levels are high.” Pilots should be keenly aware that a safe turbulence penetration airspeed varies with the aircraft’s weight which Dr. Sharman quickly points out, “The implication for aircraft bumpiness depends on the size (weight) of the aircraft.”

At the basic level EDR is really an in situ calculation. That is, it is a value determined by an aircraft while in flight. However, it is not directly measured by the aircraft like outside air temperature, for example. Instead it’s determined by using a variety of data from aircraft avionics which means aircraft can (and do) automatically report EDR in flight. Since EDR is an aircraft-independent calculation, a single engine Cessna 152 and a Boeing 747 should determine the same EDR value when flying through the same atmosphere at the same time.

It’s one thing to calculate the EDR in flight, but totally another challenge to provide a forecast for this field. That’s the job of GTG-3. Dr. Sharman points out, “From the forecasting point-of-view we cannot provide a separate forecast for every type of aircraft that is out there.” So depending on the class of aircraft you are flying, there’s a need to evaluate the EDR values properly. Below shows the EDR values that correspond to light-, medium- and heavy-weighted aircraft as they loosely relate to the vertical acceleration/deceleration (turbulence) response in that class of aircraft.

EDR Light

Eddy Dissipation Rate (EDR) scale for light-weight aircraft.

EDR Medium

Eddy Dissipation Rate (EDR) scale for medium-weight aircraft.


Eddy Dissipation Rate (EDR) scale for heavy-weight aircraft.

The new version of GTG includes a forecast for clear-air turbulence often referred to as CAT. Even more exciting, turbulence that is a direct result of mountain wave activity, or MTW, is also forecast separately. While these forecasts are not meant to predict turbulence associated with deep, moist convection, they will provide guidance of low-level terrain and thermally-induced turbulence sources.

To that end, in the ForeFlight Imagery view you’ll find three different forecasts that include clear-air turbulence (CAT), mountain wave turbulence (MTW) and one that combines the two (All). Each one of these is organized into low (1,000 ft – 13,000 ft), middle (15,000 ft -FL290) and high (FL310 – FL450) level collections as shown below.

Turbulence Selections

Turbulence forecasts in the ForeFlight Mobile Imagery view are organized by clear air turbulence (CAT), mountain wave turbulence (MTW), and forecasts that combine the two (All). Each type provides low, medium and high altitude collections beginning at 1,000 ft MSL up to and including FL450.

The GTG forecasts have been approved by the FAA for unrestricted use for preflight planning as stated below on the Aviation Weather Center website:

“GTG is generated operationally at NOAA/NCEP which is supported 24 hours a day, 7 days a week. Its use is unrestricted (meteorologists, dispatchers, GA and commercial pilots, ATC, etc).”

Keep in mind that these are automated forecasts and do not have any human input like you might find with AIRMET Tango, SIGMETs for severe or extreme turbulence and Center Weather Advisories (CWAs). When used in combination with these forecaster-generated products and pilot weather reports, GTG will provide you with the ability to minimize your exposure to dangerous turbulence and find the altitude with smoothest ride.

Decoding Wind Shear

It’s early in the morning and you’re preparing to depart on an IFR flight out of Asheville (KAVL) in western North Carolina. While finishing your breakfast you open up the ForeFlight Mobile app, insert your favorite route to your destination and tap the little suitcase to pack all of the current charts, NOTAMs, fuel prices and latest weather for your flight. While ForeFlight is packing everything that you need (love that feature!), you decide to take a peek at the latest terminal forecast for Asheville (below) and see something that doesn’t look too appealing – WS020/07040KT; ForeFlight translates this initial part of the TAF to a forecast for wind shear. Ugh!

Wind shear forecast for Asheville

A terminal forecast for non-convective low-level wind shear (LLWS) as shown in ForeFlight.

So you tap on the Imagery tab and select the Graphical AIRMETs collection and see under Tango a forecast for low-level wind shear (LLWS) covering a large area from the panhandle of Maryland southwest to northern Georgia as shown below. This area impacts a good portion of your proposed route including your departure out of Asheville. Now what? Cancel the flight since you don’t want to be fooling around with wind shear in the mountains? Perhaps, but let’s take a closer look at this particular forecast and what it really means.


A forecast for non-convective low-level wind shear (LLWS) issued by the Aviation Weather Center in the form of a Graphical AIRMET (G-AIRMET). The actual magnitude of the wind shear or direction is not provided in the G-AIRMET.

Both of these forecasts identify the potential for non-convective low-level wind shear (LLWS). This is perhaps the most misunderstood weather forecasts in aviation. Pilots hear the term wind shear and immediately equate this to severe turbulence. It is not a forecast for turbulence per se and is definitely not the same wind shear you might experience in and around areas of convection since it has nothing to do with thunderstorms. So now that we know what it is not, let’s dig a bit deeper.

Define, please?

Wind shear is defined as a marked change of wind speed and/or wind direction over a horizontal plane or within a vertical depth of the atmosphere. When the wind shear occurs near the surface, it is referred to as low-level wind shear and abbreviated LLWS. Non-convective LLWS as it appears in a TAF or within AIRMET Tango (also G-AIRMETs) is primarily a form of vertical speed shear. That is, the wind is forecast to rapidly increase with height within the wind shear layer. In addition, winds may also change direction with increasing altitude within the wind shear layer – although it is primarily a forecast for a change in wind speed.

Decode, please?

Here’s the pertinent part of the coded Asheville TAF shown above:

01008KT 6SM -SHRA BR OVC015 WS020/07040KT

Decoded, this TAF suggests that between 1200 and 1600 UTC the surface winds will be 010 degrees (true) at 8 knots with a visibility of 6 statute miles with light rain showers and mist and an overcast ceiling of 1,500 feet. Easy so far? Now the confusing part. The WS code following all of this translates to non-convective low-level wind shear. The 020 following the WS code defines the depth of the wind shear layer which is 2,000 feet above ground level (AGL) in this case. Two thousand feet is the maximum depth forecast, but you may also see 005 (500 feet), 010 (1,000 ft) or 015 (1,500 ft). But it’s also acceptable to see 018 representing an 1,800 ft depth to the wind shear layer.

The remainder of the code following the forward slash defines the wind speed and direction at the top of the wind shear layer. Therefore, 07040KT translates to a wind direction of 070 degrees (true) at 40 knots at 2,000 feet AGL. Putting it all together, the winds are expected to increase rapidly from 8 knots at the surface to 40 knots at 2,000 feet AGL. This forecast also implies that winds will also shift direction from 010 degrees at the surface to 070 degrees at 2,000 feet although there’s no way to know how or where the shift occurs within the wind shear layer. Now that you are an expert decoder of the wind shear forecast in a TAF, what does it mean to you as a pilot?

Meaning, please?

As mentioned earlier, this is not a forecast for severe turbulence as many pilots might have been taught. Forecast or not, it is a common phenomenon and you may have flown through it and did not even know it was there. In fact, in the evening and overnight hours a nocturnal temperature inversion is often the catalyst for non-convective LLWS to evolve. In the nocturnal flavor of non-convective LLWS, the sky is often clear and the winds at the surface are usually light or calm. But the air in the wind shear layer remains glassy smooth. The only thing you may notice is a change of groundspeed as you penetrate the layer.

Essentially, non-convective LLWS is a river of faster flowing air just above the surface whether it occurs during the day or night. More often than not, a surface-based temperature inversion is present and as mentioned above is the primary catalyst for this form of wind shear. Normally the temperature decreases with increasing altitude. However, with an inversion the temperature actually increases with height through some depth. Meteorologists call this a negative lapse rate. The more negative the lapse rate, the more stable the atmosphere. A stable atmosphere resists or inhibits upward or downward motion keeping the potential wind shear layer near the surface from mixing. No mixing yields no turbulence.


Profile view from the ILS approach to runway 7 at Rockford, Illinois.

Beware on approach

Even though the air may be glassy smooth, imagine the case where you are flying an instrument approach with this kind of wind shear in place. Even if the wind doesn’t change direction in the wind shear layer, you will still have to contend with the change of wind speed that is increasing rapidly with height from the surface. If the wind is right off your nose and you are flying an ILS, for example, you will notice as you intercept the localizer around 2,000 ft AGL your groundspeed will be abnormally low (you have a 40 knot headwind in the case for Asheville). As you begin to track the glideslope, your groundspeed will increase since the headwind is decreasing in the descent. This means you’ll have to increase your rate of descent to keep the glideslope needle centered.

You can also imagine this being a tailwind or a direct crosswind while on the approach. So you have be constantly changing the descent rate or heading (crab angle) into the wind. Keep in mind that non-convective LLWS comes in all shapes and sizes. There’s not a one size fits all method to handle this. In most cases, this form of wind shear is not something you should fear, but it’s something you definitely need to manage. It is probably present more than it’s forecast.

Stay tuned

Besides the shear in the overnight hours discussed above, non-convective LLWS may be associated with the following: frontal passage, lee side mountain effect, sea breeze front and Santa Ana winds just to name a few. In a future blog I will discuss the meteorology behind non-convective LLWS and provide some background when this phenomenon can become dangerous.

The Changing Of The Progs

As mentioned in the ForeFlight blog back in June, the familiar prog charts pilots use every day will be changing. Hopefully you’ve had a chance to test drive these new NDFD prog charts that were introduced in ForeFlight Mobile 7.1. Beginning this morning (September 1, 2015) the precipitation forecast on these charts will now originate from meteorologists at the local NWS forecast offices and not from meteorologists at the Weather Prediction Center (WPC). For more information, you can read the official NWS notification.

Still a forecast for precipitation coverage

The precipitation shown on the new chart represents an instantaneous precipitation forecast. That is, it shows expected likelihood and coverage of precipitation (including type) at the valid time on the chart.  Essentially the light shaded areas define at least a 15% chance and dark shaded areas define a 55% or greater chance of precipitation reaching the surface in that period. A legend in the lower-left corner designates the likelihood of precipitation (chance versus likely) as well as the precipitation type (snow, rain, mix, thunder, etc.). Nevertheless, the isobaric forecast along with high and low pressure centers and a forecast for the position of surface fronts will continue to be issued by the same meteorologists at the WPC.

Prog Chart Change

Legacy prog charts (left) are being replaced with the new NDFD Progs (right).

For better or for worse?

It goes without saying that not every change is necessarily an improvement. It’s not that the other precipitation forecasts were bad; however, given that the precipitation forecast on this new chart is generated by meteorologists at the local forecast offices, it will be more consistent with the terminal forecasts (TAFs) and the local weather forecasts from since the TAFs and local weather forecasts are issued by those same local meteorologists. Perhaps the biggest drawback of the new imagery is that the precipitation forecast now ends at the U.S. border although the isobaric forecast and forecast for surface fronts will still cross over into Canada, Mexico and coastal waters.

Here’s what we did in ForeFlight

Given that the legacy prog charts are no longer issued, we’ve moved the new prog charts from their initial home under the NDFD Progs collection to the Prog Charts collection where they will replace their legacy counterparts. Note that the extended forecast progs (Day 3 through Day 7) located in the Prog Charts collection will not be affected.

Prog Layout In ForeFlight

The result in ForeFlight is a single prog chart collection consisting of the latest surface analysis, new NDFD progs (6 to 60 hours) and the extended progs (Day 3 through Day 7).

GFS MOS Forecast Update

In this recent blog we presented a round-robin VFR flight from Oshkosh to International Falls. The concern was not the initial leg, but the return flight three days later. Would ceilings permit a VFR flight from International Falls back to Oshkosh on Saturday? The 75-hour GFS MOS forecast below provided clear guidance that a morning return would not be very likely given the IFR ceilings forecast along this route. But what really happened?

Ceiling forecast

Original 75-hour GFS MOS forecast valid at 1500 UTC for the return flight Saturday. This clearly shows that a VFR flight from International Falls to Oshkosh will not be possible in the morning.

Turns out the GFS MOS was spot on with the ceiling forecast as shown below on ForeFlight Mobile. While low IFR conditions were much more widespread than forecast, much of the region forecast to be in the marginal VFR flight category or lower were indeed at or below marginal VFR.

Actual Ceilings at 15Z

Actual ceilings at 1500 UTC as shown on ForeFlight.

How about later in the afternoon? The 81-hour GFS MOS forecast below valid at 2100 UTC suggested the low IFR ceilings would give way to VFR ceilings making a VFR flight possible later in the afternoon.

Afternoon CIGs

Original 81-hour GFS MOS forecast valid at 2100 UTC for the return flight Saturday. This guidance shows that ceilings were expected to improve later in the afternoon.

By 2100 UTC as shown below in ForeFlight, ceilings began to lift and mix out throughout the early afternoon giving rise to VFR conditions along a good portion of the proposed route of flight. However, there were some marginal VFR conditions still remaining in the vicinity of International Falls and Oshkosh with a few stations reporting ceilings slightly below 2,000 feet. It took a couple more hours before the entire route was truly VFR. Still, that’s not a bad forecast for 3 days out with an error of just a few hours.


The ceilings in the vicinity of the departure and destination airports remained slightly below VFR at 2100 UTC, but most of the route cleared as expected.

While the GFS MOS guidance won’t provide this kind of clarity every single time, it does a surprisingly good job most of the time. Give it a go on your next round-robin flight.

GFS MOS – Extended Ceiling And Visibility Forecast

Let’s say you are making a round-robin VFR flight; your plan is to leave in a couple of hours and return back home three days later. For the initial outbound leg, there’s a ton of weather guidance available to be sure you can make a safe VFR trip. This includes observational products such as ground-based radar (NEXRAD), satellite imagery, pilot weather reports and METARs, as well as forecasts such as prog charts, terminal forecasts (TAFs) and the area forecast (FA) along with AIRMET Sierra. But what about that return flight in three days? We’ll get to this trip a bit later.

No help available

The low-level SIGWX, area forecast, and terminal forecasts are fine for anticipating the weather for the next day or so, but they simply don’t extend out far enough in the future to tell you if IFR conditions might mess with your plans three days down the road. Prog charts go out to seven days, but only depict areas of precipitation out to 48 hours and say nothing about ceilings nor visibility; however, don’t cast out the prog charts just yet. A widespread low IFR event ordinarily doesn’t happen without some kind of large-scale synoptic support. So prog charts can provide some important clues.


To zero in on ceilings and visibility up to three days in advance, you’ll want to try a model-based forecast called GFS MOS (also known as the MAV). The GFS MOS ceiling and visibility forecast is available in ForeFlight Mobile’s USA Imagery collections. This forecast graphically depicts the expected ceiling and visibility over the next three days at three-hour forecast intervals for the conterminous U.S. Moreover, it’s refreshed every six hours.

Model Output Statistics, or MOS, is derived from numerical weather prediction models that meteorologists use to issue their forecasts—in this case the Global Forecast System, or GFS. This model doesn’t automatically produce a point forecast for a specific town or airport. Combined with geoclimatic data, MOS takes the “raw” model forecast and attempts to improve on it by making a more useful site-specific forecast complete with weather elements critical to pilots, such as ceiling and visibility.

MOS in several forms

MOS guidance can be displayed for specific airports, as seen in ForeFlight Mobile. However, to determine the widespread nature of the event, GFS MOS guidance can also be graphically contoured over a geographic area the size of the conterminous United States (shown below for ceiling height). Displaying the categorical ceiling height and/or visibility graphically in this way is perhaps the best approach to use MOS for extended guidance.

GFS MOS Ceiling Forecast

The categorical GFS MOS forecast for ceiling. Legend is located at the bottom of the forecast.

Definition of ceiling

Before we go any further, let’s do a quick review. A ceiling is the lowest cloud layer aloft that is reported as broken or overcast. If the sky is totally obscured (hidden), the height of the vertical visibility will be the ceiling. Ceilings are represented as above ground level, not mean sea level. So the GFS MOS forecast for ceiling is showing height above the ground. But keep in mind that ceilings can vary widely over rugged terrain.

Ceiling forecast

This forecast is a close cousin of the MOS forecast available in ForeFlight Mobile. Unlike the area forecast and TAFs that offer an absolute ceiling and prevailing ground visibility forecast, the GFS MOS guidance is a categorical forecast. It uses flight categories to include Very Low IFR (VLIFR), Low IFR (LIFR), IFR, Marginal VFR (MVFR) and VFR. The color-coded legend that depicts these categories for the contours on the map is located at the bottom of each forecast as shown below. Areas depicted in black on the map are expected to be clear below 12,000 feet AGL.

GFS MOS Ceiling Legend

Legend that exists at the bottom of each GFS MOS ceiling forecast annotated with ceiling flight categories.

Visibility forecast

Visibility is very similar. Keep in mind that this a forecast for prevailing ground visibility. Flight categories include VLIFR, LIFR, IFR, MVFR and VFR as well. Areas shown in black represent a visibility forecast greater than 6 statute miles.

GFS MOS Visibility Legend

Legend that exists at the bottom of each GFS MOS visibility forecast annotated with visibility flight categories.

Decoding the date-time stamp

Before using any forecast you must be certain how to decode the date-time stamp on the image. For the GFS MOS ceiling and visibility forecast, this is located in a banner across the top of each image like the one shown below. The date-time stamp is located on the second line of this banner. This forecast uses YYMMDD/HH as the format (annotated in white below). So in this example, the text 150828/1500 on the second line suggests the forecast is valid at 1500 UTC on August 28, 2015. The text at the end of the second line following “1500” or V075 is less important and simply states the forecast hour. In this case, it’s a 75 hour forecast—meaning that it’s a projection of what the ceiling (or visibility) will be in 75 hours from the time the GFS model was initialized. The GFS model is initialized four times daily at 0000, 0600, 1200 and 1800 UTC.


A proposed VFR round-robin flight

Back to our round-robin flight. It’s Wednesday and the plan is to depart Oshkosh (KOSH), Wisconsin this afternoon headed to International Falls (KINL), Minnesota with a return to Oshkosh three days later on Saturday morning. After examining the TAFs, area forecast and pilot weather reports, the weather is looking excellent for today’s flight. The layers overlaid on the ForeFlight Mobile Map view below include the latest satellite, current ceilings and AIR/SIGMETs. The satellite image shows a some scattered clouds in the vicinity of Oshkosh, but clear skies all the way to International Falls.

Outbound Leg

The ForeFlight Mobile Map view shows the latest satellite layer along with the AIR/SIGMETs and ceiling layers. Except for some scattered clouds in the Oshkosh area, no other weather concerns on the flight from KOSH to KINL.

So the outbound flight this afternoon has no real weather implications for a VFR flight, but what about the return leg back to Oshkosh on Saturday morning? Most of the public forecasts are showing a 30% chance of showers on Saturday as shown below, but nothing in this forecast mentions ceilings or visibility.

Public Forecast

GFS MOS comes to the rescue! This 75-hour forecast below is valid at 1500 UTC on Saturday and clearly shows that a VFR flight back to Oshkosh isn’t very likely. During the morning, a good portion of the route from International Falls to Oshkosh includes ceilings below a VFR flight category.

Return ceiling forecast

This GFS MOS categorical ceiling forecast valid at 1500 UTC on Saturday shows IFR conditions along the route.

But the news isn’t all that bad. The weather is expected to improve in the afternoon as shown in this 81-hour forecast below valid at 2100 UTC on Saturday. The entire route is forecast to be clear below 12,000 feet. Of course, it would be important to also check the forecast visibility at this time.

Afternoon ceiling forecast

This GFS MOS categorical ceiling forecast valid at 2100 UTC on Saturday shows ceilings improve significantly with skies clear below 12,000 feet along most of the proposed route.

Finding GFS MOS in ForeFlight

The GFS MOS ceiling and visibility forecasts are located in the ForeFlight weather Imagery. On the iPad tap on Imagery and then tap on the USA button on the lower left. On the left menu bar you will see selections for Ceiling Forecast and Visibility Forecast under the GFS MOS label. Forecasts on the right begin at 6 hours and run through 84 hours for both ceiling and visibility.

Check out the second half of this article series.

GFS MOS in ForeFlight

GFS MOS forecasts in the ForeFlight weather imagery.



Oh Hail! Where’s That Cockpit Weather When You Need It?

As the old saying goes, in so many ways, a picture speaks a thousand words. By now you have probably seen the chilling photo like the one shown in this media report of Delta Flight 1889 parked safely at the gate after diverting to Denver International Airport. This was the result of a nasty encounter with hail at 34,000 feet while en route from Boston to Salt Lake City last Friday evening. It makes little sense with today’s proven technology that any commercial aircraft should ever encounter such a hazardous situation and risk the lives of those on board. The crew of this Airbus A320 did an admirable job getting the aircraft safely on the ground after this encounter, but there’s another side to the story that got them into trouble in the first place. A few simple pictures in the cockpit may have made all the difference in the world to avoid putting these passengers through such a harrowing experience.

It’s truly a shame that a pilot flying a single engine Cessna 172 can have significantly more weather information available to them in the cockpit than the crew of this A320 had. We woulda, coulda, shoulda this crew’s decision to fly into this developing area of convection, but that’s not the point. The simple fact is, aircrews flying paying customers today are not equipped with matured technology that will provide them with valuable weather data to make timely decisions during any phase of flight to avoid these kinds of encounters.

Think big picture

Sure, these aircraft are equipped with onboard radar and the crews they carry are highly trained and experienced pilots who know how to make use of that radar. Even when properly used, onboard radar has limitations. It is a real-time snapshot of what’s occurring right now out in front of the aircraft, and this is critical data for tactical decisions, but is simply a microcosm of the overall energy in the atmosphere. In events like this, it is the macro picture of what’s unfolding 100 or more miles away that is often just as important. This large-scale view is better suited to help make the proper strategic decisions, especially when it comes to a developing area of severe thunderstorms. But that information needs to get to the pilot-in-command.

The view from above

Air crews can and do get help from air traffic controllers. Controllers can tout about a “hole” that the last five aircraft recently traversed. But that doesn’t necessarily tell you how fast that hole is closing up and whether or not it will still be there when you arrive. The crew may also have access to their company’s dispatchers, but in the end, the captain may not have the complete picture in his or her head…and that’s where these strategic decisions are made.

IR satellite image valid at 0045Z

This color-enhanced infrared satellite image (also available in ForeFlight Mobile) valid at 0045 UTC shows two distinct areas of convection. Dark purple and white areas denote very cold cloud tops likely from strong updrafts. Flight path of Delta 1889 taken from Flightaware is shown in black.

The squeeze play

Even before Delta Flight 1889 departed Boston’s Logan Airport, two areas of convection started to blossom in Colorado, one in northeast Colorado and the other in southeast Colorado. At 0045 UTC, you can see two distinct systems on the color-enhanced infrared satellite image shown above. As the Airbus crossed over the Mississippi River (black line is the aircraft’s approximate track taken from Flightaware) around the time this satellite image was taken, both areas of thunderstorms had been designated as severe. Nevertheless, there’s clearly a gap between these two convective systems and that appears to be where the crew was headed.

IR satellite image valid at 0115Z

This color-enhanced infrared satellite image valid at 0115 UTC shows the gap between these two areas of convection beginning to quickly fill in with a rapidly developing line of severe thunderstorms.

The gap shrinks on satellite

By 0115Z, that severe line of thunderstorms to the south moved into western Kansas. A line of storms developed on the northern extent of this area of severe thunderstorms bridging the gap shown very clearly in the infrared satellite image above. At this point the flight was still in southwest Iowa just about to cross the Nebraska State line. Even if this image didn’t become available for at least 15 to 20 minutes later at 0130Z, it was plenty of time to notice this explosive area of convection was quickly filling this gap.

IR satellite image valid at 0145Z

About 20 minutes before Delta Flight 1889 penetrated this line of storms, the gap had completely closed with the color-enhanced infrared satellite image showing very cold (high) cloud tops along the route of flight.

The gap is all gone

About 20 minutes before the crew diverted to Denver at 0205 UTC, the image above shows the gap is now completely gone as these two areas of severe convection continue to mature and merge into one mesoscale convective system (MCS). Unfortunately, the crew of this Airbus A320 did not have access to critical weather data such as this. A simple three hour loop of this color-enhanced infrared satellite image could have given the pilots enough information to recognize the gap was closing and choose a better route to prevent this kind of encounter from occurring.

The view from the ground

The local ground-based NEXRAD that is updated every five minutes had even more details that might have suggested the gap would quickly disappear. Once again, this kind of strategic weather information is not available in the cockpit of these aircraft. The 2.5 hour 0.5 degree base reflectivity loop shown below is from the Goodland, Kansas WSR-88D NEXRAD Doppler radar. The loop ends right about the time the crew diverted to Denver, shortly after being pelted by hail.


This is a loop of the 0.5 degree base reflectivity out of Goodland, Kansas. This loop shows how the gap between these two convective systems begins to quickly disappear. Radar site is in the center of this image. Source: UCAR.

The catalyst

At the beginning of this loop, notice a crescent-shaped area of low reflectivity returns appears out of the northern edge of the southern-most area of severe storms. This is called an outflow boundary. According to meteorologist and thunderstorm researcher, Dr. Charles Doswell, III, “Cold, stable air is the ‘exhaust’ of deep, moist convection, descending in downdrafts and then spreading outward like pancake batter poured on a griddle. After spreading outward, the leading edge of this outflow – a ‘gust front’ – which often has ascent associated with it, can develop new storms.”

This is a perfect description of what transpires next. This boundary continues to push north-northeast and helps to initiate a new line of thunderstorms that rapidly blossoms into a convective barrier in extreme southwest Nebraska leaving the crew little choice but to use their onboard radar to tactically locate and penetrate the “softest” part of this line.

The crew of this Airbus didn’t have any ground-based radar loop such as this available to them in the cockpit. Even so, most of the in-cockpit radar mosaics that general aviation pilots use every day is a volume product constructed from a composite of all elevation angles of the radar. Composite reflectivity tends to mask out important details such as these outflow boundaries. Unfortunately, most in-cockpit weather providers have chosen not to broadcast the 0.5 degree base reflectivity product like the one shown above. This includes the radar mosaic that can be received through the FIS-B (ADS-B) broadcast with Stratus or through the Baron Mobile Link for XM-delivered satellite weather.

VIL Loop

Vertically integrated liquid or VIL is another volume product that can show the truly nasty part of the storm. This loop clearly shows how quickly the gap disappeared between these two convective areas. Source: Plymouth State Weather Center.

More is better

Furthermore, there are other NEXRAD products that can be very useful in flight. This includes another volume product called vertically integrated liquid (VIL) shown above. As mentioned earlier, composite reflectivity looks at all elevation angles of the radar for a particular volume scan and shows the highest reflectivity return in the column. VIL is similar, but is a summation of the reflectivity in the column. VIL is often an indicator of the storms’s updraft strength and has a strong link to observed hail size.

Convective SIGMETs

At 0055 UTC, two convective SIGMETs were active each describing an area or line of severe thunderstorms. Source: NOAA.

Severe weather and aircraft don’t mix

You might say hindsight is 20/20, but this isn’t about second guessing a pilot’s decision. Instead, it’s a plea that aircrews should have more information available to them to make better informed decisions. There’s no doubt this was a risky choice; not only a risk of hail damage, but the threat of dangerous convective turbulence. The weather on both sides of the flight path consisted of severe thunderstorms. At 0055 UTC, the Aviation Weather Center (AWC) had issued two convective SIGMETs (WSTs) covering both areas of severe thunderstorms. Whether thunderstorms are severe or not is determined by the local weather forecast offices, but was echoed in the text of these convective SIGMETs. Both advisories suggested the potential for hail up to 2 inches in diameter as shown below.

WST for severe thunderstorms.

The southern most convective SIGMET was issued for a line of severe thunderstorms with hail up to 2 inches in diameter with tops above FL450. Source: NOAA.

It’s time to end this madness

It’s the middle of the second decade of the 21st century and no airplane should end up flying into a thunderstorm and encounter hail and turbulence like that of Delta Flight 1889. Was this just a case of being in the wrong place at the wrong time? Not likely. Perhaps a bit of bad timing, but it was not some kind of surprise encounter that removed the paint and dented the nose cone of this A320. Not only that, but the hail nearly shattered the pilot and co-pilot’s front windscreen making it look like something you’d see in a Hollywood movie. As Stu Ostro, a senior meteorologist at The Weather Channel, commented: “hail shafts aren’t like lightning bolts from the blue shooting way out the side of a storm.”

It’s time to start requiring commercial aircraft to have unlimited access to weather information in the cockpit – more than just onboard radar. Many aircraft equipped with Wi-Fi could easily connect to any number of ground-based weather sources and display it on a mobile device such as an iPad. Even with a device like a Stratus, the FIS-B data broadcast is still lagging way behind the available technology. So it’s time to start providing pilots with more than just basic weather data that mimics the heavy textual weather of the 1980s. There’s no doubt that getting more graphical weather data in the cockpit along with some focused training will keep professional aircrews from flying blindly into a hailstorm. Fortunately for those on board Delta Flight 1889, this flight ended well.

However, right now airlines are stuck in this information-age purgatory. Is this due to cost? Probably, but there’s no encouragement by the FAA for airlines to equip pilots with this potentially live-saving guidance. This encounter should be a wakeup call to those stakeholders in aviation safety to allow, if not require, pilots to have unfettered access in the cockpit to whatever weather data is necessary to make every flight an uneventful one. What is it going to take to make this happen? Hopefully it will not be an event that has a tragic ending. But that’s usually the way it works.

When The Radar Lies

The ground-based radar mosaic displayed on the Map view in ForeFlight Mobile combines radar data from the National Weather Service (NWS) and Environment Canada. Its primary purpose is to provide pilots with a good estimation of where precipitation is occurring and where it’s not. While there are some holes in the coverage (especially in Canada) the radar mosaic is fairly accurate most of the time. Even so, non-precipitation returns generically called ground clutter can be displayed on the radar layer producing what looks like very real areas of precipitation.

Anomalous propagation, or AP, is perhaps the most annoying form of clutter. Essentially with AP, part of the side lobes of the radar beam are ducted or bent down toward the earth during certain atmospheric conditions. This causes it to strike objects on the ground (trees, buildings, cars, etc.) and some of that power from the beam is reflected back to the radar along the same bent path and gets recorded as areas of precipitation. When this occurs you might see on ForeFlight what looks like real precipitation. In fact, it can look remarkably like real convection at times fooling even the most seasoned pilot.

ForeFlight Radar Layer With AP

Anomalous propagation (AP) on the ForeFlight radar layer near Buffalo, New York.

What to do if you suspect AP

Since AP can look remarkably like real areas of precipitation (including thunderstorms), it’s important to always examine the observational data in and around the area. This includes cross-checking surface observations (METARs) to see if precipitation or thunderstorms are being reported. Also, without clouds, it can’t rain. So if clear skies are being reported all around the area, then either the precipitation shown on the radar is very isolated or perhaps it’s erroneous. Keep in mind that automated reports only show clouds that exist below 12,000 feet AGL.

Along these lines, the visible satellite imagery in ForeFlight Mobile can also be useful to identify non-precipitation returns during the daytime hours. If precipitation exists on radar, there should be clouds in that region even if it is isolated convection. If there are no clouds, the returns on the radar are likely ground clutter or AP.

Even when the area is cloudy, AP can still exist. If this is the case and you suspect AP, try looping the radar. Most real precipitation moves and evolves over time, but AP tends to stay anchored over the same area with little noticeable movement. Moreover, the radar loop may look erratic and the intensity may change in a way that’s unnatural.

While AP can occur the U.S. it tends to occur the most in the Canadian Provinces. A favored place is on the U.S. side of Lake Erie just onshore and also in the mountains of the Pacific Northwest in British Columbia. While AP can occur anytime of the day or night, it’s often favored during the morning hours just before and after sunrise. This the time of day where the atmosphere is generally stable near the surface which is a perfect environment to allow the side lobes of the radar to be ducted.

So why can’t AP be filtered?

Filtering the radar of non-precipitation returns is like walking a fine line. If you filter too aggressively, you may remove real areas of precipitation; if you don’t filter enough, you get clutter such as AP displayed. In the U.S., filtering can be automated since the Doppler portion of the radar is available. This can be used to help filter AP and other ground clutter. While Canadian radars are Doppler radars, Environment Canada does not export the Doppler data at this time. Also in the U.S., the NEXRAD ground-based radar systems are all fitted with a dual polarization (dual pol) capability which can provide additional information to filter non-precipitation returns.

At the moment the only way to guarantee that AP from Canadian radars won’t find its way into the ForeFlight radar layer is to add a gross filter before the data reaches the display. This is accomplished by our radar provider by manually turning off the data coming from the offending radar(s). This can be risky since it means that all returns shown from this radar will be eliminated, false or not. The folks at Barons who produce the XM-delivered satellite weather also face the same issue with Canadian radars. They don’t turn off specific radars. Instead they create a manual gross filter that eliminates all returns over regions that are highly unlikely to receive precipitation.

In the end, every piece of information you use to make preflight decisions should be scrutinized even if it comes from a trusted source. Take the time to cross-check the radar layer against other sources within the ForeFlight Mobile app so you won’t be fooled.

Improved Resolution of Global Winds Aloft

With the introduction of ForeFlight Mobile 7.2 we’ve increased the resolution of the global winds and temperatures aloft. Now, flight planning calculations are more accurate than ever.

Increased temporal resolution

The horizontal and vertical resolution of the winds aloft are still the same, however, we’ve made a significant improvement by increasing the temporal (time) resolution of the forecast used within the app. At the global level, the winds aloft are still updated with a brand new forecast every 6 hours, but the forecast time step is now double the resolution at 3 hours over the entire globe.

Prior to version 7.2, winds and temperatures aloft contained a gap of 6 hours between forecast periods. That is, the winds were valid at times such as 00Z, 06Z, 12Z and 18Z or every 6 hours out to a maximum of 30 hours in the future. As you might imagine, a lot of weather can happen within a 6 hour period creating flight planning performance errors for fuel burn and time en route when interpolating over this time frame. These errors were likely more noticeable when the weather was changing rapidly along your proposed route. Now at the global level with ForeFlight Mobile 7.2 the winds will have twice the temporal resolution and will be valid at times such as 00Z, 03Z, 06Z, 09Z, etc cutting any interpolation errors essentially in half.

Even higher resolution over three continents

While the forecast time resolution was doubled throughout the world, we took it a step further and increased the temporal resolution by a factor of six over North America, Europe and Australia. That’s right, now these locations will benefit from an hourly forecast that is also updated hourly creating even more accurate results when planning a flight especially when the weather is changing rapidly. The hourly forecasts go out to a generous 12 hours in the future, then the resolution shifts to every 3 hours through 30 hours.

Winds Aloft Popover Window

In the airport popover window shown here, a single valid time is provided instead of a range. Even where the winds are available hourly, they are displayed at three hour time intervals out to 30 hours in the future.

Higher resolution winds used under the hood

Even though we’ve increased the temporal resolution, you’ll notice that in the airport popover window under the Winds tab, the winds and temperatures aloft are only shown every 3 hours (see above). This was done, in part, to keep the display of the winds consistent throughout the world and also to avoid a lot of unnecessary scrolling to get to the latter part of the forecast. Don’t worry, even though the higher hourly resolution winds are not displayed, they are indeed used in performance calculations for route planning.

The Winds Aloft layer on the Maps view will also benefit from the higher resolution. Previously, the time chosen may have been valid several hours before or after the current time. Now the Winds Aloft layer will be able to pull from the higher resolution forecast which will be valid closer to the current time. Moreover, those winds over North America, Europe and Australia will be refreshed hourly with a new forecast.

Winds Aloft Map View Layer

With the higher temporal resolution, the winds aloft layer on the Map view will use the closest forecast to the current time making it more representative of the wind speed and direction occurring right now.

By the way, if you don’t update ForeFlight Mobile to 7.2, you may see hourly forecasts displayed on the airport popover as shown below. That’s not an error; just a side-effect of progress. To get a more filtered view, you’ll need to update the app to version 7.2. Lastly, the FBWinds received through Stratus (FIS-B) haven’t changed as a result of this update. They are still the same lower resolution product as before. The FAA has no immediate plans to increase the temporal resolution of the winds received over ADS-B.

Hourly Forecasts Winds

For those customers that don’t update to version 7.2, you will see winds aloft presented hourly showing a range of times centered on the valid time of the forecast. Upgrading to 7.2 will filter the winds to 3 hour periods and show a single valid time.

What’s Up With QPF?

Areas of precipitation that are forecast along your proposed route should get your attention. These should be considered “hot spots” for concern and may add undo risk to the flight. While precipitation isn’t always problematic, even to pilots flying under visual flight rules (VFR), adverse weather elements such as thunderstorms, low IFR conditions, mountain obscuration, reduced visibility, airframe icing and turbulence tend to occur in and around areas of precipitation. So these precipitation areas are regions that pilots need to drill down a little deeper to determine what, if any, impact they may create on their planned flight.

This is the reason we introduced the 6 hour Quantitative Precipitation Forecast, or QPF, in ForeFlight Mobile 7.1. Shown below, the QPF represents excellent guidance when planning a cross country trip, whether your flight is several hours or several days in the future. You can find the 6 hour QPF under the CONUS Weather in the USA Ensembles. So let’s take a look at the advantages and limitations of the QPF.

QPF Example

This is the 6 hour QPF or Quantitative Precipitation Forecast.

Most pilots are familiar with the precipitation forecast on Prog charts. Precipitation identified on prog charts such as the one shown below is considered an instantaneous precipitation forecast. That is, the precipitation shown is valid at a single time and represents precipitation coverage or where precipitation is expected to be reaching the surface at the valid time.

Prog Chart Precipitation

The Prog chart includes an instantaneous precipitation forecast valid at a single time as shown in the lower left.

Instead of a single time, the QPF is valid over a range of time. In other words, it is the quantity of precipitation expressed in inches that is expected to reach the surface over a specific period of time. In this case, the period is six hours so the forecast is called a 6 hour QPF. The valid range of time is shown in the date-time stamp on the lower left, so this forecast is valid from 0600 through 1200 UTC as shown below. It is important to note that unlike Prog charts, the QPF does not distinguish between the type of precipitation (rain, snow, freezing rain, etc.) nor does it tell you if the precipitation is the result of deep, moist convection or thunderstorms.


Solid-filled color contours are drawn based on the expected precipitation amount (in inches) within the six hour forecast period using the scale in the lower left. Any “X” on the chart tells you the local maXimum precipitation amount (also in inches) within it’s respective contoured area. So for this forecast below in eastern Oklahoma and Texas, a maximum of 1.78” of precipitation is anticipated to reach the surface between the period beginning at 1800 UTC through 0000 UTC. In the case of wintry precipitation such as snow or ice pellets, the forecast roughly approximates the melted equivalent. Typically 12 inches of snow melted down represents about 1 inch of rain.


Also, the QPF doesn’t specify when the precipitation is expected within the valid range of time; it could fall all in the first hour, all in the last hour or it could be a continuous light rain falling throughout the entire forecast period. This is especially important to understand when the precipitation may be from convection. Often during the warm season, most of the precipitation forecast may fall within an hour or two and that could be near the beginning or end of the forecast period leaving much of the valid time free of precipitation.

The QPF offers a couple of distinct advantages over the instantaneous precipitation forecasts found on the Prog charts. Given that precipitation forecasts on prog charts represent coverage and are valid at a single time, the QPF can highlight areas of precipitation that may occur between Prog chart forecasts. For example, it is possible that an area of showers and thunderstorms may be expected to develop at 1900 UTC and dissipate by 2300 UTC. This area of precipitation would not be shown on the prog charts valid at 1800 and 0000 UTC, however, it would show up on the QPF. So the QPF is a complementary forecast to help fill in the gap in between prog chart forecasts.

Another advantage is that Instantaneous precipitation shown on prog charts stops after 48 hours. However, given that the QPF is valid over a range of time which is considerably less difficult to forecast, they provide guidance out to 3.5 days in the future – perfect for those Friday to Sunday round-robin flights.