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.

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 – 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.



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.

ForeFlight is Onboard Historic Air Race Classic

As a veteran of this and other cross-country air races, I was thrilled to participate in the Air Race Classic starting events in Fredericksburg, Virginia.


Visiting with Pilots Jessie Davidson, Topaz Grabman, and Emma Sullivan from Above All Aviation. These ladies are strategizing for a win!

The Air Race Classic has its roots in the first Women’s National Air Derby, held in 1929, when twenty female pilots set out to prove to the world that air racing was not just a sport for men. Beginning in Santa Monica, California they flew over 2,800 miles to the finish line—the National Air Races in Cleveland, Ohio. From that event, the Ninety-Nines were formed, as ninety-nine of the then 117 licensed female pilots organized to promote flying, friendship, and freedom around the world. Air Race Classic, Inc. continues the tradition of this historic race.

With airplanes and logbooks impounded for meticulous inspection by members of AWAM (Association for Women in Aviation Maintenance), anticipation and excitement built over the weekend as the 51 race teams gathered for three days of pre-race briefings. During the briefings I shared with racers how to get the most out of ForeFlight features such as our new weather imagery for better forecast planning, user waypoints, and annotating taxi diagrams with race time lines. It was fun to share my experience in what goes into creating a strategy for this race.

Each airplane is ‘handicapped’ to provide the best way possible to make all airplanes equally competitive. During the handicap flight the plane is flown full throttle on a rectangular course at a specific altitude. Vents are closed and engines are leaned for best performance. This levels the playing field and so the race becomes one of pilot skill and strategy rather than the raw speed of the fastest airplane, as it was in 1929.

As an Air Race Classic racer myself, I can tell you that the best strategy is to scrutinize the weather each day of the race and capture the best possible winds on the best possible days. Sometimes that means not flying on a day of headwinds if more favorable weather is forecast. As long as the course is completed in the four days allowed, how many of the legs are flown each day is up to each race team. There are some other fine-tuned race strategies I have used, but you’ll have to be my race partner to find out what those are! My ultimate advice: fly straight, fly fast.

2015 Air Race Classic star route

This year’s 2400 mile race forms a star pattern.

On Monday, the clock started against the 111 racers. The day VFR race legs total more than 2,400 miles. The race route is different each year; this year’s path draws a sort of star pattern, with the required timed legs going first to Hickory, NC, then Connellsville, PA, Jeffersonville, IN, Kalamazoo, MI, Lawrenceville, IL, Kirksville, MO, Union City, TN, and Gadsden, AL, and crossing the finish line at Fairhope, AL. Those who crossed the starting timeline have until 5:00 pm EDT Thursday to finish the course.

Engineers and rocket scientists who work for companies like Scaled Composites and Virgin Galactic, airline pilots, and airport managers, artists, singers, veterinarians, and retirees, and more people with fascinating backgrounds are in this race. Seventeen collegiate teams from around the country are vying not only for the coveted first place prize, but the chance to finish ahead of all other competing schools.

Unfortunately not all registered teams arrived for the start, as weather kept four race teams from reaching Fredericksburg. Another nerve-trying situation occurred at the start when Team 54, Terry Kane and Roxanne Ostrowski, returned from the starting line with a low voltage light. To remain contenders, they had three hours to secure the parts, complete repairs, and fly the timeline to start the race. I’m happy to report that they beat the clock and are still in the competition!

As the race teams navigate through the challenging race course, Team ForeFlight sends its best wishes for a safe and exciting adventure! Find out more about the race teams here, and join us as we track their progress here.

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.

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 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.

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.


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.


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.

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.