PoP Goes The Forecast

When we make a decision to depart on any round-robin flight, it’s not unusual to also factor in the expected weather on the return trip. If the return leg doesn’t look very good, what’s the sense in making a flight that may put us in a compromising position later on. But if that flight is three or more days in the future, how do you know that it’ll be safe to fly back home? That’s a difficult question to answer because at three days and beyond there really isn’t any aviation-specific weather guidance that can tell you about adverse weather elements such as low ceilings and/or low visibility, airframe icing and turbulence. That certainly leaves pilots to fend for themselves.

If you want to stack the deck in your favor and choose the best time to minimize your exposure to adverse weather you should focus in on areas of precipitation. Of course, not all precipitation events are considered hazardous, but many are, especially when flying VFR. That’s because adverse weather elements such as IFR conditions, icing, turbulence and thunderstorms tend to occur in and around areas of precipitation. So it’s a good bet that if you find yourself faced with precipitation forecast along your proposed route, you will likely encounter some form of challenging weather, even if your planned flight is several days from now.

So that’s why in ForeFlight Mobile 7.1 we introduced the 12 hour Probability of Precipitation forecast like the one shown below. It’s known as the PoP forecast and you’ll find it in the USA CONUS Imagery view under 12 HR PoP.

Probability of Precipitation forecast

This is the 12 hour Probability of Precipitation (PoP) forecast. You can find this on ForeFlight under the Imagery view in the 12 HR PoP collection.

This forecast is issued by meteorologists at the Weather Prediction Center (WPC) in College Park, Maryland. It’s the same group of forecasters that issue the familiar prog charts pilots have used for many years. It’s designed to show the forecaster’s confidence of where precipitation will likely reach the surface within a 12 hour forecast period. The higher the numbers shown on the forecast, the higher the chances (probability) precipitation will occur in that period.

It’s important to understand the date-time stamp on the forecast. Many precipitation forecasts are valid over a period of time. In this case, the period is 12 hours. The valid time in the lower left of the chart is the ending time of the 12 hour period. In this case the forecast is valid from 12Z on June 1st through 00Z on June 2nd. This basically covers the daytime hours on June 1st.

While the short-range surface prog charts generally cover the weather features and precipitation expected over the next two days, the 12 hour PoP forecast describes the medium- to long-range forecast. It starts with Day 3 (the day after tomorrow) and runs through Day 7 with 12 hour forecasts ending at 00Z (daytime) and 12 hour forecasts ending at 12Z (nighttime).

Let’s say you were planning a flight from Chicago to Atlanta during the day on June 1. Using the 12 hour PoP forecast above, there’s a good chance you’ll face challenging weather along that route. On the other hand, if you were flying from Chicago to Oklahoma City, the likelihood of any significant adverse weather is minimal.

That’s not to say that most of the significant weather may end up occurring during the late afternoon on that flight to Atlanta. So a morning flight may still be possible. But this forecast doesn’t provide that level of temporal resolution. Moreover, even though the flight to Oklahoma City looks promising, you could still face IFR conditions, icing (during the cold season), strong winds or the potential for turbulence. So you can’t become complacent.

Day 3 Prog Chart

Use the Day 3 Prog chart along with the 12 hour PoP.

In that light it’s useful to also look at the Day 3 prog chart shown above (Day 3 through Day 7 progs are now located in the Prog Charts collection). Forecasters don’t show areas of instantaneous precipitation on the Day 3 through Day 7 progs, but what you do notice is that most of the precipitation shown on the 12 hour PoP forecast is along and ahead of the stationary front that extends from the Eastern Shore of Maryland southwest to Houston, Texas. Therefore, much of this precipitation is associated with a large scale synoptic feature which means plenty of dynamic forcing and likely the reason the probabilities are so high.

Also notice that behind the front the precipitation chances decrease and become nearly zero through Illinois and Missouri. This is courtesy of a rather large area of high pressure seen on the Day 3 prog with an inverted ridge spilling down from Ontario, Canada into central Texas. Such a ridge promotes subsidence or sinking air. Air that is sinking tends to inhibit the formation of clouds and tends to keep the air stable. While you can’t rule out early morning radiation fog or some moderate thermal turbulence in the afternoon, the chances of any significant turbulence aloft is minimal under a ridge. Lastly, the pressure gradient (distance between isobars) is fairly large implying a low risk of strong and gusty winds.

Pilots have never really been all that skilled using long-range forecasts. That’s understandable since a pilot’s primary training mainly focused on making a go or stay decision moments before departure. Very little time is spent on how to analyze the weather more than 24 hours in advance. But these long-range forecasts can provide some valuable planning information especially if your timing is flexible. Or if it’s not flexible, a bad decision now might tempt you to fly when the weather is hazardous later on for your return trip. Here at ForeFlight we hope that the 12 hour PoP forecast will provide the guidance you need for that long-range flight planning.