There are many pilots that want to plan flights more than a few days in the future. Certainly understandable, especially if you are concerned about the weather on your return trip four or five days later. EZWxBrief can help with this to some degree, but much of the extended-range flight planning is all about quantifying uncertainty.
Weather forecasting isn't black and white...although some forecasts such as TAFs seem to imply that it is. Instead, it's all shades of gray. That is, when thinking about the forecast, meteorologists are rarely 100 percent certain of any weather event even a few hours in the future, much less a few days in the future. Nor are they 0 percent certain. However, the most important job of any forecaster has is to quantify their uncertainty to you. This is even more important as the forecast lead time increases. Forecasters quantify their uncertainty through a probabilistic approach...one that some pilots don't either understand or appreciate.
In the EZWxBrief static weather imagery, you will find the 12-hour Probability of Precipitation (PoP) forecast like the one shown above that is issued by forecasters at the Weather Prediction Center (WPC). This is a forecast that quantifies the meteorologist's certainty that precipitation will reach the surface in the 12 hour period identified on the chart. The actual valid time labeled on the chart is the ending time of that 12 hour period. In this case, the forecast is valid for the period from 12Z Wednesday through 00Z Thursday.
Please understand that this doesn't suggest whether the precipitation will be convective or not. Nor will it tell you the precipitation type that is expected. It simply tells you how (un)certain the forecaster is when he/she made the forecast. When you see areas of green (green is used for depicting precipitation forecasts), that means the forecaster has a much higher degree of certainty that precipitation will reach the surface during the valid period like you see in the image above in Colorado. Similarly, anywhere you see a brown color means the forecaster has a high degree of certainty that no precipitation will reach the surface during the valid period like you see above in the Pacific Northwest. Of course, then the forecaster has some uncertainty, you will typically see these depicted as red, orange, yellow and blue colors as shown in the lower Mississippi Valley area in the forecast above. This uncertainty should catch your attention that the forecast will likely change or is just difficult to forecast for a variety of reasons.
These 12 hour PoP forecasts are issued for Day 3 through Day 7, so they are meant strictly as an extended-range forecast. They are issued for periods valid from 00Z to 12Z and 12Z to 00Z which is basically a nighttime and daytime forecast, respectively. So, if there's a major convective event expected in the afternoon, it might show a high degree of certainty for the 12Z to 00Z period, but most of the morning may be very flyable. So it's important to also examine other forecasts such as the extended range prog charts and constant pressure charts to get a better idea of the big weather picture.
Last, but not least, the forecast says nothing about the potential of airframe ice, IFR conditions or turbulence. All of these adverse weather elements could be possible in regions where precipitation is not expected. However, in most cases, adverse weather tends to be more prevalent in regions where precipitation is expected to occur and this will give you a better feeling if that return trip will be possible or have some distinct challenges.
If you want to learn more about the Skew-T diagram, you can order your copy of The Skew-T log (p) and Me book that is available in soft cover or eBook format.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist