Updated: Jun 2
If you use the EZWxBrief progressive web app EZImagery, you likely have run across the Prog Charts collection. There are some high level details in the EZWxBrief Pilots Guide, but it's important to expand a bit more on this widely used weather guidance.
First, there is a similar set of prog charts that you may find on the Aviation Weather Center (AWC) website available here. However, the source of the prog charts used in EZWxBrief are those created by the Weather Prediction Center (WPC). Although the AWC site prog charts have a similar look and feel, the discussion to follow is strictly directed toward the forecasts found on the WPC website and should NOT be carried over to the AWC progs.
These prog charts come in a short and extended range variety. This discussion will focus on the short range progs which are those with a forecast lead time of 6 to 60 hours. These short range forecasts are issued twice a day and include the expected surface pressure patterns (isobars), circulation centers (highs and lows) and fronts. You may also see surface troughs, drylines and squall lines depicted as well. This forecast extends over most of North America and is produced by highly trained meteorologists at the Weather Prediction Center in College Park, Maryland.
A color mosaic depicting the type, coverage (extent) and likelihood of precipitation are extracted from the National Digital Forecast Database (NDFD) that is prepared by forecasters at the NWS local Weather Forecast Offices (WFOs) in collaboration with the WPC. The county warning areas (CWAs) for the WFOs are shown below. Consequently, the precipitation forecast depicted on these prog charts is strictly limited to the conterminous U.S. and coastal waters and does not extend into Canada or Mexico.
Here's how that precipitation forecast is built. The first distinction is that the precipitation on this chart is issued by over 100 forecasters located at the various WFOs shown above. At prescribed times throughout the day, forecasters create "grids" that get ingested into the NDFD. Below is a forecaster at the NWS Greenville-Spartanburg WFO located in Greer, South Carolina entering his grids. Once in the NDFD, these grids have a horizontal resolution of 2.5 km. One of the grids they construct is precipitation type...the same one rendered on the WPC prog charts. Although neighboring WFOs are supposed to coordinate their forecasts, you may see the precipitation forecast abruptly terminate at these boundaries.
The good news here is that in the same CWA this precipitation type forecast should match the terminal aerodrome forecasts (TAF) barring any recent amendments. That's because they are issued by the same meteorologist or WFO.
The second distinction is that the precipitation mosaic is valid at the time located on the chart. As a result, this is considered precipitation coverage. In other words, it's not a time-smeared forecast valid over a range of time, but valid at the time stamped on the chart. The precipitation on the extended range prog charts is actually valid over a range of time, however.
Now this is where dialog gets a bit more ugly. The precipitation mosaic depicted on these prog charts is shown using two color shades (light and dark) for each precipitation type representing the likelihood of precipitation reaching the surface during that time. Rain, for example, you'll see a light green and dark green color. Lighter colors represent a lower likelihood that the precipitation type depicted will reach the surface (any precipitation amount other than trace) at the valid time. Darker colors represent a greater likelihood. Please understand that this is a likelihood and not a calibrated probability forecast.
Rain (Chance) - There is a 15% to less than 55% likelihood of measurable rain (≥0.01") at the valid time on the chart.
Rain (Likely) - There is a 55% or greater likelihood of measurable rain (≥0.01") at the valid time on the chart.
Therefore, the "likely" precipitation forecast in the prog chart shown above for northwestern Georgia, indicates that there's a 55% or greater chance of measurable rain that is not generated by convection. Although neighboring WFOs are supposed to coordinate their forecasts so it appears that the forecast isn't accomplished by over a hundred forecasters, you may from time to time see the precipitation forecast terminate abruptly at these CWA boundaries. You can see an example of this from the prog chart above. Notice the dark green area stops at the Georgia and Alabama border given that the Atlanta WFO terminates at this geopolitical boundary.
The NDFD currently provides information on precipitation probability for 12-hour periods. The Precipitation Potential Index (PPI) shown on the right is used by Weather Forecast Offices to derive 12-hour Probability of Precipitation (PoP12) forecasts and provides detail on precipitation timing at up to hourly resolution. Essentially the PPI is the likelihood, expressed as a percent, of a measurable precipitation event (1/100th of an inch) at a grid point during the indicated valid time. Using the term "likelihood" rather than "probability" is a way to get around the fact that the value is not calibrated to a 1-hour period (like a POP01 would be). If it were calibrated to a 1-hour period, the values would be very much lower.
From the table above, the NDFD products and those used on the WPC prog charts combine the chance and slight chance forecasts into a single "chance" category and the likely and definite into a "likely" category. In 2014, the WPC actually had values associated with the four categories, but it was later suggested to remove these, since the probability categories are calibrated to 12-hour periods while the display of the weather grids is meant to convey the potential (likelihood) at the valid time. Otherwise, the values would be technically inaccurate. Also, the directives state that for convective situations, areal coverage can be a substitute for the probabilities, which muddies the waters even further.
For even more details you can also watch Episode 7 of EZWxChat where the prog charts are also discussed.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist