During the warm season when viewing the EZMap in the EZWxBrief progressive web app you may have noticed several different types of precipitation markers that are rendered when the Weather depiction layer is selected like those shown below. To overlay these on the map, click or tap the layer selector button in the upper left and then expand the Station Markers layer and you'll find the Weather layer is the last possible item in the Station Markers list. These icons are also rendered at the top of the EZRoute Profile and EZAirport Meteogram. Let's go through each of these different icons.
For this weather layer EZWxBrief uses what is called the Precipitation Potential Index (PPI). The PPI supports the generation of National Digital Forecast Database (NDFD) 12-hour Probability of Precipitation (PoP12) forecasts and the categorical information provided by the NDFD weather grids used to generate the precipitation forecast on the Weather Prediction Center (WPC) short range prog charts. So what you see above should generally match the short-range prog chart forecast at the valid time of that forecast.
PPI values range from 0 to 100 and resemble PoP12 values in magnitude. The PoP12 for any 12-hour period can be derived by taking the maximum PPI value within the desired period. Please note that PPI is not a calibrated probability forecast, which has different statistical characteristics. For example, as temporal resolution increases probability should decrease. In other words, a 12-hour PoP forecast will have an equal or larger magnitude than a 6-hour PoP forecast. As temporal resolution increases, the magnitude of PPI remains unaffected. Below is a table showing how the PPI for both precipitation and convection is categorized.
First and foremost, a precipitation marker implies there is a potential of precipitation within the airport's terminal area at the valid time selected on the EZDeparture Advisor. It does not imply that there's a 100% likelihood of precipitation or convection and could be as little as 15% in some cases as shown in the table above. Also, the markers are shown with a color to indicate the flight category for the airport...green indicates VFR, blue is used to represent MVFR, red is for IFR and magenta represents LIFR. Note that if precipitation does occur, the ceiling and/or visibility could be lower than forecast.
The first marker to discuss is the Rain icon pictured below. Generally speaking this indicates the potential of nonconvective rain in the airport's terminal area with the convective coverage of 14 or below (None) as indicated in the table above. Although the rain marker can be shown any time of the year to depict a stable stratiform-type rain event, in the warm season there may be deep, moist convection and thunderstorms lurking nearby. Be aware that this may just be the beginning or possibly the ending of a convective event moving through the airport's terminal area consisting mainly of a stratiform-type rain.
The next precipitation marker you may see is the potential for Rain Showers in the terminal area as shown below. The marker does not contain a lightning bolt, just a shower icon. In the tabular display notice that it says, "...showers and isolated thunderstorms." This implies there's a higher potential of rain showers and you may even see a possible thunderstorm nearby. Therefore, you should expect convective precipitation with rain showers being the dominate precipitation type over thunderstorms.
This guidance above defines a PPI range of 15-24 as shown in the table below.
On the other hand, the next marker shown below contains a lightning bolt symbol. The tabular text says, "...showers and isolated thunderstorms" which is the same text as above. This implies that the airport environment will be dominated by isolated thunderstorms with a few rain showers mixed in. In other words, there's a higher likelihood of seeing thunderstorms in the terminal area than the case above.
As the PPI for showers and thunderstorms gets higher, you will see the tabular display the Weather text mention "...showers and scattered thunderstorms" to emphasize the higher coverage of thunderstorms in and around the terminal area.
This guidance above defines a PPI range of 25-54 as shown in the table below.
As the convective threat becomes more likely in the terminal area, you will see a similar marker but the text will say, "...showers and numerous thunderstorms" in the tabular Weather field as shown below.
This guidance above defines a PPI range of 55-74 as shown in the table below.
Lastly, when the convective threat becomes extremely likely or definite, the text will say, "...showers and widespread thunderstorms" in the tabular Weather field.
This guidance above defines a PPI range of 75-100 as shown in the table below.
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Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist