Aug 115 min read

EZTip - Convective precipitation markers in EZWxBrief

During the warm season when viewing the Map 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 as shown below. To overlay these markers 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 attribute in the Station Markers list. These predominant weather icons are also rendered at the top of the Route Profile and Airport Wx Meteogram views. Let's go through each of these different precipitation icons we see during the warm season.

For this predominant weather layer EZWxBrief uses what is called the Precipitation Potential Index (PPI). A PPI such as this is similar to the guidance provided by the National Digital Forecast Database (NDFD) grids that is also used to populate the weather type forecast on the Weather Prediction Center (WPC) short range prog charts like the one shown below.

Here's the essential issue with forecasting precipitation or convection at a single point (airport) and time. It's the first day of May and imagine you are a forecaster and someone asks you how certain you on are that somewhere in the conterminous U.S. during the entire month of July there will be at least one thunderstorm reported. Your answer would be unequivocally, 100 percent! And you'd be right 100% of the time. Easy forecast. Now, if you were asked again on the first day of May, how certain you are that somewhere in the state of Oklahoma during the entire month of July there would be at least one thunderstorm reported. Again, you'd answer 100 percent and you'd be right all of the time. Another easy forecast. However, if you were asked on the first day of May, how certain you are that a thunderstorm will be reported at 8 a.m. on July 14th at the Oklahoma City airport? You'd likely answer 0 percent and you'd be right most of the time.

So you can quickly see how much the spatial and temporal (space and time) resolution matter when it comes to making a probabilistic forecast. Meteorology, after all, is not about black and white...it's all about shades of gray. So when you are trying to forecast precipitation or even thunderstorms for a specific town or airport valid at a specific time of day, this would result in incredibly low probabilities - and that would not be useful to anyone. This is the key to a Precipitation Potential Index (PPI).

PPI values range from 0 to 100 and resemble the 12-hour Probability of Precipitation (PoP12) forecast values in magnitude. The PoP12 for any 12-hour period can be derived by taking the maximum probability 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, the calibrated probability should naturally decrease. In other words, a PoP06 forecast will have an equal or (more likely) lower magnitude than a PoP12 forecast that may overlap in time. Therefore, as temporal resolution increases, the magnitude of the PPI, remains unaffected. The PPI should be considered as that hour’s contribution to the PoP12. Below is a table showing how the PPI for both precipitation and convection is categorized.

First and foremost, a precipitation marker on the EZWxBrief Map implies there is a potential of precipitation within the airport's five statute mile terminal area at the valid time selected on the EZDeparture Advisor™. It does not imply that there's a 100 percent likelihood of precipitation or convection and could be as little as a 15 percent likelihood as shown in the table above.

This is why in the airport's tabular display on the EZWxBrief Map (click or tap on the weather icon), you will see entries for the Weather line such as "Partly cloudy with a slight chance of light rain showers; Isolated thunderstorms." Essentially the PPI is in the "slight chance" category for precipitation with "isolated" as the thunderstorm category. For this example, the PPI is forecast to be in the range of 15 to 24 for both of these categories.

In the example below, notice for the Coastal Carolina Regional Airport (KEWN), there's a partly cloudy icon (daytime) with a rain shower symbol. In the tabular display, the Weather line states, "Partly cloudy with a chance of moderate rain showers; Scattered thunderstorms." This places the precipitation likelihood in the "chance" category and thunderstorm likelihood in the "scattered" category for PPI values from 25 to 54.

As a bonus, 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 (a cloud with three large drops). Generally speaking this indicates the potential of nonconvective rain within the airport's terminal area that shows a "Slight chance of light rain" in the Weather line of the tabular display. This implies that a 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 dissipating phase of a convective event moving through the airport's terminal area consisting mainly of a stratiform-type rain. Furthermore, we often see a stratiform-type region on the back side of a severe mesoscale convective system (MCS).

The next precipitation marker you may see is the potential for rain showers in the terminal area as shown below (a cloud with four forward slashed below). The marker does not contain a lightning bolt and represents a shower icon. In the tabular display notice that it says, "Partly cloudy with a chance of moderate rain showers; Scattered thunderstorms." This implies there's a higher potential of moderate rain showers in the terminal area although you may see a possible thunderstorm nearby. Therefore, you should expect convective precipitation with rain showers being the dominate precipitation type over thunderstorms. In addition to moderate rain showers, you may also see light or heavy for the intensity of showery precipitation. The intensity is a sign of a more likely convective event.

This case above defines a PPI range of 25 to 54 as shown in the table below for "chance" and "scattered."

On the other hand, the next marker shown below contains a lightning bolt symbol. The tabular Weather line text says, "Cloudy with scattered thunderstorms; Chance of heavy rain showers." This implies that the airport terminal areaw will be dominated by scattered thunderstorms with a few heavy rain showers mixed in. In other words, there's a higher likelihood of seeing thunderstorms in the terminal area than the case above. Once again, given the "chance" and "scattered" qualifiers, this fits into the PPI rain from 25 to 54.

As the convective threat becomes more likely in the terminal area, you will see a similar marker but the text will say, "Cloudy with numerous thunderstorms; Likely chance of moderate rain showers." in the tabular Weather field. This guidance defines a PPI range of 55-74 as shown in the table below.

Lastly, when the convective threat becomes almost certain, the text will say, "Cloudy with definite thunderstorms; Definite chance of heavy rain showers." in the tabular Weather field. This guidance defines a PPI range of 75-100 as shown in the table below.

In rare cases it is possible to have the precipitation category and thunderstorm category to have a different PPI range. For example, below you may see "Mostly cloudy with a chance of light rain showers; Isolated thunderstorms." This implies a PPI precipitation range of 25 to 54 with a thunderstorm PPI range of 15 to 24.