Probability of Precipitation

What are PoPs?                                                                                                                                 

Angie Enyedi, NWS Jacksonville

When the National Weather Service (NWS) refers to PoPs, we are referring to the Probability of Precipitation.  This is the likelihood, expressed as a percent, of measurable precipitation (≥ 0.01 inch) at a particular point during a specified time period.  In other words, if you learn that the NWS is advertising a 60% chance of rain for your specific location, then out of 100 days with the same atmospheric ingredients in place, we believe it will rain on 60 out of those 100 days (or a 60% chance).  When we advertise a chance of rain for a daytime period, it ranges from 6 am to 6 pm (6 pm to 6 am for nighttime periods). 

There are two main precipitation event types; convective and stratiform. Convective precipitation events are ones such as discrete showers and thunderstorms.  These convective events result in the vertical exchange of heat (energy) and moisture (mass); cold dense air sinks toward the surface which forces warm, moist air upward.  Convective events are the most common type of precipitation event we experience in the Jacksonville County Warning Area. 

The other type of precipitation event is a stratiform event.  Stratiform events are characterized by extensive horizontal development as opposed to the more vertical development characteristic of convective events. Stratiform precipitation covers large areas but exhibits relatively little vertical development, and these events are relatively continuous and uniform in intensity  (for example steady rain verses rain showers).  Our area typically experiences stratiform rain events when a slowly moving or stalled frontal boundary or storm system affects the area. 

Below is a table that defines our PoP ranges, their associated qualifying terms and the equivalent areal term we use in the forecast to try to describe the coverage of precipitation events for convective events. 

PoP Value      Qualifying Term     Equivalent Areal Term

20                    Slight Chance         Isolated                                                                                                                                                                

30, 40, 50       Chance                   Scattered

60, 70              Likely                      Numerous 

80, 90, 100     none                        none

For example, these forecasts both advertise the same chance and type of precipitation:

a.     Mostly cloudy with a 70% chance of showers.                                                                                              

b.     Mostly cloudy with numerous showers. Chance of rain 70%.

When the NWS expects less than a 20% chance of precipitation, it will not be advertised in the forecast.  If we are confident that non-measurable precipitation is possible, we may advertise drizzle, sprinkles or maybe even flurries in the forecast.

Like all of our forecasts, the NWS considers a lot of atmospheric parameters before issuing a PoP forecast.  Sometimes we get it right; sometimes we get it wrong.  There are many physical processes that still go undetected by both meteorologists and atmospheric forecast models, and these small scale processes can have a big impact on the eventuality of a precipitation event.

For example, take the precipitation event our local area experienced on November 22, 2009.  Figure 1 gives a general idea of what the NWS advertised for PoPs across the area in the forecast that was issued the afternoon prior to the event.  Notice our PoPs ranged from about 90% across our Georgia zones to around 50% across our southern Florida zones.

Post-event analysis, displayed in Figure 2, shows that the 90% PoPs verified very well across our Georgia zones, while the 70% PoPs across much of north Florida were too high.  Although many atmospheric ingredients were in place across north Florida that day to support high rain chances (including a frontal boundary, adequate moisture, strong upper level dynamics and moderate surface based instability), we believe that the infiltration of a dry slot of air during the afternoon generally along the Interstate 10 corridor, was enough to prevent significant rainfall across northeast Florida.  However, had the dry slot moved across the area a hour or two earlier, it would have likely allowed for more surface based destabilization due to increased sunshine warming moist air near the surface, thus creating strong upward motion as colder, more dense air aloft sinks downward (convection!).  If this latter scenario developed, it very well could have been one that included severe storms.

Often times, these seemingly subtle timing and phasing can result in the difference between a significant rainfall event and little to no rainfall at a particular location.  The probability of precipitation, or PoP, that the NWS advertises tries to take into account the pros and cons for and against measurable rainfall at a certain location.  So, when there is a 70% chance of rainfall as a certain location, there is also a 30% chance rainfall will not occur. 

For the chance of rainfall at your specific location, click on our interactive forecast map located on the NWS Jacksonville front webpage at:

