Explanation of "Probability of Precipitation"

The Meaning Behind The Acronym!

Background Information

Forecast discussions are no stranger to acronyms and one that is important and easy to understand is POP.  POP stands for Probability Of Precipitation which describes the chance of measurable precipitation (≥ 0.01 in) occurring at a certain location during a specific time frame, usually 12 hours.  
Often, there are misconceptions with what POP actually is.  What does the percentage mean? Some people may think, take 50 % for example, means it will rain for 50 % of the time throughout the day.  Others may believe that it will rain over 50 % of the area.  However, the correct way to interpret the percentage is that there is a 50% chance that rain will occur at any given point in the area.
So how is the probability calculated?  Forecasters use a formula which multiplies the forecaster confidence by areal coverage.  For example, if the forecaster is 80% certain that rain will develop and will cover 60% of the service area, the POP= .8 x .6= 50% chance of rain.
Another question may arise from this formula: how do forecasters determine confidence and areal coverage percentages?  Forecasters use a variety of models and personal experience to help develop these percentages.
There are a few different ways that the POP is represented; by percentages and by expressions.  For example, a forecaster might say there is a 40% probability of rain, which would be equivalent to a chance of rain or scattered rain.
Below is a table which can be used to decipher the different ways POP is represented:

Probability
Expression
Areal Coverage
0%
None
None
10%
None
Isolated
20%
Slight Chance
Isolated
30%
Chance
Scattered
40%
Chance
Scattered
50%
Chance
Scattered
60%
Likely
Numerous
70%
Likely
Numerous
80%
None
Occasional/Definite/Periods
90%
None
Occasional/Definite/Periods
100%
None
Occasional/Definite/Periods

Example 1

Let’s see if you can figure out the POP in the following situation:
Rain showers have developed in the depicted areas and will move east across Milwaukee county in the shaded regions.  Rain showers will persist across the shaded regions with the same intensity and drop a minimum of 0.01 inches of rainfall.  Looking at the map, the shaded regions cover around 40% of the service area.  So what is the POP for the entire county?  See the answer below the image.

Example 1

Answer:  The POP would be 40% because showers have already developed, the certainty is 100% and the provided areal coverage is 40%, therefore giving a POP of 40%.

Example 2
Here is another example that may be a bit trickier:
A forecaster is 60% confident that scattered thunderstorms and rain showers will develop west of Madison and move across South Central and Southeast Wisconsin with equal intensity, dropping a minimum 0.01 inches of rain.  Initial development is expected to move into the service area around 3pm and move slowly across the area, not expected to reach Milwaukee or the Lake until 8pm.  After 8pm confidence in precipitation development drops to 20%.  Using the images below depicting possible areal coverage of potential storms and rain showers, decide what the POP for Dane and Milwaukee Counties (outlined in black) will be in the afternoon (before 6pm) and evening (after 6pm).  See answers below the images.

Dane County:

Example 2 Dane

Answer:  For the afternoon portion, we know that the confidence is 60% and looking at the areal coverage from the possible storm tracks, it appears that about 90% of Dane County could be affected.  Plugging into the formula, POP= .6x.9= 50% chance for rain in the afternoon.  In the evening, visually there is maybe 10% areal coverage in Dane county with forecaster confidence of 20 %.  Therefore the evening POP will be .1x.2= less than 10%.

Milwaukee County:

Milw Example

Answer:   In the afternoon, we see 0% areal coverage depicted on the graphic which means the POP will be 0% for the afternoon.  In the evening, forecaster confidence is 60% with between 80-90% areal coverage in Milwaukee County.  The POP would therefore be .6x.8= 50%.

Another hint when figuring out the POP is to reread the given text.  The forecaster is predicting scattered thunderstorms and rain showers which, when looking back at the table, equates to between 30-50% chance.

 


 Kim Anderson, Student Volunteer, WFO Milwaukee/Sullivan

 

 


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