Precipitation Probability

We have all grown up hearing the T.V. weatherman or National Weather Service say something along the lines of, “there is a 30% chance of rain today.” But, what does this mean? A 30% chance of rain does not mean that it will rain 30% of the day or that 30% of the forecast area will see rain. It means that there is a 30% chance that rain will occur at any given point in the area.

How do forecasters come up with a probability of precipitation (PoP) then? The Peachtree City NWS office in Georgia has a great explanation of probability of precipitation.

Mathematically…

“PoP = C x A where “C” = the confidence that precipitation will occur somewhere in the forecast area, and where “A” = the percent of the area that will receive measureable precipitation, if it occurs at all.”

“So… in the case of the forecast above, if the forecaster knows precipitation is sure to occur ( confidence is 100% ), he/she is expressing how much of the area will receive measurable rain. ( PoP = “C” x “A” or “1” times “.4″ which equals .4 or 40%.)”

“But, most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80 percent of the area, the PoP (chance of rain) is 40%. ( PoP = .5 x .8 which equals .4 or 40%. )”

Confidence is a vital part of forecasting. Sometimes meteorologists may have very high confidence in a forecast if model guidance is in good agreement or what is expected to happen is meteorologically sound. Other times, confidence in a forecast can be low, especially a few days out or if there is an unusual pattern in the atmosphere.

It is very important for meteorologists to communicate confidence with their audience. The art of forecasting is far from being perfected, and sometimes the confidence of a forecast is low. If confidence is low, meteorologists should convey this.

For example: Consider a low confidence forecast regarding the onset of rainfall. One meteorologist may say, “rain is expected to begin around 5 pm, but our confidence in this is low and rain could begin several hours earlier or later,” while another meteorologist simply states, “rain is expected to begin around 5pm.” If the rain were to arrive at 2pm, even though both forecasts were wrong, the public would trust the forecaster who discussed low confidence because at least they are offering an explanation for why the rain did not begin at exactly 5pm.

As you can see, confidence plays a vital role in coming up with a probability of precipitation. Once a forecaster has decided on PoPs for an area, they use these percentages in conjunction with words like isolated, scattered, and likely to describe a forecast. These words are not loosely used, they actually have a strong correlation to PoPs.

According to the NWS:

10% chance of precipitation = slight chance, isolated, few

20% chance of precipitation = slight chance or widely scattered

30% – 50% chance of precipitation = chance or scattered

60% – 70% chance of precipitation = likely or numerous

Any PoPs 80% or higher do not have set words to describe these chances, but this is when you would just hear the terms ‘rain’ or ‘thunderstorms’ with qualifiers like heavy, periods of, areas of, or frequent, etc. to help explain the forecast.

Hopefully this explanation of probability forecasting helps to clear up a few things. Next time you hear that there is a 30% chance of rain, you will have a much better idea of what that entails.