Weather 101 – Rain Chances

Monday’s Weather 101 Question was:

When we say 20% chance of rain, what does that mean?

A. 20% chance that the Tennessee Valley will get rain.
B. 20% chance that any one spot gets rain.
C. 20% chance that the area will get .10″
D. 20% chance that the area will get .25″

B. is the correct answer. Rain chances are easily misunderstood and misinterpreted because we are trying to convey both the confidence that rain will occur and the area that rain will cover.

There is a mathematical formula for this that gives us a “Probability of Precipitation” (PoP):

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 measurable precipitation, if it occurs at all.

In the Tennessee Valley, as well as most of the South, it is very difficult to convey the intensity of summertime storms with a rain chance. On a typical summertime afternoon, the “PoP” may only be 20%; however, the few storms that develop have all of the hot, humid air they need to grow into storms that can dump up to 5 inches of rain in a small area.

Summertime storms are known as convection. Think of how chaotic the bubbles are in a pot of boiling water; that’s what the atmosphere might resemble on a hot, summer day (except on a much larger, slower scale). The chaos in the pot cannot be totally predicted, but you know that the bubbles will only develop inside that pot. The environment in the summer is a lot like that; we know the general area where storms will develop, but the individual storms are a lot like those chaotic bubbles.

PoP is a little more “telling” of the real weather situation in the Winter and Spring because most of the precipitation in those months comes along with a major storm system. Rain coverage with a storm system is usually caused by some mechanical process in the atmosphere that creates lift, and those processes are much more easily forecast since they happen on a large scale.

-Jason
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