Tuesday, September 6, 2016

Probability of Precipitation...What Does it Mean?

If you've seen the forecast, you know that there are scattered rain chances throughout the work week. Looking at the 7-day you'll see anywhere from 10-70% chance for rain, but what does that even mean?

I know many times I've been asked, "Why didn't we see rain yesterday, you said it would rain." So I thought I would take a moment to describe what meteorologists and forecasters mean when they say 'chance' for rain.

First off I'll break down one way we look at the percent chance. When a certain day has a 20% chance to see rain, it typically means isolated storms are forecast. When that is bumped to 30%, forecasters usually mean scattered rain and storms, and 60% and above storms are likely.

While that is one way to look at it, meteorologists also determine the percent chance mathematically and we call it PoPs. It stands for probability of precipitation and is defined as 'describing the chance of precipitation occuring at any given point.

Written out in an equation it looks like: POPs= C x A
C= the confidence that precipitation will occur somewhere in the forecast area.
A= % of the area that will receive measurable precipitation, IF it occurs at all.

It's a great way to express the combination of confidence of rain and precipitation and the overall coverage.

Here's an example: Lets say we are 50% sure that there will be rain somewhere in the area, and overall the ingredients for storms will be in place for 80% of the area then the Pops is:

.5 x .8 = .4 or 40%

I hope that helps next time you see the forecast littered with different percantages for rainfall to determine how likely you are to see rain!

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