Weather 101: Chance of Precipitation

It's pretty commonly accepted knowledge that weather here in Montana is ... mercurial.  And it's true.  There's a reason why when someone complains about the weather, the near-automatic response is, "Don't like the weather?  Wait 15 minutes."

So what?

One of the most maligned and apparently mis-understood parts of a weather forecast is when the forecaster gives the chance of precipitation.

OK, smart guy.  Then how does forecasting work?

First, a bit of grade school science.  When the forecast talks about precipitation, it means any form of water falling from the sky, so rain, snow, sleet, hail, etc.

These days, weather forecasting involves using satellite data along with high-level math and physics.  Whoever is doing the forecasting will use that to simulate what the weather will be in a given area for a given time frame.  And they do the simulation more than once.  If they run ten simulations, and only two say it's going to rain/snow/whatever, the report will say there's a 20% chance.

Makes sense.  So how is it being misunderstood?

According to a Popular Science article I came across, some people expect that simple percentage to mean more than is really there.

The chance of precipitation is nothing more than the chance that there will be some form of precipitation.  That's it.  But that doesn't stop some people from expecting that if it merits mentioning in the forecast, it will be significant, either in duration, area or amount deposited.

And I get that.  If the forecast said there was an 80% chance of rain, and it winds up being a 5 minute pitter-patter that doesn't accomplish anything other than changing the color of the ground, I'd be a little disappointed too.

Figuring out how much will fall and for how long is the responsibility of another branch of meteorology, and a subject for another time.