Updraft: July 8, 2008 Archive

Thirty year average rain chances

Posted at 8:26 AM on July 8, 2008 by Craig Edwards (3 Comments)

When I tell folks that there is a chance of showers on the weekend, they often question the precipitation probabilities. Five days out, I typically respond with an evasive answer, such as, I would not cancel your outdoor plans.

Historically, the climate data for the most recent thirty year average, suggests that on any given day in early to mid July in the Twin Cities there is about a 30 percent chance for measurable precipitation. We often speak of the average daily maximum and minimum temperatures, but seldom speak of the rain chances in reference to the average.

I spent a few days last month visiting with some of the younger generation of meteorologists in southern Wisconsin. One highly talented meteorologist, working out of Indiana, concurred with my befuddlement about the use of twenty percent rain chances beyond three days.

When you really think about it, forecasting a twenty percent chance of rain for Saturday night and Sunday is less than the thirty year average chance for rain. Casual listeners to a weather forecast, often hear the chance for thundershowers, without the modifier of twenty percent.

As an optimist, would it be more realistic to predict that there is an eighty percent chance of no measurable rain this weekend?
CE

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