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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

Comments (3)

We are planning a party for this Friday - three days away from now - it has to be outside and on the Weather Underground site, it indicates a 10% chance of rain. This morning, I think I heard Paul H suggest that the storms might roll into the Twin Cities Friday night. We are expecting something like 60 people and have live music planned. As I said, it has to happen outside.

I sympathize with organizations who have to make a go or no to decision on an outdoor event. I don't understand probably enough to not get worried if I see that there is ANY chance of rain.

Posted by Dan | July 8, 2008 3:49 PM

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

I'm pretty sure that's only optimistic if you want a rain free weekend. On the other hand if you've got a lawn turning crispy or just really like sitting on the porch watching a thunderstorm roll through it might be more optimistic to hope for the twenty percent.

One person's junk is another person's treasure?

Posted by Doug | July 8, 2008 5:06 PM

Got Me. Yes, you caught the unintended bias of thinking rain will ruin outoor weekend weather plans. A colleague in Indiana, during the drought and heat of 1983, once commented that we should not use the word THREAT for rain during a dry spell. We should say the HOPE for rain. Here's hoping you get doused soon. Ideally overnight on Friday!
Craig

Posted by Craig Edwards | July 8, 2008 10:40 PM

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