Why it’s harder to forecast rain probabilities during summer months

Mother Nature took aim at the Twin Cities on both Saturday and Sunday afternoon.  A cluster of thunderstorms blossomed quickly through the heart of the metro at midday on Saturday and dumped nearly two inches of rain on Golden Valley, Mn.

Heavy rain actually bypassed the some of the southern Metro with less than a tenth of an inch at the Twin Cities International Airport on Saturday.

Midwest Climate Center graphic of Saturday’s precipitation

Trying to forecast precipitation probabilities is most challenging during the summer months.  This is due to several questions:

  1. Do the models show consistency on the atmosphere initiating rain in the region?
  2. Is there a specific location that has the greatest chance for rain?
  3. Is there a featured time to the rain chance?
  4. How large of a region might be impacted by showers and storms?

I bring this up, because I am often questioned about why it rained when the chances were only 30 percent.

Here’s the forecast issued this morning by the National Weather Service for my backyard:

Scattered showers and thunderstorms, mainly after 2 pm.  Mostly sunny, with a high near 80. West northwest wind 5 to 10 mph.  Chance of precipitation is 50%.

  • Tonight Scattered showers and thunderstorms before 10pm, then isolated showers.  Mostly cloudy, with a low around 57. Northeast wind 5 to 10 mph.  Chance of precipitation is 30%.
  • Tuesday A 20 percent chance of showers and thunderstorms before 1pm.  Partly sunny, with a high near 78. East southeast wind around 5 mph.

You’ll see a lot of probabilities of 30-50 percent in the summer simply due to the nature of the scattered storms.

A 30 percent chance for rain means that your specific location has roughly a one in three chance of experiencing measurable precipitation during that time period.  In other words there is a 2 in 3 chance you will stay dry.

So far this June, there has been precipitation  at the Twin Cities International Airport 11 of the 16 days of the month.  Yet the total rainfall for June is running below normal. A large convective complex developed over Oklahoma overnight.  The IR satellite image from daybreak nicely depicts the heavy rain event.

IR Satellite image from NOAA

It looks like a showery pattern setting up for the state later in the week.  Tuesday may be the driest day of the work week.

Chanhassen National Weather Service weather story.

High temperature forecast for today from the National Weather Service:

Today’s forecast highs from the NWS.

 Craig Edwards