Probability of rain; moisture recharges SW Minnesota

After experiencing a rather dry spring, generous rainfall has occurred in June over southwest Minnesota, where totals have already exceeded over 4 inches in some locations.

Sioux Falls, South Dakota, has tallied over 4.5 inches of rain so far in June.

More rain was falling in southwest Minnesota this morning.

The water vapor satellite image from 6 a.m. depicted the extent of the cloud field associated with the rainfall.

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Radar depiction of precipitation shortly before 7 a.m. from Wunderground.com. Notice how the rain shield seems to be shrinking as it approaches east central Minnesota.

This weekend thunderstorms dumped heavy rain over portions of the Twin Cities metro. Rainfall amounts exceeded 3 inches in Watertown, Minnesota, west of the Twin Cities. Woodbury accumulated more than 1.5 inches.

Some rainfall reports of over 1 inch were posted by the National Weather Service in Chanhassen.

More showers and thunderstorms are on tap the rest of the week, with Wednesday looking to be the nicest day.

The National Oceanic and Atmospheric Administration's Weather Prediction Center paints this picture of the potential for rainfall through Thursday morning.

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The question was raised by the ground crew at Target Field recently about how to interpret the forecast of precipitation probabilities.

In reality, the highest probabilities have little to do with duration and intensity. The number is used to convey the confidence of measurable precipitation at your location during a specified time period.

If you hear a probability of precipitation is 60 percent for today, that implies the chance for measurable rain is 60 percent in your backyard. It also means there is a 40 percent chance you could miss the showers.

Forecasters also use the terms such as isolated, scattered or widely scattered.  Here's the definition of those terms as provided by the National Weather Service.

PoP Value      Qualifying Term     Equivalent Areal Term

10/20                 Slight Chance         Isolated                                                                                                                                                                

30, 40, 50         Chance                   Scattered

60, 70                Likely                      Numerous 

80, 90, 100       none                        none

Over time, forecasters apply a reliability check on the skill of providing these numbers! I did a small sample study when I worked in Indianapolis, Indiana, and found that forecasters often leaned toward a dry bias in the summer when thunderstorms are more hit and miss.

In one instance, looking at a 60 percent probability of rain, it actually rained 70 percent of the time.

See the  small bulls-eye of heavy rain that fell on Friday night and Saturday and imagine the challenge of predicting rainfall two days in advance in the summer season!