National Weather Service Weather Forecast Office

Atypical Pattern

The recent weather pattern has proved to be quite atypical in general-especially for the month of September. Between late last week and the beginning of this week, a low pressure system at the mid levels of the atmosphere did something quite unusual; it tracked from the Dakotas westward to Montana. In general, weather systems move from west to east, or a slight variation of that path. This unique happening was largely a result of a strong high pressure system over western Europe, which acted as a blocking mechanism for the low pressure system to move in the typical eastward direction across the continental United States. Several seasoned NWS forecasters do not recall seeing a low pressure system take this type of a track during the late summer months. 

Rather than experiencing a period of precipitation and cooler temperatures, the westward progression of this system brought, and continues to bring, warmer than normal temperatures and little to no precipitation to Minnesota and Wisconsin. Average temperatures have been consistently around 10 degrees above normal across the area, with highs mostly in the lower 80s. The broad area of high pressure that dominated much of the Upper Midwest also contributed to fog development for several nights in a row.

The images below show the track of the 500 mb heights (black contours), which is about half way up into the atmosphere, between Friday morning, September 11th, and Sunday night, September 13th.

Map above courtesy of the Storm Prediction Center (www.spc.noaa.gov).

 Consecutive 80° Statistics for Minneapolis/St. Paul

Stretches September 1st or After

The above table reveals how rare it is, especially for mid-September, to have such a stretch of warmth in the area.

Local Climate Web Page