Heat waves are uncommon but not unprecedented in September. The most recent one began on September 7, 2002 when, the temperature soared to a record-high 91 degrees at the National Weather Service (NWS) near Negaunee. It kept on sizzling, reaching 91 the next day and a record-shattering 93 on September 9, 2002??the hottest so late in the season. Interestingly, this heat wave does not show up in the older weather records of Iron Mountain and Ironwood. There were other heat waves earlier in the 20th century that outdid the one in 2002.
The pattern responsible for the heat is a familiar one. A big surface ridge dominated the eastern portion of the country with an upper-level trough off the West Coast and a ridge aloft covering much of the country (Images 1 above).
This heat wave set the stage for a warm ninth month in 2002. The mean temperature at the NWS came in 5.7 degrees above average. Like a typical September, it was also wet. A total of 5.71 inches of rain fell at the NWS. The average is about three-and-three-quarters inches.
The pattern evolving this September looks warm. While we will experience cool shots like the one expected late this weekend, the overall pattern looks to lean warm. Image 2 above is the upper air forecast for late next weekend. Note the dark blue over southwest Canada. That signifies a forecast trough. The yellow and green over us into eastern Canada is a forecast ridge. If this occurs, a warm mid-September is in the offing.