A gradual warm up is underway for Upper Michigan. For most residents the milder weather will be welcomed after the coldest period in four years. However, this warm up will fall far short of the February heat wave of 1991.
Rarely does the high temperature go above 50 degrees in January or February. In 1991, the mercury topped 50 degrees three days in a row at the National Weather Service (NWS) near Negaunee. From February 2-4, highs of 54, 54 and 53 degrees were recorded. It cooled back a bit for a couple of days and then new records of 48 and 51 were set on the 7th and 8th. Ironwood topped out at 49 degrees three-straight days to begin the month and Iron Mountain residents basked in 56-degree weather each day from February 3rd through 5th.
The remarkable warmth was caused by a retreat of the main jet stream into Canada (Image 1 above). The contraction of the main band of westerlies to the north bottled the cold air in far northern Canada and allowed mild Pacific air to wash over the northern United States. By the time the warm spell ended, only a trace of snow was left at the NWS. However at mid-month, a cold blast settled in, and with it, a lake-enhanced snow event. The NWS picked up over a foot of snow and it at least looked like winter again. February 1991 ended nearly six degrees above the long-term average.