Recent abnormally warm weather has resulted in a very early start to the growing season across the Great Lakes Region and much of the central and eastern USA. The trend towards warmer than normal conditions is not new, with above normal mean temperatures observed across Michigan during much of the period back to the middle of last November. The winter of 2011-2012 (December through February) was the fourth mildest on record.
The prolonged warm spell is the result of a very persistent jet stream configuration across North America that generally prevented cold, arctic-origin air masses from moving out of their high-latitude source regions southward into the central and eastern USA. This left the area under the influence of relatively mild Pacific-origin air masses. The current jet stream pattern, with deep troughing across the western USA and ridging across the east has accentuated the pattern. The result is record-breaking warmth brought northward from the Gulf States on southerly winds.
Looking like a record-breaking March
Characterizing the current warm spell as highly unusual is an understatement, both due to the magnitude of the warmth (recent high temperatures have exceeded 70ºF over many areas of the state) and due to its length. For climatological comparison, normal high temperatures during the third week of March range from the mid-30s far north to the middle to upper 40s south with lows from the mid-teens north to the upper 20s south. Thus, recent temperatures have been running in the range of 20 to 30°F or more above normal.
Short- and medium-range forecast guidance strongly suggests a general continuation of the current pattern with warmer and possibly wetter than normal weather likely through the end of the month. A map depicting forecast conditions across North America early next week is given in Figure 1.
The upper air trough over western sections of the country, ridging across the Midwest and east, and strong southerly flow all suggest maximum temperatures into the 60s and 70s statewide through at least the middle of next week. Use of the latest forecast information suggests at least the potential for mean temperatures this March to exceed the existing record (1945 in most locations) for warmest on record in the state.
Unisys, Inc.Figure 1. Projected weather conditions at approximately 5,000 feet above sea level, 8 p.m. on Monday March 19, 2012. Color contours depict temperature (°C) while winds are expressed in arrow/vector form on grid points in white (direction of arrow indicates direction, length of arrow depicts velocity). Solid white lines depict general pressure pattern and air flow.