Midwest Overview - November, 2004

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A Warm and Moist November Turns Snowy at the End

November 2004 was the 13th warmest and 17th wettest month of November for the Midwest since 1895. Ample or abundant precipitation amounts were experience in all states but parts of Minnesota and Wisconsin (Figure 1). The southern half of the region received above normal amounts of precipitation, while only northern Minnesota was truly dry, reporting less than 50% of normal (Figure 2). Unfortunately, the abundant rains were not helpful to those farmers that needed to complete late harvest and perform other fieldwork in preparation for the next growing season. Cold and warm air masses changed positions over the region frequently during the month, resulting in the normal southwest to northeast tilt of the lines of monthly average temperature in the area (Figure 3). Most of the Midwest was 3-4°F above normal during November (Figure 4). Despite the generally warm conditions, the synoptic patterns were just right to bring three substantial snowstorms to the Midwest during the last 7 days of the month. A swath of snow was evident from western Missouri to eastern Michigan, and across the northern tier of the Midwest (Figure 5). While the snow amounts were above normal in the Missouri to Michigan swath, the snowfall amounts registered in the Upper Midwest were 4-8 inches below normal generally, and even further below normal in the Lake Superior and northern Lake Michigan snow belts (Figure 6). Although the snow season started late this year, there has been rain in most of Wisconsin, Iowa, and Michigan that removed all moderate drought from the region (Figure 7, National Drought Mitigation Center). Despite a month of drying in northern Minnesota, this area had not completely used up the substantial moisture surplus from earlier in the fall; it is just beginning to get abnormally dry in Minnesota at the end of the month.
 

Fairly Quiet Start to El Niño

The Midwest experienced some of the beginning forcing of the latest El Niño event during November. Sea surface temperatures by the end of the month were warmer than normal in the central equatorial Pacific portion of the El Niño sea surface temperature anomaly field (Figure 8, Climate Diagnostics Center - CDC). There also are large areas of warmer than normal seas in both the North Pacific and North Atlantic oceans. The mid-troposphere flow showed a distinct split in the western U.S., with some storms flowing through the Southwest U.S. into the southern Midwest, and some clipper systems coming to the northern Midwest across southern Canada (Figure 9a, CDC). The mid-tropospheric heights were above normal in most of northern U.S., indicating the polar vortex stayed further north than normal during the warm month. However, one can also see the Southwest trough with lower heights than normal (Figure 9b, CDC). This split flow pattern is commonly seen during weak to moderate El Niño events. During most of the month, the dominant flow branch affecting the Midwest varied, so a mixture of drier northern clipper systems and moister southwestern lows impacted the Midwest. The southern branch became dominant at the end of the month, allowing enough cold air to enter the Midwest so that large southwestern originating storms on the 24th and 30th brought significant snows to the central Midwest.