December 1-7, 2007
Midwest Weekly Highlights - December 1-7, 2007
Winter Takes Hold
Temperatures were below normal and snowfall was above normal across most of the Midwest the first week of December with residents coping with everything from freezing rain to snow.
The entire nine-state region experienced below normal temperatures this week (Figure 1), but there was a well-defined southwest to northeast gradient in the temperature departures. Temperatures departures ranged from about 1°F below normal in southwestern Missouri to 6°F to 8°F below normal from Ohio through Michigan. A small portion of northwestern Minnesota had temperature departures from 10° to 12°F below normal. This area, with an 8 to 12 inch deep snow pack, experienced several days this week with sub-zero minimum temperatures.
Precipitation was five to seven times the average for the week from northwestern Iowa through Minnesota, and from 100 to 200 percent of normal from eastern Illinois east through Ohio, and from the Ohio River north through Michigan (Figure 2). Only southwestern Missouri and southern Kentucky were unusually dry, with less than 50 percent of normal precipitation. There was little change in the December 4 U.S. Drought Monitor for the Midwest region (Figure 3). Some precipitation did fall in the drier southern Midwest after the Drought Monitor was issued.
Much of the precipitation that fell in the Midwest this week was of the winter variety. Snowfall this week was heavy across northern portions of the region, and several inches of snow accumulated as far south as the Missouri Ozarks and the Ohio River (Figure 4). Snowfall was much above normal throughout the entire Midwest except for far southern Missouri and southern Kentucky (Figure 5). At the end of the week snow was on the ground across most of the Midwest except for extreme southern Missouri, the far southern tip of Illinois, and the western half of Kentucky (Figure 6).
Major Storm Ushers in December
For the second year in a row, the month of December began with a major storm in the Midwest with advisories, watches, and warnings posted for a large part of the region (Figure 7). A strong storm that hammered the Pacific Northwest the last two days of November was organizing over the Rockies on December 1 (Figure 8). Cold air was in place over the Midwest near the surface, while warmer air streamed in at mid-levels of the atmosphere, a perfect set-up for freezing rain. A large shield of freezing rain spread eastward through Missouri and Illinois during the morning, reaching Indiana by early afternoon. Snow fell north of the freezing rain area from across Iowa eastward through southern Wisconsin and northern Illinois and southern Michigan. Southerly winds strengthened over the Midwest as the main storm moved east and intensified, and that brought warm air northward into the southern half of the Midwest. That melted the ice over the central Midwest and brought a change from snow to freezing rain and sleet to the north. The warm air surged northward early on December 2, and high temperatures reached the low and mid 60s from central Missouri into central Illinois early in the day (Figure 9). A strong cold front racing through the Midwest brought an abrupt end to the brief period of mild weather, and temperatures plummeted into the 30s over most of this region by sunset.
By the morning of December 3 the center of the low was located over New York state, but the strong circulation extended back west through Wisconsin and Illinois. In southeastern Michigan, the high winds knocked out power for more than 8,000 customers of DTE energy in the Detroit area. A number of schools closed due to weather-related power outages. The strong winds blowing over the open waters of the Great Lakes also produced bands of lake effect snow across the Michigan Upper Peninsula and northern lower Michigan (Figure 10). The lake effect snow continued for several days as winds continued to stream across the lakes. On December 5 lake-enhanced snow developed 100 miles downwind of Lake Michigan into east-central Illinois as strong northerly winds carried moisture downstream (Figure 11).
Arctic Cold Dumps Into Midwest
Unseasonably cold air spilled into the northern half of the region behind the departing low pressure system, and the morning of December 6 was the coldest morning of the season so far (Figure 12). The temperature dropped to -15°F to -20°F over snow covered portions of northeastern Minnesota and northwestern Wisconsin, with a reading of -28°F at Diamond Lake, WI (Taylor County). Chicago's O'Hare International Airport dropped to 0°F, tying the record for December 6 set in 1972. This was the earliest occurrence of 0°F in Chicago since 1976. Light winds, clear skies, and snow cover allowed low temperatures to drop below zero in the western and northwestern suburbs of Chicago.
Next Storm Races In
As the upper Midwest shivered in the Arctic cold the morning of December 6, another storm system was organizing over the Rockies. This storm raced east through the Midwest on December 6 and December 7, laying down a swath of snow over the central third of the region (Figure 13). General snowfall amounts were from 2.0 to 3.0 inches, although some locations received as much as 5.0 inches in Iowa, and as much as 5.0 to 7.0 inches accumulated in southeastern Ohio.
SDH