Midwest Weekly Highlights - November 17-23, 2004

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Late Season Mild Weather Continues

The Midwest had relatively moist and warm conditions during the week of the 17th to the 23rd. Generally light precipitation of about half an inch covered a wide swath of the central Midwest, and heavier one inch rains fell in northern Iowa, western Wisconsin, and the upper and lower parts of the Ohio Valley (Figure 1). There were only a few areas that received less than 50% of normal precipitation for the week, including the northwestern tier of the Midwest, northwest Missouri, and parts of Kentucky (Figure 2). The entire Midwest was very warm for the whole period, with the core of the Illinois-Indiana-Ohio region over 10°F above normal, and every location in the Midwest being at least 7°F above normal (Figure 3). The precipitation in the Iowa-Wisconsin area eroded some of the abnormally dry conditions there, disconnecting the Great Lakes dry area from the Great Plains in the U.S. Drought Monitor map (Figure 4, National Drought Mitigation Center).
 

Modest Systems Make for a Moderate Week

The week started on the 17th with high pressure to the southeast circulating relatively warm and humid air from the south into the Midwest (Figure 5, College of Du Page - COD). Temperatures in the 60s where widespread, and even dew point temperatures reached the low 60s in places. This influx caused widespread fog to form during the day in the central and northern Midwest (Figure 6, COD - fog is indicated by "F" on the map). Low pressure approached the southwestern Midwest on the 18th, bringing precipitation to southeastern Missouri before moving further north to cause moderate rain in Iowa, southern Minnesota, and Wisconsin by the end of 19th (Figure 7, COD). A weak cold front was followed into the region by a western high pressure center on the 21st, creating mild and dry conditions in the Midwest until late on the 23rd. On the last day of the week, the upper air steering currents finally became stronger and a large trough formed to the west of the Midwest (Figure 8, COD). A low pressure center started moving northeastward from west Texas towards Missouri. In the meantime, the upper level trough progressed eastward, allowing cold air to begin plunging into the Midwest from Canada. By the end of the 23rd, moderate precipitation broke out in southwestern Missouri (Figure 9, NWS), and the National Weather Service began forecasting widespread snow for the next period (Figure 10, COD - white boxes indicate a winter storm watch in Missouri).