Midwest Weekly Highlights - February 22-29, 2004

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A Dry and Mild End to Climatological Winter

The period started with a weak low pressure system traveling through the northern Midwest, bringing some snow to northern Wisconsin and Michigan. However, most of the last eight days of February were dominated by high pressure slowly drifting over the area and transforming temperatures from cool to warm as it shifted eastward. Therefore, the period was quite dry, with precipitation amounts exceeding 0.4 inches restricted to the northern Great Lakes region (Figure 1). The Midwest was below average elsewhere, especially in a zone from Missouri to Ohio (Figure 2) that received only 10% or less of normal precipitation. The snowfall for February 22-29 was again restricted to the northern Great Lakes region (Figure 3), leaving much of the Ohio Valley with minimal snowfall for the entire month of February (Figure 3a). The last eight days of February were warmer than normal in the northern half of the region, and near normal in the southern third (Figure 4). The precipitation in central Wisconsin caused a small reduction in the area of severe drought status (Figure 5, National Drought Mitigation Center), but dryness was maintained elsewhere in the western Midwest, and was beginning to emerge in a zone from Illinois to Ohio.

Bookend Systems

The storm at the beginning of the period was slow moving but fairly weak, bringing an impressive shield of precipitation to the northern Midwest on the 22nd and 23rd (Figure 6, College of DuPage). Unfortunately, precipitation amounts were fairly small south of central Wisconsin and Michigan, with a few localized exceptions. After some lake effect snow added to the coverage in the Upper Penninsula of Michigan, snow depth values reached levels that were better at the end of February than in most recent years (Figure 7, Marquette NWSFO). Days of quiet weather under the dominance of high pressure came to an end on the last day of the month. A very strong low pressure system with a central pressure of 994 mb moved towards the region late in the day (Figure 8, COD); however, upper air steering patterns caused the low to move slowly and nearly south to north just west of the Midwest region (Figure 9, COD). The strength of the pressure gradient produced sufficient winds to send an elevated frontal zone far into the Midwest ahead of its arrival, causing brief but heavy convection for the first time this season in the region (Figure 10, COD). More precipitation will occur in the new month as the low pressure center and its upper-air support enter the Midwest.

February 29

An interesting sidelight to this month was the occurrence of some records on the leap day itself. While official normal conditions for leap years are usually derived from those for March 1, climate records can still only be set on the day itself. Therefore, records may seem rather small and disproportionate to those of the preceding or following day, as there is only one-fourth the number of climate observations available. Several places in Michigan either tied (54°F, Marquette), or set new temperature records (49°F, Sault Ste. Marie; 53°F, Alpena). Only 0.09 inches of precipitation at Minneapolis, MN, and 0.18 inches at Springfield, MO, were sufficient to set new daily precipitation records at both locations.