Temp departure Precip PON Surface map 12-17 Surface map 12-19

Midwest Weekly Highlights - December 15-21, 2004

Colder Weather, More Lake-Effect Snow

It was a generally dry week across the Midwest, although cold air streaming across the relatively warm waters of the Great Lakes brought more lake-effect snow to northern Indiana and the Michigan Upper Peninsula.  After two weeks of seasonably mild weather, the first outbreak of arctic air knifed through the Midwest early in the period.  Temperatures averaged well below normal over the eastern two-thirds of the region, and near to above normal in the western third (Figure 1). Daily average temperatures ranged from 8F below normal from eastern Michigan south through eastern Ohio and into eastern Kentucky, to near normal from eastern Minnesota to southwestern Missouri. From far western Minnesota to northwestern Iowa, temperatures were 4F to 7F above normal.  Precipitation this week was limited to the northern Midwest and to the lake-effect snow belts (Figure 2).

First Arctic Blast

The first surge of cold air was already plunging south out of Canada on December 15, and by December 17 the leading edge of the cold airmass had reached the Ohio River (Figure 3, Unisys). This was only a prelude, however, to Arctic air that knifed south into the Midwest on December 18-19.  By Sunday morning, December 19 the leading edge of this cold air mass had pushed almost to the Gulf coast (Figure 4).  Strong northerly winds generated by high pressure over Minnesota and a strong low over New York pulled the cold air southward.  Daytime temperatures on December 19 hovered at 0F and below over the northern Midwest and in the teens as far south as the Ohio Valley.  Low temperatures were -20F to -30F in northern Minnesota and the Michigan UP (Figure 5).  Sault Ste Marie, MI tied a record low of -20F.  The extreme cold was accompanied by sustained winds 25 30 mph which produced dangerous wind chill temperatures.  Three deaths in Minnesota were attributed to cold weather over the weekend.

Snow Machine Cranks Up Over Indiana

The strong northerly winds streaming south over the open and relatively warm waters of the Great Lakes produced heavy snow in northern Indiana.  A narrow but strong band of snow set up north to south over lake Michigan, targeting the northern Indiana shore of the lake.  The radar images from Grand Rapids, MI (Figure 6) and Romeoville (Chicago), IL (Figure 7) show this band on the morning of December 19.  The result was extremely heavy snow over a relatively small area.  Michigan City, IN piled up 26 inches of snow from this event, and Westville, IN received 20 inches, both in LaPorte County.  Outside this heavy core of snow, amounts dropped off considerably, but still were significant with South Bend receiving 8 inches of snow (Figure 8, NWS).  Lake-effect snow also occurred in the Michigan UP and in northeastern Ohio, but amounts were in the 3 to 8 inch range as the fetch of the northerly wind over Lakes Superior and Erie was considerable smaller than that over Lake Michigan.

Widespread Snow Across Upper Midwest

A low pressure system moving out of the northern plains ahead of the next Arctic air mass brought snow to a large portion of the upper Midwest on December 20-21.  Snow fell across Iowa, southeastern Minnesota, much of Wisconsin, and the northern half of lower Michigan, much to the delight of winter sports enthusiasts.  Snowfall from Iowa through southwestern Wisconsin was in the 2 to 4 inch range, while in central Wisconsin 6 to 10 inches of snow fell from La Crosse across to Green Bay (Figure 9).

Developing Storm Threatens to Snarl Holiday Travel

As the snow was tapering off over the northern Midwest, a low pressure system organizing over Texas on December 21 was threatening to spread heavy snow across much of the lower Midwest. By the evening of December 21 winter storm watches and warnings were posted from northern Arkansas through the Ohio Valley northeastward to eastern New York (Figure 10, SPC).  Snow began to fall across southern Illinois and Indiana late on December 21.  This storm and its impacts will be covered in the December 22-31 edition of the Midwest Climate Watch.

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