Midwest Weekly Highlights - December 22-31, 2009

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Major Storm Cripples Parts of the Midwest

A massive storm system affected much of the nation in the days leading up to Christmas. On the cold side of the storm, heavy snow and blizzard conditions brought holiday travel to a halt. On the warm side of the storm, heavy rain led to some flooding but milder temperatures were a change from the chill of much of December.

Temperatures the last ten days of December were near to much warmer than normal from Minnesota eastward, and normal to below elsewhere. Temperatures were as much as 9°F above normal in northern Minnesota and 4°F above normal in northern Wisconsin (Figure 1). The lower Ohio Valley was 2°F to 3°F above normal. In contrast, temperatures in eastern Missouri and eastern Iowa were 1°F to 3°F below normal. There were only a few record temperatures during this period, with some record high minimums during the first half and and some record cold readings during the last half of the period.

There was a remarkable gradient in precipitation this period, almost all as a result of the path taken by the pre-Christmas storm. Precipitation was 200 to 700 percent of normal west of a line drawn north to south along the Illinois-Indiana border (Figure 2). For much of this area precipitation was at least three times normal. East of the line, normal to slightly above normal precipitation was found in narrow bands across Michigan, Indiana, and Ohio. Precipitation in Kentucky was less than 50 percent of normal across much of the central part of the state.

Snowfall was much above normal across the western and central portions of the region (Figure 3). Snow was five to seven times normal for the period from eastern Missouri northward to eastern Minnesota. By the end of the month, snow covered the ground in most of the area north of the Ohio River, with the deepest snow over western Iowa, northern Minnesota, and the Michigan Upper Peninsula (Figure 4). The extent and amount of snow on the ground was above normal for the end of December (Figure 5). Despite heavier than normal precipitation in northern Wisconsin this period, Moderate to Severe drought conditions continued to be depicted on the U.S. Drought Monitor (Figure 6).
 

A New Major Winter Storm

On December 22 it was apparent that the pre-Christmas holiday travel period In the central U.S. was going to be impacted by a major winter storm. The low pressure system punched into the Washington coast on December 21, and then moved into the Rocky Mountains where it started to organize and gather strength (Figure 7). In anticipation of the storm a variety of winter weather advisories and warnings were issued for an area extending from the Texas Panhandle northward to the U.S. - Canadian border (Figure 8). On the morning of December 24 the center of intensifying storm was located over the Texas-Louisiana border and was forecast to move almost due north (Figure 9). Gulf moisture streamed northward on the east side of the system, fueling heavy rain from the Gulf states north through Illinois. By midday on December 24 winter storm and blizzard warnings were in effect for most of the Midwest west of the Mississippi River, and flood advisories were in effect in eastern Missouri and across all of Illinois (Figure 10).

Precipitation with this storm began as a mix of rain, drizzle and freezing in western portions of the Midwest on December 22, but gradually changed over to snow over northwestern Iowa as cold air overspread the area. Snowfall amounts of 4 to 6 inches occurred over northwestern Iowa, 4 to 7 inches over southern Minnesota, and 3 to 5 inches from southern Wisconsin into northern Illinois on December 23 (Figure 11). Further to the east there was some glazing or trees and power lines in west central, central, and east central Iowa. Colder temperatures over northern Iowa resulted in dangerously icy highways.

The storm's main push came on December 24 as it moved northward from Louisiana to a position over eastern Iowa on December 25 (Figure 12). Heavy snow and blizzard conditions occurred from western Missouri northward into southwestern Minnesota. Winds gusting to 50 mph whipped the snow into large drifts. On the eastern side of the storm, temperatures were relatively balmy for late December, with early morning temperatures in the mid to upper 40s across the southern half of Illinois and Indiana, and in the 50s in central Kentucky (Figure 13). Warm air aloft caused problems further to the north, where freezing rain and freezing drizzle mixed with snow. Many churches across central and western Michigan canceled morning Christmas services because of treacherous roads. Freezing rain and drizzle fell as far north as the Michigan Upper Peninsula. Cold air wrapped around the storm into the southern Midwest during the day, changing the rain to light snow.

The surface low pressure system was stacked below an intense upper level low, and as a result it remained nearly stationary over the central Midwest through December 26 (Figure 14). Several more inches of snow occurred across the Midwest as the low spun overhead. The low finally weakened and began to move east of the region on December 27 (Figure 15).

Snowfall totals from this storm were as much as two feet in northwestern Iowa, where 24.0 inches were measured at Spencer (Clay County) and 23.5 inches at Spirit Lake (Dickinson County). The snow at Spencer surpassed their previous record storm total of 19.0 inches set February 17-18, 1962. Storm snowfall was 12 to 18 inches of portions of Minnesota and the Michigan U. P. (Figure 16), and a deep snow cover existed over the northwestern half of the region (Figure 17).

Snow and Cold to Finish Out the Year

Much colder air coming into the Midwest on northwest winds produced significant lake effect snow in Michigan, Indiana, and Ohio in the days following the storm (Figure 18). The leading edge of Arctic air pushing through the Midwest on December 31 maintained the lake effect snow and also produced a band of several inches of snow from Iowa and northern Missouri eastward through Indiana. Although the last ten days of December were warmer than normal across much of the Midwest, the last four days of the month were significantly colder, ranging from 10°F to 12°F below normal over the deep snow in northwestern Iowa to 4°F to 7°F in the reminder of the region (Figure 19). Only western Michigan and the Michigan U. P. were near normal the last four days of December.

At the end of the month minor to moderate flooding was occurring along the lower half of the Illinois River.

SDH