April 22-30, 2008
Midwest Weekly Highlights - April 22-30, 2008
Cold and Wet West, Warm and Dry East
The last ten days of April started out in a spring-like fashion, but ended with unseasonably cold weather and in some places, heavy snow.
Average daily temperatures were generally colder than normal west of the Mississippi River, and warmer than normal east of the Mississippi. Departures from normal ranged from 8°F to 12 °F below normal in western Minnesota to 5 °F above normal in northeastern Ohio (Figure 1). The first surge of 80°F temperatures into the Midwest occurred early this week with maximum temperatures in the mid-70s as far north as the Great Lakes. However, by April 28 temperatures struggled to climb through the 40s and 50s as a strong upper low moved through the southern Great Lakes.
The precipitation pattern this week mirrored the temperature pattern, with wetter than normal weather west of the Mississippi River, and drier than normal weather east (Figure 2). Snowfall was heavy across western Minnesota and parts of central Wisconsin (Figure 3).
A Warm Start
A weak cold front pushed through the Midwest on April 22, but it was enough to kick off thunderstorms from Missouri into Iowa and western Illinois. There were a few severe storms in across Missouri, southern Iowa, and western Illinois, mostly from hail. As high pressure moved quickly into and east of the region on April 23, strong southerly winds developed and funneled warmer air on the the Midwest. The result was the warmest day of the spring to date, with maximum temperatures reaching the 80s as far north as central Missouri, Illinois, and Indiana, and with 70s occurring into the southern half of Minnesota, all of Wisconsin, and lower Michigan (Figure 4).
Severe Storms Sweep Across Midwest
The increase in warm and more humid air set the stage for some additional severe weather. A more substantial low pressure system was developing over northeastern Colorado on the morning of April 24 (Figure 5). Severe storms developed on April 23 over Iowa, with two producing tornadoes which resulted in minor damage. Severe storms occurred all along the cold front on April 25, with severe reports from the Michigan UP south through Texas. Early in the day thunderstorms over central and eastern Iowa dropped more than four inches of rain in 12 hours, causing widespread flash flooding (Figure 6). The storms continued into the mid-morning hours before moving east and dissipating. Additional storms fired up during the afternoon. One, and possibly more, tornadoes touched down near Wyocena, WI (Columbia County), damaging homes, destroying barns, and knocking out power to hundreds of homes. Several thousand homes and businesses were without power near Portage, WI (Portage County) from severe thunderstorms and a possible tornado. Hail the size of grapefruit (4.24 inches) was reported near Tomahawk, WI (Lincoln County). At least two tornado touchdowns were also reported in Iowa.
Heavy Snow Blankets Minnesota, Central Wisconsin
The low moved east to northwestern Missouri by April 25, then turned north-northeast and rapidly intensified by the time it reached western Lake Superior on April 26 (Figure 7), stacked with a strong upper level low (Figure 8). Unseasonably cold air was pulled into this system, and snow became heavy across eastern South Dakota into western Minnesota late on April 25 into April 26. Eight to 18 inches of snow fell in this area, with a number of locations setting new April snowfall records (Figure 9). Pelican Rapids, MN measured 18 inches of snow, and Fergus Falls, MN (both in Otter Tail County) measured 15 inches of snow. The heavy, wet snow made plowing of roads difficult, and forced authorities to close a portion of Interstate 94 for seven hours on April 26. Numerous vehicles were off the road, and there were 34 collisions, four resulting in injuries and one resulting in two fatalities.
A second deep upper level trough swung through the Midwest on April 28, bringing some heavy snow to portions of central Wisconsin, with lighter amounts east through Michigan (Figure 10). Whiting, WI (Portage County) received 5.5 inches, and Wild Rose, WI (Waushara County) reported 4.8 inches of snow. Showers of snow and sleet occurred as far south as central Illinois as the cold upper low spun over northern Illinois and southern Wisconsin (Figure 11).
A Chilly End
As the low departed to the northeast Canadian high pressure associated with an unseasonably cold air mass built into the Midwest. High temperatures on April 28 were generally in the 40s across the northern two-thirds of the region, and only reaching the mid to upper 50s in the far south, generally 20°F to 25°F below normal. The weather began to warm on April 29, but temperatures were still much below normal (Figure 12). On April 29 the low temperature at Chicago's O'Hare International Airport dropped to 31°F, tying the record for the date first set in 1958. In Missouri, a record low of 31°F also occurred at Springfield, breaking the old record of 34°F set in 1969. Vichy/Rolla set a record low of 30°F, breaking the old record of 34°F set in 1996. Temperatures began to approach normal across the Midwest on April 30 as winds shifted into the south ahead of the next weather system.
At the end of the month major flooding was occurring on the Mississippi River between Rock Island, IL and Burlington, IA. Moderate flooding was occurring on some rivers in Iowa, northern Illinois, and southern Wisconsin.
SDH