May 1-10, 2004
Midwest Weekly Highlights - May 1-10, 2004
Severe Weather Season Arrives in the Midwest
The period of May 1-10 started with cool temperatures, but quickly switched to warm and humid weather with synoptic patterns supportive of severe weather outbreaks. Significant severe weather events were encountered during each of the last 5 days of the period, May 6-10, and heavy rains helped to ameliorate dry conditions, especially in the southern Great Lakes area. A band of 2-4 inches of precipitation was laid down between southeastern Minnesota and southern Michigan (Figure 1). Another region of heavier precipitation included southern Missouri and southern Illinois, and the southern Midwest in general received near normal amounts of rain. Southern Michigan topped the scales at more than 300% of normal rain for the ten-day period, while parts of northern Minnesota and the Upper Peninsula of Michigan received less than 25% of normal (Figure 2). The dry areas were too far north to receive much precipitation as a frontal boundary moved back and forth across the central Midwest during the last half of the period. The northern tier of the Midwest north of the frontal boundary also had temperatures 2-6°F below normal for the period, while the southern Midwest was 1-3°F above normal (Figure 3). While the first few days of the month included some expansion of abnormal dryness through the Corn Belt (Figure 4, National Drought Mitigation Center), some of this will have been reduced by the end of the period, especially in southern Michigan.
First the Warm Temperatures...
Many places reported sub-freezing low temperatures during the mornings of May 3 and 4. On the 3rd, Rhinelander and Stevens Point, WI, actually fell below 20°F, to 19°F, tying a record in the former and setting a record in the latter (Figure 5). Several locations in Wisconsin, Iowa, and northern Illinois set new low temperature records on the 3rd, while Toledo and Mansfield, OH, set or tied records on the morning of the 4th. Temperatures began warming soon afterward, however, as a warm front moved northward across the western Midwest.. Strong warm advection brought summer temperatures to Missouri and Iowa by the 5th, when a high temperature of 95°F was recorded in Sioux City, IA, setting a record for the day. Further high temperature records were set on May 9 in Mankato, MN (91°F), and South Bend, IN (88°F).
...then the Severe Weather
The weather was quite active during the period, with a frontal boundary over the central or northern Midwest during most of the period. Storm reports were recorded each day from May 6 to May 10. During the morning on the 7th, golf ball sized hail caused substantial property damage in the Iowa City area, including at a large car dealership, and high winds overturned trucks. Hail was reported all along the northern tier of Iowa (Figure 6, Storm Prediction Center). A set of strong storms struck Iowa on the evening of the 8th (Figure 7, National Weather Service), with heavy precipitation, severe hail and winds, and several tornadoes reported west of Des Moines (Figure 8, SPC). Flash flooding was reported in eight counties in Iowa, including Guthrie, where the tornadoes occurred. On the 9th, the frontal boundary shifted northward as a warm front, bringing a large outbreak of severe weather to southern Minnesota (Figure 9, SPC). A line of strong thunderstorms hit the Minneapolis-St. Paul, MN, metro area at about 5:30 PM on Mother's Day (Figure 10, NWS). Most of the damage was done by straightline winds in excess of 70 mph, although damage patterns were being examined for suspected small tornadoes embedded in the storms. About 38,000 customers of the local utility lost power, and substantial tree losses and some building damage were apparent. Finally, on the 10th, severe weather reports were scattered, as severe thunderstorms occurred in a widespread but disorganized fashion in the eastern Midwest (Figure 11, SPC). Overall, though, the beginning of May this year was mild compared to last May, when at least 12 tornadoes swept through southwestern Missouri on May 4, 2003, killing 19 people and causing more than $400 M in damage.