September 15-21, 2006
Midwest Weekly Highlights - September 15-21, 2006
The third week of September was again active, and temperatures for the third consecutive week were on the cool side for most. The average daily temperature departure map for the Midwest shows that much of the Midwest was below average for this time of year, with the exception being across northern Lower Michigan and northern Minnesota (Figure 1). Here, daily temperature departures were very close to normal, to even 1-3°F above normal. This was not the norm, however, for the remainder of the Midwest. The coolest spot this week was in central Iowa, where temperature departures were 4-6°F below normal. Elsewhere, below normal temperatures stretched from southern Minnesota to southern Missouri, and then eastward across the remainder of the Midwest, where temperature departures were 3-4°F below normal, to 2-4°F below normal respectively.
Precipitation for the week was largely non-existent, with the exception being across the western portions of the Midwest. The heaviest precipitation this week fell across western Minnesota, northwestern Iowa, and southern Illinois (Figure 2). These locations saw precipitation totals that were 150% of normal across southern Illinois, to an astounding 200 to 400% of normal for this time of year in western Minnesota. Meanwhile, much of Wisconsin, central Missouri, northern Illinois, and lower Michigan saw precipitation totals that were only 0 to 25% of normal. Drought conditions actually worsened across northern Minnesota, where D3 categorical drought increased this week, while parts of Missouri also experienced some worsening drought (Figure 3). The extremely dry conditions across the arrowhead of Minnesota have resulted in a few wildfires, a few of which are in excess of 1000 acres. These fires were primarily located in the Superior National Forest, and burning permits are required across northeast Minnesota. Over the course of the year, over 1500 fires have been reported in Minnesota alone (Figure 4, Minnesota DNR).
Late Season Severe Weather Outbreak
Warm southerly breezes ahead of a strong low pressure system boosted temperatures to record levels on September 15 at International Falls, MN, where the maximum temperature reached a balmy 82°F, breaking the old record of 80°F set in 1968. This area of strong low pressure scooted across the upper Midwest on Saturday, September 16, sweeping a potent cold front across the region, and spawning numerous severe thunderstorms (Figure 5). Radar images initially show a few cellular storms in the Dakota's (Figure 6), and as time progressed, these storms collided and developed southward along the cold front (Figure 7). The north end of the line proved to be the most dangerous portion of the storms, where the wind shear was greatest and instability maximized ahead of the cold front. This resulted in a few tornado reports across South Dakota before heading eastward into Minnesota. At 9:55pm, CDT (radar-inferred), a tornado roared through the town of Rogers, MN in northern (Hennepin County. The tornado injured seven persons, damaged more than 300 homes, and sadly, killed a 10-year old girl. The young girl was killed when the neighbor's house she was seeking shelter in collapsed on her. According the the National Weather Service in Chanhassen, MN, the damage was determined to have been caused by an F2 tornado, with a path length of 8 miles. In all, there were over 200 reports of severe weather from this system on the 16th (Figure 8).
Radar images near the time of the event show an apparent quasi-linear convective system (QLCS, a.k.a. squall line) moving rapidly northeast across the northern suburbs of Minneapolis, MN. The analysis shows that the tornado was not immediately detected by radar until it was already descending on Rogers, MN. Thus, there was little or no time to issue a tornado warning, and sirens did not sound for Rogers, MN. Unfortunately, this has prompted much controversy across the scientific and political communities alike. Further investigations on this event are ongoing.
November-like Weather in September
With the passage of the cold front came breezy, cloudy, and cool conditions to much of the Midwest from the 18th-20th. High temperatures on the 19th failed to reach 60°F over a large portion of the Midwest (Figure 9), as a large area of upper-level low pressure spun across the Midwest (Figure 10). The cold air running over warm Lake Michigan water temperatures even resulted in the development of a few lake-effect rain showers in western Michigan (Figure 11). Finally, as the low pressure system responsible for the cloudy and cool weather shifted eastward, high pressure settled in, and when combined with clear skies and low dewpoint temperatures, the stage was set for very cool nights. Temperatures dropped into the lower 30's across northern Minnesota and much of north-central Wisconsin on the 20th, ending the growing season as frost and freeze advisories were posted (Figure 12).
Finally, the final day of the third week of September featured an area of strong low pressure located over southwestern Kansas. This system had spread widespread, generally moderate, rainfall to western Iowa and parts of Missouri on the 21st (Figure 13). This system produced several tornadoes in the Plains states, and will prove to be a major weather feature next week.
Will the month of September end on a cool note, or will we warm up before the start of October? Find out in next week's edition of the Midwest Climate Watch.
Kruk