Week 2: May 8-14, 2023

Spring in Full Swing

Between widespread warm temperatures and consistent convective precipitation, it is safe to say that the Midwest has finally settled into spring. Average temperatures were above normal for all Midwestern states and reached up to 11°F above normal in localized areas (Figure 1). Missouri had the largest temperature departure of 9.1°F above normal. Average maximum temperatures ranged from near normal to 13°F above normal (Figure 2). The distribution of well above-normal minimum temperatures was heavily centered in the west, where almost all of Iowa and Missouri recorded average minimum temperatures of at least 10°F above normal (Figure 3). This week, there were 66 high maximum and 83 high minimum temperature records broken or tied (Figure 4).

Severe weather was not sparse this week as there were 23 tornadoes, 189 hail reports, and 59 wind reports (Figure 5). The warming surface and present boundaries triggered convective precipitation and thunderstorms almost every day this week, which made for a locally wet week. Precipitation accumulations were near normal (+/- 1 inch) in much of the northern and eastern Midwest, but many areas received above-normal accumulations. A bullseye in southern Minnesota received up to four more inches than normal of precipitation (Figure 6). In fact, Le Sueur County, MN recorded a one-day total of 4.05 inches of precipitation on May 14th. This week, there were 84 precipitation records broken or tied throughout the Midwest (Figure 7).

Severe Weather 

The storms that produced tornadoes and all other threats in Iowa, Illinois, and Missouri overnight on May 7 continued into this week. The last “leg” of this storm became linear and moved east through Illinois and Missouri (Figure 8, 5/8 6am EDT, NCEI) before weakening by early afternoon.

Around 5pm EDT, cells began to pop up and form along cold fronts positioned in southern Missouri and southern Minnesota. The northern storms died off within a few hours, but the southern storms tracked east through Missouri, southern Illinois and Indiana, and Kentucky before clearing the area mid-morning on the May 9. These storms produced plenty of hail, ranging in size from quarters to baseballs (1 in-2.75 in diameter), and strong wind gusts (up to 73 mph). Missouri got a quick break before storms that had developed in Kansas propagated eastward and affected southern parts of the state overnight into May 10; again, large hail (up to 2 in diameter) and strong wind gusts (up to 78 mph) were reported. Scattered storms and precipitation occurred throughout the western Midwest all day on May 11.

Intense storms developed in Nebraska in the afternoon on the 12 and caused a record-breaking number of tornado warnings. The core of this system just barely skirted the Midwest; the remnants began to inch into the far western Midwest overnight into the 13, but severe threats were eliminated by then. Throughout late morning to early afternoon, however, cells began to develop ahead of the surface low and frontal boundaries in central Iowa (Figure 9a, 5/13 11am EDT, NCEI; Figure 9b, 5/13 11am EDT, WPC). Shortly after, storms also started to develop in northeast Missouri, along the Missouri-Illinois border. Between 2pm EDT and 8pm EDT, 21 tornadoes occurred in central Iowa and south-central Illinois. Most of these tornadoes were brief and caused little to no damage. Large hail of up to three inches in diameter was also observed, primarily in the St. Louis, MO area.

Heavy Rain for Southern Minnesota

While the typical severe threats did not affect much of the northern Midwest this week, Wisconsin and southern Minnesota got trailing precipitation from most storm systems. Additionally, lingering convective precipitation began in the morning on May 13 (Figure 10a, 5/13 8am EDT, NCEI) and continued throughout the day and into the 14 (Figure 10b, 5/14 4am EDT, NCEI), before it moved eastward and dissipated in the late afternoon (Figure 10c, 5/14 4pm EDT, NCEI). Much of this area was blanketed with flash flood warnings. For the whole week, stations in Minnesota in Watonwan, Brown, Lyon, Blue Earth, and Le Sueur Counties, to name a few, reported precipitation totals of well over 5 inches; Brown County recorded the highest weekly total of 7.82 inches of precipitation.

Streamflow and Drought Update

The heavy and consistent rain in the north caused streamflow conditions to trend towards much above normal values throughout Minnesota and Wisconsin (Figure 11). Conditions improved in southern Missouri, Kentucky, Illinois, and parts of Iowa and Indiana.

When comparing the May 9 and May 16 US Drought Monitors, slight improvement was seen (Figure 12). The Severe Drought (D2) was reduced to just a small pocket of western Iowa and a smaller area of Central Missouri. Both areas of Extreme Drought (D4), in western Iowa and central Missouri, were reduced to smaller areas, and the small section of Exceptional Drought (D4) in western Iowa was removed.