Midwest Weekly Highlights - May 25-31, 2005

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Drought Continues to Expand in the Midwest

The last week of May was drier than normal in the Midwest, adding to the deterioration of soil moisture conditions in the central and southern Midwest. Early in the week, a large amount of rain fell in northern Minnesota (Figure 1), but after this event most rain-bearing systems went around the region, and only isolated bands received adequate moisture. Almost all of the Midwest received less than 50% of normal precipitation for the week (Figure 2), which is on average one of the wettest weeks of the year normally. Most of the precipitation bearing systems were steered around the region by surface high pressure from Canada and an upper level trough, so temperatures were also 2-5°F below normal throughout the Midwest (Figure 3). The area under moderate drought status in the U.S. Drought Monitor map expanded in central Illinois and Missouri (Figure 4, National Drought Mitigation Center). Soil moisture anomalies (Figure 5) indicate that without a very substantial above normal rainfall in the first week of June, the moderate drought area in the Midwest will undergo a large-scale expansion.

Peripheral Moisture and Core Dryness

The week started on a promising note with heavy rain in northern Minnesota (Figure 6, University Corporation for Atmospheric Research) as a low pressure center moved through the state. However, upper air currents took the low to the north of the region, and the cold front trailing through the Midwest was largely dry, consisting only of a band of clouds with light showers (Figure 7, College of DuPage - COD). A large upper level trough dipped southward, keeping most of the Midwest under a cool and dry air mass (Figure 8, COD). Even when a weak low pressure center traveled through the central Midwest on the 29th, moderate precipitation dried up as the system crossed the Mississippi River and approached the D1 area in Illinois (Figure 9, NWS). Precipitation falling visibly in the vicinity of the MRCC simply evaporated before reaching the ground for the most part. Only narrow east-west bands such as one north of St. Louis received any substantial rain. On the last day of the period, a low pressure center approaching the Midwest was diverted almost due north over the Great Plains, leaving a long band of precipitation at our doorstep (Figure 10, COD), but with very little rain actually falling on the Midwest in the end. A break in the upper atmospheric pattern is predicted to bring more moisture to the area by the middle of the first week of June, which is absolutely needed to arrest the slide into a more serious drought situation.