July 11-17, 2005
Midwest Weekly Highlights - July 11-17, 2005
Dennis Helps Some, But Not All
The week of July 11-17 in the Midwest was dominated by the remnants of Hurricane Dennis. The only significant rain of the period was associated with the moisture and instability brought northward from the Gulf of Mexico into the south-central and southeastern Midwest. From 2 inches to over 4 inches of rain fell in southeastern Missouri, southern Illinois and western and southern portions of Kentucky, while 1-3 inches fell in central Illinois, southern Michigan, and many locations in the Ohio Valley (Figure 1). Rains exceeded 200% of normal over these Dennis influenced areas, but locations north and west of the Dennis influence were very dry this week, with a large portion of Iowa, Minnesota, Wisconsin, northern Missouri and northern Michigan receiving less than 25% of normal precipitation (Figure 2). The thick cloudiness associated with Dennis kept temperatures slightly below normal in the south-central Midwest, but temperatures in the northern Midwest were 6-10°F above normal (Figure 3) due to a high pressure ridge extending well into the Hudson Bay region of Canada. The combination of dryness and warmth accelerating deterioration of conditions in Wisconsin and northern Michigan, where drought status intensified (Figure 4, National Drought Mitigation Center). Most of southeastern Missouri and southern Illinois experienced 1 or 2 category improvements in drought status levels due to Dennis precipitation, but the spatial extent of amelioration was not as large as was hoped at the beginning of the week.
Six Days of Dennis
On the morning of July 11, Dennis was no longer a tropical storm, but still maintained a very symmetrical cloud mass centered above western Tennessee (Figure 5a, College of DuPage - COD). The first major bands of precipitation had already reached into the south-central Midwest at this point in time (Figure 5b, National Weather Service - NWS). For the first few days, Dennis maintained this dense cloud mass and circular organization centering on southern Illinois, and dumped some of its largest rain totals in southeastern Missouri and southwestern Illinois (Figure 6, NWS). By the afternoon of the 12th, Dennis still looked organized on satellite images (Figure 7, COD), but was no longer producing widespread precipitation. Instead, the rain was concentrated in arcing curves reminiscent of feeder bands (Figure 8, NWS). Accumulated precipitation patterns from this period are filled with arcing curves, including the zone in the central Illinois D3 drought area that received more than an inch of rain (Figure 9, NWS).
The cloud swirl of Dennis was still quite visible on July 13, centered over southern Indiana (Figure 10, COD). The accumulated precipitation over Indiana clearly displayed a similar curvature, with concentric bands of heavier and lighter precipitation totals (Figure 11, NWS). However, by the end of the 14th, Dennis could no longer maintain its elegant structure, and the dense cirrus shield aloft dissipated, revealing a more chaotic, cellular structure of convection (Figure 12, COD). The loss of the upper level cloud cover, though, allowed more sunlight to reach the very unstable air mass near the surface, firing up thunderstorms in a more quasi-random pattern. Dennis finally was kicked out of the Midwest by a dip in the upper air jet stream on the 16th (Figure 13, COD) that took it northeast out of the Ohio Valley, leaving behind a mottled pattern of precipitation in the eastern Midwest (Figure 14, COD).
The overall impact of Dennis was quite beneficial to late starting crops in the south-central Midwest, and National Agricultural Statistics Service (NASS) reports indicated some improvements in soil moisture and crop conditions. However, much of the corn crop in that area was already far enough along with fertilization to have been damaged by the drought before Dennis arrived. The soybeans should benefit much more from the Dennis, having improved soil moisture prior to pod setting and filling that usually occurs in August. Unfortunately, only the central Illinois portion of the D3 region in the Midwest received enough rain to hold its own against the drought; the rest of northern Illinois continued to miss widespread rain events, and appears to be suffering major yield losses. The differences between the forecast prior to the arrival of Dennis (Figure 15, Hydrometeorological Prediction Center) and the actual precipitation for the week (Figure 1) was very large, although the center of precipitation was located correctly.
After Dennis
On the evening of the 17th, the only strong cold front of the week entered the western Midwest, and was located across southern Minnesota and northwestern Iowa at the end of the period (Figure 16, University Corporation for Atmospheric Research). The precipitation from this incursion will be reported in the totals for next week. The drought status map for Missouri was updated on the 13th to reflect the beneficial impacts of Dennis in southeastern Missouri, but also to place more counties in central and northern Missouri into Drought Alert mode (Figure 17, State of Missouri). On the 15th, the governor of Wisconsin declared a statewide drought emergency so as to expedite permits for emergency water withdrawals. In these states, as well as almost all of Illinois, information is being gathered at the county level by the Farm Service Agency extension agents to document the drought in preparation for agricultural disaster declarations in the near future. While the front may indicate a more active weather pattern for the upper Midwest next week, it is also expected to get warmer, too. The drought in the Midwest will continue to intensify in areas missed by organized precipitation.