July 18-24, 2004
Midwest Weekly Highlights - July 18-24, 2004
Cold and Hot and Cold
Due to active weather fronts passing though the region, the week of July 18-24 started much cooler than normal in the Midwest, became very warm and humid, and then returned to very much cooler than normal conditions. Heavy precipitation occurred in a swath from northern Indiana to southern Ohio, and in more isolated areas elsewhere in the southern Midwest (Figure 1). Much of Missouri, central Illinois, Minnesota, Wisconsin, and the UP of Michigan received less than 50% of normal precipitation (Figure 2), as these areas tended to be behind cold fronts in dry air masses during much of the week. In fact, most of the Midwest received below normal amounts of precipitation. Because of the cold at the beginning and end of the period, almost all of the Midwest was 2-4°F below normal for July 18-24 (Figure 3), despite the existence of heat advisories and warnings in large parts of Missouri and Illinois at mid-period. Finally, the abnormally dry areas in Minnesota, Wisconsin, and Missouri (Figure 4, National Drought Mitigation Center) stayed pretty dry, receiving only 50% or less of normal precipitation for the week.
First Cold....
The period began with a large high pressure center moving across the region from northwest to southeast. A strong upper level ridge-trough pattern over the U.S. brought cooler than normal air deep into the southern U.S.; it was quite reminiscent of a fall or even winter upper air pattern (Figure 5, College of DuPage). On the morning of the 19th, the record low temperature was set in Rolla, MO, and tied in Paducah, KY, with temperatures of 56°F and 59°F respectively. Heavy morning fog in the region due to dew points near 60°F (Figure 6, College of DuPage) prevented any further decline in temperature to record levels elsewhere. On the northwestern side of the high pressure region, a low pressure trough triggered some long lived supercell thunderstorms, including one that moved almost due north to south across both Eau Claire and LaCrosse, WI. The path of the storms can be seen in both radar precipitation accumulations (Figure 7, NWS) and severe weather reports (Figure 8, Storm Prediction Center). Despite the relatively small width of the severe weather band, its placement over two medium sized cities led to substantial amounts of wind damage to roofs, buildings, trees, and power lines, and many hundreds of claims of hail damage to autos and windows. Hail reached up to 2.5 inches in diameter in LaCrosse, where almost 2,000 insurance claims (mostly automobile damage) were recorded with the two main local insurance companies after the storm passage, and two plant nurseries lost thousand of windows on their greenhouses.
Then Hot...
As the high pressure center moved off to the southeast, a strong influx of warm and humid air returned to the north around the west side of the high, first reaching the western Midwest, and later reaching all of the southern and eastern portions of the region. Heat advisories were issued on the 20th across large areas of Missouri, Iowa, and Illinois, and the heat index, a measure combining the effects on humans of temperature and dew point, reach 107 in Kansas City, MO, and 117 in Shenandoah, IA. Conditions were the most severe on the 21st, with St Louis, MO, reaching a heat index of 108, and many smaller towns and cities exceeding this level. Even though St. Louis maintained heat index levels of 100 or greater from 10 AM to 8 PM, the city did not experience any heat related deaths, and heat illnesses appeared to be limited to outdoor workers. Dew points exceeded 75°F over a large portion of the Midwest (Figure 9, COD), and where local conditions brought dew points into the 80s, heat index values above 110 were quite common. The highest heat index values on the 21st in each of the most afflicted states were: 121 in both Fairfield and Centralia in southern Illinois, 108 in St. Louis, MO, 104 in Evansville, IN, and 102 in Dennison, IA. In fact, Fairfield, IL, maintained a heat index of 121 for a least 3 hours, from 3 PM to 6 PM. High temperatures did not exceed 100°F, but with temperatures in the upper 90s and dew points in the lower 80s, dangerous levels of heat were present none-the-less.
With the arrival of warm and humid unstable air, thunderstorms and severe weather also occurred in the Midwest. A weak cold front provided a region for convective uplift along the Wisconsin/Illinois border and eastward. After some weaker storms passed along the border region, a large supercell thunderstorm developed rapidly over the southern suburbs of Chicago (Figure 10, NWS). This cell formed the core of a line of severe thunderstorms that rapidly developed late in the day (Figure 11, NWS) and proceed to move eastward and expand southward.. In addition to raking across northern Indiana and southern Michigan, severe storms moved southward through the Indianapolis region (Figure 12, NWS). More than 3 inches of rain fell in Indianapolis in a 9 hour period. The southern convective line continued southward all the way into Kentucky before fading. Severe weather reports were widespread in the storm path (Figure 13, SPC). The next morning, a second and stronger cold front swept across the Midwest, forming a large rain shield with severe thunderstorms embedded in its eastern edges (Figure 14, COD). Chicago was once again raked with severe winds, complicating recovery efforts from the initial storms less than 24 hours earlier. The line of thunderstorms did cause more severe event reports and damage all the way into Ohio (Figure 15, SPC).
The Chicago area and northern Indiana were the hardest hit areas over the 21st and 22nd. About 250,000 customers lost power in the Chicagoland areas served by ComEd, 200,000 on the 21st and 85,000 on the 22nd (with some overlap). More than 1500 fallen trees were removed from streets, with sizes ranging up to 24 inches in diameter. Building damage was mostly superficial, except where trees fell on houses. The 60 mph winds in Indiana knocked out power to more than 90,000, mostly in or near Gary and Lafayette. A 75 mph gust was measured at the Michigan City Lighthouse. Another 8,000 lost power in southwestern Michigan, and considerable damage occurred to an apartment complex and a trailer park that were impacted by F0 tornadoes. Finally, there were 6,000 who lost power in northwestern Kentucky, some for the third time in three week. Fortunately, no serious weather-related injuries were reported in the press.
...Then Cold Again
By mid-day on the 23rd, the cold front had moved through the region, followed by a strong Canadian high pressure center moving into the Great Plains (Figure 16, NWS). Low temperatures records were set in the northern and western Midwest on the 23rd at International Falls, MN, and Merrill, WI. The coldest day, though, followed on July 24 (Figure 17, when freezing temperatures were measured at both Tower (26°F) and Embarrass (27°F), MN. Low temperature records were set at International Falls (36°F), Duluth (41°F - tie), and St. Cloud (46°F - tie), MN; Marquette, MI (39°F); Merrill, WI (41°F - tie); and Mansfield, OH (50°F). Kansas City, MO, set a new daily minimum temperature in the middle of the day, when it reached 57°F at 12:26 PM, due to a heavy rain and cloud cover north of the cold front. Locations in Missouri (Rolla, West Plains, Columbia, and Kansas City) and Iowa (Des Moines and Ottumwa) also set records for the lowest maximum temperature on July 24; many first order stations only reached the low 70s in that area (Figure 18). A low pressure center moving south of the Midwest sent a large amount of elevated moisture above and north of a surface warm front, keeping cold air trapped at the ground yet providing the moisture necessary for heavy rain. Rain amounts of 2-3 inches fell during much of the day and evening in central Missouri, helping to keep temperatures down. Late in the day, severe thunderstorms (Figure 19, NWS) broke out near Springfield, MO, knocking out power to about 400 homes and causing other minor damage.