July 11-17, 2004
Midwest Weekly Highlights - July 11-17, 2004
Large Severe Weather Outbreak Marks Another Active Week
The week of July 11-17 started with heavy rain in northern Minnesota (noted in last week's
report), with a slowly propagating cold front leading to local heavy rains, ending with a major
severe weather outbreak on the 13th through Illinois, Indiana, and Kentucky. A second low later
in the week brought heavy precipitation to the southern Midwest on the 15th, and to north-south
bands through the eastern Midwest on the 16th. Therefore, most of the Midwest received at least
some rain, with the most active areas in Minnesota, northwestern Missouri, and southern Indiana
measuring more than 2 inch rain totals (Figure 1). These wetter areas recorded more than 200%
of normal precipitation for the week, with only the Chicago area and parts of Wisconsin having
less than 50% of normal (Figure 2). Temperatures in the Midwest were generally within a few
degrees of normal, with 3°F below normal temperatures in southern Iowa, and the about 3°F
above normal temperatures in northern Wisconsin (Figure 3). Due to a recent lack of rain,
abnormally dry conditions were identified by the U.S. Drought Monitor in the southeastern
quadrant of Missouri, while substantial drought amelioration occurred in northern Minnesota
(Figure 4, National Drought Mitigation Center). Agricultural interests are still expecting a
bumper crop in the Corn Belt, although there will be some concern if cooler and wetter than
normal conditions persist too much longer.
Heat Wave and Scattered Storms
A low pressure system entered Iowa on the 11th, drawing warm and moist air into the central Midwest and causing heavy rain and isolated severe weather as it traveled east. A substantial cluster of thunderstorms crossed Iowa on the 11th, bringing 1-2 inch rain totals across the state from northwest to southeast (Figure 5, NWS), continuing eventually through Illinois and into southern Indiana. To the north, several weak tornadoes touched down in northern Wisconsin, but did not cause serious damage. Behind the initial line of storms, another strong cluster originating in northwestern Missouri continued across the northern boundary of the state before turning south in Illinois (Figure 6, College of DuPage). The next morning, lightning from an isolated storm in St. Paul, MN, caused a fire that consumed a 60,000 gallon heavy oil tank.
A large set of counties around St. Louis were under heat advisories for the 11th through the 13th.
By the third day, a large pool of dew points over 75°F (Figure 7, COD) and mid-90s temperatures
under strong solar heating led to heat index values peaking near 108 in Kansas City, MO, 112 in
St. Louis, MO, 110 in Muscatine, IA, and 114 in Lincoln, IL. With this amount of hot and humid
air present, a strong northwesterly jet stream flow (Figure 8, COD), and highly unstable
atmosphere, the stage was set for a severe weather outbreak.
Severe Weather Outbreak on July 13
The outbreak began shortly after local noon with the formation of a single supercell thunderstorm in north-central Illinois (Figure 9, NWS). A brief tornado touchdown, and reports of severe wind and hail were associated with this storm. The NWS Storm Prediction Center correctly anticipated that more storms would form from lift caused by the outflow from the first storm (Figure 10, SPC). Additional supercells formed to the west and south of the original storm, sending towering cumulonimbus clouds to the edge of the troposphere (Figure 11, COD). The most significant event occurred in Woodford County, where an F-4 tornado (on a zero to five scale) formed. The radar reflectivity image of the storm showed a classic "hook echo" trailing on the southwestern edge of the storm (Figure 12, COD), and the radar storm relative velocity image shows a zone of rapid wind direction reversal associated with rotation (Figure 13, COD). The tornado flattened the Parsons Manufacturing plant in Roanoke. Due to the availability of strongly constructed above ground lavatories that also doubled as tornado shelters, and the recent training of employees on evacuation procedures, the plant employees survived with just a few minor injuries. The power of the storm was sufficient to carry one ton steel girders up to three-quarters of a mile. Timely National Weather Service warnings and active storm spotters were also a key, allowing the Parsons plant to initiate their severe weather procedures so that all workers were in shelters several minutes before the storm hit. The Roanoke tornado also destroyed six homes and numerous farm buildings, but again did not cause any fatalities.
Additional storms formed and moved southward, expanding by the time they reached the central part of Illinois to encompass locations from the center of the state to the Indiana border, including both Champaign and Danville in the damage path. Strong straightline winds caused substantial tree and power line damage, leaving at least 55,000 homes and business without power for one or more days. Trucks were overturned on I-57 and I-74, and some building and roof damage occurred. Many small towns reported significant damage, but no fatalities were reported as a direct result of the storms in Illinois.
The storms expanded horizontally into a classic bow echo configuration as they proceeded
southeastward through Indiana and into Kentucky (Figure 14, COD). Evansville, IN, lost power
to about 32,000 customers, and had considerable tree damage, although many of the weaker
limbs fell in a windstorm just nine days previous. Louisville, KY, had the most widespread
impacts from the straightline wind aspect of the storm, with 115,000 customers left without
power. Wind gusts of 64 mph were measured at the Louisville NWS office, while winds of up to
80 mph were recorded at the Kentucky-Tennessee border. About 1,200 trees had to be removed
from roads by public works employees, hundred of street lights were knocked out. One storm
related death was reported. The preliminary storm reports map in the shape of a fan (Figure 15,
SPC), starting with one storm and gradually fanning out to more and more storms as one moves
further southeast on the upper level winds. Eventually, the main line of storms dissipated the
following morning well south of the Midwest region and southern Tennessee and northern
Georgia.
Low Pressure Brings More Weather
The week ended with a slow moving low entering western Missouri late on the 15th and exiting Kentucky on the 17th. The heaviest rain and most severe weather occurred near Kansas City, MO, early in this period. Precipitation totals of more than 2 inches occurred north of the city (Figure 16, NWS), and tornado warnings were issued early on the morning of the 16th, waking the visiting Minnesota Twins baseball team at 4 AM. Fortunately, there were no tornadoes in that area. A large mass of rain proceeded across Missouri (Figure 17, NWS). Ahead of the low, warm, moist air circulated as far north as Wisconsin and the Great Lakes, leading to a scattered severe weather outbreak the afternoon of the 16th (Figure 18, NWS). This was organized by a trough between the southern Missouri low pressure center and another low north of the Great Lakes. Bands of heavy precipitation indicated the path of particularly strong storms or clusters of storms, such as in Iowa and Wisconsin (Figure 19, NWS), or Michigan (Figure 20, NWS). A few severe events were recorded in eastern Michigan on the 17th, and then the region cleared.