July 22, 2001 Storm Analysis
This storm developed as several multi-cell clusters, one in southwest South Dakota and the other in western Nebraska, around 5:00pm the previous day, July 21st. They traveled in an east-northeasterly direction until they merged in the early morning in western Minnesota and continued to move east-northeasterly at a slower pace than before. It entered Wisconsin around 9:00am and looked as though it would miss Southern Wisconsin entirely. Then cumulus towers began to develop from Stevens Point down southward towards Madison, and down towards Galena, Illinois, which is near Dubuque. At 1:30pm there was a solid squall line that developed from Appleton, down to Fox Lake, right through Madison and close to the Illinois border. Some areas of the squall line began to dissipate right around 2:00pm, but storms continued to develop along the outflow boundary and convergence zone where the dry, cool air of the thunderstorm affected area met the incredibly warm, moist air of the air mass ahead of it. Storms were beginning to develop by Rockford. At 2:45pm, there was a large gap forming between storms. The gap began near I-94 in Waukesha County and continued all the way to northern Walworth County. By 3:00pm, it looked almost certain that my house would be missed by the thunderstorm. At 3:22pm, the National Weather Service issued a severe thunderstorm warning for Kenosha County due to high winds and heavy rain reported in the area from a multi-cell cluster looking to make an impact on western Kenosha County. But at 3:45pm, a small line of storms began to develop from Oak Creek and Racine County to connect to the big cluster of storms that was also re-intensifying near Silver Lake in western Kenosha County. This posed a problem for Country Thunder, a music festival in western Kenosha County, where 1.24 inches of rain fell and 3 people were struck by lightning. Apparently, gusts around 40mph were reported and the storm thinned out the crowd when 12,000 people left due to the inclement weather. It ruined guitars, canceling a performance, and 12 inches of rain stood in the VIP section of the crowd. Around 4:00pm, a pulse storm formed east of I-94 very close to my house in Kenosha. The reflectivity shot up in this particular cell. It dumped heavy rains on my house where we received .23 of an inch of rain in under a half an hour. Rains became heavy at periods. I received .04 of an inch in under a minute! There may have been possible rotation in this pulse cell, but nothing developed.
The CAPE levels were quite high around midday peaking at 2674 joules per kilogram at noon. This meant there was quite a bit of energy available for convection and thunderstorm development. Earlier CAPE levels were well below 1000 j/kg which meant there was not much energy available for thunderstorms earlier in the morning.
CINS levels were also high, above 50 j/kg, which meant that there was a cap in place, allowing energy to build up in the lower levels of the atmosphere. When the cap began to weaken around midday and CINS levels dropped to 0, the moisture was released, exploding into thunderstorms. The LFC dropped as low as 3000 feet, which meant that when the moisture did explode upward, it did not have to fight warmer air, as much. Helicity numbers were in the negative numbers or extremely low for most of the time meaning those thunderstorms did not have the rotation to spawn tornadoes.
Possibly the most contributing factor to the thunderstorms exploding was the upper 70-degree dew points and extreme heat that had built up over southern Wisconsin. Moisture and instability were in place every day prior to July 22nd, but there was no source of lift. The extreme moisture could not rise and cause thunderstorms. Instead, the moisture just hung in the air making it very uncomfortable.
The dew points rose to 79.3 degrees at 11:00am July 22nd and hovered above 74 until the storm hit. 79-degree dew points indicate an incredible amount of moisture in the air, and it creates a very tropical air mass. The cloud deck that was overshooting the thunderstorms, along with a moderately strong lake breeze, helped cool the air temperature down to 82 degrees from a prior temperature of 90 degrees.
Everything was in place for thunderstorm development in Kenosha right around midday. There were incredible amounts of moisture and instability. All we needed was a source of lift, but until around 12:30pm it didn’t look like there would be any, On the radar image I noticed a very spotty, light blue line extending across south-central Wisconsin and on into northern Illinois. This indicated building cumulus towers (a convergence zone), or an area where dry air was clashing with moist, humid air.
I received a maximum wind gust of 20mph, which is 38-mph short of being severe. The gust front hit my house at 3:56pm. and the rain began at 4:03pm, when I received a hundredth of an inch of rain. There were very heavy downpours with this storm, as I received .04 of an inch of rain in less than a minute! The total rainfall at my house was 0.23 inches of rain which was significant due to the fact that it broke a four day drought and was only the second considerable rainfall (greater than 0.10 inches of rain) of the month. It also propelled our monthly total over 1 inch of rainfall.
The upper air data is put out daily by the Storm Prediction Center in the form of charts at 12 Zulu and 00 Zulu (7am and 7pm Central Time). Zulu time is set in Greenwich, England or the Prime Meridian. These upper air maps show temperature, dew point, wind direction, wind speed, current conditions, and in one map divergence, a vacuum like effect caused by separating winds.
Usually forecasters look for particular conditions for thunderstorm development. In the surface chart they’d look for moisture and moisture boundaries indicating some sort of lift. On July 22 at 7:00am as well as 7:00pm, there was plenty of moisture pooling up ahead of the storms on the surface, although it was hard to pick out moisture boundaries on the surface map. The Storm Prediction Center plots their maps in millibars, a unit of atmospheric pressure, not height, so the millibars decrease as you go up in the atmosphere. Therefore, points on one map may not all be at the same altitude.
The 925mb map at 7:00am, as well as 7:00pm, appeared much the same as the surface map, namely high moisture levels across almost all of Wisconsin and no moisture boundary. However, at 7:00am, in the 850mb there was a pocket of dry air across southwestern Wisconsin, which meant that thunderstorms would not develop or intensify in that region. You look for the same thing in the surface, 925mb, and 850mb maps. However, this meant there was a convergence of air masses across much of south-central and southeast Wisconsin. There was moist air in place everywhere in Wisconsin, except for that little pocket which may have provided lift for the storms.
No upper air map showed the boundary better than the 7:00am 700mb map which showed a dry air mass across north and western Wisconsin that would eventually interact with the moist air across the eastern and southern portions of Wisconsin. Generally you look for high winds in the area of development to help drive downdrafts in the 700 and 500mb maps, but they were relatively light at 7:00am around 25 knots. They picked up to about 40 knots at 7:00pm. In the 300mb map there was an area of divergence over much of western portions of Wisconsin drawing in moisture for thunderstorm development.
V. Damage Reports
I reviewed the damage reports generated by the Storm Prediction Center and it appears that there was no significant damage done to southern Wisconsin. The most severe report was from Hartford in Washington County estimating 55mph gusts and numerous trees down. 55mph is not quite strong enough to be classified as severe. Mainly reports came in like two trees down which is not really significant. There were three people hit by lightning Country Thunder, a country music festival, tending to a tent.