July 23, 2001 Storm Analysis
This large line of storms formed just after 12:30pm. Cells started popping up in Dodge, Jefferson, and Walworth Counties. Also, storms fired near Rockford and Freeport in Illinois near the Wisconsin-Illinois border. By 1:03pm, the storm that eventually struck northern Kenosha County was beginning to intensify over Walworth County. The DBZ readings, or the measure of reflectivity, were around 55 meaning a rain rate of around 4 inches per hour or .06 inches a minute, which is needless to say, very heavy. At 1:29pm, the cells were at their peak intensity very close to the Burlington area and the Bong State Recreation Area. The cells were beginning to dissipate when they hit I-94 in northern Kenosha County and my house. They managed to generate 41-mph wind gusts at my house though, good enough to be classified as strongest thunderstorm that actually hit my house. Other cells in the line may have been severe earlier, but had weakened when they reached my house. A rough estimate when I was spotting this storm by I-94 told me there might have been a 45-mph wind gust near I-94.
In Milwaukee on July 23rd, conditions were favorable around midday when the thunderstorms formed, but were at their peak at 11:00am when the CAPE levels were in the upper 3000’s, 3617 to be exact. This meant there was an extremely unstable environment in place and that it was probably capped earlier. Indeed the CINS levels were higher earlier and there was an incredible amount of moisture in the air on the surface. Dew points were reaching 81 degrees Fahrenheit at my house! That meant that there was a large quantity of potential instability under the cap until it weakened. The potential instability is quite like cooking something in a pot with a lid on. Pretend that the steam building up inside the pot is the moisture building up. Once you lift the lid, the steam rises out and up. Lifting the lid is like the cap weakening. The weakened cap was overwhelmed by the amount of moist air rising and thunderstorms began to form.
The LFC was also very low, which is a good thing for the rising air did not have to rise far until it reached the buoyant air that would keep it rising. The helicity levels were low, at 87, which meant that supercell development was not favored and indeed no supercells did develop.
The dew points were remarkably high well in advance of the storm as they climbed as high as 81 degrees at my house, which meant there was an extremely muggy air mass in place across much of southeast Wisconsin. There was also the extreme instability in the atmosphere across the same area. Every air parcel needs lift to reach the LFC at which point it will continue to rise unaided. This day there was no clear-cut lift. No fronts or a strong enough lake breeze. But there was a moisture boundary, or a convergence zone, right where the thunderstorms fired up.
There is an interesting fact I found out by looking at the data I’ve collected from my roof before each thunderstorm. I noticed a period of gusty winds from four hours to ten or fifteen minutes in advance at which point there was an eerie calm for fifteen to ten minutes in advance of the gust front. When this gust front hit at 2:13pm, the winds gusted to up to 41-mph, the highest gust from the storm. They were still gusty up to the point when it started to rain. Rain totals from this storm were rather disappointing, only dumping .03 inches of rain on my house.
The upper air data is put out daily by the Storm Prediction Center in the form of charts at 12 Zulu or 00 Zulu (7am and 7pm Central Time). The maps show the conditions at the surface, 925mb, 850mb, 700mb, 500mb, 300mb, and 250mb. The Storm Prediction Center graphs the data at pressure levels in the atmosphere instead of height.
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 23 at 7:00am, the surface chart showed plenty of moisture in the air, but it was hard to pick out a boundary. At 7:00pm, you could see the moisture boundary moving across northern Wisconsin. The fact that cooler temperatures and dry air were behind the boundary and moist, warm air was ahead, led me to believe that there was a cold front in place.
The 925mb map at 7:00am also showed the same high moisture levels and no discernable moisture boundary. The 7:00pm 925mb map showed the boundary a little to the southeast. This uneven, tilted moisture boundary showed that this was almost definitely a cold front. Cold fronts usually have cold air well ahead of them, but higher in the atmosphere. This cold air ahead of the front helps fuel thunderstorms well ahead of the front as well as near the front. When a warm air parcel rises and meets the cool air ahead of the front it rises un-aided. The 850mb map showed many of the same conditions as the 925mb and surface maps.
The 700mb map at 7:00am showed a moisture boundary clearly defined across central Minnesota. The 7:00pm 700mb map showed that the boundary had advanced to central Wisconsin. The 500mb, used for locating strong winds to produce downbursts, showed at 7:00am that there were only five or ten knot winds, which are pretty weak. At 7:00pm they only increased to 25 knots which is also very weak. This is probably the reason why no severe winds were generated. The 300mb and 250mb maps showed nothing favorable at either time. The jet stream appears in both of these upper air charts, but more clearly in the 250mb map. Normally, you’d look for divergence or a small dip in the jet stream. There was no divergence indicated and the dip in the jet stream was miniscule.
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. In fact, there were no reports at all anywhere near Kenosha or southeast Wisconsin.