August 7, 2001 Storm Analysis
Storms began to develop shortly after noon across much of southeast Wisconsin as a lake breeze suddenly and unexpectedly appeared and provided the necessary lift in the already warm and unstable atmosphere to produce thunderstorms. The isolated cells that formed over Racine and Kenosha Counties formed very suddenly just around 2:00pm. It started as just one cell and then quickly developed into three different cells. These storms developed in a very tropical air mass. The three cells lasted only about an hour each.
The CAPE levels were adequate for thunderstorm development around 3:00pm peaking at 2084 joules per kilogram at 3:00pm. This meant there was quite a bit of energy available for convection. 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 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 3:00pm and CINS levels dropped to 34, the moisture was released, exploding into thunderstorms. The LFC dropped as low as 4500 feet, which meant that when the moisture did explode upward, it did not have to fight warmer air, as much. Helicity numbers were around 100 for most of the day, which means that there was not enough helical flow for rotation to occur.
Possibly the most contributing factor to the thunderstorms exploding was the upper 70-degree, even 80-degree dew points and extreme heat that had built up over southern Wisconsin. The extreme moisture could not rise and cause thunderstorms because there was no lift for at least a week prior. Instead, the moisture just hung in the air making it very uncomfortable.
The dew points rose to 80.3 degrees at 2:30pm on August 7th and hovered above 79 until the storm hit. 79-degree dew points indicate an incredible amount of moisture in the air, and created a very tropical air mass.
Everything was in place for thunderstorm development in Kenosha right around 3:00pm. There were incredible amounts of moisture and instability. All we needed was a source of lift or a triggering mechanism, but until around 1:00pm it didn’t look like there would be any. Right around 1pm the wind shifted and the lake breeze took over. Normally the lake breeze doesn’t cause enough lift, but on the 7th it did. On the radar image I noticed very spotty, isolated thunderstorms across south central Wisconsin and across southeast Wisconsin. I received a maximum wind gust of 33mph, which is 25-mph short of being severe. The gust front hit my house between 2:30pm and 3:00pm and the rain began close to 3:00pm. The total rainfall at my house was 0.22 inches of rain, which was significant due to the fact that it broke a twelve-day drought.
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 August 7th 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. The 850mb showed the same thing as the 925mb and surface maps. You look for the same thing, moisture and moisture boundaries in the surface, 925mb, and 850mb maps.
The 700mb map showed dry air at mid-levels across Wisconsin that helped make the air more unstable. 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 and 7:00pm. This probably contributed to non-severe winds with the thunderstorms. There was nothing really favorable in the 300mb or 250mb maps where you generally look for divergence and/or a trough in the jet stream.
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. There were isolated, small power outages across Kenosha County, but that was all.