August 9, 2001 Storm Analysis
Right around 6:00pm thunderstorms began to develop and organize into a squall line across Milwaukee, Waukesha, Jefferson, and Rock Counties. In other words it extended from N. Milwaukee to Janesville. By 7:00pm it had already advanced to a line from Oak Creek to Lake Geneva and was getting ready to strike Kenosha County. A Kenosha middle school received a 53-mph wind gust with this storm. In fact there were warnings accompanying this storm in Kenosha, Racine, and Walworth counties, as some areas were experiencing winds in excess of 60-mph and/or hail greater than ¾ inch in diameter. This line was not a speedy line, but it wasn’t stationary either, moving at 30-mph to the southeast. There was minor damage with this storm, but not extreme damage.
The upper level environment was suggestive of thunderstorms almost eight hours in advance of the actual development. In Milwaukee, there was a capped environment, meaning moisture was building up until it was released in the early evening. The CINS levels were at 41 j/kg at 9:00am in Milwaukee indicating that there was indeed a cap as CINS is a good indicator of a cap. CAPE levels were almost at 1500 j/kg, the general level to initiate thunderstorm development and not just convection. They were at 1447 j/kg to be exact. Helicity levels were not high enough to support tornadic activity and would not reach the needed level, 150, at all on August 9th.
Later, at 5:00pm in Milwaukee, the cap had been blown off and the moisture was then allowed to rise. CINS levels were down to 0, which means there was nothing inhibiting the moisture rising. CAPE levels were at 2300 j/kg indicating a very unstable environment or a large quantity of buoyancy in the air helping the moisture rise and thunderstorms to form. The LFC, level of free convection, was hovering around 4000 feet, which helped enable thunderstorm development. Helicity levels were at 92, still too low to support tornadic activity.
In the morning in Madison, there also was a capped environment similar to Milwaukee. CINS levels were at 53 j/kg and CAPE levels were at 2346 j/kg indicating there was a tremendous amount of instability building underneath the cap in place. The LFC was at just below 5000 feet. Helicity levels were at 97 meaning a non-existent threat of tornadoes. However, as the day progressed the cap blew off, allowing moisture to rise into the incredible instability with levels reaching close to 3575 j/kg. CINS levels were at 0. The LFC had dropped to almost 3000 feet, which meant that the moisture didn’t have to rise far until it reached the buoyant air above the LFC. Helicity levels were at 101, which meant that there still was no threat of tornadoes.
On August 9th there was plenty of moisture available for convection as dew points were into the mid-70s. Dew points peaked out at 75-degrees. This moisture just needed a source of lift, which would come from the impending cold front. The three needed factors, moisture, instability, and lift were in place at the time of development, but earlier only moisture and instability were in place. All the atmosphere needed was a triggering mechanism.
When the storms did form, they let out severe gusts between 60 and 70 mph. The gust front, the point where the gusty weather begins, hit my house at 7:10pm. The gusts at my house were not even in the strong range as they peaked out at 27mph. The school I attend received a 53mph gust, which is very close to severe. Obviously I got hit by a non-severe portion of the storm.
I did receive a lot of rain, however, with totals at 0.76 inches of rain. The rain rate was as high as 3.60 inches of rain per hour! This translates to 0.06 inches per minute. With that rate I can say it must have been literally raining in sheets.
IV. Upper Air Data
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. The surface chart showed plenty of moisture with dew points in the 70s at both 12 and 00 Zulu. The surface chart showed a very distinct moisture boundary in both maps that eventually moved across Wisconsin. 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 showed a very defined moisture boundary, just like the surface chart. At 7:00pm CDT, right around the time the storm formed, the moisture boundary was right near the formation area. The 850mb map showed almost exactly the same thing as the 925mb map.
The 700mb map showed a very defined moisture boundary just like the maps before. This map showed a strengthening lower level jet stream with winds up to 45 knots. Generally, you look for high winds in the area of development to help drive downdrafts in the 700 and 500mb maps. The 500mb map also showed a low-level jet stream strengthening throughout the development area with winds up to 50 knots.
The 300mb map showed sufficient upper level divergence. This coupled with low-level convergence caused by the cold front helped provide the lift needed for thunderstorm development.
V. Damage Reports
I reviewed the damage reports generated by the Storm Prediction Center and it appears that there were some reports, but not many. Most were not of the severe nature. There was basically minor tree damage in Washington, Milwaukee, Racine, Kenosha, and Walworth Counties. There were no measured severe gusts, but the weather equipment on top of my school registered a 53-mph wind gust.