August 7, 2012 Thunderstorm in Washington County (Updated 745 pm 8/8)

Large hail stones up to 1.5 inches in diameter fell on the city of Kewaskum on Tuesday August 7, 2012, resulting in minor damage to some outside structures, vehicles, and trees.

A couple thunderstorms moved southeast out of Fond du Lac County into north-central Washington County shortly after 4 pm.  The one that affected Kewaskum briefly pulsed up to severe weather limits and generated large hail stones of 1 to 1.5 inches in diameter.  This storm also generated wind gusts up to around 45 mph as it moved southeast through the east side of West Bend.  The second storm, farther west, dropped southeast to the area between Slinger and Jackson but only generated some brief heavy rains, small hail, and wind gusts to around 45 mph.

Some small tree branches up to 2 inches in diameter were broken by the wind gusts, and a law enforcement official reported a rotten tree pushed over in the city of West Bend. 

Below are some pictures of the hail stones that were sent to John Malan, Chief Meteorologist at WTMJ4 Milwaukee.  John forwarded the pictures to our office.  The first two pictures were taken by a trained severe weather spotter.  Click on images for larger version.

 Schmidt hail pic 1  Schmidt hail pic 2  Martin hail pic 1

Below is a 4-panel Dual Pol Doppler radar composite picture with a 358 pm CDT time-stamp. Note, in the upper-left image, the thunderstorm with bright red and some white colors, reached the Kewaskum area about that time.  Click on the image for a larger version.

dual pol 4-panel 358 pm

A detailed explaination of what you see in each panel is provided below:

1.  In the upper-left is the standard Doppler radar reflectivity image (0.5 degree slice) showing some specks of white in the Kewaskum area.  The white indicates a signal return of 60 DBZ or greater - indicating the possibility of hail since hail is a good reflector of the radar signal.

2.  In the upper right is our standard storm-relative motion image.  The Kewaskum storm consists mostly of green colors which indicate motion toward the WFO Milwaukee office southeast of Sullivan in east-central Jefferson County.

3.  In the lower-left is a Correlation Coefficient (CC) image, which indicates how similar the horizontal and vertical components of the radar signal are from one radar pulse to the next.  Low values near Kewaskum indicate much variation, which implies a mix of hydrometeors, like hail and rain.

4.  In the lower-right is a Differential Reflectivity (ZDR) image, which shows the difference between the horizontal and vertical returned power within one radar pulse.  Hydrometeors that are wider than they are tall have high ZDRs and hydrometeors that are taller than they are wide have low (negative) ZDRs. Hail usually has a near zero ZDR because it tumbles when it falls and therefore appears to be spherical (horizontal and vertical dimensions are nearly equal).

We also received a report (via video) of a possible tornado in Washington County on August 7th. However, it has been determined that this was not a tornado.

We have concluded that there was no tornado in Washington County between Slinger and Jackson during the 4 PM to 5 PM time frame for the following reasons:

  1. We reviewed the video tape taken by a WTMJ4-TV News Helicopter, which was over Germantown, looking west-northwest. We saw a dust/dirt column, but could not see rotation within the column. Additionally, we could not see any connection to the cloud base above.
  2. Washington County law enforcement officials could not find any ground damage
  3. A trained severe weather spotter was located on South 60, midway between Slinger and Jackson, between 4 PM and 5 PM, Tuesday, August 7th. He did not see anything that looked like a tornado or gustnado.
  4. The WSR-88D radar at WFO Milwaukee/Sullivan did not have any low-level rotation that would suggest the possibility of tornadoes.

So, what caused formation of a dust/dirt column, gusty outflow winds from a couple of thunderstorms 5 to 10 miles north and northeast of the dust/dirt column picked up dust and dirt from the ground. State Highway 60 is under construction between Slinger and Jackson, so there is plenty of dust and dirt for winds to pick up. Wind gusts were probably in the range of 45 mph based on spotter reports when the storms moved through Washington County. Because we could not see rotation in the dust/dirt column in the WTMJ-4 Milwaukee video tape, we cannot classify the column as a gustnado.

Kapela/Brooks, WFO Milwaukee

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