Thursday's Chicago storm for the record books - DC News FOX 5 DC WTTG

Thursday's Chicago storm for the record books

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CHICAGO (FOX 32 News) -

The storm system that affected Chicago and Lake Michigan Thursday into Friday was definitely one for the record books.

This storm produced beneficial rainfall in some locations, with locally heavy rainfall totals along Lake Michigan due to lake effect/enhancement. The storm was also responsible for producing very rarely seen lake effect thunderstorms with nearly 60 cloud-to-ground lightning strikes detected in the Chicago area from these lake effect thunderstorms. Numerous waterspouts also occurred over Lake Michigan late Thursday into early Friday morning. Finally, the storm produced a prolonged period of very strong northerly winds making it one of the strongest August gales producing the highest waves ever recorded during the month of August (dating back to 1981) at the southern Lake Michigan mid lake buoy.

The rainfall with the storm was not distributed all that evenly across the area, with many areas west of Chicago picking up very light rainfall totals of less than 0.10". More significant rain fell over northwest Indiana and along the lake in northeast Illinois. A large part of the rain that fell over northwest Indiana and extreme northeast Illinois was a result of lake effect showers and thunderstorms. Between 2-4" of rain fell across Lake and northern Cook counties, especially near the beaches.

The strong pressure gradient between the unseasonably strong low pressure over the Great Lakes and a strong high pressure moving into the northern plains resulted in a very rare August gale on Lake Michigan. Fairly continuous weather records for the southern Lake Michigan mid-lake buoy date back to 1981 and can offer some insight into just how unusual this storm was. Waves at the south buoy peaked at 13.5 ft late Friday afternoon which is the highest wave height ever recorded at the south buoy during the month of August. The previous record highest wave height for August was 11.8 ft back on August 4, 1994. In fact, there were 3 hourly observations at the south buoy on Thursday that broke the previous record highest wave height.

Astonishingly, there were 9 hourly observations (2 pm through 10 pm Friday) at the south buoy that had wave heights measured at 10 ft or greater. Prior to this event, dating back to 1981 there had only been 13 hourly observations total at the south buoy in August with wave heights of 10 ft or larger. So this one record breaking gale on the lake produced nearly as many hourly observations with 10+ ft waves at the south buoy as had been recorded in the past 30 years combined for August!

It's worth noting that wave heights at the buoy are often lower than observations from ships. In fact, the ship M/V Burns Harbor reported waves of 18 ft nearby the south buoy at 7:00 PM on Friday, while at the same time the buoy was reporting 13 ft waves. Observations from the south shore of the lake near Indiana Dunes reported waves of up to 16 ft observed Friday afternoon, so while the buoy observations provide an excellent source of historical data for comparison, the wave heights recorded at the buoy in many cases are somewhat lower than those reported by ships and at the shore.

The strong north winds and large waves which occurred over Lake Michigan Thursday night and Friday caused a significant amount of sediment to be churned up and piled up over the southern end of the lake.  With generally clear skies on Saturday, visible satellite imagery reveals a noticeable discoloration of the water along the southern and western shores which is a result of sediment churned up by the high waves. The true-color, 250 meter resolution image in this story, comes from the Moderate Resolution Imaging Spectrometer (MODIS) aboard NASA's polar-orbiting Aqua satellite, and is made available by the Space Science and Engineering Center (SSEC) at the University of Wisconsin-Madison.

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