THE ANATOMY OF A STORM
Friday, July 16th, 2010 -- 8:14 AM
From the National Weather Service in La Crosse - read story [url=http://www.crh.noaa.gov/arx/?n=jul1410]here[/url]:Numerous rounds of thunderstorms moved through the region on Wednesday July 14 2010 with an endless supply of very humid and hot air. During the day, temperatures warmed to near 90 degrees and the dewpoint reached 75-80F. Dewpoint is the direct measure of the amount of water in the air (as a gas) - and at 70F - most people feel it is "very humid outside". The heat index, a direct measure of how both dewpoint and temperature impact the body and how you would truly feel the heat, was near 110F at mid-afternoon.
The first batch of storms came through in the early afternoon over northern and central Wisconsin as a squall line moved in and brought wind damage between 12pm and 2 pm. A second line of storms then formed from near Eau Claire Wisconsin into southeast Minnesota. This second phase brought more of a tornado threat and many reports of large hail, damaging winds to 75 mph, and tornadoes occurred between 3pm and 8pm. With the air still unbelievably warm and humid at 8 pm, a cold front moving through Minnesota ignited a third and final round of severe storms. This line developed over southeast Minnesota and built northeast into north central Wisconsin bringing more damaging wind reports and flash flooding. The line did fill in further south bringing some weather to Northeast Iowa and southwest Wisconsin - but they were largely spared from much of the severe weather.
These repeated rounds of rain finally took their toll and northern Monroe and southern Jackson County reported flash flooding with road closures and many roads underwater in the late evening on July 14. Street flooding also occurred in the cities of La Crosse and Winona.
The NWS issued 27 warnings beginning at noon and ending around 1 am. 72% of the Severe Thunderstorm Warnings were verified with an average of 25 minutes until the first report was received in the warning (e.g., lead time). 20% of those warnings were considered false alarms and did not have reports of severe weather within them.
http://www.crh.noaa.gov/arx/?n=jul1410
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