Damage from storms in Wisconsin...Possible heat burst

A narrow line of thunderstorms developed west of Madison Tuesday evening causing widespread wind damage between Madison and Milwaukee.  The thunderstorms were weakening, however, as they moved into south-central Wisconsin so one would think that the damage shouldn't have been that great.  If you were out yesterday afternoon you knew it was hot, but it was a dry heat.  A dry air mass was in place near the surface and as the rain from the weakening thunderstorms fell into the dry layer it evaporated and caused the strong wind gusts observed.  These features show similar characteristics of a heat burst.

What is a heat burst, you ask?

A heat burst usually begins as a thunderstorm is decaying, or weakening.  Typically the air will cool as it evaporates before reaching the ground.  In a thunderstorm that produces a heat burst, the air sinks but it rapidly warms before reaching the surface often times causing strong winds but also a quick rise in temperatures.  If a very dry air mass is present around the thunderstorm, rapid evaporation occurs which causes a significant amount of cooling of the surrounding air in a short amount of time forcing the air inside the weakening thunderstorm to sink very quickly.  This is similar to a downburst (strong winds at the surface), but a downburst contains strong winds and cool, moist air and evaporation is still occurring keeping the surrounding air cool.  In a heat burst, all the water in the sinking air is evaporated before it reaches the ground.  The air then begins to warm due to compression.  This warming sometimes slows the descent of the downdraft but if the air has sufficient momentum (or speed) built up, the hot and very dry air will still make it to the surface and will be felt as a hot, gusty wind. 

Observations recorded during a heat burst show a spike in the temperature but a drop in the dew point.  Heat bursts are actually pretty common, especially ones that produce a 10° temperature increase.  Typically this type of phenomena occurs either late at night or early in the morning as a decaying thunderstorm moves through.  The weather station near at the Sullivan/Milwaukee NWS office reported almost a 10° temperature rise coincident with sustained winds of 20 mph and gusts near 40 mph.