Wednesday Weather Update

Severe Thunderstorm Watch for Jo Daviess, Carroll, Stephenson, Winnebago, Ogle, Boone, Green, Rock and Walworth counties until 11pm.

Update 4:30 pm:  A weather watch will likely be needed for portions of Wisconsin, Iowa and maybe even northwest Illinois shortly this afternoon.  The cold front continues its steady march east and the atmosphere ahead of it will continue to destabilize.  Right now it looks like storms will form out ahead of the front around 5pm or 6pm stretching from central Wisconsin southwest into northeast Iowa.  Initially the storms will pose a large hail and damaging wind threat but as they move east into Illinois the damaging wind threat (winds greater than 60mph) will increase.  Look for these storms to begin to move into the area after 6pm or 7pm.

Update Noon: A decaying line of showers continue to move east across southeast Wisconsin and north-central Illinois as of the Noon hour.  While the initial line of thunderstorms earlier this morning weren't necessarily severe, they did produce small hail and gusty winds.   Now as this line has weakened some and begins to fall apart, the rain cooled air has produced some gusty winds that have actually produced some damage knocking down trees and power lines in Wisconsin.

Another very interesting phenomenon with these showers is that they've produced 'heat bursts' near Whitewater, WI and Milton, WI.  A heat burst is characterized by an increase in surface temperature and decrease in surface dew point.  They are usually associated with precipitation driven downdrafts that fall through a dry layer of air.  The precipitation that cooled the air has now been vaporized and as the dense air accelerates towards the surface, it quickly warms.  A wind gust of 42mph was recorded near Milton, WI (Rock Co.) as a reporting station site had a temperature jump from 75° with a dew point of 65°, to 92° and a dew point of 48°.  Another station in Whitewater, WI recorded a wind gust of 55mph and temperature jump from 78° with a dew point of 66°, to 88° with a dew point of 49°.  Notice the drier dew points!

Now for us, the cold front still sits back in western Iowa early this afternoon and with skies clearing out I'm still expecting us to remain fairly hot and humid.  Although the initial showers/storms has provided a cooler air mass at the surface.  Either way, our atmosphere will continue to destabilize with the sunshine and as the cold front moves into this unstable air mass thunderstorms will likely form in eastern Iowa up into Wisconsin.  There are already a few cumulus clouds building ahead of the front in Iowa.  The slight risk remains for today with hail, some of which could be large in the beginning of the storms, to damaging winds.  We'll likely see our storm threat increase after 3pm this afternoon as the front moves closer.