June 6, 2010 Thunderstorm over Dane County

Between 6 pm and 7 pm on June 6th a non-severe thunderstorm moved southeast through eastern Dane County dumping brief heavy rains and most likely small hail with diameters of 1/2 inch to 7/8 inch. On it's journey it merged with another thunderstorm cell that quicky developed on the east side of Madison.  The merger occurred between Interstate 90/94 and State Highway 18 just east-southeast of Madison.  The merger of these two cells resulted in the development of weak rotation in the lowest part of the merged storm. 

Below is a radar image showing the location of the merged storm east-southeast of Madison.

radar image 630 pm

As the merged storm moved southeast, Shannon Lupton, a severe weather spotter, took some pictures of the storm, including one of a weakly rotating wall cloud under the rain-free updraft base of the storm.  The rotation at the bottom of the storm was too weak to result in a tornado. However, Shannon did take some interesting pictures.

wall cloud  tstm structure

The wall cloud is an isolated lowering of the rain-free base of a storm under the updraft tower.  It signifies the location of the best inflow into the base of a storm. A good number of tornadoes and funnel clouds develop underneath or near a rotating wall cloud.  However, tornadoes and funnel clouds can develop with no rotating wall cloud present.  Some rotating wall clouds are very robust and resemble beer barrels, while others are weakly defined.  The lowering of the cloud mass that constitutes a wall cloud is due to air with higher levels of relative humidity being drawn into the inflow of the storm and its updraft tower above.  This air with higher levels of moisture is partially drawn from the downdraft air and associated rainfall.  A downdraft's rain can be seen in the left side of the left picture above.  As the air with higher levels of relative humidity is drawn and lifted into the updraft it cools and reaches the 100% relative humidty level (saturation) quickly, compared to nearby drier air.  This results in a lower cloud base or lower cloud fragments that constitute the wall cloud.


Another storm moved through the Madison area around 3:15PM and produced some small hail (1/4 to 3/4 inch in diameter).  Below are a couple photos from Tim Pierce, a trained storm spotter.  Note that his table and deck were covered white with hail stones.

TimPiercec1

 

TimPierce2

Below is the 316 pm radar image showing the cluster of storms moving through southern Wisconsin at the time.  The red arrow is pointing towards the storm which produced the hail as seen above.  There was also small hail reported with many of the other storms that moved through southern Wisconsin Sunday afternoon.

The freezing level in the atmosphere ended up only being around 8,000 feet above the ground on Sunday.  This low freezing level allowed hail to easily reach the ground.  Normally the freezing level during the months of June, July, and August ranges from 10 to 15,000 feet above the ground.  The shorter the distance between the freezing level and the ground, the less the hail stones have to travel and undergo melting.  

Ref316

Contributions....Laura Schutte, Student Volunteer



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