Storm update for Saturday evening: Recap of the morning and overnight storm update

1:50am Update: Severe Thunderstorm Warning for Carroll and Whiteside counties until 2:15am.  Line of thunderstorms crossing the Mississippi River will be capable of wind gusts to 60 mph and heavy downpours.

SATURDAY EVENING UPDATE: Severe Thunderstorm Watch extended until 5am for the following counties: Jo Daviess, Carroll, Whiteside, Stephenson, Winnebago, Boone, Ogle, McHenry, Green, Rock and Walworth counties.  Currently tracking storms in central Iowa that are moving northeast along the warm front.  This line will slowly sink east and southeast into the overnight bringing storms into northern Illinois and southern Wisconsin after midnight, likely between midnight and 2am.  An interesting note with these storms is that some of the shorter term models are indicating a weakening trend of the storms in Iowa as they move east into Illinois.  Will be interesting to see how they hold together.








8:00pm Update: After the large hail and strong winds that came through Saturday morning, additional thunderstorms will be likely through the overnight and into Sunday.  Before we recap what happened Saturday morning, let's first discuss the set up we have going into tonight.

A Severe Thunderstorm Watch is in effect for Stephenson, Jo Daviess, Carroll, Green and Rock counties until 10pm.  Thunderstorms have been firing up along a fairly pronounced warm front stretching from southwest Iowa into southern Wisconsin.  North of the front temperatures have been in the 50s and 60s where south of the front temperatures climbed into the low to middle 80s.  In fact, many places in northern Illinois recorded high temperatures this afternoon that reached 80 degrees!  The focus for thunderstorm development has been mainly along and north of the warm front in Iowa and Wisconsin and this is where the majority of storms will stay over the next few hours.  The main threats for any storms that form in Iowa will be hail.  As the storms
form, they will follow the flow within the jet stream which will track them east/northeast over time.  At the same time, the warm front is expected to slightly shift a little further south closer to the Wisconsin/Illinois border tonight.  This will then become the focus for storm development through the overnight and possibly into Sunday morning. 

As the evening progresses, the individual storms in Iowa are expected to become organized into more of a cluster of storms rather than individual storms.  This means the hail threat will decrease, but the wind and heavy rain threat will continue.  Where exactly the warm front settles is where the storms will follow into tonight.

The severe threat will stay a little further south on Sunday closer tied to the where the strongest winds will be in the upper levels of the atmosphere.  However, thunderstorms will persist through day on Sunday and will really amp up going into Sunday evening as a cold front and low pressure slide east of the Mississippi River.  Sunday is the day where we can expect the heaviest rain as winds at the surface and above flow parallel to the cold front.  This means as storms form, they have the capability to move over the same location producing the heavier rain.  Again, where the front lines up is where the heaviest rain will fall.  Right now that looks to be in northwest Illinois and southwest Wisconsin.  There, nearly the two inches of rain could fall!

Now, let's look into what happened earlier Saturday morning.  The low level jet, which are simply stronger winds about 3,000 to 5,000 ft above the surface, helped to develop storms in Iowa and Minnesota overnight Friday.  A stationary boundary near the Wisconsin/Illinois border Saturday morning then became the energy needed for those storms to intensify as they neared Jo Daviess County.  And that's exactly what they did.  Pea to quarter sized hail was reported from Galena to Freeport, but as the storm tracked over Freeport it began to show signs of intensifying into a supercell thunderstorm as it moved into an environment that had a little more shear in the atmosphere.  This means a change of wind direction with height.  Thankfully, it remained elevated otherwise we might have had a few tornado warnings issued.  Mid-level rotation within the storm strengthened producing wind gusts near 70 mph, downing trees and causing damage to many buildings around the Freeport area, likely the cause of a mini-downburst.  As the storm tracked east into Winnebago County, the strong rotation continued which caused the significant hail that fell.  While the majority of it was nickel to quarter sized, there were several reports of ping pong up to golf ball sized hail that fell around and east of Rockford.  The storm followed along the warm front into northeast Illinois before moving out over Lake Michigan.  At this point, it doesn't look like that will happen again tomorrow morning, however, we've got to watch where the warm front ends up from any overnight storms by the morning.