Thursday, July 7, 2016

Thursday Evening/Overnight Update

Many of you have been wondering about the potential for severe weather Thursday evening and overnight.  Especially given some of the stronger thunderstorms that moved through Tuesday night. 

This forecast was by no means an easy one!  But a few more pieces of the puzzle have come together to give us a little better insight into what could possibly happen.

The highest storm threat Thursday afternoon and early evening has been well to the west over western and central Iowa.  This is the area closer tied to low pressure and a cold front. 

Thursday morning was mostly dry, but by mid-morning and early afternoon showers developed.  Those showers actually helped form a 'mini' low pressure system, also known as an MCV (Mesoscale Convective Vortex) over northwest Illinois.  This MCV helped to ignite a few showers over southern Lee and DeKalb counties during the late afternoon.  Those thunderstorms are now producing flooding rainfall over the suburbs of Chicago.

Further to the west in Iowa there have been a few severe thunderstorms within a line of storms along the cold front.  However, as this line has progressed eastward it's shown some signs of weakening.  And I feel as we head further into the evening and overnight our severe threat will remain pretty low.  The majority of our weather forecast models actually have this line of thunderstorms breaking apart before crossing over the Mississippi River Friday morning.

Something that helps thunderstorms maintain their strength during the overnight (even with the loss of daytime heating) is the low level jet.  This is a nocturnal jet stream in the lower part of the atmosphere that helps maintain moisture and instability - factors needed for thunderstorms to thrive.

The low level jet is less than impressive over northern Illinois and southern Wisconsin tonight and more impressive to the south over Missouri.  There is also an upper level low and surface low that will move through Minnesota and Wisconsin tonight and Friday morning.  I think a likely scenario with thunderstorms later tonight will be the formation of two storm to the south and one to the north.  The one to the north will be closer tied to the area of low pressure while the one to the south will be closer tied to the low level jet.  This puts the Stateline right in the middle and leaves us pretty dry.

Now, as the cold front actually comes through I think there will be a few isolated thunderstorms late tonight and very early tomorrow morning.  But that should be about it.  I'm going to continue to monitor development out to the west through the evening, but if there appears to be minimal signs of further development then our overall storm threat might even be low as well.

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