An Impressive Temperature Spread Was Observed on Thursday

    Thursday saw a strong west-to-east frontal boundary take shape across the local area in the morning.  On a regional scale, this front basically remained stationary through Thursday and Thursday night, with a large temperature discontinuity along it.  Because of this, over a 30 degree temperature difference was in place across the Chicago metro area during the afternoon.

April 19th, 5 pm:  Analyzed Surface Weather Map with Temperatures as the Image

Surface Weather Map

 

3 pm Weather Map

5 pm Weather Map

7 pm Weather Map

3 pm Weather Map

5 pm Weather Map

7 pm Weather Map

The front is located just north of Interstate 80, with opposing cool northeasterly winds from lake Michigan against gusty and mild southwesterly winds to the south of the front.  The lake temperatures are in the 40s, which likely helped to modify the air above the lake to those temperatures.  This cool and more dense air likely assisted the front in moving south over upcoming hours.

The front has eased south ever so slightly by this time, with temperatures continuing to cool north of the front.  Note the 33° temperature difference across the Chicago metro from Waukegan to Joliet, and the 25° spread between Gary and Valparaiso, a distance of only 25 miles. The front continues to settle south, likely further aided by less mixing in the warm sector south of the front.  Joliet drops 22 degrees in one hour, while Valparaiso takes a 19 degree plunge.  Temperatures have dropped into the 30s just north of the Illinois/Wisconsin state line.


High Temperatures on Thursday

High Temperatures on Thursday

Text Listing of High Temperatures


 Why no Severe Weather?

    One may be wonder why with such a well-defined front in late April would severe thunderstorms not develop, and for that matter basically no thunderstorms at all and only some showers?  When meteorologists look for thunderstorm development, key ingredients are moisture, instability, and a triggering mechanism.  After that, they look for wind shear to sustain  thunderstorms and foster severe development. 

    While a focus was certainly in place at the surface on Thursday, it alone was not enough to trigger thunderstorms.  This was mainly due to the lack of deep moisture and instability.  Weather balloon launches from NWS offices in Davenport and Lincoln showed the decrease in deep moisture through the day Thursday, and the attendant lack of instability.  The Lincoln balloon launch data can be seen to the right.  Note the dryness from 650 mb to the surface.  The lack of moisture can even be seen in the weather maps above too, as dew points were only in the 30s in the far northern part of the warm sector (Pontiac, Kankakee, Rensselaer) closest to the front. This is an area ideally one would want to see pooling of higher dew points.

   In addition, an upper level triggering mechanism, or disturbance, was centered to our north in Wisconsin, which helped to produce widespread rain and even some snow there.  But for Illinois and Indiana, it was too far away to aid in bringing cooler air in aloft, which would also help to steepen lapse rates and thus increase instability (allowing air to significantly rise to potentially develop thunderstorms).

    7 pm Weather Balloon Launch Data
  



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