National Weather Service Weather Forecast Office

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

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.

    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).