Strong thunderstorms with locally heavy rains possible Saturday/Saturday night

Updated 7:45PM CDT…

For Saturday and Saturday night the National Storm Prediction Center has included the entire Chicago area in a Slight risk of severe storms (yellow-shaded area on the highlighted map above depicting a 15% chance of severe thunderstorms within 25 miles of a given location). 

The Slight risk of excessive rainfall in these strong storms continues in effect Saturday-Saturday night.

Indications now are that an up-air impulse will move across our area triggering strong to severe thunderstorms early Saturday morning. These storms are expected to weaken during the forenoon, Then later in the afternoon into the overnight hours another impulse is forecast to move over our area causing redevelopment of strong to severe storms with heavy downpours possibly leading to localized flooding. The main severe weather risk will be large hail and a few instances of damaging winds.

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Clusters of showers and thunderstorms are expected over northeast Illinois and northwest Indiana during the next 24 to 48 hours. The strongest – possibly severe  storms during this period look to occur later Saturday afternoon/Saturday night (see headlined National Storm Prediction Center outlook map that includes the Chicago area in a Marginal to Slight Risk of severe storms – yellow and green-shaded areas).

The scattered showers/thunderstorms Friday/Friday night will saturate the soils, setting the stage for localized flooding when the stronger storms occur later Saturday/Saturday night (see outline encompassing the Chicago area on the projected Excessive Rainfall map below).

A west-east-oriented frontal boundary will be positioned south of Interstate-80 during the next few days and southwesterly flow aloft will lift warm moist air over the front. At the same time periodic upper-air impulses are forecast to ripple across the area destabilizing the air below, triggering clusters of showers/thunderstorms. Timing of showers/thunderstorms is very difficult under these circumstances, but current indications are the strongest activity will occur later Saturday afternoon into the evening/overnight hours.

Excessive Rainfall outlook map Saturday/Saturday night…

Day 2 Excessive Rainfall Forecast