Severe Thunderstorm Watch in effect in Chicago, northern Illinois, Excessive Heat Warning continues

[WEATHER ALERT] A Severe Thunderstorm Watch is in effect for Chicago, northern Illinois until 8 p.m. Sunday.

A line of thunderstorms continues to develop across northwest and west central Illinois. This line is forecast to begin impacting far western portions of the metro area between 3:30 and 4 PM CDT.  All of NE Illinois and NW Indiana remain under a severe thunderstorm watch until 8 PM CDT.

Map courtesy of NWS Chicago.

Persons planning outdoor activities late this afternoon and early this evening should consider the likelihood of strong thunderstorms with a potential for damaging winds, hail, intense cloud-to-ground lightning, and heavy downpours.

It would also be prudent for boaters on southern lake Michigan to be aware of this weather situation and be prepared to reach harbor should a marine warning be issued.

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Dangerous Excessive Heat continues over much of the Chicago area, including Chicago, into the overnight hours Sunday (purple-shaded area on the headlined map). Peak heat indices will generally range from 100 to 110-degrees with a combination of afternoon high temperatures ranging from 90 to 95 and high humidity (dew-points 75 to 80-degrees).

Cloudiness will gradually thicken, and strong to severe thunderstorms will advance out of the west, as a cold front slowly approaches. Cooling showers and thunderstorms will first advance into westernmost counties around midday, so the heat index will peak early around 100-degrees in Ogle, Lee, Winnebago, Boone and DeKalb Counties, cooling as the afternoon progresses. Closer-in to the Chicago, as well as south and east, heat indexes will most likely peak between 104-110 degrees, before thunderstorms arrive later in the day or early evening.

Sunday will mark the third consecutive day with dangerous heat indices well above 100-degrees, while receiving little or no relief at night - temperatures having dropped no lower than the mid 70s to lower 80s. This is an extremely rare event for our area, and the cumulative effects of oppressive heat and humidity will take a heavy toll on individuals with respiratory or heart problems, elderly and children, unless relief is provided by air-conditioning or other cooling methods.

Cooling should occur area-wide Sunday night as the cold front passes through and eventually moves off to the east.

The National Storm Prediction Center has most of the Chicago area under a Slight Risk of Severe Thunderstorms (yellow-shaded area on the highlighted map depicting a 15% chance of severe storms within 25 miles of a given location) this Sunday afternoon/evening. The primary threat with these storms will be damaging winds. Flood-producing downpours are also possible, especially over water-soaked soils north of Interstates-88 and 290 – the National Weather Prediction Center has the Chicago area in a Marginal Risk for Excessive Flood-producing rain today and tonight (green-shaded area on the map below).

A line of strong thunderstorms is expected to develop along and ahead of a cold front as it approaches from the west today and moves through our area  -  storms gradually ending from the west Sunday night and the front eventually moving off to the east early Monday.

Excessive Rainfall outlook Sunday - Sunday night...

Day 1 Excessive Rainfall Forecast

For the latest weather updates, visit wgntv.com/weather.