Summer-level warmth is back in the Chicago area.

Temps could move within striking distance of 90-deg Friday with thunderstorm prospects increase late Friday and surge Friday night.

Thursday’s Warm Temps

Temps surge into the 80s for an eighth time this year Thursday and will do so again Friday. By comparison, there had only been 5 such days by this date a year ago.

Thursday’s 85-deg forecast Chicago high represents a 25-deg surge over Wednesday’s 60-deg max and puts this day’s temps at July levels. Though higher than Wednesday, humidities haven’t taken off.

Jet stream forecast 5pm CDT Friday. A cap may be in place thwarting t-storm development. But with the air humid and quite warm, should the cap break, t-storms will bubble up into powerful high altitude jet stream winds. Storms which do that would be able to “mix down” wind energy to the surface in storm gusts

That comes Friday amid roaring SSW winds which, even without thunderstorms, are likely to gust over 40 mph in the afternoon. The flow originates over the Gulf of Mexico–so Friday’s strengthening SSW wnds will act as a conduit for the northward transport of humid tropical air. Atmospheric moisture levels are due to “take off” as well Friday with dew points, a measure of atmospheric moisture, projected to surge well into the 60s. By evening, almost 2″ of water will be present in a column of air above Chicago. That’s a whole lot of moisture and it sets the stage for an eruption of showers and t-storms, especially Friday night.

Interactive Radar: Track Showers & Storms here

“Capping” of the atmosphere–that where a layer of warm air prevents otherwise buoyant air to produce storms—may play a role in thwarting t-storm developing during the day Friday. Once thunderstorms DO erupt Friday night, we may be past peak heating and set the stage more for some scattered heavy downpours rather than severe storms—but the warmth and humidity harbor a good deal of atmospheric energy Friday afternoon and evening and should the cap break, resulting storms could tap this energy supply and be formidable gust and potential hail producers. So the Storm Prediction Center continues to assess a “SLIGHT RISK” for severe weather Friday.

Big Rains Saturday

The National Weather Service’s Weather Prediction Center suggest this is the area in which EXCESSIVE RAINFALL is a threat in the Sat/Sat night time fram

Far and away, the more interesting weather development in the late Friday night into Saturday night time frame may be the potential for some drenching rainfall. Averaging across rainfall estimates projected by the myriad models we access to these days yields as we prepared forecasts yields general totals in the 0.85″to 1.80″ range–though individual forecast models suggest some local 2-3.50″ amounts may occur in areas swept by multiple downpours.

The atmosphere appears likely to take on a classic heavy rain set up in the Friday night through Saturday night time frame. A front is to drop south across the area shifting surface winds in from the “NNE” Saturday. This sends a layer of cooler air into the Chicago metro area. Yet modeling keeps a vigorous southerly wind flow straight out of the gulf running up and over the cool layer. Moisture levels through the atmosphere are to run at 1.5 times normal providing copious water vapor to a northbound wave which will be traveling up the southbound front to our south. This sets the stage for impressive rains–and with elevated t-storms embedded with the rain shield, the potential for downpours is real.

It wouldn’t be surprising to see the extent of rainfall and its intensity increase as we move into Saturday afternoon and night—so have the rain gear ready!

Full forecast details and more at the WGN Weather Center blog

Things Dry Out Sunday

Happily, it continues to look as if Sunday is to be dry though quite cool for the season with daytime highs likely to hold to the low and mid 60s.