Intense heat combined with very humid air is headed for the Chicago area producing a dangerous breed of heat by Wednesday afternoon into Thursday for anyone exerting themselves outdoors.

Lake breezes, like those which have come ashore in Chicago Sunday, Monday and now today as well, have shielded the area from the most intense heat.  Afternoon temps Tuesday have ranged from a hot as 92 at Joliet, Elburn and Warrenville—well away from the lake—to 74 at Lake Forest and East Chicago, IN, 73 at the Wilmette Harbor buoy and 75 at Calumet Harbor.  

The lake breezes GO AWAY as well developed—even GUSTY “WSW” winds overpower any lake cooling and allow temps to “TAKE OFF’ Wednesday. A 99-deg high would establish a new record for the date—eclipsing the current Aug 23rd record of 97 set 76 years ago in 1947.  Another record could occur Thursday if a southbound “back-door” type cold front holds off as now see s likely until late in the day. That may well permit a triple digit official high—aided by compressional warming ahead of the late day cool front. That could possibly sneak pass the record of 100-deg for the date.  Note that the predicted highs are only a matter of a degree or two from the record—so missing the record is entirely possible.  But the potential is there for a couple of record breakers.  We’ll keep monitoring.

25 states are under heat advisories or warnings across the U.S. beneath a record expanded hot air dome which may bring Chicago its first 100-deg temp in 12 years.  The last time we had AN OFFICIAL triple digit reading here was early July, 2012.

100-deg readings don’t occur often in Chicago, There have only been 62 official triple digit high temps in the 153 years since 1871 at the city’s official observation station—and at Midway Airport, where weather observations have been taken and archived since 1928, triple digit highs have occurred only 92 times.

We’re expecting peak dew point readings of 77-deg Wednesday afternoon and 74-deg Thursday afternoon. Married to predicted highs of 99-deg and 101-deg respectively—BOTH POTENTIAL RECORD BREAKERS—the peak heat index readings would soar into the 108 to 113 range Wednesday afternoon and to around 112-deg Thursday.

Several wild cards we’re monitoring which could impact temps. Were a t-storm to flare—something NOT currently expected—but the chances aren’t ZERO either—the outflow from such a storm could inhibit warming.  At the moment, the expanded dome of air which sits over the Midwest is producing a t-storm inhibiting “CAP”.  But hot, humid atmosphere’s are finicky and algorithms run by the NWS SREF put t-storm chances Wed night at 12% . That would seem to pretty well dismiss the chance of any t-storm chances—but a hot atmosphere can be one given to surprises so we’ll keep monitoring. 

The second wild card involves the late day front which would shift winds NE and crash late day temps after what appears likely to be a hot day.  Current modeling has that passing mid or late afternoon and evening which would allow plenty of time for warming. 

Much cooler and less humid air sweeps into the area this weekend—with reduced temps beginning Friday though humidities Friday may remain elevated raising the potential for a t-storm or two to “pop”