The National Weather Service had this to say three hours ago:
URGENT - WEATHER MESSAGE NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 517 AM CDT MON JUL 31 2006
HIGH PRESSURE WILL REMAIN DOMINATE OVER THE ATLANTIC OCEAN SOUTH OF 30 DEGREES LATITUDE THROUGH THURSDAY. THIS WILL CAUSE SOUTH WINDS TO BRING HOT AND HUMID AIR INTO NORTHERN ILLINOIS THROUGH LATE TUESDAY NIGHT.
...EXCESSIVE HEAT WARNING REMAINS IN EFFECT UNTIL 8 PM CDT TUESDAY...
THIS HOT AND HUMID AIR WILL PRODUCE A HEAT INDEX OF AS HIGH AS 109 TO 112. THESE ARE DANGEROUS WEATHER CONDITIONS WITH THESE NEAR RECORD HIGH TEMPERATURES. YOU COULD BE AT RISK FOR SUN STROKE ...MUSCLE CRAMPS AND HEAT EXHAUSTION. IF YOU ARE EXPOSED TO THESE CONDITIONS FOR A LONG TIME OR ARE PHYSICALLY ACTIVE YOU COULD SUFFER HEAT STROKE.
THE DURATION OF THIS HEAT EVENT IS PARTICULARLY DANGEROUS TO PEOPLE WHO ARE HEAT SENSITIVE...THE ELDERLY...THE VERY YOUNG AND THOSE WHO ARE CHRONICALLY ILL. THESE PEOPLE SHOULD STAY IN AIR CONDITIONING IF POSSIBLE OR SEEK AIR CONDITIONED SHELTERS.
TAKE EXTRA PRECAUTION IF YOU ARE GOING TO BE OUTDOORS. DRINK PLENTY OF NON-ALCOHOLIC LIQUIDS AND WEAR LOOSE FITTING...LIGHT COLORED CLOTHING. WEAR A HAT AND SUNBLOCK TO PREVENT SUNBURN. IF POSSIBLE SPEND TIME MORE TIME IN AIR CONDITIONED OR WELL VENTILATED BUILDINGS. PROVIDE WATER AND SHELTER FROM THE SUN FOR YOUR PET.
RESIDENTS OF THE CITY OF CHICAGO SHOULD CALL 3 1 1 FOR INFORMATION ON LOCATIONS OF...AND TRANSPORTATION TO...CITY COOLING CENTERS.
ELEVATED OZONE LEVELS WILL COMPOUND THIS HEALTH SITUATION CREATING UNHEALTHY CONDITIONS FOR INDIVIDUALS WITH RESPIRATORY SENSITIVITIES.
(Why the NWS still can't wrap its head around lower-case letters baffles me, but they're pretty good at predicting weather.)
At the moment, Chicago is only 28.3°C (83°F) with a heat index of 31.1°C (89°F). Of course, it's just past 8:00 am. Stay tuned.