544  
FXUS63 KLOT 162034  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
334 PM CDT TUE JUL 16 2019  
   
SHORT TERM
 
 
225 PM CDT  
 
THROUGH WEDNESDAY...  
 
WIDELY SCATTERED SHOWERS CONTINUE ACROSS MUCH OF THE AREA,  
ESPECIALLY SOUTH OF I-88. WHILE A FEW THUNDERSTORMS STILL REMAIN  
POSSIBLE OVER THE NEXT FEW HOURS BEFORE THE ACTIVITY BEGINS TO  
WANE, THE MAIN STORY WITH THESE SHOWERS WILL BE THE ISOLATED HEAVY  
RAIN RATES. THE AIRMASS IN PLACE OVER THE AREA IS OF TROPICAL  
ORIGIN AND FEATURES IN EXCESS OF 2 INCH PRECIPITABLE WATER VALUES.  
THEREFORE, EXPECT VERY EFFICIENT WARM RAIN PROCESSES WITH 2+ INCH  
PER HOUR RATES FROM THESE STORMS. LUCKILY IT APPEARS THE  
RESIDENCE TIME OF THESE HEAVY RAIN RATES OVER A GIVEN AREA WILL  
BE LIMITED AS THE CELLS MOVE NORTHEAST AROUND 15 MPH. HOWEVER, IF  
ANY CELL TRAINING OCCURS, THERE IS AT LEAST SOME SMALL RISK FOR  
SOME LOCALIZED AREAS OF EXCESSIVE RAINFALL. THIS WILL NEED TO BE  
MONITORED OVER THE NEXT FEW HOURS.  
 
OTHERWISE, EXPECT MUCH OF THIS ACTIVITY TO BEGIN TO WANE THIS  
EVENING. THERE REMAINS A SMALL CHANCE THAT A FEW STORMS DEVELOPING  
WEST OF THE AREA THIS AFTERNOON COULD MAKE IT INTO NORTH CENTRAL  
IL LATER THIS EVENING BEFORE WEAKENING, BUT CONFIDENCE ON THIS  
REMAINS LOW. AT THIS TIME WE CONTINUE TO MENTION A SLIGHT CHANCE  
FOR A FEW STORMS OVER NORTH CENTRAL IL LATER THIS EVENING.  
 
IT APPEARS THAT MOST, IF NOT ALL OF WEDNESDAY WILL REMAIN DRY  
ACROSS THE AREA. THE ONLY EXCEPTION TO THIS IS IF A FEW ISOLATED  
SHOWERS AND THUNDERSTORMS DEVELOP ALONG AN AFTERNOON LAKE BREEZE.  
DUE TO THE LOW PROBABILITIES FOR ANY STORMS TO DEVELOP ALONG THIS  
FEATURE, I HAVE CONTINUED WILL A DRY FORECAST WITH SUB 15% POPS  
WEDNESDAY AFTERNOON. OTHERWISE, EXPECT PARTLY SUNNY SKIES AND WARM  
CONDITIONS WEDNESDAY, WITH HIGHS IN THE UPPER 80S INLAND (HEAT  
INDICES INTO THE MID TO UPPER 90S). HOWEVER, AREAS NEAR THE LAKE  
WILL REMAIN A BIT COOLER DUE TO AN ONSHORE WIND WITH THE INLAND  
MOVING LAKE BREEZE.  
 
KJB  
 

 
   
LONG TERM
 
 
322 PM CDT  
 
WEDNESDAY NIGHT THROUGH TUESDAY...  
 
ATTENTION IN THIS PERIOD CERTAINLY IS THE HEAT. AN EXCESSIVE HEAT  
WATCH HAS BEEN ISSUED THURSDAY P.M. THROUGH SATURDAY P.M. FOR  
COOK COUNTY, INCLUDING CHICAGO, AS THAT IS THE MOST CONCERNING  
LOCATION BECAUSE OF THE URBAN HEAT ISLAND CORE OF CHICAGO WHERE  
LITTLE RELIEF AT NIGHTS WILL OCCUR. NO HEADLINE YET FOR ELSEWHERE,  
BECAUSE IN PART OF CONCERNS FOR MCS EFFECTS INTO THURSDAY MORNING  
WHICH ARE ALSO A CONCERN FOR CHICAGO. AT LEAST AN ADVISORY LOOKS  
VERY LIKELY, AND DO HAVE TIME FOR THAT WITH STILL 48 HOURS OUT.  
DO THINK EXCESSIVE HEAT HEADLINES ARE POSSIBLE FOR OTHER OUTLYING  
PARTS OF ILLINOIS, MAINLY NORTH CENTRAL ILLINOIS, FOR 110 HEAT  
INDEX CONCERNS ON THURSDAY AFTERNOON AND/OR FRIDAY AFTERNOON.  
CONFIDENCE WAS JUST A LITTLE BELOW ISSUANCE RIGHT NOW.  
 
THE FIRST ITEM OF INTEREST CONTINUES TO BE THE MCS POTENTIAL LATER  
WEDNESDAY NIGHT INTO THURSDAY MORNING AND ITS EFFECTS ON  
TEMPERATURES POTENTIALLY INTO THURSDAY. ORGANIZED CONVECTION  
CONTINUES TO LOOK LIKELY TO DEVELOP LATER WEDNESDAY AFTERNOON INTO  
THE EVENING ACROSS OR NEAR SOUTHERN MINNESOTA. THE PRECISE  
LATITUDE THIS OCCURS IS IMPORTANT AND THAT REMAINS UNCERTAIN, BUT  
THERE MAY BE SOME MODULATION OF EFFECTIVE AIR UPSTREAM DUE TO THE  
LATE TONIGHT INTO WEDNESDAY MORNING MCS. IN GENERAL THE TREND HAS  
CONTINUED TO BE TO LEAN TOWARD THE HIGHER-RESOLUTION GUIDANCE FOR  
HANDLING OF COLD POOL DEVELOPMENT AND PROROGATION FOR WEDNESDAY  
NIGHT, GIVEN THAT CONFIDENCE IS HIGH IN THE OTHER SYNOPTIC  
INGREDIENTS NEEDED FOR MCS DEVELOPMENT AND SUSTAINABILITY. THE  
DRIVING SHORT WAVE AND MID-LEVEL SPEED MAXIMUM LOOK TO PROGRESS  
EASTWARD ACROSS WISCONSIN LATE WEDNESDAY NIGHT INTO EARLY THURSDAY  
MORNING, BUT THE ROBUST LOW-LEVEL JET PROVIDES CORFIDI VECTOR  
MOTIONS TO THE SOUTH. WHILE THE BEST LOW-LEVEL SHEAR IS NORTH OVER  
WISCONSIN, IF THE MCS IS FULLY MATURE (WHICH IS LIKELY), WITH  
AMPLE REPLENISHING MOISTURE SOME ACTIVITY IS LIKELY TO GET INTO  
NORTHERN ILLINOIS. WHILE IT MAY NOT BE SEVERE, THOUGH THERE IS  
THAT CHANCE, IT STILL SHOULD BE PROPOGATING WITH SOME  
REDEVELOPMENT/FESTERING LIKELY THURSDAY MORNING OF ACTIVITY. THE  
BIG UNCERTAINTY IS HOW FAR SOUTH INTO THE CWA, AND FOR NOW HAVE  
COLLABORATED AN INCREASE IN STORM CHANCES ACROSS THE CWA, BUT  
PRIMARILY ALONG/NORTH OF I-88.  
 
OUTFLOW EFFECTS AND CLOUDS WILL BE FACTORS THAT COULD OFFSET HOW  
RAPIDLY WARMING OCCURS ON THURSDAY, BUT THE SYNOPTIC PATTERN OF  
STRONG SOUTHWESTERLY FLOW INTO THE REGION IS FAVORABLE FOR IT TO  
OCCUR QUICKLY. SO MESOSCALE FACTORS WILL BE KEY AND JUST TOO FAR  
OUT IN TIME TO SAY HOW MUCH. HAVE EASED TEMPERATURES SOME IN THE  
NORTHERN CWA, MAINLY THE RATE AT WHICH THEY WARM THROUGH THE  
MORNING. THE ELEVATED MIXED LAYER (EML) IS FORECAST TO ADVECT OVER  
THE AREA, WHICH WILL LIKELY PREVENT SHARP MIXING OF DEW POINTS  
FROM A MOIST LOWER ATMOSPHERE AS NOTED IN REGIONAL SOUNDINGS  
ALREADY. CERTAINLY THERE IS POTENTIAL TO SHOOT INTO THE MID 90S  
AREA WIDE, AND 110 HEAT INDEX MAY BE REACHED IN NORTH CENTRAL  
ILLINOIS IF THAT WERE TO OCCUR. THURSDAY STILL LOOKS BREEZY IN THE  
AFTERNOON, WHICH DOES KEEP THE WET BULB GLOBE TEMPERATURE (WBGT -  
INDEX FOR HEAT RELATED STRESS) DOWN SOME. FOR THURSDAY NIGHT,  
CONFIDENCE IS HIGH IN A WARM AND MUGGY NIGHT TO SET THE STAGE FOR  
A HOT FRIDAY.  
 
FRIDAY HAS PERSISTENTLY BEEN A DAY FORECAST TO HAVE THE THERMAL  
RIDGE POISED OVER THE AREA AND REPLENISHING SOUTHWEST FLOW UNDER  
NEAR FULL SUN. THE 850 MB TEMPERATURES OF 23-25C HISTORICALLY THIS  
TIME OF YEAR HAVE PROVIDED HIGHS ALL WITHIN MID 90S-LOW 100S.  
HAVE UPPER 90S FORECAST FOR ALMOST THE ENTIRE AREA. DEW POINTS  
WILL PROBABLY MIX OUT SOME IN THIS PATTERN, BUT HOW MUCH AND HOW  
GUIDANCE IS HANDLING THAT IS DIFFICULT TO SAY FOR CERTAIN,  
INCLUDING FOR SATURDAY. HEAT INDICES LOOK TO DEFINITELY WARRANT AT  
LEAST HEAT ADVISORIES, AND AGAIN THE DURATION OF THE HEAT IS A  
KEY FACTOR FOR AN EXCESSIVE HEAT HEADLINE.  
 
BY SATURDAY, THE SYNOPTIC FRONT WILL BE INCHING SOUTHWARD, AND THE  
TREND HAS BEEN BACK TO FURTHER SOUTH FOR SATURDAY. BUT THERE DOES  
NOT LOOK TO BE MUCH ACTIVITY ALONG IT INTO SATURDAY GIVEN A FAIRLY  
LAMINAR MID-LEVEL FLOW INTO THE DAY, WHICH IS WHY STILL LEANING  
UNIMPEDED HEAT ON SATURDAY, AND IT COULD HAVE TEMPERATURES THAT  
EQUAL FRIDAY'S. DEW POINTS AGAIN SHOULD MIX OUT AT LEAST  
SOMEWHAT. THE CHANCES FOR THUNDERSTORMS START TO INCREASE LATER  
SATURDAY NIGHT INTO SUNDAY AS GUIDANCE CREEPS THE BOUNDARY  
SOUTHWARD WITHIN A DAMPENING UPPER RIDGE PATTERN.  
 
MTF  
 

 
   
AVIATION
 
 
FOR THE 18Z TAFS...  
 
SCATTERED SHOWERS HAVE BEEN RAPIDLY DEVELOPING ACROSS THE AREA  
LATE THIS MORNING. EXPECT THIS TO CONTINUE TO BE THE CASE ACROSS  
MUCH OF EASTERN IL AND NORTHWEST IN AS THE REMNANTS OF BARRY MOVE  
ACROSS THE AREA. DIURNAL DESTABILIZATION MAY ALSO RESULT IN SOME  
THUNDERSTORMS, ESPECIALLY OVER THE SOUTHERN CHICAGO AREA  
TERMINALS. THE MAIN THREAT FROM THESE SHOWERS AND STORMS WILL BE  
VERY HEAVY RAINFALL. IN FACT, VISIBILITIES MAY BRIEFLY DROP UNDER  
2 MILES WITH THESE SHOWERS.  
 
THIS ACTIVITY WILL LIKELY WANE ACROSS THE AREA LATER THIS  
AFTERNOON, WITH THE EVENING AND OVERNIGHT PERIOD LIKELY REMAINING  
DRY. DRY WEATHER IS ALSO FORECAST FOR WEDNESDAY, THOUGH AN  
AFTERNOON LAKE BREEZE COULD SPARK OFF A FEW WIDELY SCATTERED  
SHOWERS OR STORMS.  
 
WINDS MAY BACK MORE SOUTH-SOUTHEASTERLY LATER THIS AFTERNOON AND  
EVENING, BUT SPEEDS WILL REMAIN LIGHT. LIGHT AND VARIABLE WINDS  
ARE EXPECTED ACROSS THE AREA TONIGHT INTO WEDNESDAY MORNING.  
THEREAFTER, A LAKE BREEZE WILL LIKELY SHIFT INLAND, RESULTING IN  
AN AFTERNOON EASTERLY WIND SHIFT AT THE CHICAGO AREA TERMINALS.  
 
KJB  
 

 
   
LOT WATCHES/WARNINGS/ADVISORIES
 
 
IL...EXCESSIVE HEAT WATCH...ILZ014...NOON THURSDAY TO 7 PM SATURDAY.  
 
IN...NONE.  
LM...NONE.  

 
 

 
 
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