330  
FXUS63 KLOT 202325  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
625 PM CDT WED SEP 20 2017  
   
SHORT TERM
 
 
1246 PM CDT  
 
THROUGH THURSDAY AFTERNOON...  
 
CONCERN IS ON CONVECTIVE DEVELOPMENT LATE THIS AFTERNOON/EARLY  
EVENING AND HOW FAR EAST/SOUTH IT WILL MAKE IT, ALONG WITH HOT  
AND HUMID CONDITIONS THAT LOOK TO PERSIST FOR THE COMING DAYS.  
 
IT IS INDEED HOT AND HUMID FOR MID SEPTEMBER, AND ORD/MDW ARE  
ALREADY ALMOST AT 90 DEGREES TODAY. WARM SOUTH WINDS AND LITTLE  
CLOUD COVER WILL ALLOW MOST LOCATIONS TO GET CLOSE TO 90 TODAY.  
CONVECTIVE TEMPERATURES ON THE RAP ARE IN THE 90S ACROSS IA,  
WHICH LOOKS TO DELAY ANY INITIAL CONVECTIVE DEVELOPMENT NEAR THE  
COLD FRONT UNTIL LATE THIS AFTERNOON. THERE IS PRETTY GOOD  
AGREEMENT THOUGH THAT THUNDERSTORMS WILL DEVELOP BETWEEN 5 AND 6  
PM NEAR THE MISSISSIPPI RIVER. THE QUESTION WILL BE WITH THE UPPER  
FORCING BEING A BIT WEAKER FARTHER EAST, AND WITH THE CONVECTION  
ARRIVING AROUND SUNSET OR SO, THAT SHOULD LIMIT THE EASTWARD  
THUNDER CONCERNS WITH THIS INITIAL DEVELOPMENT. THERE COULD BE  
SOME SPRINKLES THAT GET CLOSER TO CHICAGO BUT THE FORCING DOES NOT  
APPEAR TO OVERCOME DRY MID LEVELS.  
 
WHILE WEST/NORTHWEST IL IS FAVORED AT THIS TIME, THE LOW LEVEL  
JET WILL RAMP BACK UP IN THIS HIGH PWAT ENVIRONMENT, WHICH COULD  
LEAD TO AN UPTICK IN SHOWERS AND STORMS LATE THIS EVENING/EARLY  
OVERNIGHT HOURS. PWATS NEAR THE FRONT REMAIN ELEVATED AND DO  
INCREASE, SO IF SHOWERS AND STORMS DO GET A BIT CLOSER, SOME  
MODEST RAIN RATES COULD OCCUR IN THE FAR NORTHWEST.  
 
UPPER LEVEL HEIGHTS REBUILD TOMORROW, AND CONTINUED DEEP LAYER  
SOUTHERLY FLOW WILL LIFT THE EFFECTIVE FRONTAL BOUNDARY BACK  
NORTHWARD. THE LOWER LEVEL AIRMASS WILL BE ON PAR WITH TODAY'S IF  
NOT A BIT WARMER, AND THUS MANY AREAS WILL LIKELY TAG 90 OR EVEN  
THE LOWER 90S. THESE ARE NEAR RECORD LEVELS FOR MID TO LATE  
SEPTEMBER. WITH CONTINUED UPPER 60S TO AROUND 70 DEWPOINT, THIS  
PLACES HEAT INDICES IN THE MIDDLE OR EVEN UPPER 90S.  
 
KMD  
 

 
   
LONG TERM
 
 
THURSDAY NIGHT THROUGH WEDNESDAY...  
 
155 PM...A WARM FRONTAL BOUNDARY WILL LIFT BACK NORTH OF THE AREA  
THURSDAY NIGHT WITH HOT AND HUMID CONDITIONS CONTINUING INTO THIS  
WEEKEND. BUMPED UP HIGH TEMPS BOTH FRIDAY/SATURDAY TO NEAR  
GUIDANCE WHICH IS GENERALLY IN THE LOWER 90S FRIDAY AND AROUND 90  
SATURDAY. GIVEN HIGH TEMPS TODAY/CURRENTLY ARE AROUND 90...THESE  
FORECAST HIGH TEMPS FOR FRIDAY AND SATURDAY MAY END UP BEING A FEW  
DEGREES TOO COOL. NOT EXPECTING MUCH CHANGE TO DEWPOINTS EITHER...  
IN THE MID/UPPER 60S THUS HEAT INDEX READINGS COULD REACH THE  
MID/UPPER 90S...AT LEAST FOR A FEW HOURS DURING PEAK HEATING IN  
THE EARLY/MID AFTERNOONS. THERMALLY...CONDITIONS COOL SLIGHTLY  
SUNDAY/MONDAY BUT ITS POSSIBLE THAT WITH LITTLE CHANGE TO THIS  
PATTERN THE MODELS MAY TREND WARMER INTO EARLY NEXT WEEK. NO  
CHANGE TO HIGHS IN THE 80S SUNDAY/MONDAY...BUT THESE COULD ALSO  
END UP TOO COOL.  
 
DRY CONDITIONS EXPECTED UNTIL A COLD FRONT ARRIVES TUESDAY NIGHT  
NEXT WEEK WHICH WILL BRING THE NEXT BEST CHANCE OF RAIN. MUCH  
COOLER AIR WILL SPREAD INTO THE REGION BEHIND THIS FRONTAL  
BOUNDARY BY WEDNESDAY AFTERNOON/EVENING. THERE MAY ALSO BE AN  
ISOLATED SHOWER SUNDAY/MONDAY BUT CHANCES ARE BELOW MENTIONABLE  
FROM THIS DISTANCE. CMS  
 

 
   
AVIATION
 
 
FOR THE 00Z TAFS...  
 
PRIMARY AVIATION CONCERN THIS TAF CYCLE IS WITH SHRA/TSRA  
CHANCES. STORMS HAVE DEVELOPED FROM EASTERN IA NORTH INTO WI AHEAD  
OF A WEAK COLD FRONT. ALL INDICATIONS ARE THAT THIS ACTIVITY WILL  
DISSIPATE BEFORE REACHING THE IMMEDIATE CHICAGO TERMINALS, WITH  
BEST CHANCE OF IMPACTS BEING AT RFD. HAVE MAINTAINED A SHORT TEMPO  
AT RFD THIS EVENING TO COVER THIS THREAT. OTHERWISE, WHERE SKIES  
CLEAR OVERNIGHT, COULD SEE SOME LIGHT FOG/MVFR VSBY, ESPECIALLY IN  
THE MORE RURAL AREAS LIKE GYY/DPA. OTHERWISE, LOOK FOR VFR  
CONDITIONS AND LIGHT TO MODERATE SOUTH WINDS THROUGH THE TAF  
CYCLE.  
 
- IZZI  
 

 
   
MARINE
 
 
155 PM...STRONG LOW PRESSURE OVER WESTERN MANITOBA WILL SLOWLY  
WEAKEN TONIGHT AND THURSDAY AS A WEAK TRAILING COLD FRONT MOVES  
SOUTH ACROSS LAKE MICHIGAN TONIGHT INTO THURSDAY MORNING. THIS  
FRONT IS EXPECTED TO STALL OVER CENTRAL PORTIONS OF THE LAKE THEN  
MOVE BACK NORTH AS A WARM FRONT BY THURSDAY NIGHT. A LARGE TROUGH  
OF LOW PRESSURE WILL REMAIN NEARLY STATIONARY ACROSS THE PLAINS  
THROUGH EARLY NEXT WEEK WHICH WILL MAINTAIN SOUTHERLY FLOW ACROSS  
THE LAKE. THIS TROUGH/FRONT WILL MOVE ACROSS THE GREAT LAKES  
REGION TUESDAY NIGHT NEXT WEEK SHIFTING WINDS TO THE NORTHWEST  
WHEN A PERIOD OF 15-25KTS IS EXPECTED. CMS  
 

 
   
LOT WATCHES/WARNINGS/ADVISORIES
 
 
IL...NONE.  
IN...NONE.  
LM...NONE.  

 
 

 
 
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