515  
FXUS63 KLOT 240537  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
1237 AM CDT FRI SEP 24 2021  
   
SHORT TERM
 
 
256 PM CDT  
 
THROUGH FRIDAY NIGHT...  
 
A VERTICALLY STACKED STORM SYSTEM OVER EASTERN LOWER MICHIGAN HAS  
REMAINED JUST FAR ENOUGH EAST OF THE AREA TO KEEP THE SHOWERS  
TODAY OVER LAKE MICHIGAN AND ADJACENT AREAS OF LAKE AND PORTER  
COUNTIES IN IN. HOWEVER, THE CHICAGO METRO AREA HAS BEEN ON THE  
FAR WESTERN PERIPHERY OF THE LARGE CLOUD SHIELD ASSOCIATED WITH  
THIS SYSTEM. THIS HAS RESULTED IN A BIT A TEMPERATURE CONTRAST  
ACROSS THE AREA. LOCATIONS WEST OF THE CITY HAVE SEEN A GOOD DEAL  
OF SUN THROUGH THE DAY, THUS ALLOWING THEIR TEMPERATURES TO CLIMB  
INTO THE UPPER 60S TO AROUND 70. HOWEVER, PLACES UNDER THE CLOUD  
SHIELD, ESPECIALLY INTO NORTHWESTERN IN ARE ONLY IN THE UPPER 50S  
TO THE LOW 60S. THE LINGERING SHOWERS OVER PORTER COUNTY IN WILL  
DISSIPATE LATER THIS AFTERNOON AND SKIES WILL CLEAR AREA- WIDE,  
SETTING UP A SEASONABLY CHILLY NIGHT ACROSS THE AREA. EXPECT LOW  
TEMPERATURES TO FALL INTO THE 40S OUTSIDE OF THE CHICAGO METRO  
AREA.  
 
ON FRIDAY WINDS WILL TURN SOUTHWESTERLY AND BECOME RATHER GUSTY  
INTO THE AFTERNOON IN RESPONSE TO A STRENGTHENING SURFACE  
GRADIENT IN ADVANCE OF AN APPROACHING COLD FRONTAL TROUGH. WIND  
GUSTS ARE LIKELY TO GUST UP TO 35 MPH FROM THE SOUTHWEST DURING  
THE AFTERNOON AND EVENING. A WARMER AIRMASS WILL FILTER INTO THE  
AREA VIA THESE GUSTY WINDS, WHICH WILL SUPPORT HIGHS AREA-WIDE  
REACHING INTO THE UPPER 70S ON FRIDAY AFTERNOON.  
 
MILD AND BREEZY CONDITIONS WILL CONTINUE FRIDAY EVENING AS THE  
FRONT BEGINS TO SHIFT INTO NORTHWESTERN IL. THE FRONT WILL SHIFT  
ACROSS ALL OF NORTHERN IL BY AROUND, OR SHORTLY AFTER MIDNIGHT,  
THEN ACROSS MY NORTHWESTERN IN COUNTIES BY 4 AM. WHILE LOW-LEVEL  
MOISTURE WILL BE RATHER SPARSE TOMORROW, WITH SURFACE DEWPOINTS  
LIKELY IN THE 40S, DEWPOINTS LOOK TO POOL A BIT INTO THE LOW TO  
MID 50S ALONG THE FRONTAL BOUNDARY FRIDAY EVENING. GIVEN THE SHARP  
MID-LEVEL TROUGH WITH DECENT HEIGHT FALLS ALOFT SHIFTING  
OVERHEAD, THE EXPECTATION CONTINUES TO BE THAT A PERIOD OF SHOWERS  
WITH SOME EMBEDDED THUNDERSTORMS WILL DEVELOP WITH THE FRONTAL  
BOUNDARY. SEVERE WEATHER IS NOT ANTICIPATED, AND ANY SHOWERS AND  
STORMS SHOULD END FROM NORTHWEST TO SOUTHEAST FOLLOWING THE  
FRONTAL PASSAGE. COOLER TEMPERATURES WILL FOLLOW THE FRONTAL  
BOUNDARY INTO SATURDAY.  
 
KJB  
 

 
   
LONG TERM
 
 
110 PM CDT  
 
SATURDAY THROUGH THURSDAY...  
 
A TREND OF PLEASANT AND WARMING CONDITIONS THROUGH THE WEEKEND AND  
INTO NEXT WEEK IS EXPECTED.  
 
THE COLD FRONT THAT WILL BRING US RAIN CHANCES FRIDAY NIGHT IS  
EXPECTED TO CLEAR THE AREA BY SATURDAY MORNING AS HIGH PRESSURE  
BEGINS TO MOVE IN AND DRY US OUT. WINDS BEHIND THE FRONT WILL BE  
WESTERLY AT 10-15 MPH. MOST OF US ON SATURDAY WILL SEE SUNNY  
CONDITIONS WITH THE EXCEPTION OF A FEW CLOUDS NEAR THE LAKE WITH  
AFTERNOON HIGHS IN THE LOWER 70S.  
 
HIGH PRESSURE WILL CONTINUE TO MOVE EAST HEADING INTO SUNDAY  
WHICH WILL TURN OUR WINDS OUT OF THE SOUTH AND TRANSPORT WARMER  
AIR NORTHWARD TO THE GREAT LAKES. WINDS WILL REMAIN LIGHT THROUGH  
THE PERIOD WITH SPEEDS OF 5 TO 10 MPH. HIGH TEMPERATURES SUNDAY  
THROUGH THURSDAY WILL REACH THE MID TO UPPER 70S WITH SOME PLACES  
POSSIBLY GETTING INTO THE LOWER 80S. IN ADDITION TO THE WARM  
TEMPERATURES, HIGH PRESSURE WILL KEEP US DRY WITH NO RAIN CHANCES  
AND MOSTLY CLOUD-FREE SKIES WITH THE EXCEPTION OF A FEW PASSING  
CLOUDS FROM TIME TO TIME.  
 
YACK  
 

 
   
FIRE WEATHER
 
 
256 PM CDT  
 
SOUTHWEST WINDS SHOULD QUICKLY INCREASE LATER FRIDAY MORNING INTO  
THE AFTERNOON. CURRENTLY LOOKS LIKE A FEW HOURS OF PEAK 20FT  
WINDS AROUND 15-20 MPH WITH HIGHER GUSTS. CURRENT FORECASTS FOR  
MINIMUM AFTERNOON RH VALUES NORTHWEST OF I-55 ARE BETWEEN 30-40  
PERCENT, BUT IT IS WORTH NOTING THAT SET-UPS LIKE FRIDAY'S  
FORECAST SET-UP CAN SOMETIME SEE RH VALUES DROPPING LOWER THAN  
FORECAST DUE TO DEEP MIXING AND STRONG WINDS TAPPING INTO DRY AIR  
ABOVE THE SURFACE. THE 10-HOUR FUEL STICK MOISTURE VALUES SHOULD  
REMAIN JUST ABOVE CRITICAL THRESHOLDS AS WELL, SO AT THIS TIME IT  
APPEARS THE THREAT OF REACHING RED FLAG CRITERIA IS QUITE LOW.  
HOWEVER, THE STRONG WINDS, LOW RH, AND ANTECEDENT DRY CONDITIONS  
SHOULD RESULT IN A HEIGHTENED (MAINLY GRASS) FIRE DANGER,  
PARTICULARLY NORTHWEST OF I-55 ACROSS NORTHERN ILLINOIS.  
 
- IZZI  
 

 
   
AVIATION
 
 
FOR THE 06Z TAFS...  
 
AVIATION WEATHER CONCERNS THIS PERIOD CONSIST OF:  
 
* DEVELOPING GUSTY SOUTHWESTERLY WINDS THROUGH THIS AFTERNOON WITH  
SOME OCCASIONAL 30 KT GUSTS POSSIBLE, ESPECIALLY INVOF RFD LATE  
 
* QUICK-HITTING PERIOD OF SHOWERS THIS EVENING WITH A LOW CHANCE  
FOR A STORM  
 
* BREEZY NORTHWEST WIND SHIFT THIS EVENING WITH A COLD FRONT  
 
VFR CONDITIONS AND MOSTLY CLEAR SKIES WILL PREVAIL TONIGHT AND  
INTO THIS MORNING. WINDS WILL GRADUALLY INCREASE AND BECOME GUSTY  
THROUGH THE MORNING, WITH THE PERIOD OF STRONGEST WINDS EXPECTED  
TO OCCUR LATER IN THE AFTERNOON. LATEST GUIDANCE SUGGESTS THE  
PERIOD OF STRONGEST WINDS OCCURRING IN THE 19-00Z TIME FRAME, WITH  
A BRIEF WINDOW WHERE GUSTS COULD NEAR OR EXCEED 30 KTS, ESPECIALLY  
INVOF RFD. A QUICK-HITTING ROUND OF SHOWERS IS ANTICIPATED, WITH  
CONFIDENCE IN COVERAGE HIGH ENOUGH TO JUSTIFY TEMPO GROUPS AT ALL  
THE REGIONAL TAF SITES. INSTABILITY WILL BE LACKING WITH THIS  
SYSTEM GIVEN THE MAGNITUDE OF DRY AIR IN PLACE, BUT A FEW  
LIGHTNING STRIKES CAN'T BE RULED OUT--PROBABILITIES HOWEVER ARE  
WELL UNDER 20% AT THIS TIME.  
 
A NORTHWEST WIND SHIFT WILL OCCUR IN THE WAKE OF A COLD FRONT  
SOMETIME TOWARDS MID-LATE EVENING ALTHOUGH SOME UNCERTAINTY  
REMAINS WITH EXACT FROPA TIMING. WINDS SHOULD REMAIN A BIT  
ELEVATED/BREEZY BEHIND THE FRONT, BUT WILL GRADUALLY EASE AS THE  
STRONGEST FLOW SHIFTS OFF TO THE EAST OF THE REGION.  
 
CARLAW  
 

 
   
LOT WATCHES/WARNINGS/ADVISORIES
 
 
IL...NONE.  
IN...BEACH HAZARDS STATEMENT...INZ001-INZ002 UNTIL 4 AM FRIDAY.  
 
LM...GALE WATCH...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...5 PM  
FRIDAY TO 1 AM SATURDAY.  
 
SMALL CRAFT ADVISORY...NEARSHORE WATERS  
UNTIL 5 PM FRIDAY.  
 

 
 

 
 
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