323  
FXUS63 KLOT 200835  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
335 AM CDT WED SEP 20 2017  
   
SHORT TERM
 
 
242 AM CDT  
 
THROUGH TONIGHT...  
 
MAIN FORECAST CHALLENGE THIS MORNING IS DEALING WITH ONGOING FOG  
IN THE AREA, DENSE IN SPOTS. LATER TODAY, FOCUS WILL SHIFT OFF  
TO THE WEST AS A LINE OF THUNDERSTORMS APPROACHES THE AREA.  
 
EARLY THIS MORNING, DENSE FOG HAS DEVELOPED OVER PORTIONS OF  
NORTHWEST INDIANA AND FAR NORTHEAST ILLINOIS, PRIMARILY OUTSIDE  
THE HEART OF THE CHICAGO METRO. A DENSE FOG ADVISORY HAS BEEN  
ISSUED FOR THESE AREAS THROUGH 9 AM CDT. OVER NORTHWEST INDIANA,  
SPLIT CHANNEL FOG IMAGERY FROM GOES 16 SHOWS FOG DEVELOPING  
WESTWARD ALONG THE KANKAKEE RIVER AND LIFTING NORTH. IT'S  
POSSIBLE THIS FOG EXPANDS FARTHER WEST INTO EASTERN KANKAKEE AND  
WILL COUNTIES THROUGH THE MORNING IN WHICH CASE THE DENSE FOG  
ADVISORY MAY NEED TO BE EXPANDED. OTHERWISE, GRADUALLY INCREASING  
GRADIENT FROM THE WEST SHOULD HELP KEEP FOG FROM GETTING TOO OUT  
OF HAND FOR THE REMAINING COUNTIES.  
 
BY THIS AFTERNOON, A STRONG SHORTWAVE CURRENTLY OVER THE DAKOTAS  
WILL LIFT ACROSS MINNESOTA AND MANITOBA WHILE A SURFACE COLD FRONT  
PUSHES ACROSS THE UPPER MISSISSIPPI VALLEY. OUT AHEAD OF THE  
FRONT, TEMPERATURES WARMING INTO THE UPPER 80S AND DEWPOINTS  
NEARING 70F WILL CONTRIBUTE TO STRONG INSTABILITY. GFS INDICATES  
MLCAPE IN EXCESS OF 3000 J/KG WILL BE IN PLACE OVER PORTIONS OF  
NORTHERN ILLINOIS AND NORTHWEST INDIANA, BUT CAPPED BY 850MB TEMPS  
WARMING TO AROUND 18-20C. SHOWERS AND THUNDERSTORMS ARE EXPECTED  
TO REDEVELOP ALONG THE FRONT OVER EASTERN IA AND WESTERN WI AND  
TRACK EAST TO PORTIONS OF NORTH CENTRAL ILLINOIS THIS EVENING.  
THERE ISN'T A REALLY CLEAR SIGNAL BETWEEN THE GLOBAL MODELS AND  
CONVECTIVE ALLOWING MODELS WITH RESPECT TO INTENSITY AND COVERAGE  
OF THE STORMS THIS EVENING, AND THE CAPPING AND LACK OF UPPER  
LEVEL SUPPORT WILL BE A CONCERN. CONTINUE TO FAVOR LOWER  
COVERAGE/INTENSITY SOLUTIONS, ESPECIALLY EAST OF THE I-39 CORRIDOR  
AS THE BOUNDARY LAYER STARTS TO STABILIZE AFTER SUNSET AND  
BECAUSE OF THE SEPARATION OF THE UPPER WAVE OVER MANITOBA/ONTARIO  
WITH UPPER RIDGING IN PLACE LOCALLY. BOTTOM LINE, WHATEVER FORMS  
TO OUR WEST WILL AT SOME POINT BEGIN TO WEAKEN LATE THIS EVENING  
AS IT CROSSES THE LOCAL AREA AND THE EASTERN HALF TO TWO THIRDS OR  
SO OF THE CWA MAY SEE LITTLE IF ANY PRECIPITATION. DEEP LAYER  
SHEAR APPEARS TO BE SUB-MARGINAL TO SUPPORT ANY WIDESPREAD OR  
ORGANIZED SEVERE THREAT OVER THE AREA ALTHOUGH AN ISOLATED PULSE  
THREAT GIVEN THE STRONG INSTABILITY PRIMARILY IN THE WESTERN  
COUNTIES IS NOT OUT OF THE QUESTION.  
 
DEUBELBEISS  
 

 
   
LONG TERM
 
 
242 AM CDT  
 
THURSDAY THROUGH TUESDAY...  
 
A CLOSED UPPER LOW IS EXPECTED TO DIG FROM THE PACIFIC NW INTO THE  
INTERMOUNTAIN WEST THURSDAY AND FRIDAY WITH DOWNSTREAM  
AMPLIFICATION OF THE UPPER RIDGE OCCURRING OVER THE EASTERN HALF  
OF THE U.S. THE PATTERN BECOMES VERY BLOCKY LATE IN THE WEEK  
THROUGH THE WEEKEND WHICH WILL RESULT IN FAIRLY STAGNANT  
CONDITIONS ALLOWING A PROLONGED PERIOD OF ABOVE NORMAL HEAT AND  
HUMIDITY ALONG WITH PRIMARILY DRY CONDITIONS. FOR NOW, BOTH THE  
ECMWF AND GFS ARE IN REASONABLE AGREEMENT THAT THIS PATTERN WILL  
BEGIN TO BREAK DOWN AROUND MIDWEEK NEXT WEEK ALLOWING COOLER AIR  
BACK INTO THE REGION MODERATING TEMPERATURES CLOSER TO SEASONAL  
NORMS. IN THE MEANTIME, EXPECT AFTERNOON HIGHS AREA-WIDE IN THE  
MID TO UPPER 80S AND A FEW SPOTS MAY TAG THE 90 MARK THURSDAY  
THROUGH SATURDAY. IN ADDITION, DEW POINTS IN THE MID TO UPPER 60S  
WILL BE IN PLACE THROUGH THIS TIME FRAME.  
 
DEUBELBEISS  
 

 
   
AVIATION
 
 
FOR THE 06Z TAFS...  
 
GYY WILL BE THE MOST LIKELY SPOT FOR DENSE FOG TONIGHT. GYY  
BRIEFLY DROPPED TO 1/4SM, AND EXPECTING DENSE FOG TO RETURN TO GYY  
EARLY THIS MORNING. OTHER SITES WILL LIKELY REPORT MVFR VSBY  
EARLY THIS MORNING, AND I HAVE LOW CONFIDENCE IN LOWER VSBY. SINCE  
ORD AND MDW BRIEFLY DROPPED TO MVFR, DECIDED TO KEEP IFR IN THE  
TAFS AND ADJUST AT 09Z IF NEEDED. HOWEVER, I'M THINKING ALL THREE  
SITES WILL STAY ABOVE IFR.  
 
VFR CONDITIONS ARE EXPECTED AFTER THE FOG DISSIPATES THIS MORNING,  
AND GUSTY SOUTHERLY WINDS DEVELOP IN THE EARLY AFTN. MAX GUSTS  
WILL BE ARND 20 KT.  
 
CONVECTION FIRES ALONG A COLD FRONT ACROSS WI AND IA THIS  
AFTERNOON. RFD HAS THE BEST CHANCE IN REPORTING SHRA AND TSRA. I  
THINK THE LINE OF STORMS WILL QUICKLY DISSIPATE AS THEY ENCOUNTER  
MORE STABLE AIR ACROSS NORTHERN IL THIS EVENING. THEREFORE, THE  
ONLY TAF WITH PRECIP IS RFD WITH SHRA AND A TEMPO FOR TSRA. THE  
WINDOW OF TSRA SHOULD BE SHORT, BUT I HAVE LOW TIMING IN EXACT  
PRECIP TIME OF ARRIVAL. GUIDANCE MEMBERS VERY WITH HOW QUICKLY THE  
CONVECTION WILL ADVANCE.  
 
JEE  
 

 
   
MARINE
 
 
242 AM CDT  
 
SOUTHERLY WINDS WILL GUST TO 25 KT THIS AFTERNOON. HIGH PRESSURE  
PASSES OVER LAKE SUPERIOR TONIGHT, AND WINDS BECOME NORTH TO  
NORTHEAST OVER THE NORTHERN HALF OF THE LAKE. SOUTHERLY FLOW  
RETURNS ACROSS THE LAKE THURSDAY NIGHT DUE TO A BLOCKY PATTERN. A  
LOW PRESSURE TROUGH WILL DEVELOP FROM THE NORTHERN MISSISSIPPI  
VALLEY THROUGH THE SOUTHERN PLAINS FRIDAY WHILE A HIGH PRESSURE  
RIDGE REMAINS PARKED OVER THE EASTERN U.S. THIS PATTERN WILL HOLD  
AND RESULT IN SOUTHERLY WINDS THROUGH EARLY NEXT WEEK.  
 
JEE  
 

 
   
LOT WATCHES/WARNINGS/ADVISORIES
 
 
IL...DENSE FOG ADVISORY...ILZ005-ILZ006 UNTIL 9 AM WEDNESDAY.  
 
IN...DENSE FOG ADVISORY...INZ001-INZ002-INZ011 UNTIL 9 AM WEDNESDAY.  
 
LM...NONE.  

 
 

 
 
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