141  
FXUS63 KLOT 181210  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
610 AM CST SAT NOV 18 2017  
   
SHORT TERM
 
 
345 AM CST  
 
DEEPENING LOW PRESSURE SYSTEM MOVING THROUGH THE REGION TODAY,  
PRESENTING PLENTY OF NEAR TERM FORECAST CONCERNS. PRECIP TRENDS,  
INCLUDING MORNING THUNDERSTORMS AND MIDDAY/AFTERNOON RAIN/WET SNOW  
MIX, FALLING TEMPS AND VERY WINDY CONDITIONS ALL THE MAIN FOCI  
FOR THE NEXT 12-18 HOURS. LAKE SHORE FLOOD ADVISORY WILL BE ISSUED  
FOR INDIANA SHORE OF LAKE MICHIGAN FOR THIS AFTERNOON AND  
TONIGHT, DUE TO STRONG NORTH WINDS PRODUCING LARGE WAVES LEADING  
TO POTENTIAL FLOODING AND BEACH EROSION.  
 
SURFACE LOW PRESSURE ALONG THE KS/MO BORDER EARLY THIS MORNING,  
WILL DEEPEN IN RESPONSE TO STRONG MID-LEVEL HEIGHT FALLS  
ASSOCIATED WITH A STRENGTHENING UPPER TROUGH WHICH WILL PROPAGATE  
ACROSS THE REGION TODAY. THE LOW IS PROGGED TO LIFT QUICKLY  
NORTHEAST ACROSS CENTRAL IL AND INTO NORTHERN IN BY MIDDAY, AND TO  
LAKE ERIE BY EARLY THIS EVENING. STRONG FORCING, INCLUDING  
ISENTROPIC ASCENT OF WARM/MOIST AIR ATOP STRENGTHENING SURFACE  
STATIONARY/WARM FRONTAL TROUGH IN ADVANCE OF THE LOW AND ENHANCED  
BY UPPER DIVERGENCE IN LEFT EXIT REGION OF 140+ KNOT UPPER JET  
STREAK, IS EXPECTED TO RESULT IN RAIN EXPANDING ACROSS THE AREA  
AGAIN EARLY THIS MORNING. CURRENT RADAR AND LIGHTNING DETECTION  
DEPICTS THUNDERSTORMS OCCURRING ACROSS THE SOUTHERN TIER OR SO OF  
CWA COUNTIES WHERE RAP MESOANALYSIS INDICATED 500+ J/KG OF MUCAPE  
ALONG AND SOUTH OF THE STATIONARY FRONTAL ZONE. THUNDERSTORM  
POTENTIAL IS EXPECTED TO PERSIST ACROSS THESE SOUTHERN COUNTIES  
THIS MORNING, BEFORE INSTABILITY IS SHUNTED OFF TO THE EAST OF THE  
AREA WITH THE PASSAGE OF THE SURFACE LOW LATE THIS MORNING.  
LOCATIONS SOUTH OF A PONTIAC-KANKAKEE-RENSSELAER LINE MAY SEE  
1-1.50 INCH RAIN AMOUNTS FROM THIS CONVECTION.  
 
VARIOUS MODEL GUIDANCE DEPICTS DEFORMATION ZONE PRECIPITATION  
LINGERING ACROSS THE FORECAST AREA INTO THIS AFTERNOON IN  
ASSOCIATION WITH THE ELEVATED BAROCLINIC ZONE NORTH OF THE SURFACE  
FRONT AND LOW TRACK, WITH COOLING OF THE THERMAL COLUMN EXPECTED  
AS NORTHERLY WINDS INCREASE AND PULL COLDER AIR IN BEHIND THE  
DEEPENING LOW. MORNING TEMPERATURES IN THE LOW 40'S TO MID 50'S  
NORTH TO SOUTH WILL FALL INTO THE MID-UPPER 30'S ACROSS MOST OF  
THE AREA BY AFTERNOON, WITH FORECAST SOUNDINGS SUPPORTING A  
MIX/CHANGE TO SOME WET SNOW ESPECIALLY ACROSS NORTHERN IL PORTIONS  
OF THE CWA DURING THE LATE MORNING AND AFTERNOON. GROUND/AIR  
TEMPS ARE EXPECTED TO REMAIN MILD ENOUGH, WITH SURFACE WET-BULB  
TEMPS IN THE MID 30'S SUCH THAT LITTLE IF ANY ACCUMULATION IS  
EXPECTED, PERHAPS A SLUSHY TENTH OR TWO ON SOME GRASSY SURFACES  
WHERE PRECIP IS BRIEFLY INTENSE ENOUGH. PRECIP WILL TAPER OFF AND  
END FROM WEST TO EAST THIS AFTERNOON INTO EARLY THIS EVENING.  
 
THE OTHER NOTABLE WEATHER CONCERN WILL BE THE DEVELOPMENT OF VERY  
WINDY CONDITIONS BY LATE MORNING/MIDDAY, AS NORTHERLY WINDS  
INCREASE TO 20-30 MPH WITH GUSTS TO 40-45 MPH IN THE WAKE OF THE  
DEEPENING LOW. NOT ONLY WILL THESE WINDS MAKE FOR UNCOMFORTABLE  
WIND CHILLS IN THE 20'S, BUT THEY WILL ALSO RESULT IN LARGE WAVES  
BUILDING ON SOUTHERN LAKE MICHIGAN, WHICH WILL LIKELY CAUSE LAKE  
SHORE FLOODING ISSUES ALONG THE SOUTH (INDIANA) SHORE THIS  
AFTERNOON INTO TONIGHT. BASED ON THIS, A LAKESHORE FLOOD ADVISORY  
FOR LAKE AND PORTER COUNTIES FROM NOON TODAY THROUGH 3 AM CST  
SUNDAY MORNING.  
 
RATZER  
 

 
   
LONG TERM
 
 
346 AM CST  
 
SUNDAY THROUGH FRIDAY...  
 
DRY WEATHER IS EXPECTED THROUGH MUCH OF THE COMING WEEK, WITH  
VARIABLE TEMPERATURES AS A SERIES OF UPPER TROUGHS AND SURFACE LOW  
PRESSURE SYSTEMS MOVE THROUGH THE NORTHERN LAKES.  
 
HIGH PRESSURE BUILDS FROM THE PLAINS THROUGH THE OHIO VALLEY AND  
GULF COAST STATES SUNDAY INTO MONDAY, IN THE WAKE OF SATURDAY'S  
DEEP LOW PRESSURE SYSTEM. THIS RESULTS INITIALLY IN A CHILLY DAY  
SUNDAY, WITH WINDS STILL SOMEWHAT BLUSTERY EARLY AS THE CENTER OF  
THE HIGH PASSES SOUTH OF THE AREA. WINDS TURN SOUTHWESTERLY MONDAY  
HOWEVER, AS AN AREA OF LOW PRESSURE MOVES ACROSS SOUTHERN CANADA  
AND PASSES NORTH OF THE LAKES TUESDAY. THIS BRINGS TEMPS BACK INTO  
THE 40'S ACROSS THE CWA MONDAY-TUESDAY, BEFORE A COLD FRONT TRAILS  
ACROSS THE REGION LATER IN THE DAY TUESDAY. COLDER BUT DRY  
CONDITIONS THEN DEVELOP MID-WEEK AS HIGH PRESSURE BUILDS BACK INTO  
THE AREA FROM THE WEST.  
 
MODERATION IS THEN EXPECTED AGAIN BY THE END OF THE WEEK, WITH  
PRECIPITATION CHANCES SLOWLY INCREASING INTO THE WEEKEND, AS  
MODELS DEVELOP AN UPPER TROUGH ACROSS THE GREAT LAKES AND MIDWEST  
REGION. NIGHTTIME TEMPS IN THE MID-30'S AND DAYTIME TEMPS IN THE  
40'S WOULD SUGGEST ANY PRECIP BY FRIDAY AND SATURDAY WOULD BE MAINLY  
IN THE FORM OF RAIN.  
 
RATZER  
 

 
   
AVIATION
 
 
FOR THE 12Z TAFS...  
 
610 AM...SEVERAL FORECAST CONCERNS THIS PERIOD INCLUDING WINDS...  
CIGS AND PRECIP TIMING.  
 
COLD FRONT HAS MOVED SOUTH OF THE TERMINALS AND WINDS HAVE SHIFTED  
TO THE NORTH/NORTHEAST. FORECAST SOUNDINGS SUGGEST WINDS WILL  
REMAIN NORTH/NORTHEAST THROUGH AT LEAST MID MORNING AND THEN WILL  
SLOWLY TURN MORE NORTHERLY BY LATE MORNING AND EVENTUALLY TO THE  
NORTH/NORTHWEST THIS AFTERNOON. SPEEDS/GUSTS WILL STEADILY  
INCREASE THIS MORNING WITH SPEEDS INTO THE 15-20KT RANGE WITH  
GUSTS 25-30KTS. WINDS WILL SLOWLY BEGIN TO DIMINISH THIS EVENING  
BUT WILL LIKELY REMAIN GUSTY THROUGH MUCH OF TONIGHT.  
 
SHOWERS CONTINUE TO EXPAND IN COVERAGE ACROSS THE AREA AND THESE  
WILL CONTINUE THIS MORNING WITH A PERIOD OF PREVAILING RAIN LIKELY  
FROM LATE MORNING THROUGH EARLY AFTERNOON. AS COLDER AIR SPREADS  
INTO THE REGION TODAY...THERE IS SOME POTENTIAL FOR THE RAIN TO  
MIX WITH SNOW AS IT ENDS. CONFIDENCE IS FAIRLY LOW FOR HOW MUCH  
SNOW MAY MIX IN AND ONLY CARRIED MENTION OF RAIN/SNOW MIX AT RFD.  
SCATTERED THUNDERSTORMS ARE EXPECTED ACROSS CENTRAL IL AND THESE  
MAY CONTINUE OVER THE SAME AREAS FOR MUCH OF THIS MORNING. ITS  
POSSIBLE AN ISOLATED THUNDERSTORM MAY DEVELOP AS FAR NORTH AS THE  
INTERSTATE 80 CORRIDOR AND POSSIBLY AFFECT GYY. CONFIDENCE TOO LOW  
FOR MENTION WITH THIS FORECAST AND TRENDS WILL NEED TO BE  
MONITORED.  
 
VISIBILITIES ARE SLOWLY IMPROVING AS THE COLD FRONT MOVES SOUTH  
AND CIGS HAVE ALSO SLIGHTLY IMPROVED BUT EXPECT PREVAILING IFR  
CIGS FOR MUCH OF THIS MORNING...GRADUALLY LIFTING TO MVFR THIS  
AFTERNOON. LOWER CIGS CAN BE EXPECTED AHEAD OF THE FRONT AS IT  
MOVES SOUTH ACROSS EAST CENTRAL IL AND NORTHWEST INDIANA. MVFR  
CIGS MAY PERSIST FOR MUCH OF THIS EVENING...SLOWLY SCATTERING OUT  
FROM WEST TO EAST TONIGHT. CMS  
 

 
   
MARINE
 
 
150 AM...A COLD FRONT WILL MOVE SOUTH ACROSS LAKE MICHIGAN EARLY  
THIS MORNING SHIFTING WINDS NORTHERLY. THE GRADIENT WILL QUICKLY  
TIGHTEN AS LOW PRESSURE MOVES FROM MISSOURI THIS MORNING TO NEAR  
LAKE ERIE THIS EVENING. THESE NORTHERLY WINDS WILL INCREASE TO  
GALES BY LATE MORNING WITH 45KTS LIKELY ON THE SOUTH END OF THE  
LAKE THIS AFTERNOON. THE GALES WILL SLOWLY DIMINISH FROM NORTH TO  
SOUTH LATE TONIGHT INTO EARLY SUNDAY MORNING. A LARGE AREA OF HIGH  
PRESSURE WILL MOVE ACROSS THE SOUTHERN U.S. SUNDAY INTO MONDAY.  
AS IT PASSES SOUTH OF THE LAKE...NORTHWESTERLY WINDS SUNDAY  
MORNING WILL SHIFT SOUTHWESTERLY SUNDAY NIGHT. LOW PRESSURE WILL  
MOVE ACROSS ONTARIO MONDAY NIGHT AND THE GRADIENT WILL TIGHTEN  
AHEAD OF THIS LOW WITH A PERIOD OF SOUTHWESTERLY GALES EXPECTED  
LATE MONDAY AFTERNOON THROUGH TUESDAY MORNING. A TRAILING COLD  
FRONT WILL MOVE ACROSS THE LAKE TUESDAY AFTERNOON AND THEN LARGE  
HIGH PRESSURE WILL MOVE ACROSS THE REGION NEXT WEDNESDAY. CMS  
 

 
   
LOT WATCHES/WARNINGS/ADVISORIES
 
 
IL...NONE.  
IN...LAKESHORE FLOOD ADVISORY...INZ001-INZ002...NOON SATURDAY TO 3 AM  
SUNDAY.  
 
LM...GALE WARNING...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-  
LMZ779...9 AM SATURDAY TO 6 AM SUNDAY.  
 

 
 

 
 
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