628  
FXUS63 KLOT 221229  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
629 AM CST MON JAN 22 2018  
   
UPDATE
 
 
533 AM CST  
 
REGIONAL RADAR COMPOSITE SHOWS A REMARKABLY WELL DEVELOPED MCV  
MOVING NORTHEAST ACROSS CENTRAL IL EARLY THIS MORNING. RADAR  
ESTIMATES HAVE SHOWN HOURLY RAINFALL RATES OF 0.3-0.5" PER HOUR  
WITH THE HEAVIER RETURNS WITH SFC OBS CONFIRMING THESE RATES.  
GIVEN ELEMENTS WITH THESE RATES, STILL LOOKS REASONABLE THAT A  
SWATH OF AROUND AN INCH TO PERHAPS AN INCH AND HALF OF RAIN WILL  
FALL NEAR THE TRACK OF THE MCV, SLICING RIGHT ACROSS THE CHICAGO  
METRO AREA. GIVEN THE FROZEN GROUND AND MODERATE TO HEAVY RATES,  
ISSUED AN AREAL FLOOD ADVISORY EARLIER TO COVER THE THREAT OF ANY  
MINOR FLOODING/PONDING WITH THIS ACTIVITY EARLY THIS MORNING.  
 
THE RAIN HAS DONE A NICE JOB OF WASHING OUT THE DENSE FOG WITH  
VISIBILITIES IMPROVING SUBSTANTIALLY IN THE RAIN. ONCE THE RAIN  
ENDS, COULD SEE FOG RE-DEVELOP NORTH OF THE WARM FRONT, BUT  
CONFIDENCE ISN'T HIGH ENOUGH TO EXTEND WHAT'S LEFT OF THE DENSE  
FOG ADVISORY BEYOND ITS SCHEDULED 12Z EXPIRATION. IF DENSE FOG  
DOES REDEVELOP THIS MORNING, IT SHOULD BE TEMPORARY UNTIL THE WARM  
FRONT LIFTS NORTH TO THE WI BORDER BY AROUND MIDDAY.  
 
- IZZI  
 

 
   
SHORT TERM
 
 
252 AM CST  
 
THROUGH TONIGHT...  
 
A LITTLE SOMETHING FROM THE WEATHER DEPARTMENT FOR ALMOST  
EVERYONE THE NEXT 24-36 HOURS. ON THE DOCKET WEATHER-WISE, WE HAVE  
ONGOING DENSE FOG, SHOWERS & T-STORMS, LOCALLY HEAVY RAIN/MINOR  
FLOODING THREAT, UNSEASONABLE WARMTH, SMALL THREAT OF ISOLATED  
SEVERE WEATHER, POSSIBLE CHANGE OVER TO WET SNOW, AND WINDY/COLDER  
CONDITIONS.  
 
THIS MORNING:  
SLUG OF SHOWERS AND THUNDERSTORMS OVER MO/WESTERN IL WILL CONTINUE  
TO MOVE AND DEVELOP NORTHEAST INTO NORTHERN IL AND NORTHWEST IN  
PRIOR TO SUNRISE AND CONTINUE FOR A BIT AFTER SUNRISE. STILL LOOKS  
LIKE THERE WILL BE A SWATH OF HEAVY RAINFALL TOTALS IN EXCESS OF  
AN INCH IN LESS THAN 6 HOURS, PROBABLY EXTENDING NORTHEAST INTO  
THE CHICAGO METRO AREA. GIVEN THE FROZEN GROUND, VERY LITTLE OF  
THE RAIN WILL GET ABSORBED INTO THE GROUND AND RATHER WILL GO  
STRAIGHT TO RUN OFF. RESULT COULD BE SOME MINOR FLOODING/PONDING  
OF WATER ISSUES THIS MORNING. WIDESPREAD SHOWERS AND THUNDERSTORMS  
WILL END DURING THE MID-LATE MORNING HOURS AS A PRONOUNCED DRY  
INTRUSION PIVOTS INTO THE AREA. WARM FRONT IS SLOWLY LIFTING NORTH  
NOW, BUT IS EXPECTED TO MORE RAPIDLY SURGE NORTH INTO SOUTHERN WI  
BY MID-LATE MORNING. LOOK FOR DENSE FOG TO IMPROVE BOTH AS SHOWERS  
AND STORMS BEGIN AND ALSO AS THE WARM FRONT LIFTS NORTH. MAY SHAVE  
A FEW MORE COUNTIES OFF THE DENSE FOG ADVISORY IN ADDITION TO  
THOSE SET TO EXPIRE AT 09Z.  
 
THIS AFTERNOON:  
THE PRONOUNCED DRY INTRUSION WILL PROVIDE A BREAK IN PRECIP  
CHANCE LATE THIS MORNING INTO THE START OF THE AFTERNOON HOURS.  
BASED ON SATELLITE TRENDS UPSTREAM AND CONCEPTUAL MODEL AS WE  
BREAK INTO THE WARM SECTOR, WOULD ANTICIPATE SOME PARTIAL CLEARING  
TO TAKE PLACE WITH AT LEAST SOME BREAKS FOR SUNSHINE. HAVE  
TRENDED TEMPS UPWARD TOWARD AND A BIT ABOVE THE WARMEST GUIDANCE  
TODAY AS STOUT SOUTHERLY WINDS AND PERHAPS SOME BREAKS FOR  
SUNSHINE ALLOW TEMPS TO CLIMB WELL INTO THE 50S AREA-WIDE WITH  
PERHAPS A 60F READING HERE OR THERE THIS AFTERNOON.  
 
INGREDIENT THIS AFTERNOON LOOK LIKELY TO COME TOGETHER TO SUPPORT  
A BROKEN ARCING BAND OF CONVECTION THIS AFTERNOON AS STEEP MID-  
LEVEL LAPSE RATES/COLD MID LEVEL TEMPS PIVOT EASTWARD AND BECOME  
JUXTAPOSED OVER TOP THE NARROW WARM SECTOR CHARACTERIZED BY  
DEWPOINTS IN THE LOW/MID 50S. THE RESULT SHOULD BE A RIBBON SBCAPE  
AROUND 500 J/KG DEVELOPING JUST AHEAD OF SURFACE TROUGH. STRONG  
DCVA AND LOW LEVEL CONVERGENCE ALONG THE BOUNDARY SHOULD SUPPORT  
RAPID CONVECTIVE DEVELOPMENT DURING THE EARLY-MID AFTERNOON HOURS  
OVER WESTERN IL, WITH STORMS MOVING NORTHEAST ACROSS THE AREA  
LATE IN THE AFTERNOON INTO THE EARLY EVENING HOURS.  
 
MODEL FORECAST WIND PROFILES SUGGEST EFFECTIVE SHEAR VALUES  
SHOULD BE SUFFICIENT TO SUPPORT SOME THREAT FOR LOW TOPPED  
SUPERCELLS, THOUGH VEER-BACK-VEER PROFILE OVER NORTHERN IL ISN'T  
USUALLY PARTICULARLY FAVORABLE FOR TORNADOES. HOWEVER, STRONG  
AMBIENT VORTICITY IN THE PRESENCE OF THE APPROACHING UPPER LOW  
COULD OFFSET THE POTENTIALLY QUESTIONABLE WIND PROFILES. CERTAINLY  
DOESN'T LOOK LIKE A TORNADO OUTBREAK SET-UP, BUT DOES LOOK TO BE  
A WINDOW OF OPPORTUNITY FOR STORMS TO PRODUCE A COUPLE TORNADOES  
THIS AFTERNOON. LOW FREEZING LEVELS COULD ALLOW FOR SOME HAIL,  
MOSTLY SUB-SEVERE.  
 
TONIGHT:  
BROKEN BAND OF CONVECTION SHOULD BE IN A WEAKEN PHASE AS IT  
ROTATED NORTHEAST AND EAST OUT OF OUR CWA EARLY THIS EVENING.  
WIDELY SCATTERED SHOWERS WILL LIKELY LINGER ACROSS THE AREA  
TONIGHT WITH PRECIPITATION GRADUALLY MIXING WITH THEN CHANGING TO  
SOME WET SNOW OVERNIGHT FROM NORTHWEST TO SOUTHEAST AS COLDER AIR  
FILTERS INTO THE REGION.  
 
THERE IS A STRONG SIGNAL IN VARIOUS MODEL GUIDANCE THAT  
STRENGTHENING F-GEN ON THE NORTHWEST FLANK OF THE SYSTEM WILL  
PROMOTE AN INTENSIFYING BAND OF SNOW OVERNIGHT INTO EARLY TUESDAY  
MORNING. WHILE THERE IS STRONG AGREEMENT IN THAT SCENARIO, THE  
PLACEMENT OF THIS FEATURE VARIES WIDELY IN THE MODEL GUIDANCE. THE  
STRONG SIGNAL IS OVER SOUTHERN WI, BUT A FEW MODELS DO SUGGEST  
THAT THE INTENSE TROWAL COULD AFFECT PORTIONS OF NORTHERN IL  
OVERNIGHT. GIVEN THE PRESENCE OF VERY STEEP LAPSE RATES IN THE  
PRESENCE OF STRENGTHENING F-GEN CIRCULATION, POTENTIAL EXISTS FOR  
HEAVY SNOW AND COULDN'T EVEN RULE OUT A COUPLE LIGHTNING STRIKES  
IN THIS BAND GIVEN THE VERY STEEP LAPSE RATES. UNFORTUNATELY, DUE  
TO THE MESOSCALE NATURE OF THIS POTENTIAL BAND OF HEAVIER SNOW, IT  
REALLY IS GOING TO BE MORE OF NOWCAST TYPE ISSUE. PLAN TO  
CONTINUE WITH ACCUMS OF AN INCH OR TWO NEAR THE WI LINE LATE  
TONIGHT INTO EARLY TUES, BUT IN ALL LIKELIHOOD ACCUMULATIONS WILL  
BE MORE CONFINED TO THE F-GEN DRIVEN BAND WHERE HEAVIER PRECIP  
RATES WILL BE ABLE TO AUGMENT THE SOMEWHAT MARGINAL TEMP PROFILES  
AND ALLOW FOR ACCUMULATIONS. OUTSIDE OF THAT HEAVIER SNOW BAND  
(WHERE EVER IT MAY SET UP) NOT EXPECTING MUCH ACCUMULATION GIVEN  
THE MOSTLY ABOVE FREEZING TEMPS AND WARM/WET GROUND. WHILE THE  
MOST FAVORED AREA FOR THE HEAVIER BAND OF SNOW IS SOUTHERN WI AND  
FAR NORTHERN IL, THE HIRES WRF-NMM IS RIGHT OVER THE CHICAGO AREA,  
SO WILL NEED TO CLOSELY WATCH THE EVOLUTION OF THIS SYSTEM.  
 
- IZZI  
 

 
   
LONG TERM
 
 
252 AM CST  
 
TUESDAY THROUGH SUNDAY...  
 
ANY LINGER SNOW SHOULD END BY MID MORNING TUESDAY LEAVING JUST  
WINDY AND COLD (COMPARED TO TODAY) TEMPS IN ITS WAKE. IN REALITY,  
TEMPS TUESDAY SHOULD HOLD STEADY IN THE LOW/MID 30S WHICH IS CLOSE  
TO THE AVG HIGH FOR LATE JANUARY. WEDNESDAY LOOK DRY WITH  
SEASONABLE TEMPERATURES.  
 
ANOTHER LONG WAVE TROUGH IS FORECAST TO DIG INTO THE WESTERN CONUS  
MID-LATE WEEK WITH DOWNSTREAM AMPLIFICATION OF UPPER RIDGE OVER  
OUR AREA. SURFACE LOW IS FORECAST TO TRACK EAST ACROSS SOUTHERN  
CANADA LATE IN THE WEEK WITH BROAD AND RATHER STOUT SOUTHERLY FLOW  
DEVELOPING ACROSS THE CENTRAL AND EASTERN U.S. AND SETTING THE  
STAGE FOR ANOTHER BOUT OF UNSEASONABLE WARMTH LOCALLY. THURSDAY  
SHOULD SEE THE MODERATING TREND BEGIN, BUT BY FRIDAY IT LOOKS LIKE  
HIGH TEMPS SHOULD MAKE IT INTO THE 50S MOST AREAS. THE SOUTHERLY  
FLOW LOOKS TO BE RELATIVELY VOID OF MOISTURE THROUGH FRIDAY  
LOCALLY, SO STRATUS ISN'T MUCH OF A CONCERN AND CIRRUS SHOULD BE  
MORE PROMINENT CLOSER TO THE JET TO OUR NORTH, SO FRIDAY LOOKS  
LIKE IT COULD BE A REALLY NICE DAY WITH SUNSHINE AND TEMPS IN THE  
50S.  
 
COLD FRONT LOOKS TO MOVE ACROSS THE AREA SATURDAY AFTERNOON, SO  
AFTER A MILD START LOOK FOR A CHANCE OF SHOWERS AND A RETURN TO  
MORE SEASONABLE TEMPS LATE IN THE WEEKEND INTO EARLY NEXT WEEK.  
OPERATIONAL RUNS OF THE MEDIUM RANGE MODELS TONIGHT ARE ALL IN  
GOOD AGREEMENT ON THE LATE WEEKEND COOL DOWN (RETURN TO AVG TEMPS)  
BEING SHORT-LIVE WITH ANOTHER BOUT OF UNSEASONABLY WARM  
TEMPERATURES POSSIBLE HEADING INTO NEXT WEEK.  
 
- IZZI  
 

 
   
AVIATION
 
 
FOR THE 12Z TAFS...  
 
SHRA AND EMBEDDED TSRA ARE LIFTING NORTHWARD ACROSS THE AREA, WITH  
THE CURRENT TSRA THREAT ENDING SHORTLY FOR THE EASTERN  
TERMINALS/PRIOR TO 13Z. LOW CIGS ARE ALREADY SHOWING SIGNS OF  
IMPROVING AT THE EASTERN TERMINALS AND THIS PROCESS WILL QUICKEN  
AS A WARM FRONT LIFTS NORTH THIS MORNING. SOUTHEAST WINDS COULD  
BRIEFLY GUST IN THE 15-20 KT RANGE FOR 2-3 HOURS AS THE FRONT  
LIFTS NORTH. THIS WILL BE FOLLOWED BY SOUTH OR SOUTH-SOUTHWEST  
(170-200 DEG DIRECTION) WINDS GUSTING TO 15-20 KT FOR A FEW HOURS  
UNTIL THEY DIMINISH. NORTH OF THE WARM FRONT AT RFD, AFTER THE  
CURRENT ROUND OF SHRA ENDS, CAN'T RULE OUT FG REDEVELOPING FOR A  
FEW HOURS UNTIL THE FRONT CLEARS NORTH OF THERE BY MID DAY.  
EXPECTING CONDITIONS TO IMPROVE TO VFR ACROSS THE AREA BY THE  
EARLY AFTERNOON WITH A LULL IN PRECIPITATION. THE NEXT ROUND OF  
SCATTERED SHRA AND POSSIBLY ISOLATED TSRA LOOKS TO DEVELOP IN  
WESTERN AND NORTHWESTERN IL DURING THE MID AFTERNOON AND THEN LIFT  
NORTHEAST INTO THE EVENING. COVERED THIS ACTIVITY WITH VCSH AND  
TEMPORARY SHRA FOR ON STATION IMPACTS. DID NOT HAVE ENOUGH  
CONFIDENCE FOR TSRA INCLUSION, WHICH WILL NEED TO BE MONITORED.  
 
WITH THE SURFACE TROUGH OVERHEAD LATER THIS EVENING AND OVERNIGHT  
AND LIGHTER SOUTHWEST WINDS, SHOULD SEE LOWERING CIGS ONCE AGAIN  
AND LIKELY VSBY OUTSIDE OF ANY SHRA. SCATTERED SHRA SHOULD BE  
AROUND MUCH OF THE NIGHT. LATER TONIGHT INTO TUESDAY MORNING,  
THERE IS CONCERN FOR THE RAIN CHANGING TO SNOW AND POSSIBLY A  
NARROW BAND OF MODERATE TO HEAVY SNOW SOMEWHERE OVER NORTHERN IL  
AND SOUTHERN WI. HAVE COVERED THIS LOWER CONFIDENCE POTENTIAL WITH  
A PROB30 MENTION IN THE ORD/MDW/RFD/DPA TAFS. IF STEADY SNOW  
OCCURS AT ANY OF THE TERMINALS, THEN A SLUSHY ACCUMULATION OF WET  
SNOW IS POSSIBLE, WITH CURRENTLY RFD MOST FAVORED. CONDITIONS WILL  
IMPROVE BY MID TUESDAY MORNING WITH BRISK NORTHWEST WINDS GUSTING  
IN THE 20-25 KT RANGE.  
 
CASTRO  
 

 
   
MARINE
 
 
141 AM CST  
 
THERE ARE MULTIPLE CONCERNS THROUGH TUESDAY AS A STRONG STORM  
SYSTEM IMPACTS THE REGION. NORTHEASTERLY 35-40 KT GALES WILL  
DEVELOP ON THE NORTH HALF OF THE LAKE THIS MORNING AS THE LOW  
PRESSURE MOVES INTO SOUTHERN IA, TIGHTENING PRESSURE GRADIENT  
BETWEEN IT AND HIGH PRESSURE NORTH OF THE LAKES. MEANWHILE, A WARM  
FRONT LIFTING NORTH SPREADING UNSEASONABLY MILD AND MOIST AIR  
OVER THE SOUTH HALF WILL KEEP FAVORABLE CONDITIONS IN PLACE FOR  
DENSE FOG THROUGH MID EVENING. NORTHEASTERLY GALES WILL CONTINUE  
NORTH OF THE SURFACE LOW PATH, WHICH WILL BE OVER NORTHERN  
ILLINOIS THIS EVENING TO LAKE HURON BY TUESDAY MORNING. ONCE THE  
LOW SHIFTS EAST, GALES WILL TURN NORTHERLY ON THE NORTH HALF OF  
THE LAKE, WITH NORTH-NORTHWESTERLY GALES SPREADING DOWN THE SOUTH  
HALF. A GALE WARNING IS IN EFFECT FROM 9AM CST THIS MORNING  
THROUGH 6PM TUESDAY FOR THE NORTH HALF AND 3AM CST TO 6PM CST  
TUESDAY FOR THE SOUTH HALF.  
 
FOR THE NEARSHORE WATERS, EXPECTING THE DENSE FOG TO DISSIPATE  
THIS MORNING WITH STEADY RAIN MOVING IN AND WARM FRONT LIFTING  
NORTH. A BRIEF PERIOD OF NEAR HAZARDOUS SOUTHERLY WINDS WILL OCCUR  
THIS MORNING THROUGH EARLY AFTERNOON FOR MOSTLY THE INDIANA  
NEARSHORE. EAST-SOUTHEAST WINDS NORTH OF THE WARM FRONT COULD  
ALSO PRODUCE WAVES UP TO 3-5 FEET ALONG THE ILLINOIS SHORE FROM  
MID MORNING THROUGH EARLY AFTERNOON. WILL HOLD OFF ON SMALL CRAFT  
ADVISORY ISSUANCE WITH SOME UNCERTAINTY ON THE SPEED/GUST  
MAGNITUDE AND VERY SHORT DURATION OF THE HIGHER WINDS FOR THE  
INDIANA SHORE AND WAVES JUST SHY OF CRITERIA FOR THE ILLINOIS  
SHORE. A SMALL CRAFT ADVISORY WILL THEN BE NEEDED TUESDAY INTO  
TUESDAY EVENING FOR THE ILLINOIS SHORE AND THROUGH TUESDAY NIGHT  
FOR THE INDIANA SHORE DURING THE STRONG NORTH-NORTHWEST WINDS. A  
BRIEF PERIOD OF GALE GUSTS APPEARS POSSIBLE FOR THE INDIANA SHORE  
DURING THE MID MORNING THROUGH EARLY AFTERNOON TIMEFRAME, BUT DO  
NOT HAVE ENOUGH CONFIDENCE FOR AN ADDITIONAL GALE HEADLINE.  
 
THE NEXT PERIOD OF STRONG WIND CONCERN IS SOUTH-SOUTHWESTERLIES  
FRIDAY THROUGH FRIDAY NIGHT, WITH A PERIOD OF GALE FORCE WINDS  
CURRENTLY APPEARING PROBABLE OVER THE OPEN WATERS. WINDS WILL  
SHIFT WESTERLY AND DIMINISH BUT REMAIN ELEVATED BEHIND COLD  
FRONTAL PASSAGE ON SATURDAY.  
 
CASTRO  
 

 
   
LOT WATCHES/WARNINGS/ADVISORIES
 
 
IL...NONE.  
IN...NONE.  
LM...DENSE FOG ADVISORY...LMZ777-LMZ779 UNTIL 9 PM MONDAY.  
 
GALE WARNING...LMZ777-LMZ779...3 AM TUESDAY TO 6 PM TUESDAY.  
 

 
 

 
 
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