010  
FXUS63 KLOT 200937  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
337 AM CST SUN JAN 20 2019  
   
SHORT TERM
 
 
304 AM CST  
 
THROUGH MONDAY...  
 
ONGOING RAGGED LAKE EFFECT SNOW CONTINUES ACROSS NE IL AND NW IN  
EARLY THIS MORNING. BASED OFF A FEW SNOWFALL REPORTS COMBINED WITH  
WEBCAMS AND ASOS/AWOS VISIBILITIES, LITTLE ACCUMULATION HAS OCCURRED  
SO FAR. DECENT DIRECTIONAL SHEAR OVER THE LOWEST 3KFT IS THE LIKELY  
CULPRIT FOR THIS SOMEWHAT DISORGANIZED ACTIVITY. HOWEVER, A MORE  
DOMINANT BAND APPARENT ON RADAR AND SATELLITE IS DEVELOPING JUST  
OFFSHORE OF LAKE COUNTY EARLY THIS MORNING. EXPECTATION IS FOR THIS  
BAND TO REMAIN SOMEWHAT WELL-ORGANIZED, WITH PERIODIC DISRUPTIONS  
DUE TO THE LOW-LEVEL SHEAR, AS IT MOVES INTO COOK COUNTY OVER THE  
NEXT COUPLE HOURS. BASED OFF RADAR PRESENTATION AND THE THERMO  
PROFILE OVER LAKE MI, WOULD EXPECT SNOWFALL RATES IN EXCESS OF 1"/HR  
AT TIMES WITHIN THIS BAND.  
 
THE LES BAND SHOULD GRADUALLY SHIFT EAST THROUGH DAYBREAK,  
ULTIMATELY SETTLING OVER LAKE AND PORTER COUNTIES DURING THE DAY.  
SHORT-TERM HI-RES GUIDANCE SUGGESTS THAT THIS BAND COULD STALL FOR A  
2-4HR PERIOD OVER LAKE COUNTY THIS MORNING. IF THIS OCCURS,  
LOCALIZED SNOWFALL RATES OF 1"/HR TO POSSIBLY 2"/HR COULD OCCUR.  
WHILE WARNING CRITERIA OF 6"/12HR FOR ANY ONE LOCATION MAY NOT BE  
MET DURING THIS TIME, DID CONSIDER UPGRADING LAKE COUNTY TO WINTER  
STORM WARNING TO ACCOUNT FOR THE CONDENSED PERIOD OF HEAVY SNOW THIS  
MORNING. HOWEVER, GIVEN THE ODDITIES OF LAKE EFFECT SNOW  
FORECASTING, CONFIDENCE IN THE LES BAND ACTUALLY STALLING FOR  
SEVERAL HOURS IS TOO LOW TO UPGRADE TO A WARNING. WILL INSTEAD  
HIGHLIGHT THE POTENTIAL FOR A PERIOD OF HIGHER SNOWFALL RATES WITHIN  
THE ONGOING ADVISORY THIS MORNING.  
 
BY THIS AFTERNOON, A MID-LEVEL SHORTWAVE DIGGING SE ACROSS NE MN  
EARLY THIS MORNING WILL ALTER THE LOW-LEVEL WIND FIELD ENOUGH TO  
DISRUPT THE SINGLE LES BAND. GRADUALLY VEERING LOW-LEVEL WINDS AHEAD  
OF A SURFACE HIGH APPROACHING FROM THE NW WILL ALLOW PRIMARILY  
DISORGANIZED LES WITH AN EMBEDDED MESO-LOW OR TWO TO DRIFT BACK  
WEST, AND THEN NORTHWARD, TONIGHT. LIGHT TO BRIEFLY MODERATE SNOW IS  
EXPECTED WITH THIS ACTIVITY IMMEDIATELY ADJACENT TO LAKE MI IN BOTH  
IN AND IL. ANY REMAINING LES SHOULD EXIT THE CWA BY MID-MORNING  
MONDAY AS THE SURFACE RIDGE MOVES OVER AREA AND WINDS VEER SSE.  
 
KLUBER  
 

 
   
LONG TERM
 
 
337 AM CST  
 
MONDAY NIGHT THROUGH SATURDAY...  
 
MONDAY NIGHT THROUGH WEDNESDAY: A MID-LEVEL TROUGH MOVING ACROSS THE  
CENTRAL PACIFIC COAST TONIGHT WILL PHASE WITH A NW CANADIAN TROUGH  
OVER THE CENTRAL GREAT PLAINS MONDAY NIGHT INTO TUESDAY. AN  
ASSOCIATED SURFACE LOW WILL TRACK NE ACROSS THE CWA. GUIDANCE HAS  
COME INTO SOMEWHAT BETTER AGREEMENT ON THE TRACK OF THE LOW, WITH A  
CONSENSUS FORECAST PRODUCING A NORTHERN MO TO FAR SE WI MOTION.  
PRECIP WILL BEGIN TO SPREAD ACROSS THE CWA BY LATE TUESDAY MORNING,  
WITH WIDESPREAD PRECIP EXPECTED LATE TUESDAY AFTERNOON INTO THE  
EARLY MORNING HOURS WEDNESDAY. THERMO PROFILES SUPPORT AN INITIAL P-  
TYPE OF ALL SNOW TO TRANSITION TO LIQUID PRECIP FROM SOUTH TO NORTH  
DURING THE AFTERNOON HOURS. HOWEVER, PRECIP WILL LIKELY REMAIN  
MOSTLY IN THE FORM OF SNOW ACROSS THE NORTHERN TIER OF COUNTIES.  
WITH ABUNDANT WAA ALOFT, THE TRANSITION TO LIQUID PRECIP WILL  
PRECEDE SURFACE TEMPS RISING ABOVE FREEZING, SUPPORTING A PERIOD OF  
FREEZING RAIN DURING THE AFTERNOON INTO THE THE EARLY EVENING. GIVEN  
THE EXPECTED COLD OVER THE NEXT COUPLE DAYS, FREEZING RAIN WILL  
LIKELY CONTINUE ON COLD SURFACES EVEN AFTER SURFACE AIR TEMPS RISE A  
FEW DEGREES ABOVE FREEZING. CAA WILL COMMENCE OVERNIGHT TUESDAY INTO  
WEDNESDAY, BUT WITH THE LOSS OF DEEP MOISTURE, PRECIP MAY TRANSITION  
TO A MIX OF LIGHT SNOW AND FREEZING DRIZZLE THROUGH THE DAY  
WEDNESDAY.  
 
OVERALL, MESSY CONDITIONS ARE EXPECTED TUESDAY AFTERNOON AND NIGHT,  
WITH POTENTIAL IMPACTS ACROSS MUCH OF THE AREA. BECAUSE OF THE  
COMPLEXITIES OF THIS SYSTEM, DETAILS ON SNOW AND ICE AMOUNTS REMAIN  
DIFFICULT TO DETERMINE. HOWEVER, AT THIS TIME, THE POTENTIAL EXISTS  
FOR SEVERAL INCHES OF ACCUMULATING SNOW ACROSS THE NORTHERN TIER OF  
COUNTIES, A MIX OF ICE AND SNOW ACCUMULATIONS GENERALLY BETWEEN THE  
I-88 AND I-80 CORRIDORS (INCLUDING THE ENTIRE CHICAGO METRO), AND  
ICE ACCUMULATIONS SOUTH OF I-80.  
 
THURSDAY THROUGH SATURDAY: A MUCH COLDER PATTERN WILL SETTLE  
ACROSS THE REGION DURING THIS TIME. THE FIRST TRUE SHOT OF ARCTIC  
AIR THIS SEASON LOOKS TO CROSS THE WESTERN GREAT LAKES ON FRIDAY,  
WITH A REINFORCING SHOT SATURDAY NIGHT INTO SUNDAY. MAX TEMPS  
FRIDAY WILL LIKELY STRUGGLE TO RISE OUT OF THE SINGLE DIGITS ABOVE  
ZERO, WHILE WIND CHILLS REMAIN WELL-BELOW ZERO THROUGH THE DAY. A  
COUPLE CLIPPER SYSTEMS WILL BRING CHANCES OF SNOW DURING THIS  
TIME, WITH LIGHT SNOW POSSIBLE THURSDAY AND A MORE POTENT SYSTEM  
BRINGING BETTER CHANCES FRIDAY NIGHT INTO SATURDAY.  
 
KLUBER  
 

 
   
AVIATION
 
 
FOR THE 06Z TAFS...  
 
PRIMARY FORECAST CONCERN IS LAKE EFFECT SNOW THROUGH THE PERIOD.  
 
LAKE EFFECT SNOW SHOWERS HAVE BEEN SPREADING ACROSS NORTHEAST IL  
THIS EVENING OCCASIONALLY LOWERING VIS INTO THE 2SM RANGE. WINDS  
HAVE SHIFTED TO THE NORTH/NORTHWEST IN IL AND TO THE NORTH/  
NORTHEAST IN SOUTHWEST LOWER MI SIGNALING CONVERGENCE HAS BEGUN  
AND THIS MAY ALLOW ANY HEAVIER LAKE EFFECT BAND TO REMAIN EAST OF  
ORD AND POSSIBLY MDW...FOCUSING MORE INTO NORTHWEST IN AND GYY.  
HAVE TRENDED A BIT HIGHER WITH THIS FORECAST FOR ORD/MDW AND IF  
THE BAND DOES REMAIN EAST...FURTHER IMPROVEMENT CAN BE EXPECTED.  
THIS LAKE EFFECT SNOW MAY SHIFT EAST OF GYY BY EARLY SUNDAY  
AFTERNOON BUT AS HIGH PRESSURE MOVES ACROSS THE AREA THE BAND IS  
LEFT TO DRIFT BEFORE DISSIPATING AND THERE IS SOME CONSENSUS FROM  
THE MODELS IT MAY MOVE BACK TOWARD NOT ONLY GYY BY LATE AFTERNOON  
BUT TOWARD ORD/MDW LATER SUNDAY EVENING. CONFIDENCE IS VERY LOW  
DURING THIS TIME PERIOD BUT MAINTAINED PROB MENTION AT GYY AND  
ADDED VCSH MENTION AT ORD/MDW.  
 
PREVAILING WINDS WILL REMAIN NORTH/NORTHWEST INTO SUNDAY EVENING.  
SPEEDS/GUSTS WILL SLOWLY DIMINISH OVERNIGHT BUT SPEEDS 10-15KTS  
WILL CONTINUE SUNDAY WITH SOME HIGHER GUSTS POSSIBLE. WINDS ARE  
EXPECTED TO SHIFT LIGHT NORTHEASTERLY LATE SUNDAY NIGHT. CMS  
 

 
   
LOT WATCHES/WARNINGS/ADVISORIES
 
 
IL...WINTER WEATHER ADVISORY...ILZ014 UNTIL 9 AM SUNDAY.  
 
LAKESHORE FLOOD ADVISORY...ILZ006-ILZ014 UNTIL 9 AM SUNDAY.  
 
IN...LAKESHORE FLOOD ADVISORY...INZ001 UNTIL 9 AM SUNDAY.  
 
WINTER WEATHER ADVISORY...INZ001-INZ002 UNTIL 4 PM SUNDAY.  
 
LM...HEAVY FREEZING SPRAY WARNING...LMZ740-LMZ741-LMZ742-LMZ743-  
GARY TO MICHIGAN CITY IN UNTIL MIDNIGHT MONDAY.  
 
SMALL CRAFT ADVISORY...NEARSHORE WATERS  
UNTIL 4 AM MONDAY.  
 

 
 

 
 
VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE)  
 
FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT:  
WWW.FACEBOOK.COM/NWSCHICAGO  
WWW.TWITTER.COM/NWSCHICAGO  
WWW.YOUTUBE.COM/NWSCHICAGO  
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab IL Page
The Nexlab IN Page Main Text Page