FXUS63 KLOT 201745
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1122 AM CST TUE JAN 20 2009
.DISCUSSION...
1019 AM CST
MAIN CONCERN THIS MORNING IS ON MAGNITUDE OF LAKE EFFECT SNOW
ACCUMULATION FOR FAR NORTHEAST ILLINOIS NORTHWEST INDIANA. LAKE
EFFECT BAND HAS BECOME NEARLY STATIONARY THIS MORNING WITH MOST
INTENSE BANDING ACROSS NORTHERN LAKE COUNTY IN NORTHEAST ILLINOIS.
A MORE WIDESPREAD AREA OF LAKE EFFECT SNOW SHOWERS EXTENDS BACK
WESTWARD INTO COOK COUNTY. AT 7 AM THIS MORNING...HAD SOME REPORTS
OF 4 TO 5 INCH SNOW ACCUMULATIONS IN THE BURNAM AREA EXTENDING TO
HAMMOND. BASED ON EXPECTED ADDITIONAL 2 TO 5 INCH ACCUMULATIONS
TODAY ACROSS LAKE COUNTY INDIANA...HAVE ISSUED A LAKE EFFECT SNOW
WARNING FOR LAKE COUNTY INDIANA THROUGH 6 PM THIS EVENING. ACROSS
FAR SOUTHEAST COOK COUNTY...WILL LIKELY HAVE LOCALIZED LOCATIONS
REACH WARNING CRITERIA ACCUMULATIONS BUT EXPECTING THE GREATER
LIKELIHOOD OF THIS CLOSER TO THE INDIANA STATELINE. HAVE ISSUED A
LAKE EFFECT SNOW ADVISORY FOR COOK COUNTY TO MENTION SNOW
ACCUMULATIONS OF 4 TO 7 INCHES BY THE TIME THE SNOW BEGINS TO WIND
DOWN MID AFTERNOON. A SHEARED VORTICITY MAXIMUM DROPPING SOUTHEAST
OUT OF WESTERN ILLINOIS AND SOUTHWEST WISCONSIN WILL TRACK ACROSS
LAKE MICHIGAN THIS AFTERNOON. THE PASSAGE OF THIS SHORT WAVE WILL
ALLOW A SFC RIDGE AXIS TO BEGIN TO BUILD INTO THE AREA...ALLOWING
WINDS TO BACK MORE NORTHWEST THIS AFTERNOON AND RESULTING IN LAKE
EFFECT SNOW BAND TO BEGIN TO SHIFT BACK EASTWARD AND BECOME MORE
PROGRESSIVE. THE PROGRESSIVE NATURE OF THE BANDING THIS AFTERNOON
SHOULD LIMIT ADDITIONAL ACCUMS SOMEWHAT ACROSS PORTER COUNTY.
UPDATED FORECAST HAS BEEN SENT. 
MARSILI
&&
.PREV DISCUSSION...
330 PM CST
MAIN CONCERN FOR THE MORNING FORECAST UPDATE CONTINUES TO FOCUS ON
THE ONGOING LAKE EFFECT SNOW EVENT.  
LATEST RADAR IMAGERY SHOWS THE LAKE EFFECT SNOW BAND MOVING OVER
ERN COOK COUNTY AND FAR ERN WILL COUNTY WITH SNOW CONTINUING OVER
LAKE COUNTY IN NWRN INDIANA. SFC WIND FIELD NICELY SHOWS THE
CONVERGENCE AREA ASSOCIATED WITH THE SNOW BAND WITH NNWLY WINDS
OVER NERN IL AND NNELY WINDS OVER NWRN INDIANA. THE FIRST SEVERAL
HOURS OF THE NAM12...AND WRFARW DO NOT INDICATE THE CONVERGENCE
ZONE MOVING MUCH FARTHER INLAND THIS MORNING...SO DO NOT
ANTICIPATE ANY CHANGES TO THE CURRENT HEADLINES IN EFFECT. GIVEN
THE VERY HIGH SNOW TO LIQUID RATIOS EXPECTED...EVEN WITH LITTLE
ANTICIPATED ASSOCIATED LIQUID EQUIVALENT QPF...THE ERN AND SRN
PORTIONS OF THE METRO AREA COULD SEE UPWARDS OF 1 TO 3 INCHES OF
VERY LIGHT...FLUFFY SNOWFALL THIS MORNING. ALSO BASED ON THE SHORT
TERM WIND FORECAST...EXPECT THAT THE SNOW BAND WILL BEGIN TO SHIFT
BACK EWD AGAIN BY LATE MORNING OR AROUND NOON...ENDING THE SNOW
OVER ERN COOK/WILL COUNTIES. HOWEVER...THE LAKE PLUME WILL
CONTINUE TO DROP SNOW OVER LAKE AND PORTER COUNTIES...WITH SOME
SPREADING INTO JASPER...NEWTON AND NRN BENTON COUNTIES. STILL
EXPECT THE HEAVIEST SNOW OVER LAKE AND PORTER COUNTIES...SO WILL
NOT EXPAND THE HEADLINES INTO NEWTON OR JASPER COUNTIES AT THIS
TIME.  
THE REMAINDER OF THE SHORT TERM FORECAST PERIOD LOOKS RATHER
QUIET. THE HIGH AMPLITUDE PATTERN OVER THE CONUS WITH THE MAIN
UPPER TROUGH OVER THE ERN CONUS BECOMING SLOWLY PROGRESSIVE WHILE
THE UPPER RIDGE OVER THE ROCKIES FLATTENS CONSIDERABLY...WITH WEAK
RIDGING BUILDING OVER THE MIDDLE AND LOWER MISSISSIPPI VALLEY. THE
SHORT TERM MODELS ARE IN RATHER GOOD AGREEMENT ON KEEPING THE
UPPER MIDWEST UNDER NWLY FLOW ALOFT...WITH INCREASING HEIGHTS
ENCROACHING ON THE LOCAL AREA. THUS EXPECT SOME MODERATION TO
TEMPERATURES THROUGH THE SHORT TERM WITH DAYTIME HIGHS ACTUALLY
GETTING ABOVE FREEZING FOR THE FIRST TIME IN TWO WEEKS. THIS
WARMING TREND WILL BE SHORT LIVED AS THE MODELS ARE INDICATING
ANOTHER POLAR VORTEX PUSHING SOUTH OF HUDSON BAY BRINGING A
RESURGENCE OF ARCTIC AIR FOR THE WEEKEND. NOT EXPECTING THE EXTREME
COLD OF LAST WEEK...BUT STILL LOOKING AT HIGHS IN THE TEENS TO LOW
20S AND LOWS APPROACHING 0F FOR MUCH OF THE CWA. THE DEVELOPING
PATTERN OVER THE AREA LOOKS TO BE RATHER MOISTURE STARVED...SO DO
NOT EXPECT ANY SIGNIFICANT PCPN THROUGH THE PERIOD. 
KREIN
&&
.AVIATION...
1800 UTC...THE LAKE SNOW WILL CONTINUE TO AFFECT GARY AIRPORT TODAY. 
WILL KEEP THE FORECAST FOR IFR CEILINGS AND VISIBILITY DUE TO SNOW 
THROUGH 20 UTC. AFTER 20 UTC...THIS BAND OF LAKE EFFECT SNOW WILL 
MOVE EAST OUT OF GARY AIRPORT AREA. WE WILL FORECAST IMPROVING 
CONDITIONS TO MVFR CEILINGS. BY 23 UTC WE EXPECT VFR CONDITIONS. 
THE OTHER TAFS WILL HAVE VFR CONDITIONS THROUGH THE ENTIRE FORECAST 
PERIOD. THE WIND WILL BECOME 240 DEGREES AT 10 TO 12 KNOTS BY 15 
UTC. THERE WILL BE SOME MIDDLE LEVEL CLOUDS MOVING INTO NORTHERN 
ILLINOIS AND NORTHWEST INDIANA WEDNESDAY AFTERNOON. 
&&
.MARINE...
155 AM CST
NORTH WINDS ON THE EAST SIDE OF BUILDING HIGH PRESSURE WILL BACK
WEST LATER TODAY WITH RIDGE PASSAGE TO THE SOUTH OF THE LAKES.
WINDS BACK EVEN FURTHER TO THE SOUTHWEST BY WEDNESDAY AS A LOW
PRESSURE TROF DROPS OUT OF ONTARIO THURS NGT INTO FRIDAY. TROF
PASSAGE VEERS WINDS BACK TO THE NW FOR THE REMAINDER OF THE WEEK.
HAVE OPTED FOR ONLY INCREASING WINDS TO 25 KTS VERSUS 30 KTS
FRIDAY AND SATURDAY.
RLB
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...LAKE EFFECT SNOW ADVISORY...ILZ014 UNTIL 6 PM TUESDAY.
IN...LAKE EFFECT SNOW WARNING...INZ001-INZ002 UNTIL 6 PM TUESDAY.
LM...NONE.
&&
$$