Winter Storm in Illinois, December 24 to 25, 2002

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO IL
550 PM CST TUE DEC 24 2002

AVIATION DISCUSSION FOR 00Z TAFS

AFTER LOOKING THRU ALL AVAILABLE DATA AND LATEST 18Z RUNS...ALONG
WITH RADAR AND SFC TRENDS...LOOKS LIKE JUST SOME VERY LIGHT SNOW AT
RFD TO A LITTLE DUSTING IN CHI TERMINALS TO LITTLE HEAVIER AT GYY.
AS LATEST RADAR SHOWS...DRY LOW LEVELS ABOVE THE LAKE CLOUDS EATING
UP THE SNOW. EARLY EVE SNOW AT CHI TERMINALS JUST SOMER VERY WEAK
LAKE EFFECT...NOT SYSTEM SNOW. THIS LOWER LEVEL HAS TO MOISTEN UP
FIRST SO SOME EARLY HIGH LEVEL OMEGA WASTED. BEST HIGH LEVEL OMEGA
FROM ABOUT 03-07Z WITH COLUMN SATURATED SO ONLY A FEW HOURS OF LOWER
VIS WITH THE SNOW. HIGH LEVEL OMEGA THEN PULLS OUT SO SNOW WILL
DIMINISH TO JUST FEW FLURRIES. SHOULD BREAK DURING WED AFTERNOON.

AF

PUBLIC FORECAST DISCUSSION

UPPER LEVEL LOW PRESSURE SYSTEM OVER WESTERN MISSOURI WILL TRACK TO
THE ATLANTIC COAST BY MIDDAY WEDNESDAY...THEN DEEPEN CONSIDERABLY AS
IT ACCELERATES EAST NORTHEAST TO THE CANADIAN MARITIMES. DEFORMATION
ZONE SETTING UP JUST NORTHWEST OF THE 8H LOW TRACKING UP THE OHIO
RIVER PUTS THE HEAVIEST SNOWS OVER CENTRAL AND SOUTHERN
ILLINOIS...INDIANA...AND OHIO. NORTHERN ILLINOIS AND NORTHWEST
INDIANA WILL BE ON THE PERIPHERY OF THESE SNOWS...WITH A STRONG
GRADIENT FROM SE TO NW. GARCIA AND LEMO METHODS IMPLY THREE TO FIVE
INCH ACCUMS. TIMEFRAME OF SNOW IS SHORTER THAN A WHOLE PERIOD...BUT
THIS IS OFFSET BY SLIGHTLY STRONGER THAN NORMAL UPPER DYNAMICS...AND
BASED ON UPSTREAM TRENDS...THE SNOW ADVISORY FOR THE SOUTHEAST THIRD
OF THE FORECAST AREA REMAINS WARRANTED. FURTHER NORTH AND WEST SNOWS
WILL BE CONSIDERABLY LIGHTER...WITH JUST CHANCE POPS FOR MEASURABLE
SNOWS FAR NW.

PROGRESSIVE SYSTEM TO PULL OUT LATE OVERNIGHT...LEAVING JUST CLOUDS
AND A FEW FLURRIES MOST AREAS...WITH LIGHT SNOW TAPERING OFF
SOUTHEAST. DECENT SUBSIDENCE SHOULD CLEAR US OUT WEDNESDAY
NIGHT...ALLOWING TEMPERATURES TO COOL OFF. LONGER TERM MODELS
ADVERTISE RISING HEIGHTS AND RIDGING THIS WEEKEND...WITH
PROPORTIONATELY HIGHER TEMPERATURES.

AVIATION DISCUSSION FOR 18Z TAFS
NEW MODELS DATA/SURFACE REPORTS SUGGEST SNOW WILL BEGIN SOMEWHAT
LATER THAN EALIER THINKING. TIME SECTIONS INDICATE UVV MAX JUST
BEFORE 00Z BUT COLUMN SATURATION LAGS BY SEVERAL HOURS. HAVE MOVED
SIG SNOW BACK IN TIME TO MATCH THIS GUIDANCE. AS SYSTEM PROGRESSES
EASTWARD SNOW SHOULD TAPER TO FLURRIES THEN STOP BY/AROUND MORNING.
THOUGH VSBYS EXPECTED TO IMPROVED WITH END OF SNOW, CIGS WILL REMAIN
MVFR THROUGH THE REMAINDER OF THE TAF VALID TIME.

.CHI...SNOW ADVY TONIGHT ALG/S PNT-IKK-VPZ

JHV

ILZ003>006-008-010>014-019>023-032>033-039-INZ001>002-010>011-
019-251200-

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE CHICAGO IL
600 AM CST TUE DEC 24 2002

THIS HAZARDOUS WEATHER OUTLOOK IS FOR NORTH CENTRAL AND NORTHEAST
ILLINOIS AND NORTHWEST INDIANA.

.DAY ONE...TODAY

LIGHT SNOW IS EXPECTED TO DEVELOP FROM CENTRAL ILLINOIS INTO
NORTHERN INDIANA THIS AFTERNOON. WHILE HEAVIER ACCUMULATION WILL BE
SOUTH OF THE AREA...THERE IS SOME POTENTIAL FOR LIGHT ACCUMULATION
BY THIS EVENING SOUTH OF ROUGHLY A PONTIAC ILLINOIS TO VALPARAISO
INDIANA LINE. SNOWFALL IS EXPECTED TO INCREASE THIS EVENING...WITH 3
TO 5 INCHES TOTAL ACCUMULATION POSSIBLE IN THE AFOREMENTIONED AREA
BY WEDNESDAY MORNING. SNOWFALL AMOUNTS WILL DECREASE SHARPLY TO THE
NORTH. HOLIDAY TRAVEL MAY BE IMPACTED ACROSS SOUTHERN PORTIONS OF
THE FORECAST AREA LATE TODAY AND TONIGHT.

.SPOTTER INFORMATION STATEMENT...NO SPOTTER ACTIVATION IS
ANTICIPATED.

.DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY

DRY AND SOMEWHAT WARMER WEATHER IS EXPECTED THROUGH SATURDAY UNDER
HIGH PRESSURE. ANOTHER LOW PRESSURE SYSTEM WILL BRING A CHANCE OF
RAIN OR SNOW TO THE REGION SUNDAY NIGHT INTO MONDAY.

.SPOTTER INFORMATION STATEMENT...NO SPOTTER ACTIVATION IS
ANTICIPATED.

RATZER