FXUS63 KILX 262345
AFDILX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
545 PM CST SAT DEC 26 2009

.DISCUSSION...
ISSUED 254 PM CST SAT DEC 26 2009

A COMPLEX...VERTICALLY STACKED...UPPER/SURFACE LOW CONTINUES TO
WOBBLE OVER THE MIDWEST/GREAT LAKES REGION. THIS SYSTEM HAS
MULTIPLE DISTURBANCES PIVOTING AROUND IT...AND THESE IMPULSES ARE
CAUSING PERIODIC SNOW SHOWERS ACROSS CENTRAL AND SOUTHEAST
ILLINOIS. THIS SLOW MOVING SYSTEM WILL STILL BE WITH US FOR THE
NEXT DAY OR SO...AND THE ASSOCIATED PRECIPITATION CHANCES/AMOUNTS
ARE TODAY/S PRIMARY FORECAST CONCERN.

12Z MODELS IN GOOD OVERALL AGREEMENT...SO DO NOT REALLY HAVE A
MODEL FAVORITE. HOWEVER...MODELS HAVE BEEN HAVING TROUBLE
RESOLVING ALL THE LITTLE IMPULSES ROTATING AROUND THE UPPER LOW.
SO...HAVE FOLLOWED A BLEND OF MODEL GUIDANCE...WITH SOME TWEAKS
ADDED BASED ON THE IMPULSES EVIDENT ON WATER VAPOR IMAGERY.

SHORT TERM...TONIGHT THROUGH TUESDAY...A WAVE LIFTING NORTHEAST
THROUGH THE FORECAST AREA FROM THE BASE OF THE UPPER LOW IS
RESPONSIBLE FOR THE SNOW THAT IS CURRENTLY FALLING. SNOWFALL RATES
HAVE PICKED UP CONSIDERABLY OVER THE PAST FEW HOURS...AND WOULD
NOT BE SURPRISED TO SEE MUCH OF THE FORECAST AREA NORTH NORTH OF
I-70 PICK UP A QUICK INCH OF ACCUMULATION. THIS PARTICULAR AREA OF
SNOWFALL WILL TAPER OFF LATER THIS EVENING AS THE IMPULSE SHIFTS
NORTHEAST OF THE REGION. WHILE THE AIR MASS IS PRETTY DRY...A NICE
ISOTHERMAL LAYER THROUGH THE DENDRITIC SNOW GROWTH ZONE
/AROUND -15C/ HAS RESULTED IN SNOW TO WATER RATIOS AROUND 30 TO
1. THE SETUP LATE TONIGHT THROUGH MUCH OF THE SUNDAY DAYLIGHT
HOURS WILL BE SIMILAR AS THE LAST MAJOR IMPULSE PIVOTS THROUGH THE
REGION AT THE BASE OF THE UPPER LOW BEFORE THE LOW BEGINS TO
DEPART. HAVE BOOSTED SNOW CHANCES SUNDAY...WITH MANY AREAS LIKELY
TO PICK UP AN INCH OR TWO OF ACCUMULATION. HOWEVER... WOULD NOT BE
SURPRISED TO SEE LOCALLY HIGH AMOUNTS GIVEN THE STRENGTH OF THE
FORCING AND HIGH SNOW TO WATER RATIOS EXPECTED.

THINGS SHOULD FINALLY QUIET DOWN LATER SUNDAY NIGHT INTO TUESDAY
AS THE UPPER/SURFACE LOW SHIFT EAST OF THE REGION. THE FLOW ALOFT
IS EXPECTED TO BECOME INCREASINGLY NEUTRAL WITH TIME...WITH HIGH
PRESSURE BUILDING IN AT THE SURFACE. THIS WILL ALLOW TEMPERATURES
TO MODERATE WITH TIME...BUT FRESH SNOW COVER IN MANY AREAS WILL
KEEP TEMPERATURES DOWN A BIT FROM WHERE THEY MIGHT OTHERWISE BE...
ESPECIALLY AT NIGHT.

LONG TERM...TUESDAY NIGHT THROUGH SATURDAY...MODELS/ENSEMBLES
CONTINUE TO BE IN REASONABLE AGREEMENT IN THE MID TO LATE WEEK
PERIOD. THE UPPER FLOW IS STILL EXPECTED TO SPLIT IN THE WAKE OF
THE DEPARTING WEEKEND STORM SYSTEM...WITH THE MAIN ENERGY IN THE
FLOW STAYING TO OUR NORTH AND SOUTH. HOWEVER...A WEAK IMPULSE...
AND ASSOCIATED RISK OF SNOW...IS EXPECTED TO ARRIVE BY WEDNESDAY.
HAVE BOOSTED SNOW CHANCES UP A BIT FOR WEDNESDAY...BUT THE SPREAD
IN MODEL TIMING STILL SUGGESTS THIS WAVE COULD ARRIVE AS LATE AS
WEDNESDAY NIGHT OR EARLY THURSDAY. IN ANY CASE...THE BETTER
PRECIPITATION CHANCES WILL STAY MORE CLOSELY TIED TO TO THE
SOUTHERN STREAM OF THE JET NEAR THE GULF COAST. TEMPERATURES ARE
EXPECTED TO MODERATE UNTIL THE WAVE PASSES AROUND MIDWEEK...WITH
COOLER READINGS RETURNING AS UPPER TROFFING BECOMES BETTER
ESTABLISHED BEHIND THE WAVE.

BAK
&&

.AVIATION...
ISSUED 545 PM CST SAT DEC 26 2009

AREA OF SNOW IS BEGINNING TO WANE WITH THE BACK EDGE STARTING TO
MOVE THROUGH TAF SITES...STARTING AT PIA FIRST. THE BACK EDGE WILL
PROGRESS THROUGH THE SITES THIS EVENING...GETTING TO CMI LAST AT
AROUND 3Z. BASED ON OBSERVATIONS TO THE WEST...SHOULD SEE VFR CIGS
AND VIS FOR SOME TIME TONIGHT AND INTO OVERNIGHT. THE NEXT ROUND
OF SNOW WILL ARRIVE EARLY MORNING AND CONTINUE INTO THE EARLY
AFTERNOON HOURS...BASED ON CURRENT MODEL FORECASTS. SO EXPECTING
IFR VIS AND CIGS JUST ABOVE 1KFT...EXCEPT FOR BMI...TOMORROW
DURING THE HEAVIEST SNOWFALL. THEN MVFR CIGS WILL ARRIVE FOR THE
AFTERNOON AFTER THE SNOW ENDS. SOUTHWESTERLY WINDS WILL BECOME
WESTERLY AS THIS CURRENT SHORT WAVE MOVES THROUGH THIS EVENING.
THEN WILL EXPECT WEST TO NORTHWEST WINDS TOMORROW.

AUTEN
&&

.ILX WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL 9 PM CST THIS EVENING FOR 
ILZ027>031-036-037.

FLASH FLOOD WATCH THROUGH SUNDAY MORNING FOR ILZ051.

&&

$$