FXUS63 KILX 220522
AFDILX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
1122 PM CST MON DEC 21 2009

.DISCUSSION...
ISSUED 857 PM CST MON DEC 21 2009

LATEST NAM/RUC MODELS SUGGEST EXPECTED BAROCLINIC/FRONTOGENETIC
ZONE OVER NORTHERN ILLINOIS TONIGHT IS SETTING UP A BIT FURTHER
SOUTH AND/OR IS BROADER THAN PREVIOUS FORECASTS. A FAIRLY POTENT
SHORT WAVE IS CURRENTLY EVIDENT ON WATER VAPOR IMAGERY MOVING EAST
SOUTHEAST ALONG THIS THERMAL GRADIENT. IN LIGHT OF THIS NEW MODEL
DATA...AS WELL AS RECENT RADAR TRENDS UPSTREAM...WILL BE BUMPING
UP SNOW CHANCES A BIT FURTHER SOUTH INTO CENTRAL ILLINOIS
OVERNIGHT. THAT BEING SAID...THE BULK OF THE ACCUMULATING SNOW
SHOULD STAY OVER NORTHERN ILLINOIS.

OTHERWISE...FORECAST IS IN GOOD SHAPE OVERALL. ONLY PLAN ON MAKING
MINOR TWEAKS...WHICH INCLUDES NUDGING LOW TEMPERATURES UP TO
NEARLY STEADY LEVELS.

BAK
&&

.AVIATION...
ISSUED 1122 PM CST MON DEC 21 2009

WEAK HIGH PRESSURE WILL PULL AWAY FROM THE CENTRAL ILLINOIS
TERMINALS...TO BE REPLACED BY AN APPROACHING AREA OF LOW PRESSURE
FROM THE PLAINS. LIGHT SOUTHEAST WINDS EXPECTED TO PERSIST THROUGH
THE NIGHT. THE WINDS SHOULD TURN EAST AND INCREASE TO 10 TO 15 KTS
ON TUESDAY AS THE DEVELOPING SYSTEM DRAWS CLOSER. EXPECT
PREDOMINANTLY MVFR CONDITIONS TO PERSIST AS LOW LEVEL MOISTURE
REMAINS TRAPPED AHEAD OF THE APPROACHING STORM. IN ADDITION...A
DISTURBANCE ALOFT WILL LIKELY PRODUCE A BIT OF LIGHT SNOW AT
KPIA/KBMI AND POSSIBLY KCMI OVERNIGHT. AS THE LOW CLOUD DECK
THICKENS BY TUESDAY EVENING...THERE WILL BE A RISK OF DRIZZLE AS
WELL. AT THIS TIME...SURFACE TEMPERATURES SHOULD BE JUST ABOVE
FREEZING TUESDAY EVENING AS THIS DRIZZLE IS FALLING. HOWEVER...THE
THREAT FOR FREEZING DRIZZLE WILL NEED TO BE MONITORED.

BAK
&&

.PREV DISCUSSION...
ISSUED 120 PM CST MON DEC 21 2009

SHORT TERM...TONIGHT THROUGH THURSDAY

MAIN FORECAST CONCERN THIS PERIOD WILL REVOLVE AROUND THE MAJOR
STORM SYSTEM EXPECTED TO TAKE SHAPE ACROSS THE SRN PLAINS LATE TUE
INTO WEDNESDAY...AND THEN SWING NNE INTO THE MIDWEST XMAS EVE.

OPERATIONAL MODELS IN DECENT AGREEMENT THRU MOST OF THIS PERIOD
EXCPT AFTR 60 HRS WHERE THE NAM IS MUCH SLOWER TO CLOSE OFF THE
STRONG SRN PLAINS STORM SYSTEM...WHILE THE GFS SEEMS TO HAVE
SETTLED DOWN AND WAS SHWG MUCH BETTER CONSISTENCY WITH THE TRACK
OF THE H5 AND SFC LOWS. STILL FEEL THE NAM-WRF MODEL IS HANDLING
THE LOW LEVEL THERMAL PROFILES BETTER INTO WEDNESDAY SO WL TEND
TO SIDE MORE WITH THAT MODEL THRU 48 HRS. INITIAL SHORTWAVE ENERGY
AND ISENT ASCENT APPRCHS THE AREA LATE TNT INTO TUE MRNG WITH THE
BULK OF THE STRONGER LIFT AND 850-700 MB FRONTOGENETICAL FORCING
JUST TO OUR NORTH. LOOKS AS IF THE MORE SIGNIFICANT SNOWS WILL
REMAIN JUST NORTH OF OUR AREA. FORECAST SOUNDINGS FROM BOTH THE
NAM AND GFS NOT SUPPORTING ANY SIGNIFICANT PRECIP AS ANY DEEP
MOISTURE WHICH WOULD SUPPORT ICE CRYSTAL GROWTH IS NOT PRESENT...
BRINGING MORE MORE FLURRIES OR EVEN SOME FREEZING DRIZZLE TWDS
DAWN ACRS THE NORTH...ROUGHLY ALONG AND NORTH OF INTERSTATE 74.

MODELS CONTINUE TO ADVERTISE INCREASING ISENT LIFT ACRS THE AREA
WITH A DECREASE IN PRES DEFICITS DURING THE DAY TUESDAY...LEADING
TO SOME LIGHT RAIN OR DRIZZLE...WITH OUR NRN AREAS STILL IN THE
THREAT FOR SOME LIGHT FREEZING RAIN OR DRIZZLE...WITH SOUNDINGS
SUPPORTING MORE FREEZING DRIZZLE THAN ANYTHING ELSE. STILL NOT
SEEING ANY GREAT INCREASE IN MOISTURE ABOVE 800 MB ACRS THE FAR
NORTH TO SUPPORT FREEZING RAIN. THAT WL BEGIN TO CHANGE LATE TUE
NIGHT AND ESP ON WEDNESDAY AS THE STORM SYSTEM ROLLS INTO THE SRN
PLAINS WITH IMPRESSIVE LOW LEVEL THERMAL AND MOISTURE ADVECTION
PUSHING NWD INTO CENTRAL IL. BY THAT TIME...FORECAST SOUNDINGS
SUPPORT RAIN ACRS ALL OF OUR AREA WITH STEADY OR SLOWLY RISING
TEMPS THRU WEDNESDAY NIGHT AND XMAS EVE. 

WITH THE TRACK OF THE SFC LOW EXPECTED TO PUSH TO OUR WEST...ALL OF
OUR FCST AREA WILL BE ON THE WARM SIDE OF THIS SYSTEM SO OTHER THAN
PSBLY A BRIEF PERIOD OF SOME FREEZING RAIN OR SLEET AT THE ONSET OF
THE MORE SIGNIFICANT PRECIP ON WED ACRS OUR FAR NRN COUNTIES...IT
APPEARS THIS SYSTEM WL BE MOSTLY A RAIN MAKER. AND SOME OF THE RAIN
MAY BE HEAVY ACRS CENTRAL IL WITH THUNDERSTORMS A DISTINCT PSBLTY
INTO THU AFTN AND EVENING. WILL FRESHEN UP THE CURRENT SPS THAT
IS OUT AND EMPHASIZE A WARMER AND WETTER TREND TO THE UPCOMING
STORM.

LONG TERM...FRIDAY THROUGH MONDAY

WINDY AND COLDER WEATHER WILL FOLLOW ON XMAS DAY WITH SCATTERED
SNOW SHOWERS AS THE INTENSE STORM SYSTEMS SHIFTS NORTH ACRS LK
MICHIGAN. MODELS...ESP THE LATEST ECMWF...WERE BACKING OFF A BIT
ON THE LOW LEVEL COLD ADVECTION BEHIND THE SYSTEM FOR THE
WEEKEND...BUT AM NOT PLANNING ON MAKING ANY SIGNIFICANT CHANGES TO
OUR GOING TEMPS FOR THE REMAINDER OF THE HOLIDAY WEEKEND WITH DAY-
TIME HIGHS IN THE 20S AND EARLY MRNG LOWS RANGING FROM THE MID
TEENS NORTH TO THE LOW 20S SOUTH.

MEDIUM RANGE MODELS IN DECENT AGREEMENT FOR A CHANGE WITH THE 
SPLIT FLOW EXPECTED TO PREVAIL WITH THE NRN STREAM FORECAST TO
BE FAR ENOUGH TO OUR NORTH TO KEEP THE VERY COLD AIR BOTTLED
UP ACRS CANADA...WHILE THE ACTIVE BRANCH OF THE SRN STREAM WL 
CONTINUE TO BRING SYSTEMS IN ACRS THE WEST COAST...AND THEN 
TRACK EWD INTO THE SRN PLAINS. RIGHT NOW...IT APPEARS THE SRN
STREAM ACTION WILL REMAIN SOUTH OF THE FCST AREA INTO EARLY 
NEXT WEEK BUT THAT IS SUBJECT TO CHANGE. 

SMITH

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$