FXUS63 KLOT 111754
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1150 AM CST FRI DEC 11 2009

.DISCUSSION...
335 AM CST

PRIMARY FORECAST CHALLENGES THIS MORNING REVOLVE AROUND TEMPERATURES 
THE NEXT SEVERAL DAYS AS WELL AS SMALL PRECIP CHANCES/TYPE SAT NIGHT 
INTO SUN MORNING AND AGAIN SUNDAY NIGHT INTO MONDAY. 

WARM AIR ADVECTION ALREADY UNDERWAY AS HEART OF THE ARCTIC AIRMASS 
RETREATS NORTHEASTWARD INTO CANADA. DESPITE THE FRESH SNOWPACK 
STEADY W TO SW WINDS HAVE HELPED HOLD TEMPS STEADY MOSTLY IN THE 
SINGLE DIGITS ABOVE ZERO OVER SNOWPACK AND IN THE TEENS SE COUNTIES 
WHERE LESS SNOW COVER EXISTS. EVEN WITH THE SOMEWHAT MILDER START 
TODAY AND FULL SUNSHINE EXPECT THAT THE SNOW PACK WILL STILL RETARD 
OUR WARM UP WITH DEEP SNOW COVERED AREAS PROBABLY NOT REACHING 20F 
THIS AFTERNOON.

TEMP FORECAST TONIGHT WILL AGAIN BE TRICKY AS WARM AIR ADVECTION 
KEEPS TEMPS WARMING ALOFT...HOWEVER CLEAR SKIES AND LIGHTER WINDS 
THIS EVENING MAY ALLOW FOR A RAPID CRASH IN TEMPERATURES OVER THE 
SNOW PACK AND HAVE CONTINUED TO UNDERCUT GUIDANCE. STILL WORRIED 
THAT IF WE DECOUPLE SUFFICIENTLY THIS EVENING THAT TEMPS COULD BE 
EVEN COLDER THAN WHAT I HAVE IN THE GRIDS...BUT DIDN'T WANT TO GET 
TOO CARRIED AWAY WITH TEMPS AS SOUTHERLY GRADIENT IS PROGGED TO 
TIGHTEN OVERNIGHT. 

WARM AIR ADVECTION WILL CONTINUE THROUGH THE DAY SATURDAY IN ADVANCE 
OF A WEAK SHORTWAVE RIDING THE SUBTROPICAL JET TOWARD THE REGION
SAT NIGHT INTO SUNDAY. FORECAST SOUNDINGS SUGGEST TOP-DOWN
SATURATION WILL BE TAKING PLACE HOWEVER IT ALSO APPEARS THAT BY
THE TIME THE LOW LEVELS HAVE MOISTENED SUFFICIENTLY FOR PRECIP TO
REACH THE GROUND THAT WE COULD BE LOSING MID/UPPER LEVEL MOISTURE
AND THUS ICE CRYSTALS IN THE CLOUDS. OVERALL DOESNT LOOK TO BE ALL
THAT IMPRESSIVE OF A PRECIP EVENT WITH A TRACE TO COUPLE/FEW
HUNDREDTHS OF AN INCH EVENT APPEARING TO BE THE WORST CASE
SCENARIO. P-TYPE ALSO NOT STRAIGHT FORWARD WITH WARM ADVECTION
TENDING TO PUSH TEMPERATURES ABOVE FREEZING ALOFT...THOUGH
EVAPORATIVE COOLING WILL BE WORKING TO COUNTERACT THAT. GIVEN TOP-
DOWN SATURATION WOULDNT BE SURPRISED IF WE SAW A BIT OF
GRAUPEL...FREEZING SPRINKLES OR FLURRIES. ASSUMING LOW LEVELS
SATURATE FULLY...FORECAST SOUNDINGS SUGGEST A STRATUS DECK WILL
LINGER INTO SUNDAY MORNING WITH SOME LIGHT DRIZZLE OR FREEZING
DRIZZLE A POSSIBILITY.

NEXT MORE SIGNIFICANT NORTHERN STREAM TROUGH FORECAST TO LEAD TO SFC 
CYCLOGENESIS OVER THE CENTRAL HIGH PLAINS SUNDAY NIGHT. LOW IS THEN 
FORECAST TO TRACK INTO WISCONSIN ON MONDAY WITH FAIRLY STRONG WARM 
AIR ADVECTION IN ADVANCE OF THIS SYSTEM LIKELY LEADING TO A NICE 
REBOUND IN TEMPS. MEX GUIDANCE HAS OUR SOUTHERN CWA CLIMBING INTO 
THE 50S...WITH H8 TEMPS EVEN MILDER ON THE 00Z ECMWF. HAVE NUDGED 
HIGHS MONDAY UP INTO THE 40S OVER THE URBAN AND NON-SNOW COVERED 
AREAS WITH MID/UPPER 30S ELSEWHERE...THOUGH GIVEN LATEST MODEL 
GUIDANCE THESE TEMPS MAY EVEN BE CONSERVATIVE.

GFS BRINGS IN ANOTHER SHOT OF BITTERLY COLD AIR BEHIND THIS FRONT 
TUESDAY INTO WEDNESDAY. MEX GUIDANCE KNOCKING HIGHS BACK DOWN INTO 
THE TEENS AND LOWS IN THE SINGLE DIGITS. GEM/ECMWF/DGEX ALL MUCH 
LESS 
INTENSE WITH THE ARCTIC INTRUSION SO HAVE NUDGED GOING FORECAST 
TEMPS DOWN SLIGHTLY THOUGH KEPT THEM WELL ABOVE WHAT THE OUTLIER GFS 
MODEL WOULD SUGGEST IS POSSIBLE. 

NO CHANGES MADE TO GOING FORECAST BEYOND TUESDAY...

IZZI

&&

.AVIATION...
1150 AM CST

RE: 1800Z TAFS...AN EXTENSIVE RIDGE OF HIGH PRESSURE FROM THE TN 
VLY...NWWD ACROSS UPR MS VLY WILL DOMINATE WX THROUGH TONIGHT.  THIS 
WILL MAINTAIN GENLY VFR CLR CONDITIONS.  EXCEPTION WILL BE OVR 
PORTIONS OF EXTRM NRN IL THIS AFTN AS SCT V BKN AREA OF MID LVL 
CLOUDINESS NOW OVR IA AND NWRN IL SHIFTS EWD ACROSS REGION. VFR CIGS 
TO DVLP SATURDAY MORNING AS WARM AIR ADVECTION KICKS IN ON BACK SIDE 
OF RETREATING RIDGE AND WEAK SHORT WAVE APCHS FROM THE CNTRL PLAINS. 

MAIN REASON FOR CHG GROUPS IN FCST THRU NXT 24 HRS WILL BE DUE TO 
BACKING OF WIND AS RIDGE AXIS PASSES ACROSS REGION LATE THIS AFTN 
AND THIS EVENING.  SFC PRESSURE GRADIENT CONTINUES TO WEAKEN WITH 
APCH OF THIS RIDGE...SO WSWLY FLOW IN 10-15 KT RANGE WILL BE IN THE 
PROCESS OF TURNING SLY WITH SPEEDS BLO 10 KTS THIS EVENING.  AS SYS 
TO W APCHS ON SATURDAY...SLY WINDS WILL AGAIN BEGIN TO INCR...WITH 
FQT GUSTS NR 20 KTS DVLPG AFT 18Z SAT. 

MERZLOCK

&&

.MARINE...
318 AM CST

A BROAD AREA OF HIGH PRESSURE CENTERED OVER THE SOUTHEASTERN
US WILL DRIFT TOWARD THE MID ATLANTIC STATES OVER THE NEXT COUPLE
OF DAYS. THE NORTHWESTERN EXTENT OF THIS RIDGE WILL MOVE OVER LAKE
MICHIGAN AND ALLOW WEST WINDS TO SUBSIDE SUBSTANTIALLY BY LATE
FRIDAY. WINDS TURN SOUTH TO SOUTHWEST AND THE PRESSURE GRADIENT
TIGHTENS AGAIN BY SATURDAY AFTERNOON AS A TROUGH OF LOW PRESSURE
DEVELOPS OVER THE NORTHWESTERN GREAT LAKES. THIS TROUGH CROSSES
LAKE MICHIGAN ON SUNDAY AND WINDS AGAIN VEER WEST TO
NORTHWEST...THEN EVENTUALLY AROUND TO THE EAST BY MONDAY MORNING
AHEAD OF THE NEXT APPROACHING LOW PRESSURE SYSTEM. DEPENDING ON
THE EXACT STRENGTH AND TRACK OF THIS SYSTEM AS THE LOW PRESSURE
CENTER CROSSES THE LAKE LATE MONDAY OR EARLY TUESDAY...WINDS COULD
AGAIN INCREASE TOWARD GALE THRESHOLDS.

LENNING

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...GALE WARNING...LMZ080-LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-
     LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ777-LMZ779-LMZ868-
     LMZ870-LMZ872-LMZ874-LMZ876-LMZ878 UNTIL MIDNIGHT SATURDAY.

     SMALL CRAFT ADVISORY...NEARSHORE WATERS 
     UNTIL MIDNIGHT SATURDAY.

&&

$$