FXUS63 KLOT 010932
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
332 AM CST TUE DEC 1 2009

.DISCUSSION...
300 AM CST

MAIN CONCERN FOR THE OVERNIGHT UPDATE CONTINUES TO FOCUS ON THE
UPCOMING WINTRY PCPN EVENT SETTING UP FOR WEDNESDAY NIGHT INTO FRIDAY.

THE EXPECTED SLOPPY WEATHER WILL BE PRECEDED BY A RELATIVELY CALM
PERIOD TODAY THROUGH WEDNESDAY MORNING. DURING THIS TIME...MOSTLY
CLEAR SKIES WITH SLIGHTLY ABOVE NORMAL TEMPERATURES.

BY WEDNESDAY MORNING...THE MODEL GUIDANCE IS RATHER CONSISTENT ON
THE SYNOPTIC SCALE IN DEVELOPING A STRONG SRN STREAM CLOSED LOW
AND LIFTING IT OUT OF NORTHERN OLD MEXICO TODAY AND TRACKING ACROSS
CNTRL TONIGHT. AT THE SAME TIME...A STRONG NRN STREAM SHORT WAVE
WILL DIG OUT OF CENTRAL CANADA AND INTO THE NRN PLAINS. ONE OF THE
PRIMARY CONCERNS FOR THE IMPACT ON THE LOCAL AREA WILL BE THE
DEGREE OF PHASING THAT MAY OCCUR WITH THESE TWO SYSTEMS. THE MOST
RECENT MODEL RUNS ARE MAINTAINING MORE SEPARATION TO THE STREAMS
THAN WAS SEEN IN YESTERDAY'S RUNS...WHICH WOULD IMPLY THAT THE NRN
STREAM SYSTEM WILL BE MOVE OF A CLIPPER-TYPE SYSTEM...SKIRTING THE
UPPER GREAT LAKES...WHILE THE SRN STREAM SYSTEM TAKES A MORE
EASTERLY TRACK THROUGH THE OHIO VALLEY AND INTO THE NERN CONUS.
THE NAM/GFS/ECMWF ARE ALL CONSISTENT ON MAINTAINING SEPARATION THE
THE STREAMS AND HAVE SIMILAR SOLUTIONS FOR THE NRN STREAM. THERE
IS SOME DIFFERENCE IN THE TRACK OF THE SFC REFLECTION OF THE SRN
STREAM SYSTEM. BY WEDNESDAY EVENING...THE GFS IS THE FARTHEST
EAST...STAMPING OUT THE LOW CENTER OVER SRN KENTUCKY AND THE ECMWF
IS THE SLOWEST...KEEPING THE LOW CENTER OVER THE WRN
KENTUCKY/TENNESSEE BORDER. THE NAM IS MORE OF A MIDDLE OF THE ROAD
SOLUTION AND SPLITS THE DIFFERENCE BETWEEN THE OTHER TWO MODELS
EXAMINED. HAVE GENERALLY TRENDED WITH THE MIDDLE OF THE ROAD NAM
FOR THE SRN STREAM. HOWEVER...EVEN AT ONLY 36-48 HOURS OUT...EVEN
THE RELATIVELY MINOR DIFFERENCES BETWEEN THE MODELS CAN MAKE THE
DIFFERENCE BETWEEN A PCPN EVENT OF MORE RAIN OR MORE SNOW. GIVEN
THE TRACK OF THE SYSTEM...AND THE EXPECTED RELATIVELY WARM
CONDITIONS TODAY...EXPECT THAT THE BULK OF THE PCPN WILL BE MORE
LIQUID THAN FROZEN...PARTICULARLY AT THE ONSET OF THE EVENT.
THUS...IF THE SYSTEMS EVOLVE AS EXPECTED...THE HEAVIEST PCPN OVER
THE CWA WILL BE RAIN OVER THE SERN LOCATIONS...WHILE LIGHT SNOW OR
A RAIN/SNOW MIX SHOULD PREVAIL OVER THE THE REMAINDER OF THE
CWA...PRODUCING LITTLE OR NO ACTUAL ACCUMULATION.

HOWEVER...THIS IS WHERE THE NRN STREAM SYSTEM CAN REPRESENT A REAL
FLY-IN-THE-OINTMENT. IF THE SYSTEMS REMAIN UNPHASED AS
EXPECTED...THE MAIN SRN STEAM SFC LOW WILL GO THROUGH IT'S
SIGNIFICANT DEEPENING FARTHER EAST THAN IF THERE IS A GREATER
DEGREE OF PHASING. GREATER PHASING...AND STRONGER DEEPENING
FARTHER WEST MIGHT DRAW MORE COLD AIR INTO THE REGION AND FASTER.
THUS THE CHANGE-OVER FROM LIQUID TO SOLID WOULD BE FASTER...AND
SOME ACCUMULATION COULD BE POSSIBLE. AS IS ALWAYS THE CASE WITH
EVALUATING A RAIN-SNOW LINE...SMALL CHANGES AND DISTANCES CAN MAKE
A BIG DIFFERENCE...AND THE SITUATION WILL CERTAINLY BEAR WATCHING.

THE PCPN SHOULD BEGIN TO WINDS DOWN BY LATE THURSDAY. AS THE TWO
SYSTEM EXIT THE REGION...LARGE SCALE TROUGH WILL CARVE OUT OVER
THE CNTRL CONUS. THIS WILL KEEP A CHANCE OF FLURRIES OVER THE
REGION AS MUCH COOLER AIR OVERSPREADS THE MIDWEST AND GREAT LAKES
REGIONS THURSDAY AND INTO FRIDAY.  

KREIN

&&

.AVIATION...

0600 UTC TAFS...PRIMARY AVIATION CONCERN NEXT 24-30 HOURS WILL BE
WINDS...BOTH AT SURFACE AND EVEN JUST OFF THE SURFACE TONIGHT. SW
WINDS STILL MAINTAINING SOME GUSTINESS THIS EVENING AS MODERATELY
STRONG (FOR THIS TIME OF DAY) PRESSURE FALLS TAKE PLACE OVER LOWER
MI IN ADVANCE OF DEPARTING SURFACE LOW. AS THE PRESSURE FALL
MAXIMUM MOVES EAST OVERNIGHT LOOK FOR WINDS TO BACK...SLACKEN AND
LOSE THEIR GUSTINESS. RECENT ACARS SOUNDINGS FROM ORD SHOW 50KT
WINDS AT THE TOP OF THE NOCTURNAL INVERSION AROUND 2K FT...SO HAVE
GONE AHEAD AND INCLUDED LLWS IN THE TAFS. LOW LEVEL JET WILL
DECREASE PRIOR TO SUNRISE WITH THREAT OF LLWS ALSO SUBSIDING. SSW
WINDS WILL BEGIN TO INCREASE AGAIN TUESDAY AFTERNOON AS THE NEXT
SURFACE LOW MOVES INTO THE UPPER MISSISSIPPI VALLEY. COLD FRONT
PROGGED TO BE MOVING THROUGH THE AREA TOWARD THE END OF THE 30
HOUR TAF PERIOD.

IZZI

&&

.MARINE...
240 AM CST

AS THE STRONG PRESSURE GRADIENT SHIFTS OFF TO THE EAST EARLY THIS 
MRNG...GALE FORCE WINDS WIND DOWN. A WEAK SFC RIDGE WILL BUILD 
ACROSS THE LAKES THIS MRNG...PASSING TO THE EAST OF THE LAKES THIS
EVENING AND PERMITTING ANOTHER SFC LOW TO APPROACH THE LAKES
TONIGHT. THIS WEAKENING LOW SHUD SLIP PAST THE SAULT NEAR MIDDAY
WEDNESDAY BEFORE MERGING WITH A MORE INTENSE LOW MOVING UP FROM
THE LOWER MISSISSIPPI VALLEY. THIS NORTHEAST MOVING LOW WILL KICK
OUT OF THE LOWER MISSISSIPPI VALLEY EARLY WEDNESDAY...MERGING WITH
THIS SOUTHERN ONTARIO LOW BY WEDNESDAY EVENING AS IT REACHES THE
OHIO VALLEY. AT THIS POINT...THE LOW DEEPENS AND CONTINUES
NORTHEAST TO LAKE ONTARIO...BACKING WINDS TO THE NORTH AND
INCREASING TO 30 KNOTS OVER LAKE MICHIGAN. STRONG WINDS CONTINUE
INTO THURSDAY UNTIL THE LOW ADVANCES TO NEW ENGLAND IN THE
AFTERNOON.

WINDS HAVE ALREADY WEAKENED BELOW SMALL CRAFT ADVISORY VALUES WHILE 
GALE FORCE WINDS HAVE SUBSIDED OVER THE MIDDLE OF THE LAKE. THUS WILL
CANCEL THE GALE AND SCA BEFORE LAKES ISSUANCE THIS MRNG.

RLB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$