FXUS63 KLOT 300409
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1009 PM CST SUN NOV 29 2009

.DISCUSSION...
240 PM CST

FORECAST CONCERNS TODAY REVOLVE LARGELY ON FIRST BOUT OF TRUE 
WINTER-LIKE WEATHER CURRENTLY PENCILED IN FOR A MID TO LATE WEEK 
ARRIVAL...JUST IN TIME FOR THE START OF METEOROLOGICAL WINTER.

BUT FIRST...SHORTWAVE RESPONSIBLE FOR OUR THUNDERY START TO THE DAY 
CONTINUES TO MOVE QUICKLY EASTWARD ACROSS ILLINOIS EARLY THIS 
AFTERNOON AS LARGER SCALE TROUGH DIGS INTO THE UPPER MISSISSIPPI 
VALLEY. SURFACE FRONTOGENESIS TAKING PLACE WITH FRONTAL BOUNDARY 
JUST GETTING READY TO CLEAR OUR FAR SE COUNTIES. WITH LEAD SHORTWAVE 
MOVING QUICKLY EASTWARD EXPECT PRECIP TO HAVE CLEARED THE CWA BY 00Z 
WITH DRY WEATHER TONIGHT. STRATUS DECK IS NOT OVERLY IMPRESSIVE 
UPSTREAM BUT COULD STILL SEE THIS FILL IN A BIT LATER THIS AFTERNOON 
INTO TONIGHT...WHICH IF IT DOES WOULD HOLD TEMPS UP A BIT TONIGHT 
MORE IN LINE WITH SLIGHTLY MORE MODERATE MAV TEMPS. 

A WEAK SHORTWAVE IS PROGGED TO RIDE THE NW FLOW INTO THE REGION 
LATER MONDAY AFTERNOON AND EVENING. STRENGTHENING WARM AIR ADVECTION 
IN ADVANCE OF THIS FEATURE EXPECTED TO LEAD TO SOME RAIN/SNOW 
BREAKING OUT MAINLY TO THE NORTH AND NORTHEAST OF THE CWA...BUT 
COULD BE CLOSE ENOUGH TO JUSTIFY SOME VERY SMALL POPS FOR OUR NE-ERN 
MOST COUNTIES MON EVENING. IF PRECIP DOES SNEAK INTO THE AREA IT 
CERTAINLY DOESNT LOOK LIKE ANY BIG DEAL. 

HEIGHTS WILL QUICKLY RISE IN THE WAKE OF THIS WAVE MONDAY NIGHT WITH 
FAIRLY IMPRESSIVE WARM AIR ADVECTION TAKING PLACE. STRONG WESTERLY 
FLOW FROM H8-H5 SHOULD BE QUITE EFFECTIVE IN ADVECTING CHINOOK 
WARMED AIR EASTWARD...BUT BIG QUESTION FOR OUR HIGH TEMPS IS HOW 
MUCH OF THIS WARM AIR ALOFT WILL BE MIXED DOWN TO THE SURFACE GIVEN 
LOW SUN ANGLE AND ONLY MODEST SFC WAA. GIVEN THE EXPECTATION OF 
CONSIDERABLE SUNSHINE...DID GO A BIT ABOVE MOS WITH HIGHS MOST AREAS 
POKING ABOVE 50F FOR WHAT WILL LIKELY BE ONE LAST GASP OF MILD-ISH 
AIR BEFORE THE BOTTOM STARTS DROPPING OUT. 

FIRST COLD FRONT SHOULD MOVE ACROSS THE AREA LATE TUES NIGHT INTO 
EARLY WED WITH AN INITIAL SHOT OF COLD AIR ADVECTION BRINGING TEMPS 
BACK DOWN CLOSER TO SEASONAL AVERAGES AND MARKING THE START OF THE 
MULTI-DAY DOWNWARD SLIDE IN TEMPS. FROPA LOOKS TO BE UNEVENTFUL  
WITH COLUMN FAIRLY DRY AND FORCING NOT OVERLY STRONG ALONG THE 
FRONT. 

THINGS GET MUCH MORE COMPLEX FROM THIS POINT ONWARD AND CONSEQUENTLY 
CONFIDENCE IN THE FORECAST BEGINS TO FALL ALMOST AS FAST AS THE 
TEMPS WILL BE FALLING! THE PROBLEM IS HOW THE MODELS HANDLE THE CUT 
OFF LOW CURRENTLY OVER THE NORTHERN BAJA CALIFORNIA AND TIMING OF 
PHASING OF THIS UPPER LOW/SOUTHERN STREAM SYSTEM WITH THE POLAR 
JET...BOTH THINGS THAT THE MODELS ARE NOTORIOUS FOR HANDLING POORLY. 
THIS GO AROUND APPEARS TO BE NO DIFFERENT WITH A FAIRLY WIDE 
SPECTRUM OF SOLUTIONS FOR WHEN THE UPPER LOW MOVES EAST AND HOW SOON 
IT PHASES WITH AMPLIFYING NORTHERN STREAM TROUGH. 

GENERAL MODEL CONSENSUS IS FOR UPPER LOW TO TRACK EAST AND NOT START 
TO PHASE WITH NORTHERN STREAM JET UNTIL WED NIGHT ONCE IT IS SAFELY 
EAST OF OUR CWA...WITH SURFACE LOW PROGGED TO TRACK NEAR THE SPINE 
OF THE APPALACHIANS BEFORE SECONDARY SURFACE CYCLOGENESIS TAKES 
PLACE NEAR THE EAST COAST. THIS SOLUTION...WHICH IS SUPPORTED BY THE 
12Z ECMWF/12Z GFS/12Z NOGAPS...WOULD SAFELY KEEP MOST IF NOT ALL OF 
THE SYNOPTIC SNOWS SE OF OUR CWA. HAVE GENERALLY FOLLOWED THIS LINE 
OF THINKING BUT STILL AM STILL SOMEWHAT LEARY SINCE MODELS TEND TO 
KICK UPPER LOWS OUT TOO QUICKLY...AND SHOULD THE UPPER LOW MOVE OUT 
SLOWER IT WOULD BE MORE LIKELY TO GET PICKED UP BY AND PHASE WITH 
THE AMPLIFYING NORTHERN STREAM TROUGH FARTHER WEST AND POSE MORE OF 
A SNOW THREAT TO OUR CWA. THE 12Z GEM/12Z UKMET AND A HANDFUL OF 
MEMBERS FROM THE 12Z GFS ENSEMBLE SUITE SUGGEST THAT THIS IS A 
POSSIBILITY...SO CANNOT COMPLETELY RULE OUT AN ACCUMULATING SNOW 
DURING THE WED NIGHT/THURSDAY TIME RANGE. UNTIL THERE IS BETTER 
MODEL CONSENSUS JUST DECIDED TO CONTINUE WITH LOW CHANCE/SLIGHT 
CHANCE POPS FOR SNOW WED NIGHT INTO THURSDAY. 

IN ADDITION...AMPLIFYING NORTHERN STREAM TROUGH SHOULD ALLOW THE 
COLDEST AIR MASS THUS FAR THIS SEASON TO SPILL SOUTHWARD INTO THE 
AREA THURSDAY INTO FRIDAY. H8 TEMPS ARE PROGGED TO DROP INTO THE 
TEENS BELOW ZERO CELSIUS...WHICH SHOULD THIS VERIFY WOULD RESULT IN 
HIGHS NOT CLIMBING OUT OF THE 20S BY FRIDAY AND POSSIBLY FALLING 
INTO THE 20S THURSDAY AFTERNOON DEPENDING ON THE TIME OF ARRIVAL OF 
THE SECONDARY PUNCH OF VERY COLD AIR. WHILE THE AIR MASS ARRIVING 
WILL BE THE COLDEST OF THE SEASON...IT ACTUALLY COULD BE MUCH WORSE. 
COMPARED TO LAST YEAR OR EVEN "NORMAL" THERE IS A RELATIVE LACK OF 
SNOW COVER UPSTREAM WHICH WILL ALLOW THE COLD AIR MASS TO MODIFY AS 
IT MOVES SOUTH AND TAKE A BIT OF AN EDGE OFF THE BITE.

DEEPENING TROUGH MOVING INTO THE AREA WITH STRONG COLD AIR ADVECTION 
SHOULD RESULT IN STEEPENING LAPSE RATES AND WITH CYCLONIC LOW LEVEL 
FLOW...SET THE STAGE FOR NUMEROUS FLURRIES AND POSSIBLY SOME 
SCATTERED SNOW SHOWERS THURSDAY INTO FRIDAY AS WELL...EVEN IF THE 
SYNOPTIC SNOWS MISS US TO THE SOUTH/EAST AS IS CAUTIOUSLY EXPECTED.

FINALLY...AS SURFACE LOW INITIALLY DEVELOPS AND DEEPENS TO OUR SOUTH 
AND VERY COLD AIR SPILLS INTO THE AREA IT LOOKS POSSIBLE THAT THERE 
COULD BE A WINDOW OF OPPORTUNITY FOR LAKE EFFECT SNOW TO AFFECT OUR 
NW IN COUNTIES AND MAYBE EVEN SNEAK WEST INTO FAR NE IL FOR A TIME 
WED NIGHT OR THURSDAY. AS SFC LOW MOVES NORTHEAST LOW LEVEL FLOW 
WILL BACK AND CAUSE ANY LAKE EFFECT SNOW TO SHIFT EAST INTO THE MORE 
TYPICAL LAKE EFFECT SNOW BELT AREAS OF NC IN AND SW LOWER MI. 

TO SUM THINGS UP...

AT THIS POINT...WITH A LOW DEGREE OF CONFIDENCE...HAVE OPTED TO 
FOLLOW THE MAJORITY (ALBEIT A SOMEWHAT SMALL MAJORITY) OF THE MODEL 
GUIDANCE WITH THE POWERFUL STORM MOVING UP THE EAST COAST AND 
LARGELY BYPASSING OUR CWA WITH ITS ACCUMULATING SYNOPTIC SNOWS. 
WOULDN'T BE SURPRISED TO SEE MODEL SOLUTIONS WAFFLE AROUND MORE OVER 
THE NEXT COUPLE OF DAYS BEFORE COMING TO MORE OF A CONSENSUS. SO 
BOTTOM LINE IS THAT WE WILL NEED TO KEEP A WEARY EYE ON THE FORECAST 
FOR LATER THIS WEEK.

IZZI

&&

.AVIATION...
1009 PM CST

0600 UTC TAFS...MVFR CIGS HAVE GENERALLY IMPROVED TO VFR ACROSS
THE TAF REGION AS DRIER LOW LEVEL AIR CONTINUES TO MOVE INTO THE
AREA. WINDS SHOULD START TO SLACKEN OFF OVERNIGHT AS A RIDGE AXIS
OF HIGH PRESSURE MOVES INTO THE REGION. AS THE NEXT SYSTEM DROPS
DOWN FROM CANADA INTO THE GREAT LAKES ON MONDAY...THE SFC PRESSURE
GRADIENT WILL AGAIN TIGHTEN UP WITH WESTERLY WINDS LIKELY GUSTING
UP INTO THE 15-20KT RANGE. SOME LOW END VFR CIGS APPEAR POSSIBLE
MONDAY AFTERNOON WITH THIS SYSTEM APPROACHING FROM THE NORTH...BUT
ANY ASSOCIATED PRECIPITATION IS EXPECTED TO STAY NORTHEAST OF THE
TAF REGION.

EXPECT VFR CONDITIONS THROUGH OUT THE FORECAST PERIOD.

HALBACH

&&

.MARINE...
305 PM CST

SMALL CRAFT ADVISORY CANCELLED AS WINDS WILL NOT BE STRONG ENOUGH
BEHIND THE COLD FRONT

N TO NW WINDS TO BACK SOMEWHAT TONIGHT AS THE LOW CURRENTLY OVER
LAKE HURON CONTINUES TO MOVE EAST AND THE LARGE ANTICYCLONE CENTERED
IN THE NORTHERN ROCKIES AND CENTRAL HIGH PLAINS SETTLES SSE. A
WEAK COLD FRONT/TROF WILL ROTATE INTO THE UPPER GREAT LAKES LATE MON
AND MON NIGHT VEERING WINDS BACK TO WNW BEFORE BACKING TO SW AND S
AHEAD OF THE LOW TO TRACK ACROSS FAR SOUTH CENTRAL CANADA TUE.
ASSOCIATED STRONG COLD FRONT TO CROSS LAKE MI LATE TUE NIGHT.
WHILE STRONG CAA ADVECTION TO OCCUR ACROSS THE LAKES THE CANADIAN
LOW WEAKENS IN FAVOR OF STRONGLY DEEPENING LOW MOVING NORTHEAST
ACROSS THE SE U.S. SO ANTICIPATE NW WINDS BEHIND THE STRONG COLD
FRONT TO REMAIN BELOW GALE FORCE. 

TRS

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 6 AM MONDAY.

&&

$$