FXUS63 KLOT 160934
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
334 AM CST WED DEC 16 2009
.DISCUSSION...
334 AM CST
MAIN FORECAST CHALLENGES THIS MORNING REVOLVE AROUND TEMPS WITH
SLOWLY MODIFYING ARCTIC AIR MASS THEN SNOW CHANCES WITH CLIPPER AND
LAKE EFFECT LATE THURS NIGHT INTO THE FIRST HALF OF THE WEEKEND.
ARCTIC AIR MASS HAS CERTAINLY BECOME ENTRENCHED OVER THE AREA EARLY
THIS MORNING WITH TEMPS AROUND ZERO OVER THE SNOWPACK AND GENERALLY
BETWEEN 5 AND 10 ABOVE ELSEWHERE. STILL APPEARS AS THOUGH SFC HIGH
WILL DRIFT ACROSS THE REGION TODAY RESULTING IN LIGHT WINDS AND WEAK
ADVECTION IN THE BOUNDARY LAYER. SO DESPITE THE CONSIDERABLE
MODIFICATION OF THE AIR MASS ABOVE THE BOUNDARY LAYER SUSPECT THAT
MUCH OF THAT WARMING WILL NOT BE FELT AT THE SURFACE GIVEN LOW SUN
ANGLE...PARTICULARLY OVER THE SNOW PACK.
TEMP FORECAST REMAINS TRICKY TONIGHT AS HIGH PRESSURE WILL STILL BE
IN THE VICINITY AND SHOULD BE CLOSE ENOUGH FOR WINDS TO GO NEARLY
CALM THIS EVENING. OTHER THAN THE INCREASE IN CIRRUS ITS HARD TO
FIND GOOD REASONING FOR NOT SEEING AT LEAST MODEST FALL IN TEMPS
THIS EVENING. CONTINUED TREND OF UNDERCUTTING GUIDANCE TEMPS
TONIGHT...ESPECIALLY OVER SNOWPACK BUT ADMITTEDLY IT MAY BE A BIT
COOLER THAN GOING GRIDS WOULD SUGGEST. FOLLOWED SAME LINE OF
REASONING FOR TEMPS THURSDAY WITH GUIDANCE SEEMINGLY STILL TOO MILD
GIVEN INCREASING/THICKENING CLOUD COVER AND WEAK SFC ADVECTION.
ATTENTION THEN TURNS TO THE FAIRLY VIGOROUS LOOKING SHORTWAVE IN
WATER VAPOR OFF THE BRITISH COLUMBIA COAST. MODEL TRENDS OVER THE
PAST 24 HOURS HAVE BEEN TOWARD THIS WAVE EMERGING EAST OF THE
ROCKIES AS SHARPER AND MUCH MORE VIGOROUS TROUGH...WITH THE 00Z GFS
AND SEVERAL OF ITS ENSEMBLE MEMBERS ACTUALLY CLOSING OFF A
CIRCULATION AT 500MB AS THE SYSTEM MOVES ACROSS OUR AREA FRIDAY.
WRF-NAM IS MUCH WEAKER WITH THIS SYSTEM...BUT GIVEN THE
ECMWF...UKMET AND GEM SUPPORT FOR THE GFS'S MORE VIGOROUS SOLUTION
HAVE GENERALLY LEANED MUCH MORE HEAVILY ON THAT GUIDANCE CAMP FOR
THE FORECAST THIS MORNING. ALSO WOULD TEND TO EXPECT GLOBAL SPECTRAL
MODELS TO HANDLE SYSTEMS STILL OVER THE POORLY SAMPLED PACIFIC
BETTER THAN THE WRF-NAM.
ALL THAT BEING SAID...IT APPEARS AS THOUGH THE STRONG CLIPPER SYSTEM
WILL BRING SNOW TO THE AREA POSSIBLY STARTING AS EARLY AS THURSDAY
NIGHT AND LASTING THROUGH THE DAY FRIDAY. GIVEN THE PROGGED STRENGTH
OF THE SYSTEM WOULD ANTICIPATE A SWATH OF ADVISORY CRITERIA SNOWFALL
TO OCCUR TO THE NORTH OF THE TRACK OF THE SYSTEM. TOO SOON TO SAY
WHETHER THAT SWATH OF HEAVIER SNOWFALL WILL END UP OVER...JUST NORTH
OF...OR JUST SOUTH OF OUR CWA AT THIS POINT...BUT IT DOES LOOK LIKE
IT WILL BE IN OUR GENERAL NEIGHBORHOOD. GIVEN WHAT IS STILL A FAIR
AMOUNT OF UNCERTAINTY WITH THIS SYSTEM HAVE ONLY BUMPED UP POPS TO
HIGH END CHANCE FOR NOW...WHICH IS STILL CONSIDERABLY HIGHER THAN
NEIGHBORING OFFICES.
ONE EXCEPTION TO THE CHANCE POPS WAS NEAR LAKE MICHIGAN WHERE FLOW
SHOULD TURN OFF THE LAKE DURING THE DAY FRIDAY. THERMODYNAMICALLY IT
LOOKS LIKE A LAKE EFFECT SET-UP IS MARGINAL DURING THE DAY
FRIDAY...THOUGH WOULDNT BE SURPRISED TO SEE A BIT OF LAKE
ENHANCEMENT TO THE SYNOPTIC SNOWS DURING THE DAY FRIDAY. BY FRIDAY
NIGHT AS SYSTEM SNOWS PULL OUT THE GFS AND ECMWF BOTH SHOW LAKE
EFFECT SET-UP BECOMING MUCH BETTER THERMODYNAMICALLY WHILE LOW LEVEL
FLOW WOULD KEEP LAKE EFFECT MACHINE AIMED AT NE IL AND POSSIBLY FAR
NW IN FRIDAY NIGHT. FAR TOO EARLY TO START TOSSING OUT NUMBERS FOR
ACCUMS FROM LAKE EFFECT...BUT DOES APPEAR AS THOUGH THERE IS A
THREAT FOR AT LEAST SOME ACCUMULATING LAKE EFFECT FRIDAY NIGHT INTO
AT LEAST EARLY SATURDAY MORNING.
NO CHANGES MADE TO GRIDS BEYOND SATURDAY AS MODELS CONTINUE TO
STRUGGLE WITH LARGE SCALE BLOCKING PATTERN AND HOW IT EVENTUALLY
BREAKS DOWN.
IZZI
&&
.AVIATION...
06Z TAFS...NO CONCERNS...VFR THRU THE PERIOD. A LARGE RIDGE OF
HIGH PRESSURE EXTENDS FROM MINNESOTA SOUTH TO THE ARKLATEX. THIS
RIDGE WILL MOVE EAST ACROSS ILLINOIS WEDNESDAY...REACHING THE
EASTERN GREAT LAKES BY THURSDAY MORNING. WESTERLY WINDS 5 TO 10
KTS WILL SLOWLY DIMINISH TO 5 KTS OR SO BY MORNING...BECOMING
SOUTHWEST DURING THE MORNING THEN BECOMING LIGHT AND VRB BY
EVENING.
A BAND OF MID CLOUDS EXTENDS FROM CENTRAL MINNESOTA SOUTHEAST
INTO CENTRAL WISCONSIN AND IS MOVING SOUTHEAST. EXPECT SOME OF
THIS CLOUD DECK TO REACH THE TERMINALS BETWEEN 08Z-10Z...BUT FOR
SIMPLICITY WILL JUST INCLUDE SCATTERED MENTION WITH THE START OF
THIS FORECAST. THIS AREA OF MID CLOUDS WILL SHIFT EAST BY MID/
LATE WEDNESDAY MORNING LEAVING PRIMARILY CIRRUS FOR THE REMAINDER
OF THE PERIOD. CMS
&&
.MARINE...
151 AM CST
HIGH PRESSURE CENTERED OVER THE MID MISSISSIPPI VALLEY THIS
MORNING...WILL DRIFT EAST ACROSS THE WESTERN GREAT LAKES TODAY
ALLOWING WINDS AND WAVES TO DIMINISH. THE HIGH WILL CONTINUE
MOVING EAST TO THE EASTERN GREAT LAKES AND SOUTHERN ONTARIO
THURSDAY. WEAK LOW PRESSURE WILL THEN MOVE SLOWLY EAST FROM THE
MID MISSISSIPPI VALLEY ACROSS THE OHIO VALLEY FRIDAY AND SATURDAY
WHILE HIGH PRESSURE SITS OVER ONTARIO AND MANITOBA CANADA. LOW
PRESSURE WILL MOVE FROM EASTERN QUEBEC WESTWARD INTO THE GREAT
LAKES SUNDAY AND MONDAY...BRINGING INCREASING NORTHWEST WINDS TO
LAKE MICHIGAN.
ALLSOPP
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 6 AM WEDNESDAY.
&&
$$
