Heavy Snow in Alaska, December 20 to 21, 2005

SOUTHEAST ALASKA FORECAST DISCUSSION
1100 AM AST TUE DEC 20 2005

.SYNOPSIS...
.SHORT RANGE...DEEP LOW HAS HAS LEAST 2 CENTERS. LEADING TRIPLE
POINT LOW ALREADY INTO EASTERN GULF AND NOT HANDLED BY PROGS.
PARENT 965 MB LOW...ABOUT 4 MB DEEPER THAN PROGS...LIFTING NORTH
FROM WEST COAST CHARLOTTES INTO THE SOUTHEAST GULF THIS EVENING
THEN INTO CENTRAL GULF BY WEDNESDAY MORNING AND FILLING TO AROUND
970 MB. THE ASSOCIATED OCCLUSION WRAPPED COMPLETELY AROUND LOW
NOW PUSHING ALOFT OVER NORTHERN PANHANDLE. SECOND WRAP GAINING
STRENGTH OF QUEEN CHARLOTTE ISLANDS AND FORECAST TO SLAM INTO
SOUTHERN THIS EVENING AND RUN NNW ALONG OUTER COAST TO NEAR SITKA
BY MIDNIGHT. SINCE FRONT ALOFT PAGY WILL LIKELY MAINTAIN BRISK N
WINDS AND WILL EXTEND WIND ADVISORY INTO TONIGHT.

LEADING OCCLUSION ONLY PRODUCING A FEW HUNDRETHS OF PRECIP DUE TO
STRONG EASTERLY COMPONENT TO FLOW...HOWEVER DEEP WARM LAYER ALOFT
RESULTING IN POCKETS OF FREEZING RAIN AND SLEET OVER NORTHERN
PANHANDLE THIS MORNING. THE EVENT SHOULD BE SHORT LIVED AS IT
MOVES THROUGH PAYA-PAHN AND PAGY MIDDAY AND SHOULD BREAK UP
INVERSION.

SECOND WRAP DIFFERENT STORY. STRONG 60-75 KT LOW LEVEL JET MOVING
INTO SOUTHERN ZONES THIS EVENING WILL LIKELY INCREASE WINDS TO
STORM IN CLARENCE AND WARNING LEVELS WITH PEAK GUSTS AROUND 75
MPH FOR PUBLIC. HAVE ISSUED HWA THAT WILL PROBABLY BE UPGRADED TO
A HHW WITH AFTERNOON ZONES. RAIN WITH THIS WILL BE HEAVY AT TIMES
AND TOTAL 1-3 INCHES OVERNIGHT FOR SOUTHERN.

WINDS AND RAIN WILL SPREAD NORTH THROUGH INNER CHANNELS TONIGHT
BUT SHOULD WEAKEN TO GALES AND WIND ADVISORY FOR REMAINING INNER
CHANNELS ZONES LATER TONIGHT AND FOR FAR NORTHERN ZONES WEDNESDAY
MORNING. ONE BIG QUESTION THAT REMAINS IS WEATHER OR NOT UPPER
LEVELS WILL COOL ENOUGH BEHIND FIRST OCCLUSION TO SUPPORT SNOW
FOR NORTHERN LYNN CANAL TONIGHT. FOR NOW I AM LEANING TOWARDS A
FEW INCHES OF SNOW UNTIL SOUTHERLIES ARRIVE.

.LONG RANGE...WARM AND GENERALLY WET.  WARM MARITME ONSHORE
FLOW WITH WARM BOUNDARY LAYER WILL HAVE LITTLE COOLER AIR TO
OVERRUN THROUGH CHRISTMAS WEEKEND EXCEPT FOR THE VICINITIES OF
YAKUTAT...HAINES...AND SKAGWAY.  SYNOPTIC SCALE CIRCULATION
AROUND LOW ALOFT OVER CHAIN AND AK PEN IS HIGHLY PERSISTENT
ALONG WITH RIDGE ALOFT NW CAN. THE RIDGE ALOFT STAYS TOO FAR
E OF OUR FORECAST AREA TO SUPPORT THE TRACK OF ARCITC HIGHS
NEAR OUR FORECAST DOMAIN.

GREATEST MODEL SOLUTION SPREAD IS SEEN FOR SAT...THE GFS
SOLNS CHANGING FROM RUN TO RUN AND ENSEMBLE SPREAD IS LARGE
AND UNIFORM OVER PANHANDLE...SOLNS DO GEN HAVE HAVE
A LOW TRACKING NORTH ALONG THE CHARLOTTES 12Z SAT BUT
VARIETY OF PRESSURE CENTERS.  CONSENSUS FOR THIS LOW IS HARD
TO REACH WITH DETERMINISTIC GUIDANCE SO CLIMATOLOGY WEIGHED IN
PROGRESSIVELY FROM FRIDAY ONWARD.  BEST ESTIMATE IS LOW
TO TRACK OVER KETCHKAN BEFORE FILLING AND REDEVELOPING IN
NORTHERN B.C. SUN...WITHOUT PULLING AN ARCTIC AMS INTO THE
PANHANDLE EXCEPT POSSIBLY FOR N AND W OF HAINES AND SKAGWAY
SAT PM.

.AJK...WATCHES/WARNINGS/ADVISORIES
PUBLIC...WSW FOR FZRA 20 AND 25 UNTIL NOON...18-19 TIL 3 PM NUMEROUS WIND
ADVISORY THROUGH TONIGHT HWA 6PM-6AM 23, 27, AND 28
MARINE...GALE COASTAL AND SOUTHERN INNER CHANNELS TODAY STORM 36, 41,
42..GALES REMAINDER TONIGHT GALES COASTAL AND NORTH CHANNELS ENDING IN
MORNING

MJM JT