Blizzard in Alaska, December 21 to 23, 2011

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
520 AM AKST WED DEC 21 2011

.ANALYSIS AND UPPER LEVELS...
THE MORNING WEATHER MAP ONCE AGAIN SHOWS A LONGWAVE TROUGH CENTER
OVER THE BERING SEA WITH SOUTHERLY FLOW PREVAILING ACROSS MAINLAND
ALASKA. YESTERDAYS LOW AND FRONT THAT IMPACTED SOUTHERN ALASKA HAS
PASSED THROUGH MOST OF THE FORECAST AREA....LEAVING IN ITS WAKE
MODERATELY STRONG SOUTHERLY WINDS THROUGHOUT THE COOK INLET AND
PRINCE WILLIAM SOUND REGIONS. AN AREA OF PARTIAL CLEARING IS EVIDENT
ON SATELLITE IMAGERY MOVING FROM KODIAK ISLAND TO CENTRAL COOK INLET
BEHIND THE MAIN UPPER TROUGH MOVING THROUGH THE AREA. MEANWHILE...A
RAPIDLY INTENSIFYING LOW IS EVIDENT WELL SOUTH OF THE ALASKA
PENINSULA.

OUT WEST...THE WEAKENING UPPER LEVEL ANCHOR LOW OF THE PAST SEVERAL
WEEKS CONTINUES TO SPIN IN THE CENTRAL BERING SEA. THIS FEATURE IS
RESPONSIBLE FOR THE WIDESPREAD SNOW SHOWER ACTIVITY ACROSS THE BERING
SEA AND ALEUTIAN REGION THAT CONTINUE THIS MORNING.

.MODEL DISCUSSION...
THE MODELS HAVE COME INTO GOOD AGREEMENT ON THE GENERAL EVOLUTION OF
THE LARGE-SCALE PATTERN SHIFT UNDERWAY THIS MORNING. THE ECMWF HAD A
SLIGHT EDGE IN RUN-TO-RUN CONSISTENCY OVER THE OTHER MODELS...AND WAS
THE MODEL OF CHOICE FOR BOTH DOMAINS FOR THE SHORT TERM FORECAST. THE
LATEST HPC GUIDANCE WAS USED FOR THE EXTENDED FORECAST. ALL MODELS
NOW SHOW A PRONOUNCED COOL DOWN FOR MAINLAND ALASKA BEGINNING TODAY
AND CONTINUING THROUGHOUT THE CHRISTMAS WEEKEND.

.SHORT TERM FORECAST...
SOUTHCENTRAL...WITH WIDESPREAD PRECIPITATION FINALLY CLEAR OF MOST OF
THE AREA THE BIGGEST CHALLENGE TODAY WILL BE THE STRONG SOUTHERLY
WINDS PLAGUING MUCH OF THE AREA THIS MORNING. AN AREA OF PRESSURE
RISES IN THE WAKE OF THE FRONT THAT WENT THROUGH OVERNIGHT IS
CURRENTLY MOVING UP COOK INLET. THUS EXPECT GUSTY WINDS TO CONTINUE
THROUGH THE MORNING HOURS...SLOWLY TAPERING OFF BY NOON IN MOST
AREAS. THE REGION WILL CATCH A BREATHER THIS AFTERNOON THROUGH
THURSDAY AFTERNOON AS THE OVERALL STORM TRACK SHIFTS EASTWARD INTO
THE GULF OF ALASKA. THE FIRST BIG LOW TO MOVE THROUGH THIS NEW
PATTERN WILL BE A WEAKENING NORTH PACIFIC LOW THAT NOW APPEARS THAT
IT WILL MOVE UP TOWARD PRINCE WILLIAM SOUND THURSDAY NIGHT. WHILE
THERE IS STILL SOME UNCERTAINTY WITH HOW FAR NORTH AND WEST THE LOW
WILL MOVE THIS HAS THE POTENTIAL TO CREATE SIGNIFICANT SNOWFALL IN
THE ANCHORAGE BOWL AND MATANUSKA VALLEY. SUBSEQUENT MODEL RUNS WILL
BE MONITORED CLOSELY AS ANY EASTWARD DEVIATION IN THE LOW TRACK WOULD
HAVE A DRAMATIC IMPACT ON SNOWFALL POTENTIAL FOR THE ANCHORAGE AREA.

SOUTHWEST...SNOW SHOWERS PERSISTING IN THE IMMEDIATE VICINITY OF THE
LOW CENTER CURRENTLY NORTH OF BETHEL. THESE SHOULD BE WINDING DOWN
THIS MORNING...KICKING OFF A PRONOUNCED DRYING AND COOLING TREND FOR
THE SOUTHWEST MAINLAND. SKIES WILL BECOME PARTLY CLOUDY TONIGHT AND
TEMPERATURES WILL BEGIN TO SLOWLY COOL OFF THROUGH THE END OF THE
WEEK. THE FLOOR FALLS OUT THIS WEEKEND AS BITTERLY COLD AIR RETURNS
TO THE AREA BY CHRISTMAS DAY.

BERING SEA AND ALEUTIANS...RELATIVELY QUIET WEATHER WILL CONTINUE AS
THE MAIN STORM TRACK REMAINS TO THE SOUTH OF THE ALEUTIAN ISLANDS.
THE COOL AIRMASS AROUND THE REGION WILL SEE SEVERAL REINFORCING SHOTS
OF COLD AIR...ESPECIALLY HEADING INTO THE WEEKEND. THE ONLY ORGANIZED
LOW THAT THREATENS THE AREA MOVES UP FROM THE SOUTH AND CLIPS THE
CENTRAL AND EASTERN ALEUTIANS WITH SOME MODERATELY GUSTY WINDS AND A
BIT OF SNOW FOR THE DAY THURSDAY. OTHERWISE...EXPECT ISOLATED TO
SCATTERED SNOW SHOWERS TO CONTINUE FOR MOST AREAS FOR AT LEAST THE
NEXT FEW DAYS DUE TO COLD AIR MOVING OVER THE RELATIVELY WARM BERING
SEA WATER.

.LONG TERM FORECAST... AS ADVERTISED A PATTERN SHIFT FOR THE ALASKAN
DOMAIN COMMENCES TODAY AS CLOUDY SKIES BEGIN TO BREAK AND COOLER AIR
BEGINS TO TRICKLE BACK INTO THE MAINLAND. IT LOOKS LIKE SEASONABLY
COOL WILL BECOME BITTERLY COLD FOR THE MAINLAND BY THE END OF THE
WEEKEND AS A VERY COLD UPPER LOW MOVES DOWN FROM THE ARCTIC AND TAKES
UP RESIDENCE OVER THE WESTERN COAST OF ALASKA.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...HEAVY SNOW WARNING 145
MARINE...GALE 120 130 132 155 170 172 185
         HEAVY FREEZING SPRAY 185
FIRE WEATHER...NONE

AMD DEC 11