Blizzard in Alaska, December 29 to 30, 2010

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ANCHORAGE AK
600 AM AKST WED DEC 29 2010

...UPDATED TO ADD MENTION OF WIND CHILLS IN KUSKOKWIM DELTA ZONE...

.MODEL DISCUSSION...

MODELS ARE CONTINUING TO STRUGGLE AS USUAL WITH MAJOR WEATHER PATTERN
CHANGES. MOST OF THE MAJOR DIFFERENCES CENTER AROUND THE DEVELOPMENT
OF SURFACE LOWS DEVELOPING ALONG THE STRONG EAST-WEST ORIENTED WARM
FRONTAL BOUNDARY THAT IS SITUATED FROM THE CENTRAL ALEUTIANS TO JUST
SOUTH OF THE GULF OF ALASKA. THE EUROPEAN (EC) IS AN OUTLIER IN
DEVELOPING A TRIPLE POINT LOW ABOUT ONE HUNDRED MILES SOUTHEAST OF
DUTCH HARBOR EARLY THIS AFTERNOON AND MOVING IT NORTH AND EAST ALONG
THIS BOUNDARY INTO BRISTOL BAY BY THURSDAY MORNING. THE EC THEN
DEVELOPS A SECOND TRIPLE POINT LOW SEVERAL HUNDRED MILES SOUTH OF THE
ALASKA PENINSULA LATE TONIGHT AND MOVES IT TO NEAR KODIAK ISLAND AT
976 MB THURSDAY EVENING. THE GFS AND NAM DEVELOP JUST ONE LOW SOUTH
OF THE ALASKA PENINSULA...AND THEN MOVES IT NORTH AND EAST TO NEAR
THE EC POSITION AT 970 MB BY THURSDAY EVENING. THE EC HAS BEEN THE
MORE CONSISTENT MODEL FROM RUN TO RUN...AND ALSO FEATURES A SUPERIOR
DATA ASSIMILATION SCHEME WHICH CERTAINLY HELPS WITH SUCH A COMPLEX
WEATHER SYSTEM. THUS...THE EC CONTINUES TO BE THE PREFERRED MODEL FOR
THIS FORECAST PACKAGE THROUGH AT LEAST SATURDAY AFTERNOON. THE NAM
WAS USED FOR THE FIRST 36 HOURS DUE TO GOOD AGREEMENT WITH THE EC ON
THE EAST SIDE BEFORE TIMING AND PLACEMENT ISSUES WITH LOW PRESSURE
SYSTEMS ARISE.

.ANALYSIS AND UPPER LEVELS...

AN UPPER LEVEL LOW CONTINUES TO DIG SOUTH AND WEST FROM THE
NORTHWESTERN MAINLAND TOWARDS THE WESTERN BERING OVER THE NEXT COUPLE
OF DAYS. SEVERAL VIGOROUS DISTURBANCES WILL MOVE FROM THE NORTHWEST
PACIFIC NEAR JAPAN NORTHEASTWARD TO THE GULF OF ALASKA AND THEN INTO
THE SOUTHERN MAINLAND...BRINGING A VERY ACTIVE WEATHER PATTERN TO THE
STATE THROUGH AT LEAST THE BEGINNING OF NEXT WEEK. THE STRONG
WEST-EAST WARM FRONTAL BOUNDARY FROM THE CENTRAL ALEUTIANS TO THE
SOUTHERN GULF WILL SERVE AS THE FOCAL POINT FOR SURFACE LOWS TO
DEVELOP AS THESE DISTURBANCES MOVE NORTH AND EAST TOWARDS THE WESTERN
GULF AND SOUTHCENTRAL ALASKA.

.SHORT TERM FORECAST...

SOUTHCENTRAL...

BASED ON KENAI RADAR IMAGERY AND THE MODEL QPF FIELD...THE NAM
CONTINUES TO HAVE A VERY GOOD HANDLE ON THE CURRENT SNOW EVENT ACROSS
THE UPPER COOK INLET REGION. AN INVERTED TROUGH AT THE SURFACE IS
INTERACTING WITH A VIGOROUS DISTURBANCE ALOFT TO PRODUCE STEADY LIGHT
SNOW. THE SNOW WILL CONTINUE THROUGH MID MORNING BEFORE SLOWLY
DISSIPATING INTO EARLY AFTERNOON WITH ACCUMULATIONS OF ABOUT 2 TO 4
INCHES IN THE ANCHORAGE BOWL. THE DISTURBANCE WILL SLOWLY SHIFT NORTH
THROUGH THE DAY AND BRING STEADY LIGHT SNOW TO THE UPPER SUSITNA
VALLEY AS WELL AS THE PRINCE WILLIAM SOUND REGION LATER THIS
AFTERNOON INTO THIS EVENING.

ATTENTION THEN TURNS TO THE NEXT SYSTEM...WHICH WILL ARRIVE INTO
SOUTHCENTRAL BY EARLY THIS EVENING. IT APPEARS THERE WILL BE AN
INITIAL SURGE OF SNOW UP THE INLET WITH GENERALLY LIGHT ACCUMULATIONS
BEFORE A PERIOD OF SOUTHEASTERLY FLOW AND DOWNSLOPING WILL GENERALLY
KEEP CEILINGS HIGHER OVER WEST-FACING SLOPES (SUCH AS THE ANCHORAGE
AREA) AND TEMPERATURES RISING THROUGH THE OVERNIGHT HOURS TONIGHT. AT
THE SAME TIME...THIS SOUTHEASTERLY FLOW WILL INDUCE A STRONG UPSLOPE
COMPONENT ACROSS THE NORTHWESTERN SUSITNA VALLEY...WHICH SHOULD HELP
TO PRODUCE PERIODS OF MODERATE TO LOCALLY HEAVY SNOWFALL BEGINNING
SOMETIME LATE TONIGHT OR THURSDAY MORNING AND PERSISTING THROUGH MOST
OF THE DAY. THIS MAY EVENTUALLY REQUIRE A WINTER WEATHER ADVISORY FOR
SNOW AT SOME POINT...BUT TIMING THE ARRIVAL OF THE SNOW IS SOMEWHAT
PROBLEMATIC DUE TO THE LARGE DIFFERENCES BETWEEN THE EC...NAM...AND
THE OUTLIER GFS MODEL SOLUTION. ALONG THE COAST...EXPECT TO SEE WARM
ENOUGH TEMPERATURES FOR SNOW TO BECOME RAIN AS THE STRONG FRONT
MOVING THROUGH THE GULF MOVES INLAND LATE TONIGHT OR EARLY TOMORROW.

MAIN AREAS OF CONCERN FOR HIGH IMPACT WEATHER CONDITIONS ALONG THE
COAST ARE:  1) THE PORTAGE AREA...AS IT APPEARS THE SNOW COULD BE
ACCOMPANIED BY BLIZZARD CONDITIONS INITIALLY BEFORE THE WARMER AIR
ARRIVES AND CHANGES THE SNOW OVER TO RAIN BY MID THURSDAY AFTERNOON.
WILL ISSUE A BLIZZARD WATCH WITH THE MORNING PACKAGE FROM THIS
EVENING THROUGH THURSDAY AFTERNOON FOR AREAS NORTH OF MOOSE PASS. 2)
THOMPSON PASS...WHERE A TIGHTENED PRESSURE GRADIENT AND A PERIOD OF
HIGHER INTENSITY SNOW COULD HELP PRODUCE BLIZZARD CONDITIONS ANYTIME
BETWEEN LATE THIS AFTERNOON AND LATE WEDNESDAY NIGHT. CONFIDENCE IS
NOT AS HIGH IN REACHING BLIZZARD CONDITIONS THERE BUT WILL USE STRONG
WORDING FOR VISIBILITIES REDUCED TO NEAR ONE HALF MILE AT TIMES. WILL
LEAVE TO THE DISCRETION OF THE DAY CREW WHETHER TO ISSUE AN
ADVISORY(FOR BLOWING SNOW) OR WARNING (OR ISSUE NOTHING) AS THE EVENT
DRAWS CLOSER.

SOUTHWEST MAINLAND...

MAIN FORECAST CONCERN IS HOW MUCH SNOW AND/OR FREEZING RAIN MIGHT
FALL ACROSS THE ALASKA PENINSULA...BRISTOL BAY AND KUSKOKWIM VALLEY
ZONES THROUGH THE NEXT FEW DAYS. IF THE PREFERRED EC MODEL PANS
OUT...SNOW AMOUNTS COULD APPROACH ADVISORY CRITERIA FOR PORTIONS OF
THE MOUNTAINS...ESPECIALLY SOUTHEAST FACING SLOPES OF THE AHKLUN AND
KUSKOKWIM MOUNTAIN RANGES. FREEZING RAIN MAY BE A PROBLEM AS EARLY AS
THIS MORNING ALONG THE ALASKA PENINSULA AS THE FRONTAL BOUNDARY WORKS
ITS WAY NORTH. STRONG OVERRUNNING JUST NORTH OF THE FRONTAL BOUNDARY
WITH A SHALLOW BUT STRONG COLD POOL NEAR THE SURFACE COULD HELP
PRODUCE ISOLATED POCKETS OF FREEZING RAIN THAT COULD BECOME MORE OF A
CONCERN ACROSS THE BRISTOL BAY ZONE DURING THE DAY ON THURSDAY.

OTHERWISE...PERSISTENT NORTH WINDS OF 15 TO 30 MPH WITH VERY COLD
TEMPERATURES ON THE ORDER OF 5 BELOW TO 20 BELOW HAS NECESSITATED THE
ISSUANCE OF A WIND CHILL ADVISORY FOR THE KUSKOKWIM DELTA ZONE
THROUGH THURSDAY MORNING FOR WIND CHILLS OF 30 BELOW TO 50 BELOW.
TEMPERATURES WILL LIKELY MODERATE SOME ON THURSDAY...BUT THERE IS A
CHANCE THAT TEMPERATURES COULD DROP ONCE AGAIN THURSDAY NIGHT AND
BRING ANOTHER ROUND OF ADVISORY LEVEL WIND CHILLS TO THE ZONE.

BERING SEA/ALEUTIANS...

THE BLIZZARD WARNING FOR THE PRIBILOFS WAS MOVED UP LATE LAST EVENING
AS IT APPEARS HEAVIER PRECIPITATION WILL REACH SAINT GEORGE BY EARLY
THIS MORNING. AS THE INTENSITY OF THE SNOW INCREASES...EXPECT
VISIBILITIES TO GO DOWN TO NEAR ZERO THIS MORNING AND STAY THAT WAY
THROUGH EARLY THURSDAY AFTERNOON. ELSEWHERE...EXPECT RAIN MOST PLACES
AS THE FRONT HAS MOVED ACROSS UNALASKA ISLAND AND INITIATED THE
CHANGEOVER THERE...AND WILL DO THE SAME THIS MORNING ACROSS THE
SOUTHERN ALASKA PENINSULA...WITH PERHAPS A BRIEF PERIOD OF FREEZING
RAIN BEFORE TOTAL CHANGEOVER TO RAIN.

THE STRONG LOW SOUTH OF THE CENTRAL ALEUTIANS THIS MORNING WILL MOVE
EAST AND WEAKEN THURSDAY SOUTHEAST OF DUTCH HARBOR. ON THE BACK SIDE
COLD ADVECTION SNOW SHOWERS WILL PERSIST ACROSS MOST OF THE ISLAND
CHAIN THROUGH THE REST OF THE WEEK...WITH PERIODS OF MORE STEADY SNOW
OVER THE OPEN WATERS OF THE BERING AND THE PRIBILOFS WHERE WARM
ADVECTION WILL CONTINUE BETWEEN THE UPPER TROUGH OVER THE WESTERN
BERING AND THE SURFACE LOW TOWARDS THE ALASKA PENINSULA.

.LONG TERM FORECAST...

THE ACTIVE WEATHER PATTERN LOOKS TO CONTINUE THROUGH THE EXTENDED
PERIOD...BUT WITH LOTS OF MODEL UNCERTAINTY AT THIS POINT WILL KEEP
FORECASTS SOMEWHAT TEMPERED A BIT TOWARDS CLIMATOLOGY AS DEPICTED BY
HPC GUIDANCE. TEMPERATURES LOOK TO AVERAGE ABOVE NORMAL ACROSS
SOUTHCENTRAL WITH ABOVE NORMAL PRECIPITATION FOR NEXT WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...WIND CHILL ADVISORY...155
         BLIZZARD WATCH...125
         BLIZZARD WARNING...195
MARINE...GALES...120 125 130 140 172 179 180 185
         STORMS...175 176
         HVY FRZG SPRY...140 160 165 179 185
FIRE WEATHER...NONE.

CC DEC 10