Blizzard and Heavy Snow in Alaska, December 20 to 21, 2011

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
530 AM AKST TUE DEC 20 2011

.ANALYSIS AND UPPER LEVELS...
NO SIGNIFICANT CHANGES IN THE UPPER LEVEL PATTERN SINCE
YESTERDAY...WITH LONGWAVE TROUGH ENTRENCHED OVER THE BERING SEA AND
BROAD SOUTHERLY FLOW ACROSS MAINLAND ALASKA. A WEAK LOW MOVING
THROUGH THE PATTERN MOVED NORTHWARD UP COOK INLET OVERNIGHT BRINGING
A QUICK SHOT OF SEVERAL INCHES OF SNOW TO THE WESTERN
KENAI...ANCHORAGE...AND THE MATANUSKA VALLEY. THIS SYSTEM IS RAPIDLY
FALLING APART AS IT RACES NORTHWARD AND OUT OF THE AREA. ALL EYES ARE
NOW TURNING TO THE MUCH STRONGER LOW CROSSING INTO THE EASTERN BERING
SEA THIS MORNING WITH AN ASSOCIATED FRONT APPROACHING THE EASTERN
GULF. SATELLITE IMAGERY SHOWS THE SYSTEM HAS LOST JET SUPPORT AND IS
OCCLUDED...SUGGESTING THAT THE LOW IS PROBABLY AT OR NEAR MAX
INTENSITY. FURTHER WEST...A POOL OF VERY COLD AIR ALOFT HAS STEEPENED
LAPSE RATES AND IS HELPING GENERATE SOME MODERATE TO HEAVY SNOW
SHOWERS OVER THE WESTERN AND CENTRAL ALEUTIANS.

.MODEL DISCUSSION...
MODELS ARE IN GOOD AGREEMENT ON BRINGING THE MAIN LOW CURRENTLY NEAR
DUTCH HARBOR TO THE NORTHEAST AND ONSHORE NEAR TOGIAK AROUND 3PM THIS
AFTERNOON. SIMILARLY...ALL MODELS AGREE ON THE BASICS OF THE
ASSOCIATED FRONT MOVING TOWARD THE NORTH GULF COAST...CREATING HIGH
WIND AND BLIZZARD ISSUES FROM PRINCE WILLIAM SOUND OVER TO THE
ANCHORAGE AREA. BY WEDNESDAY MORNING THIS LOW WILL A DISTANT MEMORY
AS THE LONGWAVE PATTERN CHANGE BEGINS IN EARNEST. THIS IS WHERE THE
MODELS START TO BREAK DOWN SIGNIFICANTLY...ESPECIALLY OVER THE BERING
SEA. A GENERAL BLEND TOWARD THE GFS AND ECMWF WAS USED TO UPDATE THE
SHORT TERM FORECAST...WITH SOME OF THE CANADIAN GEM MODEL USED IN THE
BERING REGION. FOR DAYS 3-7 HPC GUIDANCE WAS USED EXCLUSIVELY DUE TO
VERY LOW CONFIDENCE IN ANY PARTICULAR MODEL SOLUTION.

.SHORT TERM FORECAST...
SOUTHCENTRAL...SNOW TAPERING OFF EARLY THIS MORNING ACROSS THE
MATANUSKA AND SUSITNA VALLEYS WITH LITTLE ADDITIONAL ACCUMULATION
EXPECTED. A BREAK IN THE WEATHER WILL BE EXTREMELY SHORT LIVED AS
CLOUDS AND WINDS INCREASE IN EARNEST BY THIS AFTERNOON FOR MANY
AREAS. AS MENTIONED EARLIER THIS SYSTEM HAS OCCLUDED...THUS AM
EXPECTING MOST AREAS TO REMAIN SNOW AS THE FRONT IMPACTS THE AREA
THIS EVENING. HAVE ISSUED A BLIZZARD WARNING FOR PORTAGE VALLEY AND
EASTERN TURNAGAIN ARM AS THE INTENSITY OF THE SNOWFALL SHOULD BE
ENOUGH TO LOWER VISIBILITIES TO 1/4 MILE EVEN IF NEAR-FREEZING
TEMPERATURES LIMIT HOW MUCH THE SNOW IS ACTUALLY PICKED UP AND BLOWN
BY THE WIND. CONFIDENCE IS ALSO HIGH THAT THOMPSON PASS WILL SEE
PLENTY OF WIND AND SNOW...THUS BLIZZARD WATCH HAS BEEN UPGRADED TO A
WARNING. IN ADDITION...BROAD SOUTHERLY FLOW ALOFT SHOULD ALLOW MOST
PARTS OF THE SUSITNA VALLEY TO SEE SIGNIFICANT SNOW TOTALS FROM THIS
STORM...MORE THAN JUST THE NORMAL AREAS WEST OF THE PARKS HIGHWAY.
THUS HAVE ISSUED A WINTER WEATHER ADVISORY FOR SNOW STARTING THIS
EVENING AND RUNNING THROUGH WEDNESDAY MORNING. MEANWHILE...STILL SOME
UNCERTAINTY IN THE ANCHORAGE AREA HOW MUCH STRONG WINDS AT HIGHER
ELEVATIONS WILL MAKE IT DOWN TO THE LOWER HILLSIDE AND EAST
ANCHORAGE. AT THIS POINT IT STILL LOOKS LIKE SOME SIGNIFICANT WINDS
WILL IMPACT THESE AREAS...BUT WIND SPEEDS SHOULD BE COMPARABLE OR
SLIGHTLY LOWER THAN THE STORM THIS PAST SUNDAY. HOWEVER...AS THE
FRONT CLEARS THE AREA AROUND MIDNIGHT TONIGHT WINDS SHOULD SHIFT TO
THE SOUTH AND PICK UP TO NEAR 50 MPH ACROSS ALL OF THE ANCHORAGE BOWL
BEFORE GRADUALLY DIMINISHING.

SOUTHWEST...LOW STILL ON TRACK TO MAKE LANDFALL NEAR TOGIAK THIS
AFTERNOON. SHOULD SEE A PERIOD OF STRONG WINDS AND HEAVY SNOW AHEAD
OF THE FRONT IN BRISTOL BAY DURING THE DAY WHICH SHOULD CREATE SOME
LOCALIZED BLIZZARD CONDITIONS INLAND. AS SUCH...THE BLIZZARD WATCH
HAS BEEN UPGRADED TO A WARNING WITH THE MORNING FORECAST PACKAGE.
WILL PROBABLY ALSO HAVE SOME BLOWING SNOW ISSUES ACROSS PARTS OF THE
KUSKOKWIM DELTA AS WELL...THOUGH BLIZZARD CONDITIONS ARE NOT EXPECTED
AT THIS TIME. ONCE THE CURRENT LOW MOVES OUT OF THE AREA LATE TONIGHT
THE AREA SHOULD CATCH A BREAK FROM THE SIGNIFICANT WEATHER THROUGH AT
LEAST THE WEEKEND AS THE LONGWAVE PATTERN SHIFT GETS UNDERWAY.

BERING SEA AND ALEUTIANS...THE LOW MOVES THROUGH THE EASTERN BERING
TODAY AND SPREADS MAXIMUM GALE-FORCE WINDS THROUGHOUT THE AREA BEFORE
WINDS DROP AND CONDITIONS IMPROVE TONIGHT. WIDESPREAD SNOW SHOWERS
WILL PERSIST ACROSS THE REGION AS COLD AIR ALOFT CREATES SOME PRETTY
IMPRESSIVE ATMOSPHERIC LAPSE RATES. THE NEXT WEATHER-MAKER FOR THE
REGION POTENTIALLY MOVES UP TOWARD THE CENTRAL ALEUTIANS ON
THURSDAY...THOUGH THERE REMAINS CONSIDERABLE MODEL DISAGREEMENT ON
HOW FAR NORTH THE STORM WILL MAKE IT.

.LONG TERM FORECAST...
AFTER THIS STORM THE WELL-ADVERTISED PATTERN SHIFT WILL BEGIN TO TAKE
SHAPE. PLENTY OF DETAILS REMAIN TO BE WORKED OUT STARTING THIS
WEEKEND...HOWEVER THE GENERAL TREND AMONG ALL THE MODELS IS A RETURN
TO SEASONABLY COOL TEMPERATURES ACROSS ALL PARTS OF THE
MAINLAND...WITH THE POTENTIAL FOR DRAMATICALLY COLDER TEMPERATURES
ARRIVING BY EARLY NEXT WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...HIGH WIND WARNING 101
         BLIZZARD WARNING 125 131 161
         SNOW ADVISORY 145
MARINE...STORM 120 150 155
         GALE 125 129 130 132 138 140 160 165 170
         HEAVY FREEZING SPRAY 185
FIRE WEATHER...NONE

AMD DEC 11