Blizzard and High Wind in Alaska, November 3, 2011

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
345 PM AKDT WED NOV 2 2011

.ANALYSIS AND UPPER LEVELS...UPPER LEVEL PATTERN IS PROGRESSIVE BUT
AMPLIFIED. WATER VAPOR IMAGERY SHOWS A STRONG RIDGE EXTENDING FROM
THE NORTH CENTRAL PACIFIC UP INTO THE EASTERN BERING WITH HIGH
AMPLITUDE TROUGHS ON EITHER SIDE OF IT. THE SUBTROPICAL JET RIDES
UP AND OVER THIS RIDGE WITH THE STRONGEST WINDS ON THE DOWNSTREAM
SIDE. THIS IS CAUSING THE LOW THAT AFFECTED SOUTHCENTRAL AND THE
GULF YESTERDAY TO MAKE A RAPID EXIT ACROSS THE NORTHEAST PACIFIC.
THE PRESSURE GRADIENT REMAINS QUITE TIGHT ACROSS SOUTHCENTRAL...
PARTIALLY DUE TO ARCTIC AIR ADVECTING ACROSS THE COASTAL MOUNTAIN
RANGES CAUSING LOWER PRESSURE ON THE LEE SIDE. SYNOPTIC SUBSIDENCE
ON THE POLEWARD SIDE OF THE SUBTROPICAL JET AND MORE FOCUSED
SUBSIDENCE WITH A SHORT-WAVE DROPPING THROUGH SOUTHCENTRAL THIS
MORNING ARE CONTRIBUTING TO MIXING DOWN OF STRONGER WINDS FROM
ALOFT. SO AFTER WINDS DIMINISHED A BIT OVERNIGHT THEY HAVE INCREASED
THIS MORNING IN MANY LOCATIONS.

THE NEXT MAJOR SYSTEM IS BEGINNING TO TAKE SHAPE OUT ACROSS THE
WESTERN ALEUTIANS AND BERING SEA. A MODERATE STRENGTH LOW APPROACHING

THE WESTERN ALEUTIANS HAS BECOME VERY ELONGATED DUE TO THE STRONG
SOUTHERLY JET JUST TO ITS EAST. EXPECT TO SEE A TRIPLE POINT LOW
DEVELOP THIS AFTERNOON WELL NORTH OF THE PARENT LOW AND STRENGTHEN
WITH THE HELP OF THE UPPER JET.

.MODEL DISCUSSION...THERE ARE SOME MINOR POSITION DIFFERENCES WITH
TRACK OF LOWS WHICH WILL MAKE LITTLE DIFFERENCE TO THE FORECAST.
OTHERWISE...MODELS ARE IN VERY GOOD AGREEMENT AND SHOW GOOD
RUN-TO-RUN CONSISTENCY. FORECAST CONFIDENCE IS ABOVE AVERAGE
FOR THIS NEXT STORM SYSTEM.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...WINDS WILL DIE DOWN RAPIDLY ACROSS MOST OF
SOUTHCENTRAL ALASKA THIS AFTERNOON AND EVENING AS HIGH PRESSURE
BUILDS IN FROM THE WEST...HOLDING ON LONGEST ALONG COASTAL LOCATIONS.

A POWERFUL BERING SEA STORM WILL SEND A STRONG FRONT INTO THE GULF
AND SOUTHCENTRAL DURING THE DAY THURSDAY SO EXPECT CONDITIONS TO
RAPIDLY DETERIORATE. THE GREATEST IMPACT FROM THIS STORM WILL BE
STRONG WINDS. THE COMBINATION OF A STRONG FRONT AND TRACK OF THE
BERING SEE LOW TO SOUTHWEST ALASKA IT IS A CLASSIC SET-UP FOR HIGH
WINDS ALONG WESTERN TURNAGAIN ARM AND HIGHER ELEVATIONS THURSDAY
AFTERNOON AND NIGHT...SO WILL ISSUE A HIGH WIND WARNING WITH THE
NEW FORECAST PACKAGE. ALTHOUGH WINDS ARE EXPECTED TO REMAIN BELOW
WARNING CRITERIA OVER THE REST OF THE LAND AREA...WIDESPREAD GALES
WILL AFFECT THE MARINE AREAS. WITH A COLD AIR MASS IN PLACE
PRECIPITATION WILL START AS SNOW IN MOST AREAS. THE COMBINATION OF
SNOW WITH INCREASING WINDS WILL RESULT IN AREAS OF LOW VISIBILITIES
IN BLOWING SNOW. HOWEVER...THE STRONGEST WINDS WILL OCCUR IN COASTAL
LOCATIONS WHERE TEMPERATURES WILL WARM UP THE QUICKEST...SO EXPECT
THESE LOW VISIBILITIES TO BE SHORT-LIVED. THE WARMER AIR WILL ALSO
CHANGE SNOW TO RAIN ALONG THE COAST. THE SUSITNA VALLEY HOWEVER WILL
REMAIN ALL SNOW AND STRONG SOUTH TO SOUTHEAST LOW LEVEL WINDS WILL
ENHANCE THE SNOWFALL OVER PORTIONS OF THE VALLEY. A WINTER WEATHER
ADVISORY WILL BE ISSUED TO HIGHLIGHT THIS THREAT. THERE WILL BE SOME
INITIAL OVERRUNNING AHEAD OF THE FRONT WHICH COULD BRING SOME LIGHT
SNOW TO ANCHORAGE AND THE MAT VALLEY...BUT EXPECT STRONG SOUTHEAST
WINDS TO RAPIDLY DEVELOP AND CAUSE DOWNSLOPE DRYING. SO EXPECT
LITTLE OR NO SNOW ON THE LEE SIDE OF THE CHUGACH AND KENAI MOUNTAINS.

A TRIPLE POINT LOW WILL DEVELOP NORTHEAST OF KODIAK ISLAND THURSDAY
NIGHT WHICH WILL CAUSE LOW LEVEL FLOW TO BACK OVER INTERIOR
SOUTHCENTRAL AND DIMINISH SNOWFALL OR IN SOME AREAS CUT IT OFF
COMPLETELY. AS HAS OCCURRED WITH THE LAST COUPLE SYSTEMS IN THIS
AMPLIFIED FLOW ARCTIC AIR WILL MOVE IN BEHIND THE TRIPLE POINT
LOW BRINGING STORM FORCE WINDS TO THE WESTERN GULF AND LIKELY SOME
SNOW TO THE REST OF SOUTHCENTRAL AS WE HEAD INTO THE WEEKEND.

SOUTHWEST ALASKA...STRONG WINDS AND MODERATE TO HEAVY PRECIPITATION
WILL ARRIVE ALONG THE SOUTHWEST COAST LATE THIS EVENING INTO THE
OVERNIGHT HOURS AND SPREAD INLAND THURSDAY. PRECIP WILL INITIALLY
START AS SNOW WHICH WILL CAUSE BLOWING SNOW ISSUES. WARMER AIR WILL
QUICKLY MOVE INTO THE BRISTOL BAY COASTAL REGION SHORTLY AFTER PRECIP
ARRIVES WHICH WILL WARM TEMPERATURES TO NEAR FREEZING AND PREVENT
BLIZZARD CONDITIONS. THE WARMER AIR WILL TAKE LONGER TO REACH THE
KUSKOKWIM DELTA COASTAL AREA...ESPECIALLY THE NORTHERN COAST UP TO
AROUND HOOPER BAY. THEREFORE...PLAN TO UPGRADE TO A BLIZZARD WARNING
FOR THE COAST. FOR INTERIOR SOUTHWEST ALASKA THERE WILL BE A AS MUCH
AS 9 TO 12 HOURS OF SNOW ACCUMULATION PRIOR TO ENOUGH WARM AIR MOVING
INTO TO CAUSE MIXING WITH RAIN OR A COMPLETE CHANGEOVER SO WILL
INCREASE TO ADVISORY AMOUNTS FOR THESE AREAS (6 INCHES OR MORE).
STRONG WINDS WILL ALSO MAKE IT INTO INTERIOR BRISTOL BAY AHEAD OF
THE FRONT. WITH TEMPERATURES EXPECTED TO HOVER NEAR FREEZING FOR
MUCH OF THIS TIME WILL GO WITH A WINTER WEATHER ADVISORY OVER A
BLIZZARD WARNING. COLD AIR IS HARD TO SCOUR OUT OF THE KUSKOKWIM
VALLEY SO AS WARM AIR OVERRIDES THIS COLD AIR COULD ALSO SEE SOME
FREEZING RAIN...BUT THE BULK OF PRECIP WILL BE IN THE FORM OF SNOW.
THE LOW WILL TRACK ACROSS SOUTHWEST ALASKA TO THE INTERIOR ON FRIDAY
SO ARCTIC AIR WILL MOVE BACK IN AND CHANGE ANY REMAINING PRECIP TO
SNOW.

BERING SEA/ALEUTIANS...AN ALREADY STRONG SOUTHERLY LOW LEVEL JET
AHEAD OF THE COLD FRONT WILL STRENGTHEN FURTHER THIS AFTERNOON
AND EVENING AS NEWLY FORMED TRIPLE POINT LOW DEEPENS. THE NAM
MODEL IS STRONGEST WITH 90 KT AT AROUND 850 MB...BUT THE OTHER
MODELS AREN'T FAR BEHIND. WITH UPWARD VERTICAL MOTION DON'T EXPECT
THESE WINDS TO REACH THE SURFACE BUT IT GIVES AN IDEA OF JUST HOW
STRONG THE LOW LEVEL WINDS WILL BE. MOUNTAIN WAVES AND GAP WINDS
WILL ADD TO ALREADY STRONG SYNOPTIC WINDS TO PRODUCE DANGEROUS
HIGH WINDS FROM THE EASTERN ALEUTIANS TO SOUTHERN ALASKA PENINSULA
AS THE FRONT MOVES THROUGH. WINDS ON THE BACK SIDE OF THE LOW WILL
ALSO BE QUITE STRONG DUE TO WEAK COLD ADVECTION AND RAPID PRESSURE
RISES ON THE ORDER OF 2 TO 3 MB PER HOUR. THERE IS POTENTIAL FOR
THE EASTERN ALEUTIANS AND SOUTHERN ALASKA PENINSULA TO REACH HIGH
WIND CRITERIA AGAIN...BUT WILL WAIT UNTIL THE CURRENT WARNINGS ARE
OVER BEFORE CONSIDERING ISSUING NEW WARNINGS.

AS THIS FIRST STORM EXITS ANOTHER WILL ENTER THE WESTERN BERING ON
FRIDAY...BUT IT IS EXPECTED TO BE CONSIDERABLY WEAKER DUE TO A LESS
AMPLIFIED UPPER JET.

.LONG TERM FORECAST...UPPER PATTERN LOOKS MORE ZONAL THROUGH THE
EXTENDED PERIOD WHICH MEANS STORMS WILL MOVE EVEN FASTER FROM WEST
TO EAST ACROSS THE REGION BUT WON'T BE AS STRONG AS THE ONES
WE ARE CURRENTLY EXPERIENCING. ACTIVE WEATHER WILL CONTINUE.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...HIGH WIND WARNING 131 181 185.
         BLIZZARD WATCH 155.
MARINE...STORM WARNING...155 165 170 171 172 179 180 185.
         GALE WARNING...120 125 127 128 130 132 138 140 141
                        150 160 175 176.
FIRE WEATHER...NONE.

SEB NOV 11