Storm Warning 171, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
130 PM AKDT WED OCT 27 2010

.ANALYSIS AND UPPER LEVELS...STORY OF THE DAY IS MAJOR STORM WHICH
HAS SLOWED TO A CRAWL OVER THE NORTHWEST BERING SEA JUST OFF THE
COAST OF RUSSIA. A 185 KT UPPER JET HAS NOW MOVED OFF TO THE EAST
AND IS HELPING DRIVE THE FRONTAL SYSTEM EASTWARD TO MAINLAND ALASKA.
THE FRONT IS CURRENTLY JUST OFFSHORE OF THE SOUTHWEST ALASKA COAST.
RADAR IMAGERY SHOWS A SOLID BAND OF PRECIPITATION PROGRESSING INLAND.
THE AIR MASS IN PLACE IS COLD ENOUGH TO SUPPORT MAINLY SNOW AS THE
PRECIPITATION FIRST STARTS BUT WARMER AIR WITH THE FRONT WILL HELP
CHANGE THINGS TO MOSTLY RAIN COASTAL REGIONS. SOUTHCENTRAL ALASKA
IS QUIET FOR THE MOMENT OUT AHEAD OF THE SYSTEM WITH JUST SOME
SHOWERS ALONG THE COAST AND A FEW AREAS OF STRATUS AND FOG INLAND.
COLDER AIR IS FEEDING BACK INTO THE SYSTEM ACROSS THE WESTERN BERING
RESULTING IN A SHOWERY REGIME BEHIND THE FRONT.

.MODEL DISCUSSION...MODELS REMAIN IN GOOD AGREEMENT THROUGH FRIDAY.
DIFFERENCES DEVELOP WITH LOW MOVING ALONG THE ALEUTIANS FRIDAY NIGHT
AND SATURDAY AS WELL AS WITH MAJOR LOW DEVELOPING OVER NORTH CENTRAL
PACIFIC SATURDAY. SOLUTIONS ARE CLOSE ENOUGH THOUGH TO ENABLE USE
OF AN AVERAGE POSITION AND STRENGTH FOR NOW.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...UPPER LEVEL DIVERGENCE IN THE LEFT EXIT REGION
OF THE STRONG UPPER JET WILL AID IN DEVELOPMENT OF A TRIPLE POINT
LOW SOUTH OF THE ALASKA PENINSULA TONIGHT. THIS LOW WILL THEN TRACK
TO WESTERN PRINCE WILLIAM SOUND THURSDAY AFTERNOON. THE PRESENCE OF
THIS LOW WILL RESULT IN EVEN STRONGER WINDS AS THE FRONT CROSSES
THE GULF AND SET UP A BARRIER JET ALONG THE NORTH GULF COAST THURSDAY
AFTERNOON. THEREFORE...WILL INCREASE WINDS A LITTLE BIT FROM CURRENT
FORECAST. ALSO...WITH A STRONG PUSH BEHIND THE SYSTEM THE FRONT WILL
PUSH ONSHORE...SO EXPECT WINDS TO BRIEFLY MAKE IT INTO CORDOVA/PRINCE
WILLIAM SOUND/PASSAGE CANAL BEFORE THE FRONT FALLS APART ALONG THE
COASTAL RANGE THURSDAY EVENING.

THE AIR MASS ALONG COASTAL REGIONS OF SOUTHCENTRAL (INCLUDING
ANCHORAGE) STILL HAS A DECENT ABOVE FREEZING LAYER AS PER THE 12Z
ANCHORAGE SOUNDING. THEREFORE...EXPECT RAIN TO DOMINATE IN THESE
REGIONS AS THE FRONT MOVES IN TONIGHT. THE EXCEPTION WOULD BE
VALDEZ WHERE ENOUGH COLD AIR DRAINS DOWN THE VALLEYS TO INITIALLY
SUPPORT SNOW OUT AHEAD OF THE FRONT. HIGHER ELEVATIONS INCLUDING
TURNAGAIN PASS IN WESTERN PRICNE WILLIAM SOUND MAY ALSO SEE A
LITTLE SNOW. HOWEVER...THE PRIMARY AREA FOR SNOW ACCUMULATION
WITH THIS SYSTEM WILL BE THE SUSITNA VALLEY...ESPECIALLY UPSLOPE
AREAS ALONG THE ALASKA RANGE. A WINTER WEATHER ADVISORY HAS BEEN
ISSUED FOR AREAS WEST OF THE PARKS HIGHWAY AS WELL AS NORTH OF
TALKEETNA FOR TONIGHT THROUGH THURSDAY MORNING. ENOUGH WARM AIR
MAY THEN WORK IN THURSDAY AFTERNOON TO MIX WITH OR CHANGE PRECIP
TO RAIN. IN ANY CASE  AS TRIPLE POINT LOW MOVES TO WESTERN PRINCE
WILLIAM SOUND LOW LEVEL FLOW WILL BACK TO THE NORTH ACROSS THE
SUSITNA VALLEY WHICH WILL CAUSE PRECIP TO DIMINISH OVER THE
WESTERN VALLEY. PRECIP WILL THEN SPREAD NORTH AND EAST LATE
THURSDAY AS FLOW SHIFTS TO WEST TO SOUTHWEST BEHIND THE SYSTEM.
OUT AHEAD OF THE TRIPLE POINT LOW WARMER AIR WILL ADVECT NORTHWARD
ACROSS PRINCE WILLIAM SOUND AND THE COPPER RIVER BASIN SO WITH
THE EXCEPTION OF THOMPSON PASS DON'T EXPECT ANY SIGNIFICANT SNOW
ACCUMULATION. THERE IS STILL A SHOT OF SNOW FOR ANCHORAGE AND THE
MATANUSKA VALLEY THURSDAY NIGHT AS COLDER AIR WORKS BACK IN. IF
SO...IT WILL BE A QUICK SHOT OF SNOW BECAUSE THE JET WILL QUICKLY
BRING AN END TO PRECIP BY EARLY FRIDAY MORNING.

SOUTHWEST ALASKA...PRECIP WILL CONTINUE TO PUSH ONSHORE
TONIGHT...FALLING MAINLY AS SNOW OVER THE SOUTHERN KUSKOKWIM VALLEY
AND RAIN DOWN ACROSS BRISTOL BAY DUE TO ADVECTION OF MUCH WARMER
AIR. PRECIP WILL HANG ON THROUGH THURSDAY MORNING OVER INLAND
BRISTOL BAY AND THE EASTERN KUSKOKWIM VALLEY INTO THURSDAY MORNING
BECAUSE SOUTHERN PORTION OF FRONT WILL MOVE FASTER THAN NORTHERN
PORTION. OTHERWISE...ONSHORE FLOW WILL BRING A STEADY SUPPLY OF
SHOWERS OFF THE BERING THROUGH THE END OF THE WEEK. COLDER AIR ALOFT
WILL MOVE BACK IN...SO PRECIP WILL MIX WITH OR CHANGE BACK TO SNOW.

HIGH SURF ADVISORIES REMAIN IN EFFECT ALONG THE SOUTHWEST COAST
THROUGH FRIDAY AFTERNOON FOR MINOR BEACH EROSION DURING AFTERNOON
HIGH TIDES. THIS LOOKS ON TRACK WITH LATEST STORM SURGE GUIDANCE
INDICATING WATER LEVELS AROUND ONE FOOT ABOVE MEAN ASTRONOMICAL
TIDES.

BERING SEA/ALEUTIANS...STRONG WESTERLIES AND HIGH SEAS WILL PERSIST
ACROSS THE REGION THROUGH THE REMAINDER OF THE WEEK AS THE LOW VERY
SLOWLY WEAKENS OVER THE NORTHERN BERING SEA. AIR ALOFT WILL COOL
ENOUGH TONIGHT AND THURSDAY TO SUPPORT A MIX OF SNOW AND RAIN
SHOWERS.

.LONG TERM FORECAST...ALTHOUGH MODELS DISAGREE ON DETAILS OF EXTENDED
THERE IS GOOD CONSISTENCY AND AGREEMENT ON DEVELOPMENT OF A MAJOR
LOW OVER NORTH CENTRAL PACIFIC THIS WEEKEND WHICH WILL THEN MOVE
INTO THE GULF EARLY NEXT WEEK. THIS WILL DRAG ARCTIC AIR SOUTHWARD
ACROSS THE BERING SEA AND MAINLAND ALASKA LEADING TO COLDER
TEMPERATURES REGION-WIDE AND INCREASING POTENTIAL FOR SNOW AT SOME
POINT NEXT WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...STORM WARNING 120 130
         GALE WARNING...125 129 132 140 150 155 160 165 170 171 172
                        175 176 179 180 185.
FIRE WEATHER...NONE.

SEB OCT 10