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000
FXAK68 PAFC 180131
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
430 PM AKST TUE DEC 17 2013

.ANALYSIS AND UPPER LEVELS...WEAK RIDGING OVER THE BERING WITH WEAK
UPPER LEVEL FLOW HAS MOVED OVER THE EASTERN BERING AND EASTERN
ALEUTIANS. AN ELONGATED EAST TO WEST ORIENTATED FRONT FROM SOUTH OF
THE ALEUTIAN CHAIN WILL MOVE OVER THE WESTERN AND CENTRAL ALEUTIANS
BY THIS EVENING. THIS FRONT WILL PUSH INTO THE CENTRAL BERING AND
EASTERN ALEUTIANS BY TOMORROW MORNING AS A COMPLEX LOW SOUTH OF THE
ALEUTIAN CHAIN PROGRESSES NORTHEASTWARD. FAVORABLE UPPER LEVEL
SUPPORT FROM A 160+KT UPPER LEVEL JET WILL ASSIST IN THE DEVELOPMENT
OF MULTIPLE TRIPLE POINT LOWS OVER THE EASTERN ALEUTIANS AND WESTERN
GULF. WHILE THE LOCATION AND INTENSITY OF THESE TRIPLE POINT LOWS
REMAIN A FORECAST CHALLENGE...A TRANSITION FROM A MORE STABLE
OFFSHORE FLOW TO A LESS STABLE ONSHORE FLOW WITH WIDE SPREAD SNOW AND
WINDY CONDITIONS CAN BE EXPECTED FOR MUCH OF THE SOUTHERN MAINLAND ON
WEDNESDAY. AS THE TRIPLE POINT LOW FORMS IN THE WESTERN GULF LATE
WEDNESDAY EVENING.

.MODEL DISCUSSION...MODELS HAVE BEEN A CHALLENGE TODAY. THE 12Z RUNS
INITIALIZED WELL BUT QUICKLY DIVERGE AS THEY STRUGGLE TO MANAGE THE
ENERGY FROM THE VARIOUS TRIPLE POINT LOWS THAT WILL DEVELOP ALONG THE
FRONT OVER THE ALEUTIANS...AND THEN STRUGGLE WITH THE TRIPLE POINT
LOW AND MOVEMENT WHEN A LOW DEVELOPS IN THE GULF OF ALASKA TOMORROW
NIGHT. WHEN LOOKING AT FIVE DIFFERENT 12Z MODELS FOR WEDNESDAY NIGHT
THERE ARE FIVE DIFFERENT SOLUTIONS FOR THE LOW IN THE BRISTOL BAY
AREA. WHILE THE 18Z MODEL RUNS GENERALLY ARE SEEN AS LESS
RELIABLE...THE 18Z NAM AND GFS HAVE BOTH JUMPED SIGNIFICANTLY TOWARD
THE SOLUTION THE 12Z GEM MODEL WAS SHOWING. HOPEFULLY THIS IS
INDICATING THAT THE MODELS ARE BEGINNING TO LOCK ONTO THE MAIN
FEATURES AND WILL CONVERGE ON A COMMON SOLUTION SOON. THERE IS ENOUGH
CONFIDENCE IN THIS THAT THE FORECASTS ARE BEING ADJUSTED TOWARD THIS
END.

.SHORT TERM FORECAST...
SOUTHCENTRAL...THE TROUGH OF LOW PRESSURE IN THE NORTHERN GULF IS
WEAKENING AND PUSHING EASTWARD. THIS WILL BRING DOWN THE PRESSURE
GRADIENT ALONG THE NORTH GULF COAST AND DIMINISH THE WINDS OVERNIGHT.
THE FRONT ALONG THE ALEUTIANS WILL PUSH NORTHWARD AND EASTWARD AND
DEVELOP A LOW THAT MOVES INTO BRISTOL BAY TOMORROW NIGHT...AND THEN
ANOTHER LOW WILL DEVELOP IN THE LEE OF THE ALEUTIAN RANGE IN THE
NORTHWEST GULF THURSDAY AS A TROUGH ALOFT MOVES OVER THE REGION. THIS
WILL BRING A QUICK BURST OF SNOW TO KODIAK ISLAND TOMORROW BEFORE
TURNING TO RAIN. THE NORTH GULF COAST WILL STAY SNOW AS THE FRONT
ELONGATES AND HAS A HARD TIME DISLODGING THE RIDGE IN THE EASTERN
INTERIOR AND YUKON. THIS WILL INCREASE THE PRESSURE GRADIENT ALONG
THE NORTH GULF COAST BRINGING FULL GALES TO THE AREAS AS WE AS
SNOWFALL. THIS HAS PROMPTED A BLIZZARD WATCH TO BE ISSUED FOR THE
EASTERN KENAI PENINSULA. WHILE LAST NIGHT IT LOOKED LIKE THE BLIZZARD
POSSIBILITY WAS MOSTLY OVER PORTAGE VALLEY AND EASTERN TURNAGAIN ARM
IT NOW LOOKS LIKE THERE IS A CHANCE BLIZZARD CONDITIONS COULD DEVELOP
FROM MOOSE PASS TOWARD SEWARD AS WELL. THIS IS NOT A SURE THING BUT
CONDITIONS ARE LOOKING MORE FAVORABLE FOR THIS TO HAPPEN. SOUTHERN
AND SOUTHEAST PRINCE WILLIAM SOUND ARE ALSO AT RISK FOR BLIZZARD
CONDITIONS AS THE FRONT MOVES INTO THE AREA WEDNESDAY NIGHT.
THE SOUTHERN KENAI PENINSULA COULD SEE SNOW AND BLOWING
SNOW...ESPECIALLY ALONG THE HOMER BLUFF AND NEAR SELDOVIA. HOWEVER
THE SELDOVIA AREA MAY WARM UP ENOUGH TO TURN THE PRECIPITATION TO
RAIN. FARTHER NORTH ON THE KENAI PENINSULA AND FARTHER NORTH THROUGH
THE ANCHORAGE AND MAT-SU AREA THERE WILL BE SOME SNOWFALL BUT IT WILL
BE TRUNCATED BY THE DOWNSLOPING OFF THE KENAI AND CHUGACH MOUNTAINS.
THE WRENCH IN THE WORKS HERE IS THAT THE UPPER LEVEL FLOW IS A MOIST
SOUTHWEST FLOW WHICH COULD OVERRIDE SOME OF THE DOWN SLOPE EFFECT.

SOUTHWEST....AS THE FRONT OVER THE ALEUTIAN ISLANDS IS MAKING
PROGRESS NORTHWARD AS IT ERODES THE RIDGE OF HIGH PRESSURE OVER THE
BERING SEA. A LOW WILL DEVELOP ALONG THIS FRONT NEAR ADAK OVERNIGHT
AND THEN A TRIPLE POINT LOW WILL DEVELOP NEAR DUTCH HARBOR
TOMORROW...MOVING INTO EASTERN BRISTOL BAY BY TOMORROW NIGHT. THIS
FRONT WILL BRING A QUICK SHOT OF SNOW BEFORE CHANGING PRECIPITATION
TO RAIN OVER THE ALEUTIAN ISLAND...PRIBILOF ISLANDS...AND ALASKA
PENINSULA. IN BRISTOL BAY THINGS ARE A LITTLE DIFFERENT AS THE FRONT
WILL STALL IN THAT AREA WHICH MAY KEEP THE PRECIPITATION SNOW FOR
MOST OF THE NIGHT TOMORROW NIGHT. THEREFORE THERE HAS BEEN A BLIZZARD
WATCH ISSUED FOR ZONE 161 AS THERE WILL BE STRONG WINDS IN THE AREA
ALONG THIS FRONT AS WELL.
FARTHER WEST IN THE KUSKOKWIM DELTA THE WINDS WILL BE LESS AND ANY
PRECIPITATION SHOULD BE LIGHTER.

.LONG TERM FORECAST...THE OVERALL FLOW PATTERN FOR THE SOUTHERN
MAINLAND WILL REMAIN PROGRESSIVE THROUGH MID WEEK. A BROAD TROUGH
OVER THE BERING WILL BRING SLIGHTLY WARMER AND WET SOUTHWEST FLOW
OVER THE SOUTHWEST MAINLAND THROUGH MOST THE WEEKEND. A TRANSITION TO
A COLDER NORTHWESTERLY FLOW OVER THE SOUTHWEST MAINLAND WILL BEGIN
SUNDAY MORNING AS THE TROUGH PUSHES EASTWARD OVER THE SOUTHWEST
MAINLAND AND WESTERN GULF. THE UPPER LEVEL RIDGE OVER SOUTH CENTRAL
WILL MOVE EASTWARD AS THE UPPER LEVEL TROUGH MOVES INTO THE GULF LATE
SUNDAY MORNING. BY MID WEEK THE PATTERN SHIFTS AGAIN TO SHOW A HIGH
AMPLITUDE RIDGE OVER THE EASTERN MAINLAND AND A TROUGH OVER THE
WESTERN AND CENTRAL BERING. THE OVERALL FLOW PATTERN FOR THE SOUTHERN
MAINLAND WILL REMAIN PROGRESSIVE THROUGH MID WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...BLIZZARD WATCH 121 125 135 161.
MARINE...GALE 119 120 130 132-138 150-165 171 178-180 185.
         STORM 170 172-177.
         HEAVY FREEZING SPRAY 121 126-131 139 160 181 185.
FIRE WEATHER...NONE.

DEC 13