Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 230155
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
455 PM AKST THU JAN 22 2015

.ANALYSIS AND UPPER LEVELS...
THE STAGE IS SET FOR A SIGNIFICANT LONG DURATION SNOWFALL ALONG
THE POPULATION CORRIDOR OF SOUTHCENTRAL ALASKA...FROM THE WESTERN
KENAI PENINSULA TO ANCHORAGE AND THE MAT-SU. A WARM OCCLUDED FRONT
IS TRACKING NORTHWESTWARD ACROSS THE GULF OF ALASKA TOWARD
SOUTHCENTRAL ALASKA THIS MORNING. PRECIPITATION HAS REACHED THE
GULF COAST AND WILL CONTINUE TO SPREAD INLAND ACROSS ALL OF
SOUTHCENTRAL THIS AFTERNOON AND EVENING. WARMER AIR IS QUICKLY
CHANGING PRECIPITATION OVER TO RAIN ALONG THE COAST...BUT MUCH
COLDER LAND INLAND WILL PRODUCE ALL SNOW. MEANWHILE...A COLD UPPER
TROUGH IS DIGGING SOUTHWARD OVER SOUTHWEST ALASKA. THIS WILL CAUSE
THE OCCLUDED FRONT TO STALL AND PREVENT ABOVE FREEZING AIR FROM
MAKING ITS WAY ACROSS THE KENAI PENINSULA INTO COOK INLET. ANOTHER
KEY FEATURE IS A SHORT-WAVE TROUGH MOVING OUT OF RUSSIA. IT IS
POISED TO DROP INTO THE TROUGH OVER SOUTHWEST ALASKA AND
ULTIMATELY HELP KICK IT EASTWARD.

OUT WEST AN UPPER RIDGE DOMINATES MUCH OF THE BERING SEA. COLD AIR
WITH THE DIGGING TROUGH OVER SOUTHWEST ALASKA IS ADVECTING SOUTHWARD
ON THE EAST SIDE OF THE RIDGE ACROSS THE EASTERN BERING SEA/SOUTHERN
ALASKA PENINSULA/EASTERN ALEUTIANS PRODUCING STRONG GUSTY WINDS.
WIND CHILL ADVISORIES WERE ISSUED THIS MORNING FOR THE KUSKOKWIM
DELTA AND A PORTION OF THE SOUTHERN KUSKOKWIM VALLEY WHERE AIR
TEMPERATURES ALL ALL BELOW ZERO.

&&

.MODEL DISCUSSION...
MODELS HAVE COME CLOSER TOGETHER WITH KEY FEATURES AFFECTING THE
MULTI-PHASED SNOW EVENT OVER SOUTHCENTRAL...THOUGH THERE ARE STILL
A FEW SUBTLE DIFFERENCES WHICH AFFECT THE TIMING/PLACEMENT OF
HEAVIEST SNOW THROUGH SATURDAY. THE 12Z NAM HAD BEEN AN OUTLIER
IN KEEPING ANCHORAGE AND THE MAT VALLEY UNDER THE HEAVIER DEFORMATION
BAND OF SNOW TONIGHT INTO FRIDAY...BUT THE 18Z RUN IS NOW IN
AGREEMENT WITH OTHER SOLUTIONS IN SHIFTING THIS WESTWARD OVER COOK
INLET. THUS...STILL EXPECT A BREAK IN SNOWFALL IN THESE AREAS
SOMETIME LATER TONIGHT THROUGH FRIDAY MORNING BEFORE HEAVIER SNOW
MOVES BACK IN. THE EXACT TIMING OF THIS VARIES FROM MODEL TO MODEL.
THERE IS NOW VERY GOOD AGREEMENT THAT UPPER TROUGH WILL AMPLIFY ON
SATURDAY AND BECOME NEGATIVELY TILTED BEFORE PUSHING THROUGH
SOUTHCENTRAL ALASKA. THIS WILL EXTEND THE SNOW EVENT FARTHER OUT
IN TIME THAN CURRENTLY ADVERTISED.

ANOTHER KEY MODEL DIFFERENCE WORTH MENTIONING IS THE EXTENT OF
WARMING FROM THE VALDEZ AREA NORTHWARD INTO THE COPPER RIVER
BASIN. THERE IS A HUGE SPREAD IN LOW LEVEL THICKNESSES WITH SOME
MODELS KEEPING THE AIR MASS COLD ENOUGH FOR ALL SNOW AND OTHERS
CHANGING IT OVER TO ALL RAIN (OR FREEZING RAIN). ALTHOUGH THE
AIR MASS BEHIND THE OCCLUDED FRONT MOVED IN FROM THE NORTHEAST
PACIFIC IT IS NOT THE DEEP TROPICAL AIR MASS WE SAW A WEEK AGO.
ALSO...THE INTENSITY OF PRECIPITATION WILL BE MODERATE FOR MUCH
OF THE TIME. THEREFORE...TEND TO BELIEVE SOMETHING MORE LIKE THE
COOLER SOLUTIONS. EVEN SO...VALDEZ WILL BE RIGHT ON THE EDGE AND
SHOULD SEE A PERIOD OF FREEZING RAIN.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE BIG CHANGE WITH THIS AFTERNOONS FORECAST PACKAGE WILL BE TO
EXTEND THE WINTER WEATHER ADVISORIES FOR THE WESTERN
KENAI/ANCHORAGE/MATANUSKA AND SUSITNA VALLEYS OUT THROUGH SOMETIME
SATURDAY AFTERNOON. THIS NOW LOOKS LIKE THERE MAY BE APPROXIMATELY
THREE DIFFERENT PHASES TO THE EVENT. FIRST IS THE INITIAL SNOW
MOVING IN THIS AFTERNOON AND EVENING WITH THE WARM OCCLUDED FRONT.
THIS WILL GENERALLY BE LIGHT SNOW...ALTHOUGH THERE MAY BE BRIEF
PERIODS OF MODERATE SNOW.

SECOND IS THE DEFORMATION BAND WHICH SETS UP AS THE OCCLUDED
FRONT MEETS THE TROUGH (AND ASSOCIATED COLD AIR) DIGGING OVER
SOUTHWEST ALASKA. THERE IS INCREASING CONFIDENCE THAT THIS HEAVIER
BAND OF SNOW WILL BE CENTERED OVER COOK INLET AND THE SUSITNA
VALLEY LATE TONIGHT THROUGH FRIDAY MORNING. THIS WILL CAUSE SNOW
TO LIGHTEN UP...OR COMPLETELY END FOR A WHILE...FROM THE INTERIOR
KENAI TO ANCHORAGE AND THE EASTERN PORTION OF THE MATANUSKA
VALLEY. THE HEAVIEST SNOW AT THIS POINT WILL BE FROM
KENAI/SOLDOTNA NORTHWARD ACROSS MOST OF THE SUSITNA VALLEY. AS THE
UPPER SHORT-WAVE EXITING RUSSIA THIS MORNING MOVES THROUGH THE
UPPER TROUGH IT WILL BEGIN TO SHIFT EASTWARD...PUSHING THIS BAND
BACK OVER ANCHORAGE AND THE MAT VALLEY SOMETIME FRIDAY AFTERNOON
OR EVENING.

THE THIRD PHASE OF THE EVENT WILL KICK IN SATURDAY AS THE UPPER
TROUGH BECOMES NEGATIVELY TILTED AND COLD ADVECTION REALLY KICKS
IN. IT IS UNCLEAR WHETHER SNOW WILL LIGHTEN UP BETWEEN DEFORMATION
BAND AND ARRIVAL OF COLD ADVECTION. IT MAY JUST BE ONE CONTINUOUS
PERIOD OF MODERATE OR HEAVY SNOW. THE HEAVIEST SNOWFALL FOR THIS
PHASE WILL BE ALONG AND NEAR THE WESTERN SLOPES OF THE CHUGACH
AND TALKEETNA MOUNTAINS.

AS ELUDED TO IN THE MODEL DISCUSSION ABOVE ONE AREA OF LOW
FORECAST CONFIDENCE IS PRECIP-TYPE IN THE VALDEZ AREA AND ON IN
TO THE COPPER BASIN. THE COPPER BASIN WILL GET DOWNSLOPED BEHIND
THE OCCLUDED FRONT LATER TONIGHT...SO DO NOT EXPECT MAJOR PROBLEMS
THERE. VALDEZ HAS POTENTIAL FOR A FAIRLY LONG PERIOD OF FREEZING
RAIN. A WINTER WEATHER ADVISORY IS IN EFFECT THROUGH FRIDAY
AFTERNOON TO HIGHLIGHT THIS THREAT. IF IT TURNS OUT THAT PRECIP
STAYS MAINLY SNOW THEN SNOW ACCUMULATIONS WILL NEED TO BE
INCREASED SIGNIFICANTLY.

AS THE UPPER TROUGH PROGRESSES EASTWARD SATURDAY AND SATURDAY
NIGHT CONDITIONS WILL BECOME FAVORABLE FOR HEAVY PRECIPITATION IN
NORTHERN TO EASTERN PRINCE WILLIAM SOUND. ALTHOUGH THERE IS QUITE
A BIT OF UNCERTAINTY IN PRECIP-TYPE IN VALDEZ OVER THE NEXT 24 TO
36 HOURS IT APPEARS THE AIR MASS WILL COOL ENOUGH BY LATE SATURDAY
TO ENSURE ALL SNOW. AS GET CLOSER IN TIME WILL HAVE TO EVALUATE
THE POTENTIAL FOR A HEAVY SNOW WARNING IN THE VALDEZ TO THOMPSON
PASS AREA.

IN THE WAKE OF ALL THIS PRECIPITATION...VERY COLD AND DRY
CONDITIONS WILL SETTLE IN ACROSS SOUTHCENTRAL AS WE HEAD THROUGH
THE WEEKEND.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
NORTHEASTERLY FLOW IS BRINGING DOWN COLDER TEMPERATURES FROM THE
NORTH WHICH IS BRINGING COLD WIND CHILLS TO THE AREA. THIS WILL
LAST INTO FRIDAY UNTIL WINDS DIE DOWN. HOWEVER...TEMPERATURES WILL
REMAIN COLD THROUGH THE WEEKEND. THE NORTHEASTERLY FLOW SHOULD
ALSO KEEP SKIES CLEAR THROUGH MUCH OF THE WEEKEND.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
COLD NORTHEASTERLY FLOW OVER THE BERING AND ALEUTIANS IS BRINGING
LIGHT SNOW SHOWERS THROUGHOUT THE AREA. AS HIGH PRESSURE MOVES
INTO THE BERING FROM THE WEST FLOW WILL SHIFT TO BECOMING MORE
SOUTHERLY. THE SOUTHERLY WINDS WILL ALLOW FOR WARMER AIR TO MOVE
BACK OVER THE AREA. THIS IS ESPECIALLY TRUE WITH A FRONT THAT
WILL MOVE IN OVER THE WESTERN ALEUTIANS LATE SATURDAY.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
SNOW WILL BE WINDING DOWN ACROSS SOUTHCENTRAL AK BY SUNDAY AS THE
UPPER TROUGH PUSHES THROUGH THE AREA. VERY COLD TEMPERATURES ARE
THEN IN STORE THROUGH MUCH OF NEXT WEEK AS THE COLDEST AIR SO FAR
THIS SEASON FILTERS IN BEHIND THE TROUGH...WITH LOW TEMPERATURES
POTENTIALLY DIPPING BELOW ZERO ACROSS MUCH OF THE AREA BY SUNDAY
NIGHT. MOSTLY DRY CONDITIONS CAN BE EXPECTED ACROSS THE SOUTHERN
MAINLAND WITH OFFSHORE FLOW IN PLACE AT THE SURFACE. MEANWHILE...
A CHANCE OF RAIN AND SNOW WILL PERSIST ACROSS THE ALEUTIANS
THROUGH MUCH OF THE WEEK AS A BROAD NORTH PACIFIC LOW PASSES SOUTH
OF THE CHAIN.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WINTER WEATHER ADVISORY 101 111 121 131 141 145.
MARINE...GALE WARNING 130 138 150 155 160 165 170 172 173 174 175
                176 179 180.
         HEAVY FREEZING SPRAY 160 165 180 185. FIRE
WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...CB



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