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000
FXAK68 PAFC 162313
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
315 PM AKDT THU MAY 16 2013

.ANALYSIS AND UPPER LEVELS...
A LONGWAVE TROUGH REMAINS CENTERED OVER THE WEST COAST OF ALASKA WITH
BROAD SOUTHWESTERLY FLOW IN PLACE OVER THE SOUTHERN AND EASTERN
MAINLAND. A FAIRLY WEAK LOW HAS MOVED NORTHWARD INTO THE GULF OF
ALASKA...BRINGING WIDESPREAD RAIN/SNOW TO MOST LOCATIONS EAST OF THE
ALASKA RANGE. FURTHER WEST...A VIGOROUS SHORTWAVE DROPPING INTO THE
LONGWAVE TROUGH IS USHERING IN UNSEASONABLY COLD AIR TO THE SOUTHWEST
MAINLAND AND IS EN ROUTE TO SOUTHCENTRAL ALASKA TOMORROW. AFTER A
PERIOD OF RAIN AND SNOW EARLIER THIS MORNING THE INCREASING
CROSS-BARRIER WINDS HAVE ALLOWED A CLASSIC DOWNSLOPE PATTERN TO
DEVELOP OVER THE COOK INLET REGION THIS AFTERNOON. STRONG WINDS ARE
BLOWING THROUGH TURNAGAIN ARM AND THE PRINCE WILLIAM SOUND COASTLINE
IS SEEING SUSTAINED HEAVY PRECIPITATION.

.MODEL DISCUSSION...
THE MODELS REMAIN IN GOOD OVERALL SYNOPTIC AGREEMENT. THE NAM
CONTINUES TO BE FASTER THAN THE OTHER SOLUTIONS PROGRESSING THE
MIDLEVEL TROUGH TO THE EAST. THIS IS RELATED TO THE FACT THAT THE NAM
DOES NOT DEVELOP A WEAK CLOSED LOW IN THE BASE OF THIS TROUGH LATE
FRIDAY NIGHT AS THE OTHER MODELS DO. THIS ALSO MEANS THE NAM BRINGS
THE COLD AIR INTO THE ANCHORAGE AREA FASTER THAN THE OTHER MODELS.
THE GFS AND ECMWF ARE CLOSER TOGETHER WITH A SLOWER SOLUTION FOR THE
ARRIVAL OF THE COLD AIR AND DEVELOPMENT OF A WEAK CLOSED UPPER LOW.
THE ECMWF IS A TOUCH FASTER THAN THE GFS...AND REPRESENTS A GOOD
MIDDLE-GROUND SOLUTION FOR THE PRODUCTION OF THE FORECAST TODAY.

.SHORT TERM FORECAST...
SOUTHCENTRAL...DOWNSLOPING WILL COME TO THE END BY LATE THIS EVENING
AS SURFACE LOW PRESSURE MAKES PROGRESS NORTHWARD TOWARD PRINCE
WILLIAM SOUND. THIS SHOULD ALLOW RAIN (MIXED WITH SNOW AT HIGHER
ELEVATIONS) TO RETURN TO THE WESTERN KENAI...ANCHORAGE...AND MAT-SU
AREAS. COLD AIR BEGINS TO BLEED INTO THE AREA AFTER MIDNIGHT
TONIGHT...AND SHOULD BE ENOUGH TO PRODUCE A RAIN/SNOW MIX AT SEA
LEVEL FROM ANCHORAGE NORTHWARD DURING THE DAY TOMORROW. BY TOMORROW
AFTERNOON THE COLD AIR ADVECTION WILL INTENSIFY DRAMATICALLY AS THE
UPPER TROUGH APPROACHES FROM THE WEST. THIS SHOULD LEAD TO INCREASED
PRECIPITATION RATES AND A CHANGE FROM RAIN/SNOW MIX TO ALL SNOW EVEN
AT SEA LEVEL. HIGHER ELEVATIONS WILL SEE THE THE MOST
ACCUMULATION...BUT MEASURABLE ACCUMULATION IS LIKELY EVEN AT LOWER
ELEVATIONS. CONSIDERING THE EXTREME RARITY OF AN EVENT OF THIS
MAGNITUDE AT THIS TIME OF YEAR...WILL BE ISSUING SNOW ADVISORIES FOR
THE ANCHORAGE AND MAT-SU REGIONS WITH THE UPCOMING FORECAST PACKAGE.
BY SATURDAY MORNING...SNOW WILL TAPER OFF AS MOST OF THE DYNAMIC
SUPPORT SHIFTS AWAY FROM THE AREA AS THE UPPER LOW DEVELOPS SOUTH OF
THE KENAI PENINSULA AND COLD AIR CONTINUES TO MOVE IN FROM THE WEST.

SOUTHWEST MAINLAND...UNSEASONABLY COLD AIR MOVE IN FROM WEST TO EAST
THROUGH FRIDAY AFTERNOON. FRONTAL PASSAGE WILL BE MOSTLY DRY ACROSS
THE REGION...WITH THE MOST SIGNIFICANT PRECIPITATION OCCURRING OVER
THE FAR EASTERN BRISTOL BAY AND KUSKOKWIM VALLEY AREAS. THESE AREAS
WILL SEE RAIN TURN TO SNOW TONIGHT AND LIKELY SEE SOME SNOW
ACCUMULATION BEFORE SNOW ENDS FRIDAY EVENING. A RIDGE OF HIGH
PRESSURE BUILDS IN FROM THE WEST LATE SATURDAY...CLEARING OUT CLOUDS
AND MODERATING TEMPERATURES.

BERING SEA AND ALEUTIANS...COLD AIR BEHIND THE FRONT WILL DELIVER A
GLANCING BLOW TO THE EASTERN BERING SEA THROUGH SATURDAY MORNING
BEFORE A RIDGE MOVES BACK INTO THE AREA. WIDESPREAD FOG AND STRATUS
WILL LIKELY CONTINUE THROUGH THE WEEKEND AT MANY LOCATIONS. A LOW
PRESSURE CENTER AND ASSOCIATED FRONT WILL MOVE INTO THE WESTERN AND
CENTRAL ALEUTIANS SATURDAY NIGHT WITH GALE-FORCE WINDS AND RAIN.

.LONG TERM FORECAST...AFTER COLD TEMPERATURES EARLY THIS
WEEKEND...LONG RANGE MODELS ARE IN GOOD AGREEMENT ON A WARMING TREND
EARLY NEXT WEEK (ALTHOUGH TEMPERATURES WILL STILL STAY AT OR BELOW
NORMAL FOR THIS TIME OF YEAR. THIS WILL BE ACCOMPANIED BY A MORE
PROGRESSIVE WEATHER PATTERN THAT IS CENTERED MORE OVER THE BERING SEA
THAN THE CURRENT STORM TRACK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...SNOW ADVISORY 101 111 145
MARINE...GALE 120 130 132 150
         HEAVY FREEZING SPRAY WARNING 180
FIRE WEATHER...NONE

DIXON MAY 13