Small Craft Advisory 924, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
500 AM AKST THU NOV 21 2013

.ANALYSIS AND UPPER LEVELS...

THE UPPER LEVEL LOW ACROSS THE ALASKA MAINLAND HAS STALLED AS A
SHORTWAVE OVER THE EASTERN BERING SEA CUTS OFF THE UPPER LEVEL LOW
FROM THE MEAN FLOW. THIS HAS CAUSED THE LOW TO RETROGRADE TOWARD THE
KUSKOKWIM DELTA/EASTERN BERING SEA EARLY THIS MORNING. MOISTURE THAT
WAS PREVIOUSLY INHIBITED FROM ENTERING THE REGION WILL NOW HAVE FREE
ACCESS TO ADVECT NORTHWARD TOWARD THE ALASKA MAINLAND BEHIND A WARM
FRONT. THIS WARM FRONT CURRENTLY EXTENDS FROM SOUTH OF PORT HEIDEN
EASTWARD NORTH OF KODIAK AND INTO THE NORTHERN GULF. THE UPPER LEVEL
TROUGH HAS BECOME WELL ESTABLISHED OVER THE ALASKA MAINLAND AND WILL
ALLOW MOISTURE FROM THE SUBTROPICS TO REPLACE THE DRY AIR MASS
CURRENTLY SITUATED OVER THE STATE. SNOW SHOWERS HAVE ALREADY BEGUN
ACROSS THE BRISTOL BAY AREA AS MOISTURE SURGES AHEAD OF THE WARM
FRONT AND INTERACTS WITH THE SHORTWAVE AND UPPER LEVEL LOW OVER THE
KUSKOKWIM DELTA/WESTERN BERING SEA.

.MODEL DISCUSSION...

MODELS ARE IN GOOD AGREEMENT THROUGH 36 HOURS AS LOW PLACEMENT AND
THE EXTENT OF WARM AIR ADVECTION BEGINS TO CREATE FORECAST CHALLENGES
PAST 36 HOURS. THE GFS...WHICH HAD BEEN THE MOST CONSISTENT MODEL HAS
NOW BECOME A BIT OF AN OUTLIER IN REGARDS TO THE PLACEMENT OF THE LOW
AS IT MOVES INLAND. THE ECMWF AND NAM ARE IN BETTER AGREEMENT WITH
THE LOW MOVING INLAND NEAR THE ALEUTIAN RANGE AND MOVING UP TOWARD
COOK INLET. THE GFS REMAINS FURTHER EAST WITH A LOW PLACEMENT OVER
THE KENAI PENINSULA. WITH THE CONSENSUS OF MODEL SOLUTIONS FURTHER
WEST...IT APPEARS A DONE DEAL THAT WARM AIR WILL MAKE ITS WAY INTO
ANCHORAGE ON FRIDAY CAUSING A PERIOD OF FREEZING RAIN. MODELS ARE IN
BETTER AGREEMENT WITH A TRIPLE POINT LOW DEVELOPING IN THE GULF OF
ALASKA THIS EVENING AND MOVING INLAND EARLY FRIDAY. THIS WILL AID IN
BRINGING ANOTHER SURGE OF MOISTURE TO THE NORTHERN GULF COAST AND
SOUTHCENTRAL ALASKA. THE NAM IS THE MOST AGGRESSIVE MODEL IN REGARDS
TO THE STRENGTH AND DEPTH OF THE WARM AIR THAT WILL MOVE IN EARLY
FRIDAY. IT RAISES 850 MB TEMPERATURES UP TO +3 TO +4 DEGREES CELSIUS
OVER ANCHORAGE AND THE MATANUSKA VALLEY WHILE THE GFS/ECMWF/UKMET ALL
HAVE 850 TEMPS NO HIGHER THAN +1 DEGREE CELSIUS FOR ANCHORAGE AND THE
MATANUSKA VALLEY. THUS...CONFIDENCE HAS CONTINUED TO INCREASE WITH
THE POTENTIAL FOR FREEZING RAIN ACROSS ANCHORAGE AND THE MAT VALLEY
ON FRIDAY.

.SHORT TERM FORECAST...

SOUTHCENTRAL...A WARM FRONT WILL APPROACH FROM THE SOUTH DURING THE
DAY. PRECIPITATION WILL BEGIN TO FALL ALONG THE GULF COAST EARLY THIS
AFTERNOON...WITH PRECIPITATION MOVING ACROSS ANCHORAGE AND THE MAT-SU
VALLEY BY THIS EVENING. PRECIPITATION WILL START AS SNOW ALONG THE
GULF COAST THIS AFTERNOON BUT WILL QUICKLY CHANGE OVER TO RAIN OR A
RAIN/SNOW MIX BY LATE THIS AFTERNOON/EARLY THIS EVENING. BLOWING SNOW
WILL ALSO BE POSSIBLE AS THE PRESSURE GRADIENT BEGINS TO TIGHTEN AND
PERIODS OF HEAVY SNOW OCCURS. THIS WILL LEAD TO VISIBILITIES ONE HALF
MILE OR LESS AT TIMES. OVER ANCHORAGE AND THE MATANUSKA
VALLEY...LIGHT SNOW WILL START IN THE MID TO LATE AFTERNOON. WITH THE
LOW EXPECTED TO MOVE INLAND WEST OF THE INLET...MODELS ARE BEGINNING
TO INDICATE THAT SOME SEMBLANCE OF DOWN-SLOPING WILL OCCUR OVER
ANCHORAGE AND THE MATANUSKA VALLEY AS STOUT EAST/SOUTHEAST WINDS AT
850 MB WILL BE PRESENT. THIS WILL COME INTO PLAY AS THE WARM FRONT
MOVES INLAND AND TEMPERATURES ALOFT BEGIN TO WARM ABOVE FREEZING ON
FRIDAY. SNOW WILL TRANSITION TO FREEZING RAIN EARLY FRIDAY MORNING AS
850 TEMPERATURES WILL RISE TO +1 TO +2 DEGREES CELSIUS. THIS WILL
BRING A PERIOD OF FREEZING RAIN TO ANCHORAGE AND THE MATANUSKA
VALLEY. IF THE AFOREMENTIONED DOWN-SLOPING SHOULD OCCUR...IT WOULD
REDUCE THE QPF RECEIVED FROM THE FREEZING RAIN. REGARDLESS...FREEZING
RAIN IS ALMOST A CERTAINTY AT THIS POINT EARLY FRIDAY MORNING THROUGH
EARLY FRIDAY AFTERNOON.

ALL PRECIPITATION WILL QUICKLY TRANSITION BACK TO SNOW FRIDAY EVENING
AS COLD AIR WRAPS AROUND THE UPPER LEVEL LOW OVER THE EASTERN BERING
SEA. WITH AMPLE MOISTURE LEFT OVER THE REGION FOLLOWING THE PASSAGE
OF THE WARM FRONT...PERIODS OF MODERATE TO HEAVY SNOWFALL WILL BE
SEEN ACROSS THE SOUTHCENTRAL. SCATTERED SNOW SHOWERS SHOULD CONTINUE
THROUGH SUNDAY AS THE UPPER LEVEL LOW WILL CONTINUE TO SPIN OVER
SOUTHERN ALASKA. BY THE END OF THE SHORT-TERM...THE UPPER LEVEL LOW
WILL GET RE-ABSORBED BY THE LONG-WAVE PATTERN AND BRING AN END TO THE
PRECIPITATION FOR THE TIME BEING. NEAR NORMAL TEMPERATURES WILL BE
OBSERVED FROM FRIDAY ONWARD.

SOUTHWEST...WARM AND MOIST AIR CONTINUES TO SURGE NORTH ACROSS THE
BRISTOL BAY AREA AHEAD OF A WARM FRONT LOCATED OVER THE ALASKA
PENINSULA. THIS HAS LEAD TO SCATTERED SNOW SHOWERS MAINLY SOUTH OF
DILLINGHAM THIS MORNING WITH ONLY MINOR ACCUMULATIONS. THE
PRECIPITATION WILL CONTINUE TO SPREAD NORTH THROUGHOUT THE DAY...WITH
SNOW MOVING INTO THE KUSKOKWIM DELTA BY LATE THIS AFTERNOON. BLOWING
SNOW WILL BE SEEN ACROSS SEVERAL LOCATIONS IN THE BRISTOL BAY AND
LOWER KUSKOKWIM VALLEY AREAS AS GUSTY WINDS IN EXCESS OF 25 MPH WILL
BE OBSERVED. THIS WILL CREATE VISIBILITY RESTRICTIONS TO LESS THAN
ONE HALF MILE AT TIMES. ADDITIONAL ACCUMULATIONS OF 5 TO 10 INCHES
WILL BE POSSIBLE ACROSS THE BRISTOL BAY AND LOWER KUSKOKWIM VALLEY
AREAS...WITH 1 TO 3 INCHES OF ADDITIONAL SNOWFALL IN THE KUSKOKWIM
DELTA. SNOW SHOWERS WILL CONTINUE THROUGH SATURDAY AS THE UPPER LEVEL
LOW CONTINUES TO SPIN OVER THE AREA. THIS LOW WILL MOVE EASTWARD ON
SUNDAY AS IT GETS RE-ABSORBED BY THE LONG-WAVE PATTERN...THUS
BRINGING AN END TO THE SNOW.

BERING SEA AND ALEUTIAN ISLANDS...WITH THE UPPER LEVEL LOW SPINNING
OVER THE EASTERN BERING SEA...SCATTERED SNOW SHOWERS WILL BE SEEN
ACROSS THE EASTERN HALF OF THE BERING SEA. SNOW SHOWERS WILL TAPER
OFF ON SATURDAY AS THE UPPER LEVEL LOW SLIDES EASTWARD TOWARD
SOUTHCENTRAL ALASKA. AT THE SAME TIME...A NORTH PACIFIC LOW WILL
SKIRT SOUTH OF THE WESTERN AND CENTRAL ALEUTIANS. THIS WILL BRING AN
INCREASED CHANCE FOR PRECIPITATION TO THE REGION THROUGH THE WEEKEND.
MODELS ARE CURRENTLY HOLDING MOST OF THE WARM AIR SOUTH OF THE
ALEUTIAN ISLANDS WHICH COULD ALLOW FOR A RAIN/SNOW MIX THROUGH THE
WEEKEND. COLD AIR WILL WRAP AROUND THIS LOW AS IT MOVES EAST OVER THE
WEEKEND CREATING SCATTERED SNOW SHOWERS OVER THE WESTERN BERING SEA.

.LONG TERM FORECAST...

AN ACTIVE WEATHER PATTERN WILL CONTINUE INTO THE LONG TERM AS A
SERIES OF NORTH PACIFIC LOWS WILL IMPACT THE ALEUTIAN ISLANDS...THE
BERING SEA AND GULF COAST. THE UPPER LEVEL PATTERN WILL BE IN
FLUX...WITH LOW AMPLITUDE LONG-WAVE FEATURES. MODELS ARE STRUGGLING
TO DISTINGUISH HOW THE UPPER LEVEL PATTERN WILL EVOLVE WHICH WILL BE
A KEY FACTOR IN DETERMINING PRECIPITATION TYPE ACROSS SOUTHCENTRAL
AND SOUTHWEST ALASKA AS A SERIES OF LOW IMPACTS THE REGION.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...BLIZZARD WARNING...131
MARINE...STORM WARNING...119 130 131 138
         GALE WARNING...120 125 127 128 129 132 137 139 140 150 155
                        160 165 172 176 177 178 181 185
         HEAVY FREEZING SPRAY WARNING...121 126 127 128 130 139 140
                                        141 160 180 181 185
FIRE WEATHER...NONE.

MMC NOV 13