Small Craft Advisory 1029, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
420 PM AKST TUE DEC 23 2014

.ANALYSIS AND UPPER LEVELS...
A STORM FORCE LOW HAS BOTTOMED OUT AROUND 954 MB NORTH OF THE
WESTERN ALEUTIANS. THE LOW HAS NEARLY STALLED WHILE THE FRONTAL
SYSTEM IS VERY SLOWLY WEAKENING AS IT SPREADS NORTH AND EAST
ACROSS THE BERING AND ALEUTIANS. SOME OF THE STRONGEST WINDS WITH
THIS SYSTEM ARE BEING OBSERVED ALONG THE WESTERN ALEUTIANS. AS
THE LOW DRIFTS NORTH AND WEST AND BEGINS TO WEAKEN THIS AFTERNOON
WINDS WILL BEGIN TO SLACKEN. THE AIR MASS OUT AHEAD OF THE FRONTAL
SYSTEM IS COLD ENOUGH FOR PRECIPITATION TO START AS ALL SNOW FROM
THE PRIBILOF ISLANDS TO THE SOUTHERN ALASKA PENINSULA...BUT
SURFACE TEMPERATURES ARE GENERALLY ABOVE FREEZING SO EXPECT LITTLE
OR NO ACCUMULATION BEFORE CHANGING TO ALL RAIN. ANOTHER MUCH
WEAKER STORM SYSTEM OVER THE NORTHWEST PACIFIC IS POISED TO BE
PULLED INTO THE CIRCULATION OF THE FIRST SYSTEM...CAUSING A
NORTHWARD CURVE TOWARD THE ALEUTIANS.

OUT AHEAD OF THE ALEUTIAN SYSTEM A COUPLE WEAK OUT OF PHASE UPPER
LEVEL SHORT-WAVES ARE CROSSING SOUTHWEST ALASKA AND SOUTHCENTRAL
ALASKA. THERE IS A BROKEN UP BAND OF LIGHT SHOWERS NOTED ON RADAR
OVER INTERIOR SOUTHWEST ALASKA. OTHERWISE PRECIPITATION IS VERY
SPOTTY. THE BIGGER PROBLEM IS FOG AND STRATUS WHICH DEVELOPED
OVERNIGHT. FOG HAS BECOME LOCALLY DENSE ACROSS PORTIONS OF
SOUTHCENTRAL ALASKA. WITH A SHORT-WAVE RIDGE SET TO BUILD IN
BEHIND THE UPPER WAVES DON'T EXPECT A WHOLE LOT OF IMPROVEMENT
UNTIL THE FRONTAL SYSTEM OUT WEST ARRIVES ON WEDNESDAY.

&&

.MODEL DISCUSSION...
A DYNAMIC PATTERN WITH STRONG FAST MOVING SYSTEMS WILL PROVIDE
A VERY CHALLENGING FORECAST AREA-WIDE OVER THE NEXT FEW DAYS.
NUMERICAL GUIDANCE IS REFLECTIVE OF THIS WITH GENERALLY POOR
RUN-TO-RUN CONSISTENCY. THERE IS INITIALLY HIGH CONFIDENCE WITH
CONTINUED EASTWARD PROGRESSION OF THE SURFACE OCCLUDED FRONT AND
ASSOCIATED UPPER LEVEL WAVE. MODEL DIFFERENCES ARISE WITH
DEVELOPMENT OF A TRIPLE POINT LOW OVER THE NORTHWEST GULF ON
WEDNESDAY. WITH WEAK LOW LEVEL BAROCLINICITY/FAST FLOW ALOFT/AND
INCREASING TILT OF THE UPPER TROUGH WITH TIME PREFER A WEAKER LOW
AND FARTHER NORTH TRACK AS DEPICTED BY THE 12Z ECMWF AND 18Z NAM.
THIS PRIMARILY AFFECTS THE WIND FORECAST OVER THE NORTHERN
GULF/PRINCE WILLIAM SOUND WITH LITTLE IMPACT ON THE PRECIPITATION
FORECAST FOR SOUTHCENTRAL.

MODEL SPREAD CONTINUES TO GROW WITH EACH SUCCESSIVE STORM SYSTEM
MOVING FROM THE PACIFIC ACROSS THE ALEUTIANS INTO THE BERING SEA
LEADING TO GENERALLY LOW CONFIDENCE IN THE EXACT TRACK OF THE
STORMS.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE SOUTH END OF AN UPPER LEVEL SHORT-WAVE OVER SOUTHWEST ALASKA
WILL CROSS SOUTHCENTRAL ALASKA TONIGHT. WITH VERY WEAK VERTICAL
MOTION AND LOW LEVEL RIDGING BEGINNING TO BUILD OVER SOUTHCENTRAL
EXPECT IT TO COME THROUGH MOSTLY DRY. IT SEEMS THE BEST CHANCE FOR
A FEW SNOW SHOWERS WILL BE THE NORTHERN SUSITNA VALLEY...INITIALLY
AIDED BY LOW LEVEL UPSLOPE FLOW THIS EVENING. OTHERWISE...WITH A
SHORT-WAVE UPPER LEVEL RIDGE BUILD IN BEHIND IT EXPECT AREAS OF
FOG...DENSE IN PLACES...TO PERSIST UNTIL THE ARRIVAL OF THE
FRONTAL SYSTEM CURRENTLY CROSSING THE BERING SEA.

EVEN WITH SOME UNCERTAINTY IN THE STRENGTH AND TRACK OF A TRIPLE
POINT LOW OVER THE NORTHERN GULF ON WEDNESDAY THERE IS HIGH
CONFIDENCE IN THE PRECIPITATION FORECAST AS THE OCCLUDED FRONT
AND UPPER WAVE MOVE THROUGH THE REGION. WITH DEEP SOUTHERLY FLOW
AND MODERATE TO STRONG VERTICAL MOTION AHEAD OF THE UPPER TROUGH
MOST AREAS WILL SEE SOME PRECIP WEDNESDAY AFTERNOON THROUGH
WEDNESDAY NIGHT...SO WILL UP POPS ACCORDINGLY.

THE NEXT LOW MOVING INTO THE BERING WILL SEND ANOTHER FAST MOVING
FRONT AND A NOTABLE WEAKER UPPER LEVEL TROUGH TO SOUTHCENTRAL
SOMETIME THURSDAY. THERE IS MORE UNCERTAINTY IN THE TIMING OF
THIS...BUT THERE ARE ENOUGH SIMILARITIES WITH THE FIRST SYSTEM
THAT ANOTHER (LIGHTER) PRECIP EVENT IS LIKELY.

THESE LOW PRESSURE SYSTEMS WILL OCCLUDE WAY OUT WEST WHICH MEANS
THE LOW LEVEL AIR MASS WILL NOT CHANGE A WHOLE LOT OVER
SOUTHCENTRAL THE NEXT COUPLE DAYS. THUS...THE PRIMARY PRECIP-TYPE
WILL BE SNOW...WITH THE EXCEPTION OF THE IMMEDIATE GULF COAST.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...

MAIN IMPACTFUL CONCERN THE NEXT COUPLE OF DAYS WILL BE WITH THE
POTENTIAL FOR BLOWING SNOW FOR THE KUSKOKWIM DELTA AND THE WESTERN
CAPES OF THE BRISTOL BAY ZONE. SEASONABLY CHILLY TEMPERATURES
COMBINED WITH AN APPROACHING FRONT AND MOISTURE ABOVE THE SURFACE
WILL RESULT IN A CONTINUOUS BAND OF SNOW MOVING ONSHORE OF THIS
REGION TONIGHT. WINDS WILL GUST TO 25 TO 30 MPH AND COMBINED WITH
THE DRY CHARACTER OF THE SNOW WILL LIKELY RESULT IN VISIBILITY
REDUCTIONS TO ONE-HALF MILE AND POTENTIALLY TO ONE-QUARTER MILE AT
TIMES. A WINTER WEATHER ADVISORY FOR BLOWING SNOW WILL BE ISSUED
WITH THE AFTERNOON FORECAST PACKAGE GIVEN THE INCREASING
CONFIDENCE OF GREATLY REDUCED VISIBILITIES IN BLOWING SNOW
ESPECIALLY FOR COASTAL AREAS OF THE KUSKOKWIM DELTA COVERING THE
PERIOD FROM MID-EVENING TONIGHT THROUGH LATE MORNING WEDNESDAY.

BLOWING SNOW CONDITIONS WILL IMPROVE FROM SOUTH TO NORTH ACROSS
THE KUSKOKWIM DELTA DURING THE MORNING HOURS...THOUGH THIS MAY BE
TEMPORARY. IF LOW PRESSURE DEVELOPS JUST NORTH OF BETHEL ALONG
THE STRETCHING/WEAKENING FRONT AS THE EC AND CANADIAN REGIONAL
MODELS SUGGEST...THEN A STRONGER ONSHORE PUSH OF WINDS AND A RE-
DEVELOPING OF SNOW MAY OCCUR AND RESULT IN BLOWING SNOW WITH
REDUCED VISIBILITIES ALONG THE KUSKOKWIM RIVER.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
LOW PRESSURE HAS REACHED PEAK INTENSITY OVER THE WESTERN BERING
SEA...AND WINDS HAVE BEGUN A SLOW SUBSIDING TREND BUT WILL STILL
REMAIN STRONG FOR A WHILE YET. WILL CANCEL THE HIGH WIND WARNING
WITH THE AFTERNOON PACKAGE AS A RESULT. A SECOND SYSTEM WILL
ROTATE ACROSS THE WESTERN AND CENTRAL BERING TONIGHT THROUGH
WEDNESDAY RESULTING IN PERIODS OF RAIN AND BLUSTERY CONDITIONS WITH
SOME SNOW ALONG THE OUTER FRINGES OF THE MAIN PRECIPITATION
SHIELD.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...

THE BERING SEA WILL REMAIN IN AN ACTIVE WET AND WINDY PATTERN
THROUGH EARLY NEXT WEEK AS A LONGWAVE TROUGH REMAINS CENTERED OVER
THE WESTERN BERING. A SERIES OF NORTH PACIFIC SHORTWAVES AND
ASSOCIATED DEEP SURFACE LOWS WILL ROTATE AROUND THE BASE OF THE
TROUGH...AND BECOME REDIRECTED BACK INTO THE CENTRAL/EASTERN
BERING BY A HIGHLY AMPLIFIED BLOCKING RIDGE THAT SETS UP OVER THE
MAINLAND BY THE END OF THIS WEEK. MODELS CONTINUE TO STRUGGLE
WITH THE TRACK AND STRENGTH OF THESE SYSTEMS OUT WEST...WITH THE
ECMWF MAKING QUITE A LARGE JUMP TODAY REGARDING THE TRACK OF THE
LOW APPROACHING THE WESTERN ALEUTIANS BY FRIDAY AFTERNOON. IN
COMPARISON TO THE 00Z ECMWF RUN...THE 12Z ECMWF SOLUTION
SUBSTANTIALLY SLOWED DOWN AND STRENGTHENED THE LOW...AND STALLS IT
OUT OVER THE WESTERN ALEUTIANS THROUGH MUCH OF THE WEEKEND. EVEN
THOUGH THE 18Z NAM AND GFS HAVE BOTH TRENDED TOWARD THAT SLOWER
AND STRONGER 12Z EC SOLUTION...FORECAST CONFIDENCE STILL REMAINS
LOWER THAN AVERAGE GIVEN THE CURRENT MODEL DIFFERENCES AND
INCONSISTENCIES.

FURTHER EAST...THIS PATTERN WILL YIELD DEEP SOUTH TO
SOUTHWESTERLY FLOW OVER THE SOUTHERN MAINLAND AS THE HIGHLY
AMPLIFIED RIDGE SETS UP OVER THE MAINLAND THIS WEEKEND. THIS
SHOULD KEEP SHOWERY CONDITIONS FOCUSED OVER MUCH OF THE AREA...AS
EACH WAVE TRACKS FROM THE NORTH PACIFIC INTO THE EASTERN/CENTRAL
BERING. THESE SYSTEMS WILL ACT AS A CONTINUOUS CONVEYOR BELT
BRINGING WARMER TEMPERATURES OVER THE SOUTHERN MAINLAND FROM THE
NORTH PACIFIC. WARMER TEMPERATURES ARE EXPECTED TO BUILD OVER THE
WEEKEND AND PERSIST INTO EARLY NEXT WEEK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...STORMS 177 178 185
         GALES 150 155 160 165 170 172-176 179 180 181
         HEAVY FREEZING SPRAY 181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CC
LONG TERM...TP