Small Craft Advisory 317, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
519 AM AKDT SAT APR 11 2015

.ANALYSIS AND UPPER LEVELS...

THE REGIONAL PATTERN CONTINUES TO BE DICTATED BY AN ACTIVE NORTH
PACIFIC JET STREAM...WITH A WEAKENING UPPER LOW OVER THE EASTERN
GULF OF ALASKA. THIS GULF LOW IS RESPONSIBLE FOR MOST OF THE
PRECIPITATION ACROSS SOUTHERN ALASKA. OVER THE BERING SEA...BROAD
NORTHWEST FLOW AND COLD AIR ADVECTION ORIGINATING FROM EASTERN
SIBERIA CONTINUES TO POUR SOUTHEASTWARD INTO THE NORTHEAST PACIFIC
OCEAN. DEEP INSTABILITY ASSOCIATED WITH THE WARM OCEAN WATERS AND
VERY COLD AIR IS SUPPORTING AREAS OF SIGNIFICANT MARITIME
CONVECTIVE SNOW SHOWERS OVER THE BERING AND ALEUTIAN CHAIN. SOME
OF THAT UNSTABLE AIR WAS RESPONSIBLE FOR HELPING SPARK OF NUMEROUS
CONVECTIVE SHOWERS OVER THE MAINLAND YESTERDAY
AFTERNOON...INCLUDING ISOLATED THUNDERSTORMS NEAR THE ANCHORAGE
BOWL...QUITE A RARITY FOR APRIL.

BY LATE WEEKEND...A SECONDARY UPPER TROUGH AXIS NEAR KODIAK
ISLAND WILL BEGIN MOVING TOWARD THE NORTHERN GULF COAST AND
SOUTHERN ALASKA...SPREADING MORE SNOW SHOWERS OVER PORTIONS OF THE
REGION.

&&

.MODEL DISCUSSION...

THE MODELS ARE IN EXCELLENT AGREEMENT WITH THE LARGE SCALE PATTERN
HEADING INTO EARLY NEXT WEEK...AND WITH THE GENERAL PATTERN
HEADING INTO MIDWEEK. THE LARGEST DISCREPANCIES CONTINUE TO
REVOLVE AROUND SMALLER SCALE MESOSCALE LOWS IN THE BERING SEA
PROPAGATING WITHIN THE DEEPER NORTHWEST FLOW...AND PRECIPITATION
PATTERNS OVER SOUTHERN ALASKA AS AFTERNOON CONVECTI0N ADDS TO
TEMPORAL AND SPATIAL UNCERTAINTY. AS A WHOLE...THE ECMWF AND
REGIONAL CANADIAN MODELS WERE FAVORED FOR THE WESTERN AND EASTERN
FORECAST DOMAINS.

&&

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

A WEAKENING LOW IN THE NORTHERN GULF OF ALASKA WILL CONTINUE TO
STREAM SUFFICIENT MOISTURE INTO SOUTHERN ALASKA TO SUPPORT THE
DEVELOPMENT OF DAYTIME CONVECTIVE SHOWER ACTIVITY....ESPECIALLY
OVER THE HIGHER MOUNTAIN RANGES. WEAKER INSTABILITY AND INCREASING
MID LEVEL MOISTURE WILL DIMINISH INSTABILITY COMPARED TO
YESTERDAY...BUT WILL REMAIN SUFFICIENT ENOUGH TO SUPPORT A FEW
VIGOROUS SNOW SHOWERS...ESPECIALLY OVER THE KENAI MOUNTAINS AND
NEAR THE TALKEETNA MOUNTAINS. STEERING FLOW WILL BE WEAKER THAN
YESTERDAY...SO THE MOST INTENSE SHOWERS WILL GENERALLY REMAIN
ALONG THE NEAR THE MOUNTAINOUS TERRAIN. FROM THE BARREN ISLANDS
SOUTHWARD...WEST TO NORTHWEST WINDS WILL REMAIN STRONG INTO THE
AFTERNOON BEFORE THE LOW WEAKENS AND MOVES NORTH OVERNIGHT.

ON SUNDAY...A TROUGH AXIS NEAR KODIAK ISLAND WILL LIFT NORTHWARD
ACROSS THE NORTHERN GULF COAST INTO SOUTHERN ALASKA...AND THIS
WILL SUPPORT WIDESPREAD ACCUMULATING SNOW SHOWERS ACROSS COASTAL
LOCATIONS...ESPECIALLY FROM PRINCE WILLIAM SOUND EASTWARD. SHOWER
ACTIVITY WILL PUSH INTO INLAND AREAS AS WELL...BUT THEY WILL BE
LESS INTENSE AND LESS WIDESPREAD AS THEY MOVE INLAND. BUILDING
HIGH PRESSURE ALONG THE COAST...AS WELL AS DEEPENING PRESSURE
AROUND COOK INLET...WILL SUPPORT GUSTY EVENING AND OVERNIGHT GAP
WINDS. TURNAGAIN ARM WINDS WILL EVENTUALLY MAKE IT INTO SOUTH AND
WEST ANCHORAGE BY SUNDAY EVENING.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (TONIGHT THROUGH SUNDAY)...
AN UPPER LEVEL TROUGH APPROACHING THE SOUTHWEST COAST OVERNIGHT
WILL BRING CONTINUED SNOW SHOWERS...ESPECIALLY FROM SOUTHERN
BRISTOL BAY INTO THE KUSKOKWIM DELTA. THE HEAVIEST SHOWERS ARE
EXPECTED OVER BRISTOL BAY...AND ESPECIALLY TO THE EAST OF KING
SALMON WHERE A FEW INCHES OF NEW SNOWFALL IS POSSIBLE BY MORNING.
NORTHWESTERLY WINDS STRETCHING FROM THE KUSKOKWIM DELTA COAST INTO
THE AKPENN WILL DIMINISH LATE TONIGHT AS THE GULF LOW CONTINUES
TO WEAKEN AND THE PRESSURE GRADIENT DIMINISHES. THIS MAY CREATE A
FEW ISOLATED AREAS OF BLOWING SNOW THIS EVENING...BUT WITH LITTLE
ADDITIONAL ACCUMULATION EXPECTED THESE ARE NOT EXPECTED TO BE LONG
LIVED. FOR THIS WEEKEND...THE WEAKENING UPPER LEVEL TROUGH WILL
MOVE JUST SOUTH OF THE AREA WHICH WILL KEEP MOSTLY CLOUDY SKIES
AND SHOWERS IN THE AREA...BUT NOT AS HEAVY OR WIDESPREAD AS THE
LAST 2 DAYS. THE MAJORITY OF THESE SHOWERS SHOULD OCCUR DURING THE
AFTERNOON-EVENING HOURS AS DIURNAL SURFACE HEATING SUPPLIES SOME
ADDITIONAL INSTABILITY TO THE ATMOSPHERE.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (TONIGHT THROUGH SUNDAY)...
A WEAK SHORTWAVE MOVING THROUGH THE ALEUTIANS WILL BRING SNOW
SHOWERS TO THE AREA INTO TOMORROW MORNING...WITH ACCUMULATIONS
LIMITED MOSTLY TO THE BERING SEA COAST. NORTHWESTERLY FLOW WILL
ALSO CONTINUE THROUGHOUT THE REGION THIS WEEKEND...BUT WITH LIGHT
ACCUMULATIONS AND DIMINISHING WINDS EXPECTED SIGNIFICANT AREAS OF
BLOWING SNOW ARE NOT LIKELY. THIS PATTERN WILL THEN BEING TO
CHANGE LATE SUNDAY AS THE NEXT LOW APPROACHES THE WESTERN
ALEUTIANS FOR EARLY NEXT WEEK.

&&

.LONG TERM FORECAST (MONDAY THROUGH FRIDAY)...

A RAPIDLY DEVELOPING AREA OF LOW PRESSURE WILL MOVE FROM THE NORTH
PACIFIC AND INTO THE EASTERN GULF OF ALASKA ON MONDAY AND MOVE
INLAND OVER THE ALASKA PENINSULA MONDAY AFTERNOON. MODELS ARE
STRUGGLING WITH THIS SYSTEM AS THE LONGWAVE TROUGH INTERACTS WITH
WITH A WEAK UPPER LEVEL LOW NEAR KODIAK ISLANDS AND CREATES A
CLOSED UPPER LOW CENTER IN THE EASTERN GULF. THE REASON FOR THE
VOLATILITY IS THAT THE INTERACTION BETWEEN THE TWO UPPER LEVEL
FEATURES IS TOO DIFFICULT TO RESOLVE THIS FAR OUT...BUT THE OVERALL
SYNOPTIC PATTERN PAINTS THE SAME PICTURE. ON THE BACK SIDE OF THE
SFC LOW MOVING TOWARD THE PANHANDLE...DECENT PRESSURE RISES WILL
BE OBSERVED WHICH WILL LEAD TO SLIGHT AMPLIFICATION OF THE SFC
RIDGE OVER THE NPAC AND SOUTHERN GULF. THIS HAS RESULTED IN THE
SLOW DOWN OF THE STRONG LOW PRESSURE SYSTEM THAT WILL BE SKIRTING
SOUTH OF THE ALEUTIANS ISLANDS. THE BIGGEST CHANGE IN THE PAST 24
HOURS HAS BEEN THE SLOWING DOWN OF THE SYSTEM SOUTH OF THE ALEUTIANS
WHICH WILL ALLOW THE LOW TO MOVE INTO THE GULF OF ALASKA VS MOVING
ACROSS THE SOUTHERN GULF TOWARD THE PANHANDLE. WITH MODELS NOW
SHOWING THIS SOLUTION FOR THE PAST 2 SYNOPTIC MODEL RUNS AND WITH
GOOD AGREEMENT AMONGST ENSEMBLE AND DETERMINISTIC GUIDANCE...IT
APPEARS THAT A DECENT SNOWFALL EVENT FOR SOUTHCENTRAL ALASKA COULD
BE ON THE HORIZON.

HOWEVER...IT IS DIFFICULT TO BUY IN ON A WIDESPREAD SIGNIFICANT
SNOWFALL EVENT AT THIS TIME. TEMPERATURES WILL BE COLD ENOUGH
(HIGHS IN THE 30S AND LOWS IN THE UPPER 20S TO LOWER 30S) AND 850
TEMPERATURES BETWEEN -5 AND -10 DEGREE C. THE BIG QUESTION IS THE
TRACK OF THE LOW AS IT MOVES INTO THE GULF TO SEE HOW MUCH OF THE
PRECIPITATION WILL SPREAD INLAND. AS OF YESTERDAY WITH THE MORE
SOUTHERLY TRACK...IT APPEARED THAT DOWNSLOPING WOULD EAT UP A
MAJORITY OF THE PRECIPITATION. HOWEVER...MODELS NOW SHOW A MORE
NORTHERLY TRACK THAT PUTS THE LOW NEAR THE "SWEET SPOT" THAT COULD
POSSIBLY BRING DECENT SNOW ACCUMULATION THROUGH MUCH OF SOUTHCENTRAL
ALASKA. THE FORECAST PACKAGE NOW REFLECTS A HIGHER CHANCE FOR
PRECIP ALONG THE NORTHERN GULF COAST BUT HAVE HELD OFF ON BRINGING
HIGHER POPS INTO ANCHORAGE AND THE MAT-SU VALLEY UNTIL SOME OF THE
UPPER LEVEL FEATURES BEGIN TO TIP THEIR HAND. THIS IS A SITUATION
THAT WILL BE MONITORED CLOSELY IN THE COMING DAYS SO MAKE SURE TO
KEEP AN EYE OUT FOR SUBSEQUENT FORECAST PACKAGES/DISCUSSIONS.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE...130 132 138 150 178
         HEAVY FREEZING SPRAY...179 180
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
LONG TERM...MMC