Small Craft Advisory 158, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
515 AM AKST THU FEB 19 2015

.ANALYSIS AND UPPER LEVELS...
AN UPPER LEVEL AMPLIFIED RIDGE IS POSITIONED NEAR 50N 140W AND
EXTENDS THROUGH KODIAK ISLAND INTO THE BERING STRAIT REGION. ON
THE BACKSIDE OF THIS RIDGE THERE IS A BROAD SOUTHERLY FLOW THROUGH
THE BERING SEA AND THE ALEUTIAN CHAIN...EXCEPT FOR A CLOSED LOW
NEAR 48N 173W. AT THE SURFACE THERE IS A GALE FORCE NORTH PACIFIC
LOW SOUTH OF THE CENTRAL ALEUTIANS WITH A FRONTAL BOUNDARY
EXTENDING THROUGH NIKOLSKI INTO THE EASTERN BERING SEA. SATELLITE
IMAGERY IS SHOWING THE LARGE COMMA SHIELD SPREADING INTO THE
CENTRAL TO EASTERN ALEUTIANS AND SPILLING OVER INTO THE EASTERN
BERING SEA. THIS SYSTEM WILL BRING MORE WARM AIR THROUGH THE
ALEUTIANS AND BERING SEA. THE SURFACE OBSERVATIONS ALONG THE
CENTRAL/EASTERN ALEUTIANS ARE REPORTING TEMPERATURES IN THE LOWER
40S WITH STEADY RAIN THIS MORNING. ASCAT SCATTEROMETER WINDS ARE
SHOWING SOUTHEASTERLY GALE FORCE WINDS ON THE NORTH PACIFIC SIDE
OF THE EASTERN ALEUTIANS. SOUTHWEST ALASKA CONTINUES WITH ONSHORE
FLOW BRINGING STRATUS THROUGHOUT THE AREA WITH PATCHY FOG THIS
MORNING. THERE IS A WEAK DISTURBANCE ALOFT OVER THE KENAI
PENINSULA AND IS REFLECTED AT THE SURFACE WITH A WEAK LOW JUST
SOUTH OF CAPE SAINT ELIAS. THIS SYNOPTIC SYSTEM IS SUPPORTED BY A
50 KNOT WESTERLY JET STREAK OVER THE NORTHERN GULF. THIS SYSTEM
WILL LOSE ITS SUPPORT AND DISSIPATE BY THIS AFTERNOON.

&&

.MODEL DISCUSSION...
THE MODELS WERE IN DECENT AGREEMENT WITH THE WEAK LOW IN THE
NORTHERN GULF BEFORE TRANSITIONING TO A SOUTHEASTERLY FLOW
THROUGH THIS WEEKEND. THE MODELS STRUGGLED WITH ONLY THE GEM-REG
SHOWING A LOW FORMING SOUTH OF KODIAK ISLAND. WHILE...THE
ECMWF/GFS AND THE NAM MODELS WERE ONLY SHOWING A TROUGH OVER THE
AREA. IN THE BERING SEA AND THE ALEUTIANS THIS MORNING...A NORTH
PACIFIC LOW IS LOCATED JUST SOUTH OF ATKA. THERE IS AN ASSOCIATED
WEATHER FRONT WITH THIS SYSTEM EXTENDING THROUGH THE CENTRAL
ALEUTIANS INTO THE CENTRAL BERING SEA TODAY. THE MODELS THEN
STRUGGLE WITH THIS SYSTEM AS IT MOVES EASTWARD AND CROSSES OVER THE
EASTERN ALEUTIANS BY SATURDAY. THE ECMWF/GFS KEEP THIS SYSTEM AS
A BROAD LOW AS IT MOVES THROUGH THE BERING SEA. THE PREFERRED
MODELS OF CHOICE WAS THE NAM/GEM-REG THROUGH 36 HOURS BEFORE
TRANSITIONING TO ECMWF/GFS BLEND FOR REST OF SHORT TERM. FORECAST
CONFIDENCE IS MODERATE TO HIGH.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (THURSDAY AND FRIDAY)...

THE DRAMATIC AMPLIFICATION OF THE NORTHEAST PACIFIC RIDGE HAS
BEGUN TO NOSE INTO THE GULF OF ALASKA TODAY. THIS BROAD-SCALE
SUBSIDENCE WILL LEAD TO A SUNNY DAY TODAY ACROSS LARGE PORTIONS
OF SOUTHCENTRAL ALASKA (EXCLUDING KODIAK ISLAND). PATCHY FOG THAT
DEVELOPED AROUND COOK INLET SHOULD BURN OFF DURING THE MORNING IN
THE FACE OF UNFILTERED LATE-FEBRUARY SUNSHINE. THE RIDGE AXIS OF
THIS BUILDING RIDGE WILL PASS OVER THE AREA TONIGHT...USHERING IN
THE BEGINNING OF ANOTHER DRAMATIC ROUND OF SOUTHERLY FLOW AND WARM
AIR ADVECTION. CURRENT THINKING REMAINS UNCHANGED THAT THIS WARM
AIR ADVECTION WILL HELP TO STRENGTHEN SUBSIDENCE TEMPERATURE
INVERSIONS AND WILL LIKELY LEAD TO FOG DEVELOPMENT ONCE AGAIN
TONIGHT...POSSIBLY EVEN MORE WIDESPREAD THAN THE NIGHT JUST
FINISHED (WEDNESDAY NIGHT).

THE NEXT FRONT APPROACHES FROM THE SOUTH DURING THE DAY
FRIDAY...WITH ASSOCIATED RAINFALL STARTING THE DAY OVER KODIAK
ISLAND THEN SPREADING UP TO THE EASTERN KENAI PENINSULA BY LATE
IN THE AFTERNOON. THE JET STREAM WILL BE MORE SOUTHERLY WITH THIS
APPROACHING STORM COMPARED TO A MORE SOUTHEASTERLY ORIENTATION
DURING THE STORM 2 DAYS AGO...THUS EXPECT THE TYPICAL DOWNSLOPE
AREAS FROM THE KENAI UP THROUGH ANCHORAGE AND THE MAT-SU TO GET IN
ON THE PRECIP MORE THAN USUAL. THE GOOD AND BAD NEWS IS THAT
PLENTY OF WARM AIR WILL ALSO SURGE INTO THE AREA AND SO PRECIP AT
VIRTUALLY ALL LOCATIONS IS EXPECTED TO BE REGULAR RAIN DURING THIS
STORM...INSTEAD OF THE DREADED FREEZING RAIN SO COMMON WHEN COLD
AIR IS TRAPPED IN THE LEE OF THE MOUNTAINS.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
A STOUT NORTH PACIFIC FRONT WILL MOVE INTO SOUTHWEST ALASKA THIS
AFTERNOON...AND WILL MAKE EASTWARD PROGRESS THROUGH SATURDAY
TOWARD THE LOWER KUSKOKWIM VALLEY. WITH SOME LOW LEVEL COLD AIR IN
PLACE...THE INITIAL WARM FRONT MAY SUPPORT SOME SNOW ACROSS PARTS
OF BRISTOL BAY AND THE LOWER KUSKOKWIM DELTA...BUT THIS SHOULD
QUICKLY CHANGE TO RAIN ACROSS MOST LOCATIONS. THE ONE HOLD OUT
AREA WILL BE ACROSS THE NORTHWEST DELTA AS WELL AS ACROSS THE
LOWER KUSKOKWIM VALLEY WHERE COLDER AIR MAY LINGER LONGER. A FEW
ISOLATED POCKETS OF FREEZING RAIN CAN NOT BE RULED OUT ACROSS THE
KUSKOKWIM VALLEY. THE STRONGEST WINDS WILL BE ACROSS THE BRISTOL
BAY REGION...ESPECIALLY NEAR THE ALEUTIAN RANGE AND THROUGH THE
GAPS.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THERE IS A WEATHER FRONT EXTENDING THROUGH THE CENTRAL ALEUTIANS
WITH A LARGE SWATH OF MOISTURE AND PRECIPITATION SPILLING OVER
INTO THE CENTRAL TO EASTERN BERING SEA. THIS WILL BE ACCOMPANIED
BY WARMER TEMPERATURES AND LOW STRATUS WITH RAIN ALONG THE CHAIN
AND A WINTRY MIX TOWARDS THE PRIBILOF ISLANDS THIS MORNING. THE
PRIBILOF ISLANDS WILL CHANGE OVER TO ALL RAIN BY MIDDAY. THE NORTH
PACIFIC LOW SOUTH OF ATKA THIS MORNING WILL GRADUALLY MOVE
EASTWARD AND BECOME ELONGATED RUNNING PARALLEL WITH THE CHAIN.
THIS SYSTEM WILL CONTINUE TO BRING MORE MOISTURE AND
PRECIPITATION ALONG THE EASTERN ALEUTIANS BEFORE SPILLING OVER
INTO THE EASTERN BERING SEA. THE WESTERN BERING SEA WILL REMAIN
WITH NORTHERLY WINDS WITH SCATTERED SHOWERS THROUGH FRIDAY.

&&

.LONG TERM FORECAST (SATURDAY THROUGH WEDNESDAY)...
ON SATURDAY...THE NORTH PACIFIC DELUGE WILL BE IN FULL SWING AS A
NORTH PACIFIC TROUGH SPREADS WARM MARITIME AIR INTO SOUTHERN
ALASKA. HEAVY RAIN WILL BE IMPACTING MOST OF THE GULF COAST TO
KODIAK ISLAND...WITH RAIN SHOWERS ACROSS INLAND LOCATIONS. SOME
ISOLATED AREAS OF SNOW WILL BE POSSIBLE ACROSS PORTIONS OF
SOUTHWEST ALASKA...AS WELL AS ACROSS THE COPPER BASIN. A SUBTLE
SHORTWAVE DISTURBANCE ALONG THE JET STREAM WILL HELP SUPPORT THE
DEVELOPMENT OF PERIODS OF ACCUMULATING RAIN ACROSS MOST OF
SOUTHCENTRAL BY SATURDAY AFTERNOON AND EVENING...WITH RAIN SHOWERS
PERSISTING INTO SUNDAY. EVENTUALLY THE NORTH PACIFIC TROUGH WILL
MOVE EAST LATE SUNDAY THROUGH MONDAY...WITH PRECIPITATION ENDING
FROM WEST TO EAST ACROSS SOUTHERN ALASKA. THERE WILL BE AT LEAST A
ONE DAY BREAK IN THE RAIN-FEST...WITH SLIGHTLY COOLER AIR
INFILTRATING ALASKA TUESDAY. THEREAFTER...ALL THE GLOBAL NUMERICAL
GUIDANCE IS HINTING AT A FASTER AND MORE PROGRESSIVE/LESS
AMPLIFIED UPPER AIR PATTERN. AS IS TYPICAL IN THESE PATTERNS...THE
TIMING OF UPPER LEVEL SHORTWAVE TROUGHS IS LESS CERTAIN...SO
FORECAST CONFIDENCE DROPS RAPIDLY BY MIDWEEK. HOWEVER...THERE DOES
SEEM TO BE A REPRIEVE FROM THE ANOMALOUSLY WARM WEATHER REGIME BY
MIDWEEK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES 119 120 130 132 136-138 150 155 160 165 170 175-177
               180.
         STORM 131.
         HEAVY FREEZING SPRAY 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...AD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD/JRA
LONG TERM...JRA