Small Craft Advisory 1001, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
544 AM AKST TUE DEC 15 2015

.ANALYSIS AND UPPER LEVELS...

THE LARGE SCALE PATTERN ACROSS THE REGION CONTINUES TO BE
DOMINATED BY GENERAL UPPER LEVEL TROUGHING STRETCHING FROM THE
WESTERN BERING TO THE PANHANDLE OF ALASKA. THERE IS A SPLIT POLAR
AND SUBTROPICAL JET STREAM THIS MORNING...WHICH IS VERY WELL
DEFINED ON WATER VAPOR SATELLITE IMAGERY...WHICH IS KEEPING THE
MAIN THERMAL DRIVEN BAROCLINIC ZONE SEPARATED FROM THE RICH
SUBTROPICAL MOISTURE SOURCES TO THE SOUTH. EMBEDDED WITHIN THIS
TROUGH IS A SERIES OF SHORTWAVE TROUGH DISTURBANCES PROPAGATING
THROUGH THE FLOW. THE FIRST IS A WEAKENING AND ELONGATED SHEARED
TROUGH AXIS MOVING THROUGH SOUTHCENTRAL AND THE NORTH GULF. THE
SECOND IS A FAST MOVING COMPACT BUT SHARP PV ANOMALY TRACKING INTO
EASTERN BRISTOL BAY...AND THE THIRD IS A SLOW MOVING SHORTWAVE
TROUGH ROUNDING THE BASE OF THE LARGER TROUGH NEAR 45N SOUTH OF
ADAK. THE MAIN FEATURE OF INTEREST IS THE COMPACT PV ANOMALY AND
ASSOCIATED JET STREAK RAPIDLY TRACKING INTO EASTERN BRISTOL BAY.
ALTHOUGH OF LOW AMPLITUDE AND DISCONNECTED FROM DEEP SUBTROPICAL
MOISTURE...FOCUSED AND STEADY STATE ASCENT ABOVE THE LEVEL OF NON-
DIVERGENCE (ABOVE 500 HPA) IS FORCING STRONG LOW LEVEL
CYCLOGENESIS. A RECENT ASCAT SATELLITE PASS AT 0600Z CAUGHT STORM
FORCE WINDS SOUTH OF THE FAR WESTERN ALASKA PENINSULA IN MARINE
ZONE 150. MOREOVER...BUOY 46075 IN THAT SAME ZONE OBSERVED STORM
FORCE WIND GUSTS. LIGHTNING STRIKES HAVE BEEN OBSERVED AS
WELL...ALONG THE REGION OF ENHANCED MID LEVEL ASCENT. THIS SYSTEM
WILL RAPIDLY MOVE INTO SOUTHWEST ALASKA THIS MORNING...THEN INTO
SOUTHCENTRAL TODAY INTO THE EARLY EVENING.

&&

.MODEL DISCUSSION...

THE MODELS ARE IN EXCELLENT AGREEMENT WITH THE GENERAL SYNOPTIC
PATTERN THROUGH THE WEEKEND...AND ARE IN VERY GOOD AGREEMENT WITH
A DEEP LOW DEVELOPING IN THE GULF OF ALASKA WEDNESDAY AND
THURSDAY. THEY ARE IN LESSER AGREEMENT WITH A FAST MOVING LOW
CURRENTLY MOVING INTO BRISTOL BAY THIS MORNING...ESPECIALLY WITH
THE DEVELOPMENT OF THREE LOWS AS THE INITIAL SURFACE LOW SPLITS
OFF AND SECONDARY LOWS DEVELOP ACROSS COOK INLET AND PRINCE
WILLIAM SOUND TODAY. AS A RESULT...THERE IS LESS CONFIDENCE IN
PRECIPITATION AND PRESSURE GRADIENT PATTERNS. THIS IS MOST NOTABLE
AROUND COOK INLET AND ANCHORAGE...WHERE THE EFFECT OF SOUTHEAST
WINDS SWEEPING THROUGH ANCHORAGE WILL IMPACT PRECIPITATION TYPE
AND AMOUNT. WITH TEMPERATURES ALREADY SPIKING INTO THE LOW 30S
ACROSS ANCHORAGE THIS MORNING...THERE IS THE POSSIBILITY OF A
MIXED OF RAIN AND SNOW...AND POSSIBLY GRAUPEL (SEE SHORT TERM
FORECAST FOR SOUTHCENTRAL). OVERALL...THE CANADIAN RDPS WAS
FAVORED FOR PRESSURE...WINDS...AND PRECIPITATION...WITH THE GFS
AND ECMWF FAVORED BEYOND 36 HOURS TO CAPTURE THE LARGE LOW IN THE
GULF OF ALASKA...AND ANOTHER STRONG LOW MOVING INTO THE BERING SEA
THURSDAY AND FRIDAY.

&&

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

FORECAST CHALLENGES REMAIN WITH THE CURRENT WEATHER PATTERN EVEN
FOR THE SHORT TERM. ALREADY THIS MORNING...TEMPERATURES ARE
SEVERAL DEGREES WARMER ACROSS ANCHORAGE THAN FORECASTED BY ANY
CURRENT MODEL...MOST LIKELY DUE TO LIGHT SUBSIDENCE WINDS
OVERNIGHT AND A VERY LIGHT SOUTHEASTERLY WIND. IN ADDITION...LIGHT
CONVECTIVE SHOWERS ARE ALREADY APPROACHING THE KENAI PENINSULA
THIS MORNING EARLIER THAN ANTICIPATED. AS THE ASSOCIATED UPPER
LEVEL DISTURBANCE PROGRESSES NORTHWARD DURING THE DAY TODAY...TWO
WEAK LOW PRESSURE SYSTEMS ARE EXPECTED TO DEVELOP OVER COOK INLET
AND PRINCE WILLIAM SOUND. AS A RESULT THE PRESSURE GRADIENT WILL
BE WEAKER...AND SOUTHEAST DOWNSLOPING WINDS WILL BE LESS STRONG
THAN PREVIOUSLY EXPECTED. IN ADDITION...THE MID LEVEL JET WILL BE
IN A FAVORABLE POSITION TO SUPPORT PRECIPITATION ACROSS ANCHORAGE
THIS AFTERNOON. HOWEVER...AS TEMPERATURES ARE ALREADY NEAR
FREEZING THIS MORNING...LIMITED SNOWFALL ACCUMULATION IS EXPECTED.
IN ADDITION...PRECIPITATION IS EXPECTED TO BE QUITE CONVECTIVE IN
NATURE...RESULTING IN A WIDE SPREAD OF PRECIPITATION AMOUNTS.

IN CONTRAST...MORE STEADY PRECIPITATION IS EXPECTED ACROSS THE
EASTERN GULF OF ALASKA...AS PERSISTENT ONSHORE FLOW DEVELOPS. WITH
COOLER TEMPERATURES IN PLACE...SNOW IS EXPECTED TO BE THE
PREDOMINANT PRECIPITATION TYPE FOR VALDEZ. BY WEDNESDAY A MORE
ORGANIZED LOW IS EXPECTED TO APPROACH KODIAK ISLAND AND BRING
HEAVY RAINS TO KODIAK AND STORM FORCE WINDS TO THE GULF...BUT
REMAIN SOUTH OF THE MAINLAND.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
A RATHER SMALL BUT POTENT AREA OF LOW PRESSURE WILL MAKE LANDFALL
ALONG THE NORTHERN BRISTOL BAY COAST EARLY THIS MORNING THEN
CONTINUE INLAND TO THE NORTHEAST. THE LOW IS BRINGING ENOUGH WARM
AIR OUT AHEAD OF IT THAT PRECIPITATION ALONG THE ALASKA PENINSULA
FROM ROUGHLY KING SALMON SOUTH WILL START THE MORNING OFF AS RAIN
OR A MIX OF RAIN AND SNOW BUT THEN QUICKLY MIX OVER TO ALL SNOW AS
THE COOLER POST FRONTAL AIR MASS PUSHES IN. FURTHER NORTH AN AREA
OF MODERATE TO HEAVY SNOW WILL FOLLOW THE LOW INLAND. WITH THE
TIGHT PRESSURE GRADIENT ASSOCIATED WITH THE COMPACT LOW EXPECTING
TO SEE STRONG SOUTHWESTERLY WINDS INCREASE RATHER RAPIDLY AROUND
THE BRISTOL BAY AREA IMMEDIATELY FOLLOWING THE LOW PASSAGE.

SNOW SHOWERS WILL TAPER OFF RAPIDLY THIS EVENING WITH COLD
ADVECTION AND SOME PARTIAL CLEARING DROPPING TEMPERATURES
SUBSTANTIALLY TONIGHT. THE NEXT SYSTEM IMPACTING THE AREA WILL BE
AN AREA OF LOW PRESSURE SOUTH OF KODIAK ISLAND ON WEDNESDAY.
CURRENTLY MODELS ARE KEEPING THIS LOW AND ASSOCIATED FRONTAL
SYSTEM FAR ENOUGH SOUTH TO ONLY SPREAD PRECIPITATION ACROSS THE
ALASKA PENINSULA AND UP TO THE BRISTOL BAY COAST.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE BERING REMAINS UNDER THE INFLUENCE OF A BROAD UPPER LEVEL
TROUGH AS THE LARGE REMNANT SURFACE LOW CONTINUES TO FILL AND
WEAKEN AS IT LIFTS NORTH TOWARDS RUSSIA. THE NEXT RATHER STRONG
INCOMING FRONTAL SYSTEM WILL BEGIN PUSHING INTO THE WESTERN
ALEUTIANS WEDNESDAY AFTERNOON AND EVENING THEN SWING INTO THE
WESTERN BERING WEDNESDAY NIGHT.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
THERE IS DECENT AGREEMENT WITH THE MODELS MOVING A STORM-FORCE LOW
INTO THE WESTERN BERING THURSDAY. THE EC IS A LITTLE BIT FASTER
THAN THE OTHERS BUT FROM AN IMPACTS STANDPOINT THEY ALL HAVE THE
FRONT FROM THIS SYSTEM MAKING IT TO THE WEST COAST FRIDAY BEFORE
FALLING APART AT THE SURFACE...WITH A WEAKENING FRONT ALOFT
MOVING INLAND. THE PRIMARY IMPACT FROM THIS STORM WILL BE
WIDESPREAD GALE-FORCE OR STRONGER WESTERLY WINDS ON THE SOUTH SIDE
OF THIS LOW ACROSS MUCH OF THE SOUTHERN HALF OF THE BERING SEA.

THE LOW APPROACHING KODIAK ISLAND FROM THE SOUTH IN THE SHORT TERM
WILL REACH PEAK STRENGTH WEDNESDAY NIGHT...THEN RETREAT SOUTH AND
EAST ACROSS THE NORTHEAST PACIFIC WHILE SLOWLY WEAKENING THURSDAY
INTO FRIDAY. REGARDLESS OF WHICH TRACK OCCURS...STRONG GALE FORCE
WINDS WILL IMPACT MOST OF THE MARINE WATERS...ESPECIALLY FOR INNER
COASTAL WATERS FROM COOK INLET THROUGH THE BARREN ISLANDS TO
SHELIKOF STRAIT...AND EAST ALONG THE NORTH GULF COAST THROUGH
WEDNESDAY NIGHT. THE EC SOLUTION WAS PREFERRED FOR THE WINDS FOR
WEDNESDAY NIGHT INTO THURSDAY...WITH AN ENSEMBLE MODEL APPROACH
FOR POPS AND QPF.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...HEAVY FREEZING SPRAY 185.
         GALE WARNING 119 120 130 131 132 136 137 138 139  150 155
                      160 165 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...EN
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...CC