Small Craft Advisory 918, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
455 PM AKST THU NOV 19 2015

.ANALYSIS AND UPPER LEVELS...

THE MAJOR WEATHER FEATURE INFLUENCING THE WEATHER ACROSS THE AREA
TODAY IS A LARGE AND MATURE LOW OVER THE EASTERN BERING SEA WITH A
SPRAWLING ASSOCIATED FRONT MOVING THROUGH SOUTHWEST ALASKA AND
INTO SOUTHCENTRAL ALASKA. TEMPERATURES HAVE WARMED TO ABOVE
FREEZING OVER MOST OF THE KUSKOKWIM DELTA AND THE BRISTOL BAY
ZONE AS STRONG SOUTHERLY WINDS ARE MOVING THROUGH THE
AREA...CHANGING MOST PRECIPITATION OVER TO RAIN.
MEANWHILE...STRONG WINDS ON THE SOUTHERN PERIPHERY OF THE LARGE
CIRCULATION ARE PRODUCING VERY STRONG WIND GUSTS AROUND DUTCH
HARBOR AND THE EASTERN ALEUTIANS INTO THE AKPEN. THESE WILL ALL BE
DIMINISHING WITH TIME AS THE ENTIRE WEATHER SYSTEM PROGRESSES
QUICKLY EASTWARD.

TO THE EAST OF THE ALASKA RANGE...WIDESPREAD SNOWFALL IS EVIDENT
ON THE KENAI AND MIDDLETON RADARS AS A SIGNIFICANTLY WARMER
AIR MASS MOVES IN FROM THE SOUTH AND IS PRODUCING WIDESPREAD
ISENTROPIC ASCENT. THIS IS QUICKLY TRANSITIONING TO HEAVIER AND
MORE DYNAMICALLY-DRIVEN PRECIPITATION AS THE FRONT IS BEGINNING TO
CROSS THE ALASKA AND ALEUTIAN RANGES INTO COOK INLET. HEAVY SNOW
HAS ALREADY BEGUN TO FALL ACROSS PARTS OF THE SUSITNA VALLEY WHERE
OROGRAPHIC LIFT IS ADDING TO ALREADY IMPRESSIVE VERTICAL MOTIONS.
MEANWHILE...STRONG SOUTHEAST WINDS ALONG/AHEAD OF THE FRONT HAVE
MADE THEIR INITIAL APPEARANCE AROUND KACHEMAK BAY WILL BEGIN TO
RE-ORIENT THE PRESSURE GRADIENT AROUND COOK INLET OVER THE NEXT 6
HOURS.

&&

.MODEL DISCUSSION...

MODELS ARE FINE-TUNING THE TIMING OF THE FRONTAL PASSAGE EXPECTED
IN COOK INLET THIS EVENING BUT REMAIN LARGELY ON THE SAME PAGE.
ADDITIONALLY...THERE IS INCREASING AGREEMENT REGARDING A SECOND
STORM THAT WILL TRACK RAPIDLY INTO AND THROUGH THE AREA FRIDAY
NIGHT AND SATURDAY. THE ECMWF HAD BEEN AN OUTLIER IN PREVIOUS RUNS
WITH A MORE NORTHERN TRACK THROUGH UPPER COOK INLET THAT WOULD
HAVE RESULTED IN RAIN OR A RAIN/SNOW MIX IN MANY OF THE POPULATION
CENTERS. LATEST RUNS HAVE NOW TRENDED A BIT SOUTHWARD...AND IT NOW
LOOKS LIKE THE WESTERN KENAI TO ANCHORAGE AND UP INTO THE MAT-SU
VALLEYS WILL BE COLD ENOUGH FOR MOST SNOW DURING THIS TIME
PERIOD...WITH ACCUMULATION OF SNOW LOOKING INCREASINGLY LIKELY.
HI-RESOLUTION MODELS (GEM-REGIONAL AND NAM) ARE USEFUL AND WERE
USED IN THE FORECAST PREPARATION TODAY.

&&

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

WARM AIR AND SOUTHERLY WINDS WILL WORK THEIR WAY UP COOK INLET
THIS EVENING ASSOCIATED WITH THE AFOREMENTIONED STRONG FRONT.
WINDS THROUGH WESTERN TURNAGAIN ARM HAVE ALREADY BECOME QUITE
STRONG...AND WILL REACH THEIR PEAK INTENSITY SHORTLY BEFORE
FRONTAL PASSAGE AROUND 9 PM THIS EVENING. SNOWFALL WILL CONTINUE
TO INCREASE IN INTENSITY THROUGH THE EVENING ACROSS THE SUSITNA
VALLEY AND ALSO OVER THE NORTHERN GULF AND PRINCE WILLIAM SOUND.
SNOWFALL IN THE ANCHORAGE BOWL AND OVER MUCH OF THE NORTHERN KENAI
WILL REMAIN GENERALLY LIGHT UNTIL THE FRONT ITSELF APPROACHES THIS
EVENING. AS THE FRONT PASSES THE AREA A BAND OF HEAVY
PRECIPITATION...LIKELY IN THE FORM OF SNOW...SHOULD MOVE ACROSS
THE AREA AND HAS THE POTENTIAL TO QUICKLY DROP A FEW INCHES OF
SNOW BEFORE GUSTY SOUTH/SOUTHEAST WINDS MOVE INTO ANCHORAGE FROM
TURNAGAIN ARM AND WARM TEMPERATURES TO NEAR THE FREEZING MARK. AT
THIS SAME POINT TEMPERATURES WILL ALSO RISE ABOVE FREEZING THROUGH
TURNAGAIN ARM...ENDING THE BLOWING SNOW THREAT...THOUGH HEAVY SNOW
WILL CONTINUE THROUGH TURNAGAIN PASS.

THE NEXT STRONG FRONTAL SYSTEM WILL PUSH INTO THE WESTERN GULF
FRIDAY AFTERNOON BRINGING STORM FORCE WINDS WITH IT. THE
ASSOCIATED SURFACE LOW WILL TRACK UP SHELIKOF STRAIT EARLY FRIDAY
EVENING THEN CONTINUE ALONG THE SOUTHEASTERN KENAI PENINSULA COAST
TO ARRIVE IN PRINCE WILLIAM SOUND LATE FRIDAY NIGHT OR EARLY
SATURDAY MORNING. WITH THE STRONG WESTERLY JET SUPPORT AND GOOD
COLD ADVECTION BEHIND THE SYSTEM AM EXPECTING TO SEE MIXED
PRECIPITATION TURNING RAPIDLY OVER TO SNOW FRIDAY EVENING. WILL
NEED TO WATCH QPF AMOUNTS CLOSELY FOR FRIDAY NIGHT AS THE CURRENT
MODEL TRACKS FOR THIS LOW ARE FAVORABLE FOR BRINGING ANOTHER
SIGNIFICANT ROUND OF SNOW INTO SOUTH CENTRAL ALASKA.

&&

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

THE STRONG FRONT WHICH MOVED INLAND TODAY IS SHIFTING EAST...WITH
STEADY RAIN AND SNOW TURNING MORE TO RAIN AND SNOW SHOWERS AS A
COLD UPPER LOW BEHIND THE FRONT MOVES INLAND TONIGHT. SHOWERS WILL
BE HEAVY AT TIMES...WITH A FEW ISOLATED THUNDERSTORMS POSSIBLE
ACROSS THE BRISTOL BAY COAST. A FAST MOVING LOW WILL MOVE INTO SOUTHWEST
FROM THE NORTH PACIFIC FRIDAY AFTERNOON AND EVENING. MOST OF THE
RAIN AND SNOW WILL IMPACT THE BRISTOL BAY AND LOWER KUSKOKWIM
VALLEY REGIONS WITH STRONG NORTHWEST WINDS KICKING UP BEHIND THIS
LOW. MUCH COLDER AIR WILL MOVE BACK INTO THE REGION...WITH TEMPS
AGAIN FALLING BELOW FREEZING REGION WIDE. SCATTERED SNOW SHOWERS
WILL REMAIN ACROSS THE REGION THROUGH SATURDAY AS NORTHWEST FLOW
CONTINUES.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...

A VERTICALLY STACKED LOW OFF OF NUNIVAK ISLAND IS WEAKENING AS
THE UPPER LEVEL JET EVOLVES INTO A MORE DIRECTLY SOUTHWEST TO
NORTHEAST ORIENTED FLOW. THIS IS EFFECTIVELY DIMINISHING THE UPPER
LEVEL DIVERGENCE SUPPORTING THE STORM...THUS CONTINUED WEAKENING
OF THE STORM IS EXPECTED. ALSO WITH THE STORM BEING VERTICALLY
STACKED...ANY TEMPERATURE GRADIENTS THAT MAY KEEP THE STORM STRONG
HAVE LONG BEEN MIXED OUT BY THE BROAD CIRCULATION. THE STORM IS
GENERATING INSTABILITY SOUTH OF THE LOW AS POLAR AIR MOVES OVER
THE WARMER WATERS OF THE BERING...PRODUCING LOCALIZED STORMS THAT
ARE PRODUCING OCCASIONAL LIGHTNING.

THE NEXT BIG WEATHER MAKER FOR THE BERING IS A NORTH PACIFIC LOW
CURRENTLY DEVELOPING SOUTH OF THE WESTERN ALEUTIANS. THE UPPER
LEVEL JET SUPPORT FOR THIS STORM IS MUCH MORE FAVORABLE AS THE JET
INTENSIFIES AND ROUNDS THE NORTHERN PERIPHERY OF AN UPPER LEVEL
RIDGE. THIS ARC SHAPE TO THE JET STREAK FAVORS ADDITIONAL
SUPPORTING UPPER LEVEL DIVERGENCE FOR THAT STORM IN THE RIGHT
ENTRANCE REGION. ADDITIONALLY...THE LOW IS TRACKING ALONG A VERY
ROBUST BAROCLINIC ZONE...ALSO KNOWN AS AN AREA WITH A STRONG
TEMPERATURE GRADIENT. THE LOWER LEVEL TEMPERATURE CONTRASTS
BETWEEN THE WARM AIR SOUTH AND EAST OF THE LOW AND THE POLAR AIR
OVER THE BERING NORTH OF THE LOW FUELS THE STORM. STORM FORCE
WINDS ARE EXPECTED MAINLY SOUTH OF THE LOW...PRIMARILY ON THE
NORTH PACIFIC SIDE OF THE ALEUTIANS AND ALASKA PENINSULA LATE
TONIGHT THROUGH FRIDAY EVENING. ANOTHER FAR WEAKER LOW MOVES INTO
THE WESTERN BERING SATURDAY MORNING...WITH A FEW GALES POSSIBLE
WITH THAT FEATURE. ANOTHER VERY STRONG LOW ENTERS THE WESTERN
BERING SUNDAY.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
BY THE START OF THE MIDTERM...SOUTHCENTRAL ALASKA WILL BE ON THE
BACKSIDE OF THE LOW WHICH WILL BE MOVING INTO SOUTHEASTERN
ALASKA...KEEPING THE REGION IN A COLD AND SHOWERY PATTERN THROUGH
SUNDAY EVENING. AS THIS LOW MOVES FURTHER EAST...GALE FORCE WINDS
OVER THE NORTH CENTRAL GULF COAST AND ADJACENT COASTAL WATERS
WILL FALL BELOW GALE FORCE BY SUNDAY AFTERNOON. TO THE WEST...A FAST
MOVING LOW WILL RACE ACROSS THE BERING SEA INTO THE Y-K DELTA ON
SUNDAY...BRINGING ANOTHER SHOT OF SHOWERS TO THE SOUTHWEST ALONG
WITH GALE FORCE WINDS OVER PORTIONS OF THE SOUTHWESTERN COASTAL
WATERS.

THE MAIN STORY IN THE MID TO LONG TERM IS THE LARGE LOW ENTERING
THE FORECAST AREA NEAR SHEMYA ON SUNDAY MORNING. MODELS HAVE BEEN
FAIRLY GOOD AGREEMENT FOR SEVERAL DAYS ON THE GENERAL STRENGTH AND
TRACK OF THIS LOW. FOR THIS FORECAST...AT DAY 3 A BLEND OF THE GFS
AND EC WAS USED BEFORE TRANSITIONING TO THE WPC FORECAST AT DAY 4.
WINDS ALONG THE FRONT ON SUNDAY INTO MONDAY ARE EXPECTED TO BE
STORM FORCE AS THE LOW INTENSIFIES INTO THE 965-970 MB RANGE
BEFORE MOVING TO THE NORTH CENTRAL BERING SEA NEAR ST MATTHEW
ISLAND BY MONDAY EVENING. THE ASSOCIATED FRONT WILL SWEEP RAPIDLY
EASTWARD THROUGH THE BERING SEA...PASSING THROUGH THE PRIBILOFS
AND THE CENTRAL ALEUTIANS SUNDAY NIGHT BEFORE REACHING THE
SOUTHWEST MAINLAND ON MONDAY MORNING. AS DISCUSSED PREVIOUSLY...THIS
SYSTEM IS EXPECTED TO HAVE A DEEP SOUTHERLY COMPONENT TO
IT...WHICH WILL POTENTIALLY BRING HEAVY PRECIPITATION ALONG THE
FRONT AS IT MOVES INTO THE ALASKA PENINSULA AND THE SOUTHWEST
MAINLAND. THIS FRONT IS EXPECTED TO PUSH INTO SOUTHCENTRAL BY
TUESDAY...LIKELY BRINGING SNOW INLAND WITH RAIN IN COASTAL AREAS.
WITH MODELS IN GOOD AGREEMENT...THE FORECAST TRENDED TO MOSTLY WPC
FOR THE DAY 4-6 WITH SOME BLENDING OF THE GFS ALONG THE FRONT OVER
THE BERING.

WEDNESDAY INTO THURSDAY THE FORECAST BECOMES LESS CERTAIN. GLOBAL
MODELS ARE IN GROSS AGREEMENT IN MAINTAINING THE RIDGE OVER THE
NORTHEASTERN PACIFIC WHILE AT THE SAME TIME BRINGING A LOW UP
FROM THE CENTRAL PACIFIC ALONG THE WESTERN SIDE OF THE RIDGE INTO
THE ALASKA PENINSULA. DURING THE SAME TIME PERIOD...THE REMNANTS
OF EX-TYPHOON IN-FA ARE EXPECTED TO MOVE INTO THE WESTERN BERING
SEA AND ALONG THE ALEUTIAN CHAIN. MODELS ARE IN COMPLETE
DISAGREEMENT ON THE TIMING...STRENGTH...AND LOCATION OF THE
REMAINS ON IN-FA. WITH THIS IN MIND...THE FORECAST FOR DAYS 6-7
USED THE WPC FORECAST WHICH SHOWS SOME SORT OF CYCLONE OVER THE
CENTRAL BERING SEA AND DOES NOT INCLUDE THE LOW MOVING ALONG THE
WESTERN SIDE OF THE RIDGE. CONFIDENCE AT THIS TIME IS LOW IN THE
DAY 6-7 FORECAST DUE TO THE WIDE MODEL SPREAD IN THE HANDLING OF
IN-FA.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...STORM WARNING 119 120 131 132 150.
 GALE WARNING 125 130 136 137 138 139 141 155 160 165 170 172 180
185.
 HEAVY FREEZING SPRAY 181.

&&

SYNOPSIS AND MODEL DISCUSSION...AD
SOUTHCENTRAL ALASKA...JR
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA/JW
LONG TERM...RF