Small Craft Advisory 1033, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
424 PM AKST WED DEC 24 2014

.ANALYSIS AND UPPER LEVELS...
A VERY ACTIVE PATTERN IS IN PLACE ACROSS THE ALASKA REGION WITH
A VERY LARGE UPPER LEVEL CLOSED LOW IN THE VICINITY OF THE
WESTERN ALEUTIANS/FAR WESTERN BERING SEA AND A VERY WINTER-LIKE
200+ KT SUBTROPICAL JET TO THE SOUTH. THIS IS PROVIDING FAVORABLE
CONDITIONS FOR CYCLOGENESIS OVER THE NORTHWEST PACIFIC. THE
DEVELOPING SYSTEMS THEN GET PULLED NORTHWARD ACROSS THE ALEUTIANS
ON THE EAST SIDE OF THE UPPER LOW...WHILE THE FRONTAL SYSTEMS AND
UPPER LEVEL WAVES (WHICH HELPED INDUCE CYCLOGENESIS) CONTINUE
EAST- NORTHEAST TO MAINLAND ALASKA. THE RAPID DEVELOPMENT AND
MOVEMENT OF FEATURES IN THIS PATTERN PROVIDES QUITE A FORECAST
CHALLENGE FOR FORECAST TIMING.

THE FIRST DEEP LOW WHICH LED THE WAY INTO THIS PATTERN IS NOW
WEAKENING OVER THE FAR SOUTHWEST BERING SEA. THE OCCLUDED FRONT
IS STRETCHED OUT ACROSS THE NORTH CENTRAL BERING SEA THEN
SOUTHEASTWARD ACROSS MAINLAND ALASKA. SATELLITE IMAGERY IS
BEGINNING TO SHOW SIGNS OF A WEAK TRIPLE POINT LOW FORMING NEAR
KODIAK ISLAND. THE HEAVIEST PRECIP IS OVER INTERIOR SOUTHWEST
ALASKA AND THE WESTERN GULF OUT AHEAD OF A FAIRLY STRONG UPPER
LEVEL SHORT-WAVE. A STRONG VORT-MAX OVER THE SOUTHWESTERN GULF
IS BRING DRIVEN EASTWARD WHILE THE REST OF THE TROUGH LIFTS
NORTH-NORTHEASTWARD. THIS IS LEADING TO DEAMPLIFICATION OF THE
TROUGH AS IT APPROACHES SOUTHCENTRAL ALASKA. RADAR IMAGERY SHOWS
LIGHT SNOW GRADUALLY OVERSPREADING SOUTHCENTRAL FROM SOUTHWEST
TO NORTHEAST...BUT NOT ALL OF THIS IS REACHING THE GROUND. IN
PARTICULAR THERE IS A SHALLOW LAYER OF SOUTHEAST FLOW ACROSS THE
KENAI AND CHUGACH MOUNTAINS WHICH IS PRODUCING SOME WEAK
DOWNSLOPE DRYING IN THE LOWEST LEVELS. AS MORE INTENSE SNOW MOVES
IN THIS EVENING EXPECT TO SEE MORE OF THE AREA FILL IN WITH LIGHT
SNOW BRIEFLY BEFORE THE TROUGH MOVES PAST.

A MODERATE LOW WHICH CROSSED THE ALEUTIANS OVERNIGHT HAS NOW
SLOWED ITS FORWARD MOTION AND IS SITTING JUST NORTH OF THE
WESTERN ALEUTIANS. IT IS SENDING ANOTHER FRONTAL SYSTEM EASTWARD
ACROSS THE ALEUTIANS AND SOUTHERN BERING SEA. COLD AIR FEEDING IN
BEHIND THIS SYSTEM HAS DESTABILIZED THE ATMOSPHERE WITH CONVECTION
NOTED IN SATELLITE IMAGERY SOUTH OF THE WESTERN ALEUTIANS. THIS IS
AN AREA TO WATCH AS IT MOVES INTO THE FORECAST AREA BECAUSE OF THE
POTENTIAL FOR STRONG AND GUSTY WINDS WITH A WELL MIXED BOUNDARY
LAYER.

A THIRD LOW HAS FORMED ON THE SOUTH SIDE OF THE DEEP LOW OVER THE
SOUTHWEST BERING SEA AND IS POISED TO EJECT EASTWARD ACROSS THE
WESTERN ALEUTIANS.

&&

.MODEL DISCUSSION...
CONSIDERING THE VERY PROGRESSIVE NATURE OF THE PATTERN MODEL
SPREAD IS RELATIVELY LOW FOR THE FIRST 48 HOURS OF THE FORECAST.
THERE ARE DEFINITELY SOME DIFFERENCES WITH DEPICTION OF SMALLER
SCALE FEATURES...ESPECIALLY IN TIMING.

MODELS ARE ALL VERIFYING WELL THIS MORNING WITH CURRENT FEATURES
BUT DIFFERENCES QUICKLY DEVELOP THIS AFTERNOON AND EVENING...
PRIMARILY WITH LOCATION OF THE TRIPLE POINT LOW IN THE GULF AND
TIMING OF THE UPPER TROUGH MOVING THROUGH SOUTHCENTRAL. PREFER
THE HIGHER RESOLUTION MODELS WHICH TRACK THE TRIPLE POINT LOW
FARTHER NORTH RESULTING IN STRONGER WINDS ALONG THE NORTH GULF
COAST AND INTO PRINCE WILLIAM SOUND. WITH PROGRESSIVE FLOW ALOFT
PREFER THE FASTER MOVEMENT OF THE UPPER TROUGH.

THE GEMGLB AND GEMREG ARE OUTLIERS WITH DEVELOPMENT OF A TRIPLE
POINT LOW NEAR THE EASTERN ALEUTIANS THIS AFTERNOON. THE UPPER
LEVEL TROUGH IS LOW AMPLITUDE AND THERE ISN'T MUCH LOW LEVEL
BAROCLINICITY IN THAT AREA...SO WILL COMPLETELY DISCOUNT THIS
SOLUTION. ALL MODELS HAVE TRENDED STRONGER WITH THE WINDS BEHIND
THE OCCLUSION...POSSIBLY PICKING UP ON THE LOW LEVEL INSTABILITY
MENTIONED ABOVE. WILL INCREASE WINDS IN THIS AREA AND CARRY THEM
ALL THE WAY INTO BRISTOL BAY.

NUMERICAL GUIDANCE IS CONVERGING ON A COMMON SOLUTION WITH TRACK
OF THE NEXT PACIFIC LOW CROSSING THE WESTERN ALEUTIANS THURSDAY
NIGHT...SO FORECAST CONFIDENCE HAS INCREASED. THE BIGGER CHALLENGE
WILL BE A SERIES OF THREE UPPER LEVEL SHORT-WAVES WHICH QUICKLY
TRACK FROM THE ALEUTIANS TO MAINLAND ALASKA THROUGH FRIDAY. EACH
SOLUTION IS JUST A LITTLE DIFFERENT IN TRACK AND AMPLITUDE OF
THESE FEATURES WHICH MAKES IT HARD TO FORECAST SNOW AMOUNTS. THE
FORECASTS WILL ADEQUATELY REFLECT THE HIGH PROBABILITY OF PRECIP
AS THESE WAVES MOVE ACROSS THE REGION. SNOW AMOUNT OVER SOUTHWEST
AND SOUTHCENTRAL WILL HAVE TO BE FINE-TUNED JUST PRIOR TO EACH
EVENT WHEN MODEL GUIDANCE TENDS TO CONVERGE ON KEY DETAILS OF
THE FEATURES.

&&

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

AN INITIAL WAVE MOVING ACROSS THE REGION WILL BRIEFLY BRING SOME
SNOW SHOWERS TO INTERIOR ZONES TONIGHT...WITH A MIX OF RAIN AND
SNOW ALONG THE COAST. PRECIPITATION WILL TAPER OFF OVERNIGHT
BEFORE THE NEXT WAVE ARRIVES WITH SNOW SPREADING NORTHEAST ACROSS
THE KENAI PENINSULA AND INTO SOUTHCENTRAL. A LOW LEVEL SOUTHWEST
JET WILL ENHANCE SNOWFALL IN UPSLOPE FLOW...WITH GREATEST AMOUNTS
EXPECTED OVER HIGHER ELEVATIONS OF THE SUSITNA VALLEY. A WINTER
WEATHER ADVISORY FOR 6 TO 14 INCHES OF SNOW HAS BEEN ISSUED FOR
THAT AREA. SIMILAR AMOUNTS OF SNOWFALL ARE EXPECTED AROUND PRINCE
WILLIAM SOUND...WITH LESSER AMOUNTS FOR THE REST OF SOUTHCENTRAL.
SNOW WILL BEGIN TO END LATE THURSDAY NIGHT AS THE NEXT SHORTWAVE
RIDGE ARRIVES. ON FRIDAY...ANY LINGERING SNOWFALL ACTIVITY NEAR
PRINCE WILLIAM SOUND AND ACROSS THE COPPER RIVER BASIN WILL ALSO
END BENEATH THE BUILDING RIDGE.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
AS THE FRONT OVER THE SOUTHWEST MAINLAND TRACKS FURTHER INTO THE
INTERIOR THIS EVENING...A TRIPLE POINT LOW WILL BEGIN TO DEVELOP
OVER THE EASTERN BERING AND QUICKLY TRACK INLAND OVERNIGHT. THIS
FEATURE WILL BE THE FIRST IN A SERIES OF WAVES THAT WILL KEEP
SHOWERY CONDITIONS AND GUSTY WINDS IN THE FORECAST FOR PARTS OF
THE SOUTHWEST MAINLAND THROUGH THE END OF THE WEEK. MODELS ARE IN
GOOD AGREEMENT WITH A FAIR AMOUNT OF INSTABILITY DEVELOPING OVER
THE SOUTHWEST COAST THURSDAY MORNING AND SPREADING INLAND THROUGH
FRIDAY AFTERNOON...HELPING TO PRODUCE EVEN GUSTIER WINDS AT THE
SURFACE OVER PARTS OF THE BRISTOL BAY AREA EARLY TOMORROW MORNING.
WEAK HIGH PRESSURE WILL BUILD OVER THE ALASKA PENINSULA FRIDAY
MORNING...CAUSING SHOWERY CONDITIONS OVER THE AREA TO TEMPORARILY
DIMINISH. THIS BREAK WILL BE SHORT LIVED THOUGH WITH THE FRONT
OVER THE EASTERN BERING BEGINNING TO TRACK INLAND FRIDAY
AFTERNOON...BRINGING ANOTHER ROUND OF GUSTY WINDS AND
PRECIPITATION TO THE SOUTHWEST COAST.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
AS THE LOW OVER THE WESTERN BERING BROADENS OUT ACROSS MUCH OF
THE BERING TONIGHT...EXPECT GUSTY WINDS TO PERSIST OVER PARTS OF
THE ALEUTIAN CHAIN THROUGH TOMORROW AFTERNOON. PRECIPITATION OVER
THE AREA SHOULD REMAIN PREDOMINANTLY RAIN...BUT WITH SOME COLDER
AIR BEING ADVECTED IN OVER THE WESTERN ALEUTIANS/BERING...A MIX OF
RAIN AND SNOW WILL BECOME MORE PREVALENT. A DEVELOPING LOW OVER
THE NORTH PACIFIC TOMORROW MORNING WILL DEEPEN AS IT TRACKS
THROUGH THE WESTERN ALEUTIANS AND INTO THE SOUTHWESTERN BERING BY
FRIDAY AFTERNOON. STRONG GUSTY WINDS ARE EXPECTED TO DEVELOP OVER
THE WESTERN BERING AND ALEUTIANS TOMORROW NIGHT AND PERSIST
THROUGH FRIDAY AFTERNOON. THE FRONT ASSOCIATED WITH THIS SYSTEM
WILL PUSH NORTH AND EAST THROUGH THE WESTERN AND CENTRAL
BERING/ALEUTIANS FRIDAY MORNING...AND THEN TRACK OVER THE EASTERN
BERING/ALEUTIANS BY FRIDAY AFTERNOON BEFORE DISSIPATING. GUSTY
SOUTHERLY WINDS AND RAIN ARE EXPECTED TO TRAVERSE THROUGH THE
ALEUTIAN CHAIN FROM WEST TO EAST...AS THE FRONTAL SYSTEM
PROGRESSES THROUGH THE AREA.

&&

.LONG TERM FORECAST (SATURDAY THROUGH WEDNESDAY)...

THE OVERALL SYNOPTIC PATTERN WILL SHIFT SIGNIFICANTLY STARTING
THIS WEEKEND AS A DEEP AREA OF LOW PRESSURE OVER THE WESTERN
ALEUTIAN COLLIDES WITH A STOUT NORTHEASTERN PACIFIC RIDGE. THIS
WILL RESULT IN A LARGE SCALE AMPLIFICATION OF THE RIDGE...WHICH
WILL RAPIDLY BUILD ACROSS PARTS OF THE ALASKA MAINLAND. THIS WILL
LIKELY PUSH A CHINOOK ACROSS SOUTHCENTRAL ALASKA (POSSIBLY THE
ALASKA PENINSULA TOO DEPENDING ON THE OVERALL DEVELOPMENT OF THE
RIDGE) AND BRING VERY UNSEASONABLY WARM TEMPERATURES. WIDESPREAD
RAINFALL (POTENTIALLY HEAVY AT TIMES) WILL EXTEND FROM KODIAK
ISLAND AND ALONG THE NORTHERN GULF COAST. WITH TEMPERATURES
WARMING INTO THE UPPER 30S TO POTENTIALLY MID 40S INLAND...
PRECIPITATION TYPE WILL LIKELY TRANSITION TO PURELY RAINFALL FOR
INLAND LOCATIONS INCLUDING ANCHORAGE AND PARTS OF THE MAT-SU
VALLEY. WHILE MODELS PREVIOUSLY HINTED AT THE RIDGE BEGINNING TO
FLATTEN OUT DURING THE BEGINNING/MIDDLE OF NEXT WEEK...MODELS NOW
SUGGEST ADDITIONAL SHORTWAVE ENERGY ENTERING THE UPPER LEVEL
TROUGH OVER THE BERING SEA WILL AMPLIFY THE TROUGH AND IN TURN
AMPLIFY THE UPPER RIDGE.

MODELS ARE IN FAR LESS AGREEMENT OVER THE BERING SEA AND SOUTHWEST
ALASKA. WHILE AN OVERALL STORMY PATTERN IS ON TAP WITH MULTIPLE
AREAS OF LOW PRESSURE TRAVERSING THE BERING SEA...THE STORM TRACKS
AND STRENGTH OF THESE LOW IS IN QUESTION. THIS IS DUE TO MULTIPLE
KEY FEATURES BEING HANDLED DIFFERENTLY BY BOTH DETERMINISTIC AND
ENSEMBLE GUIDANCE. NEVERTHELESS...A PERIOD OF SNOW RAIN IS LIKELY
ACROSS MUCH OF THE BERING SEA AND SOUTHWEST ALASKA WITH PERIODS OF
WINDS RANGING FROM 25 TO 35 KT.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...SNOW ADVISORY 145.
MARINE...GALE 150 155 160 165 170 172 173 174 175 176 177 178 180
              185.
         HEAVY FREEZING SPRAY WARNING 181 185.
FIRE WEATHER...NONE.

&&

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