Small Craft Advisory 163, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
538 AM AKST THU FEB 18 2016

.ANALYSIS AND UPPER LEVELS...
WATER VAPOR SATELLITE LOOKS PRETTY INTERESTING THIS MORNING. THE
DRIVING FEATURE OF THIS PATTERN LOOKS TO BE A SHEARING OUT LOW
PRESSURE SYSTEM NEAR THE BERING STRAIT WHILE ITS FRONT STRETCHES
SOUTHEASTWARD THROUGH THE MAINLAND AND INTO THE GULF. ANOTHER
SYSTEM IS DEVELOPING ALONG THIS FRONT SOUTH OF KODIAK WITH A NICE
DRY INTRUSION SHOWING UP AS WELL. A VERY COLD AIR MASS ALOFT IS IN
PLACE WITH A CONVECTIVE LOOKING SIGNATURE TO THE SOUTH OF THE
ALASKA PENINSULA STARTING TO WRAP INTO THE KODIAK SYSTEM. USUALLY
THIS OPEN CELLULAR LOOK IS EASY TO SPOT ON INFRARED IN THE CLOUD
TOP TEMPERATURES BUT THE FACT THAT IT IS SHOWING UP ON IN WATER
VAPOR HIGHLIGHTS THE DEPTH OF THIS INSTABILITY. THE 00Z/18 PASN
RAOB SHOWS THIS COLD POOL ALOFT NICELY WITH 500 MB TEMPS OF
-44C...IMPRESSIVE EVEN IN WINTER. THE KODIAK SYSTEM EXTENDS A NICE
DEFORMATION ZONE/TROUGH INTO THE SOUTHWESTERN MAINLAND WHERE
STEADY SNOW HAS BEEN FALLING ALL NIGHT. OTHERWISE PRETTY QUIET
CONDITIONS OVER THE REMAINDER OF THE MAINLAND...GENERALLY AVERAGE
TEMPERATURES AND HIGH CLOUDS STREAMING IN FROM THIS SYSTEM.
FURTHER OUT WEST...ANOTHER STRONGER LOW PRESSURE IS DEVELOPING IN
THE WESTERN BERING SEA WITH FRONTAL BOUNDARY EXTENDING INTO THE
NORTHERN PACIFIC.

&&

.MODEL DISCUSSION...
GUIDANCE SEEMS TO HANDLE THE SYNOPTIC EVOLUTION OF THE PATTERN
QUITE SIMILARLY...HOWEVER THE MESOSCALE DETAILS ARE EVER SO
SLIGHTLY DIFFERENT...WHICH WILL HAVE A BIG IMPACT IN A FEW AREAS.
THE FEATURE IN QUESTION IS A STALLING OUT FRONTAL BOUNDARY AND
TRIPLE POINT LOW DEVELOPING ALONG SAID BOUNDARY. WHILE THE MODELS
AGREE UPON A NARROW BAND OF SNOW ACROSS THE CENTRAL BERING SEA...THE
EXACT LOCATION IS NOT WELL AGREED UPON AND VERY CLOSE TO THE
PRIBILOF ISLANDS. SOME SOLUTIONS WOULD BRING ADVISORY LEVEL SNOW
TO THE AREA. THE ONE PLACE THERE IS GOOD MODEL AGREEMENT IS OVER
UMNAK ISLAND WHERE GUIDANCE IS HITTING IT PRETTY HARD WITH SNOW
AND WIND TONIGHT INTO FRIDAY.

&&

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

THE LOW IN THE GULF HAS SLID UNDERNEATH THE RIDGE OVER THE EASTERN
HALF OF THE STATE. THIS HAS SITUATED THE LOW IN A POSITION TO
GIVE QUITE VARIED WEATHER TO KODIAK ISLAND. THE MAJORITY OF THE
ISLAND IS IN THE WARM SECTOR OF THE LOW AND WILL SEE RAIN. THE
MOST SOUTHERN PART OF THE ISLAND IS IN THE COLD SECTOR WHICH IS
BRINGING GUSTY NORTHWEST WINDS AND SNOWFALL TO THE REGION. AS THE
DAY PROGRESSES AND THE LOW PULLS SOUTHWARD IT SHOULD BRING THE
WARM SECTOR OVER ALL OF THE ISLAND AND TURN ALL LOWER ELEVATION
PRECIPITATION TO RAIN. THE RIDGE OVER THE EASTERN PART OF
MAINLAND ALASKA WILL CLEAR OUT THE MID AND UPPER LEVEL CLOUD COVER
FROM THE COPPER RIVER BASIN TO THE NORTHERN KENAI PENINSULA BY
LATER TODAY. THE COPPER RIVER BASIN WILL ONCE AGAIN BE SUBJECT TO
A STRONG INVERSION WHICH WILL NOT COMPLETELY BREAK. THIS WILL KEEP
A LARGE REGION OF STRATUS OVER THE BASIN AGAIN TODAY. SOME
LOCATIONS WILL BREAK OUT INTO SOME SUNSHINE BUT THE MAJORITY OF
THE BASIN WILL REMAIN IN THE STRATUS. THE HIGHER LEVEL CLOUDS OVER
NORTHERN COOK INLET HAVE KEPT THE FOG OUT OF THE ANCHORAGE AND
MAT-SU AREAS THIS MORNING.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
THE DEFORMATION BAND THAT SET UP OVER BRISTOL BAY AND THE LOWER
KUSKOKWIM VALLEY YESTERDAY WILL CONTINUE TO BRING STEADY SNOWFALL
TO THOSE AREAS INTO EARLY THIS AFTERNOON. BY LATE THIS MORNING
CONDITIONS SHOULD BEGIN TO IMPROVE FROM NORTH TO SOUTH AS THE BAND
BEGINS TO DIMINISH...WITH SNOW PERSISTING THE LONGEST OVER THE AKPEN
WHERE WESTERLY FLOW FROM THE WATERS OF BRISTOL BAY WILL PROVIDE
ADDITIONAL UPSLOPE FLOW. WHILE THESE WESTERLY WINDS WILL ALSO BE
FAIRLY GUSTY...THEY ARE NOT EXPECTED TO BRING SIGNIFICANT BLOWING
SNOW CONCERNS TO THE AREA DUE TO THE RELATIVELY WARM ONSHORE FLOW.
WEAK RIDGING WILL BRING PARTIAL CLEARING OF THE CLOUD COVER
TONIGHT...BUT THIS WILL THEN QUICKLY RETURN BY FRIDAY AFTERNOON AS
AN UPPER LEVEL TROUGH APPROACHES KODIAK ISLAND FROM THE
SOUTH...BRINGING A CHANCE OF RAIN AND SNOW TO BRISTOL BAY INTO
SATURDAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
MOST OF THE ACTIVITY OVER THE AREA WILL CENTER AROUND A BAROCLINIC
ZONE THAT WILL DEVELOP ALONG A WEAKENING FRONT FROM THE PRIBILOF
ISLANDS TO THE CENTRAL ALEUTIANS THIS AFTERNOON. THIS IS NOW
EXPECTED TO BRING SIGNIFICANT SNOWFALL TO THE PRIBILOF ISLANDS
INTO FRIDAY AFTERNOON...HOWEVER EXACT ACCUMULATIONS WILL BE
DEPENDENT ON WHERE BANDING OF HEAVY SNOWFALL WILL DEVELOP. AS A
RESULT A SNOW ADVISORY WAS ISSUED THIS MORNING TO COVER A
CONSERVATIVE FORECAST FROM THIS BAND...HOWEVER THESE ACCUMULATIONS
MAY NEED TO BE CHANGED SIGNIFICANTLY IF THE LOCATION OF THE
HEAVIER SNOWFALL CHANGES.

A SECONDARY CONCERN FROM THIS BAND EXISTS OVER THE CENTRAL
ALEUTIANS AS HEAVY SNOW FROM THE BAROCLINIC ZONE HAS THE
POTENTIAL TO COMBINE WITH GALE-STORM FORCE WINDS TO BRING BLIZZARD
CONDITIONS TO THAT AREA ON FRIDAY. DUE TO THE UNCERTAINTY IN
WHERE EXACTLY THE HEAVIEST SNOWFALL WILL OCCUR A BLIZZARD WATCH
WAS ISSUED THIS MORNING. THERE IS REASONABLE CONFIDENCE HOWEVER
THAT AREAS FROM UNALASKA EAST WILL LARGELY BE SPARED FROM BLIZZARD
CONDITIONS...AND MAY EVEN STAY AS A RAIN/SNOW MIX WITH MINIMAL
SNOW ACCUMULATION FROM THIS EVENT.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...

THE MAIN CHALLENGE IN THE LONG TERM FORECAST SURROUNDS THE UPPER
LOW...MOVING SOUTH OF THE ALASKA PENINSULA FRIDAY NIGHT. THIS
SYSTEM IS PROJECTED TO INITIALLY MOVE A WEAK FRONT ACROSS THE
WESTERN GULF AND KENAI PENINSULA DURING THE EARLY MORNING HOURS ON
SATURDAY. SURFACE RIDGING ACROSS SOUTHCENTRAL COMBINED WITH
DOWNSLOPING DUE TO STRONG LOW LEVEL EASTERLY WINDS...INHIBITS RAIN
POTENTIAL ACROSS SOUTHCENTRAL WITH THIS FIRST SYSTEM. A SECOND
FRONT DEVELOPS QUICKLY BEHIND THE FIRST AND PUSHES INTO THE
SOUTHERN GULF LATE SATURDAY MORNING. AT THE SAME TIME...THE UPPER
LEVEL JET STREAM BEGINS TO SHIFT INTO A FAVORABLE PATTERN TO BRING
A SATURATED PACIFIC AIR MASS ACROSS THE KENAI AND SOUTHCENTRAL.
THE FRONT IN THE SOUTHERN GULF PUSHES TO THE NORTH COAST IN THE
EVENING BRINGING GUSTY EASTERLY WINDS AND PRECIPITATION TO THE
COASTAL AREAS AND PRINCE WILLIAM SOUND. SEVERAL STRONG UPPER LEVEL
WAVES WILL MOVE NORTH SPREADING PRECIPITATION NORTH ACROSS
SOUTHCENTRAL INTO SUNDAY. THE COLDER AIR MASS ALONG THE COAST AND
JUST INLAND IS EXPECTED TO MODERATE QUICKLY TO A WARMER MARITIME
ENVIRONMENT WHICH WILL CHANGE SNOW OVER TO RAIN OR A MIX OF SNOW
AND RAIN DURING THIS EVENT.

TO THE WEST...NORTHERLY WINDS AND SHOWERY CONDITIONS PREVAIL
ACROSS THE BERING THROUGH THE WEEKEND. THE MAIN IMPACTS INITIALLY
WILL BE ACROSS THE EASTERN BERING WITH GALE WINDS AND MIXED
PRECIPITATION AS A LOW SOUTH OF THE ALASKA PENINSULA TRACKS INTO
BRISTOL BAY. A SECOND SYSTEM PUSHES A FRONT INTO THE WESTERN
ALEUTIANS LATE SUNDAY AFTERNOON SPREADING GUSTY EAST WINDS AND
PRECIPITATION INTO THE CENTRAL ALEUTIANS/SOUTHERN BERING BY EARLY
MONDAY.

AN ACTIVE WEATHER PATTERN CONTINUES THROUGH MID WEEK AS THE
ALEUTIAN LOW TRACKS EASTWARD AND ANOTHER DEVELOPING EASTERN
PACIFIC LOW TRACKS TOWARD THE GULF ON TUESDAY.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WINTER WEATHER ADVISORY...161 185
         BLIZZARD WATCH...195
MARINE...HEAVY FREEZING SPRAY...185
         STORM...150 172 173 174 178
         GALE...130 155 160 170 175 176 177 179 185

&&

SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DEK
LONG TERM...KH/MMC