Small Craft Advisory 234, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
449 PM AKDT MON MAR 14 2016

.ANALYSIS AND UPPER LEVELS...
A BROAD TROUGH OVER THE CENTRAL BERING STRETCHES INTO THE EASTERN
ALEUTIANS BRINGING SNOW SHOWERS TO THE REGION. THESE ARE
ESPECIALLY APPARENT ON THE WESTERN EDGE OF THE TROUGH WERE COLD
AIR ADVECTION WITHIN THE NORTHWESTERLY FLOW IS PRODUCING
WIDESPREAD SNOW SHOWERS. OVER THE SOUTH MAINLAND...RIDGING
CONTINUES TO BRING SUNNY SKIES AND LIGHT WINDS TO MOST
AREAS. THIS IS HOWEVER BEGINNING TO DISSIPATE IN ADVANCE OF AN
ARCTIC TROUGH THAT IS BEGINNING TO DIVE SOUTHWARD INTO THE
INTERIOR. THIS TROUGH WILL CONTINUE ON THIS TRACK TO BRING MORE
ACTIVE AND COOLER WEATHER TO THE SOUTHCENTRAL BY MID WEEK. OVER
THE EASTERN GULF A WEAKENING LOW IS BEGINNING TO FILL AS IT MOVES
TOWARDS THE PANHANDLE.

&&

.MODEL DISCUSSION...
THE MODELS ARE ALL IN GENERALLY GOOD SYNOPTIC AGREEMENT INTO THE
MIDDLE OF THIS WEEK...BUT THERE REMAIN SOME SMALL BUT CRITICAL
DIFFERENCES THAT ARE IMPACTING FORECAST CONFIDENCE. THE MOST
SIGNIFICANT FEATURE FOR SOUTHCENTRAL IS THE ARCTIC TROUGH DIVING
INTO THE SOUTHWEST MAINLAND FOR TUE-WED. THE MODELS ARE IN VERY
GOOD AGREEMENT WITH THE TRACK OF BOTH THE UPPER LEVEL LOW AND A
WEAK LOW MOVING INTO THE NORTH GULF COAST TUE NIGHT...AND AS A
RESULT ARE IN REASONABLE AGREEMENT IN THE PLACEMENT OF SNOWFALL.
HOWEVER SMALL CHANGES IN TRACK OF THIS LOW WILL HAVE A
SIGNIFICANT IMPACT ON THE SNOW FORECAST. IN PARTICULAR...THIS WILL
BE IMPORTANT FOR BOTH ANCHORAGE AND THE MATANUSKA VALLEY WHICH ARE
EXPECTED TO BE ON THE VERY WESTERN EDGE OF THE SNOWFALL AND COULD
MEAN THE DIFFERENCE BETWEEN MINIMAL ACCUMULATIONS AND SEVERAL
INCHES OF SNOWFALL. OVER THE BERING THERE ARE ALSO DIFFERENCES IN
THE TRACK OF A WEAK LOW APPROACHING THE UNALASKA AREA FROM THE
PRIBILOFS THAT ARE SIGNIFICANT ENOUGH THAT IT IS UNCERTAIN AT THIS
POINT WHETHER THE LOW WILL TRACK JUST EAST OR WEST OF UNALASKA
ISLAND. THE AFTERNOON PACKAGE USED A BLEND TO BRING THIS LOW
ESSENTIALLY OVER UNALASKA...SO SMALL CHANGES TO THE EXPECTED TRACK
WILL HAVE SIGNIFICANT DIFFERENCES TO THE AREAS SNOW AND WIND
FORECAST.

&&

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

THE PRIMARY CONCERNS FOR THE SOUTHERN MAINLAND IS THE UPPER LEVEL
TROUGH THAT IS DIGGING SOUTH FROM THE ARCTIC INTO SOUTHWEST ALASKA
AND WEAK LOW PRESSURE BACKING TO THE WEST INTO PRINCE WILLIAM
SOUND TOMORROW. THE UPPER TROUGH IS ON THE DRY SIDE AND WE
CURRENTLY DON'T EXPECT MUCH MOISTURE ADVECTION FROM THE EAST INTO
NORTHERN COOK INLET. THE UPPER FLOW IS MORE SOUTHERLY WITH THIS
TROUGH AND THAT WILL SERVE TO PUSH THE MOISTURE UP THE SUSITNA VALLEY.
THE BIG IF WITH THIS SCENARIO IS IF THE LOW IN PRINCE WILLIAM
SOUND DOES BACK ACROSS THE CHUGACH MOUNTAINS AND BRING WITH IT
AMPLE MOISTURE TO FEED A PROLONGED SNOW EVENT OVER THE KENAI
PENINSULA TO SUSITNA VALLEY REGION. THIS WILL BE WATCHED CLOSELY
OVER THE NEXT COUPLE OF DAYS. AT THIS TIME THE HEAVIES SNOW IS
EXPECTED TO BE FROM CHULITNA NORTH UP TOWARD DENALI PARK. THE BEST
CHANCE FOR ACCUMULATING SNOW ACROSS THE ANCHORAGE BOWL AND MAT
VALLEY STILL APPEARS TO BE TUESDAY NIGHT TO WEDNESDAY MORNING.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
AN UPPER LEVEL ARCTIC TROUGH DIGGING SOUTHWARD ACROSS WESTERN
ALASKA WILL BE SLOW TO PASS...NOT EXITING THE ENTIRE REGION UNTIL
MIDDAY WEDNESDAY. WEAK LIFT OUT AHEAD OF THIS TROUGH WILL INTERACT
WITH SOME LOW TO MID LEVEL MOISTURE AND WEAK INSTABILITY SITTING
OVER BRISTOL BAY AND THE KUSKOKWIM VALLEY TO PRODUCE ISOLATED TO
SCATTERED SNOW SHOWERS THROUGH TUESDAY NIGHT. MEANWHILE...COLD AND
DRY AIR WILL ADVECT IN WITH THE TROUGH RESULTING IN GRADUAL
CLEARING AS THE TROUGH PROGRESSES SOUTH AND EASTWARD THE NEXT 36
HOURS. THE KUSKOKWIM DELTA WILL SEE THE STRONGEST COLD AIR
ADVECTION...WITH GUSTY WINDS AND WIND CHILLS DROPPING BELOW ZERO
TONIGHT THROUGH TUESDAY NIGHT. DRY AND COOL CONDITIONS WILL SETTLE
IN ACROSS THE ENTIRE REGION BY WEDNESDAY...LIKELY CONTINUING
THROUGH THE REST OF THE WEEK.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
A BROAD UPPER TROUGH OVER THE BERING SEA AND ALEUTIANS WILL BEGIN
TO PROGRESS SOUTHWARD TONIGHT IN RESPONSE TO AMPLIFICATION OF THE
UPSTREAM PATTERN OVER THE NORTHWEST PACIFIC/WESTERN BERING. NUMEROUS
SNOW SHOWERS AND GUSTY WINDS WILL CONTINUE FOR MUCH OF THE
ALEUTIANS AND BERING SEA AS A SERIES OF UPPER WAVES MOVE THROUGH
THE LARGER TROUGH. THERE IS ONE NOTABLY STRONGER SHORT-WAVE WITH
CORRESPONDING SURFACE LOW SITTING SOUTHEAST OF THE PRIBILOF ISLANDS.
AS THE UPSTREAM RIDGE BUILDS IN FROM THE WEST PRESSURE GRADIENTS
WILL TIGHTEN ACROSS THE CENTRAL TO EASTERN ALEUTIANS RESULTING IN
SOLID GALE FORCE WINDS. THIS WILL COMBINE WITH STEADY SNOW. WITH
TEMPERATURES NEAR OR ABOVE FREEZING DO NOT EXPECT ANY ISSUES WITH
BLOWING SNOW. THIS SYSTEM AND THE REMAINS OF THE UPPER TROUGH WILL
EXIT INTO THE PACIFIC TUESDAY NIGHT WITH COLD ADVECTION QUICKLY
COMING TO AN END. THIS WILL LEAD TO DIMINISHING WINDS AND OVERALL
QUIETER WEATHER ACROSS THE EASTERN HALF OF THE BERING SEA AND THE
ALEUTIANS/ALASKA PENINSULA.

FOCUS WILL QUICKLY SHIFT TO A BOMBING LOW OVER THE NORTHWEST PACIFIC
TONIGHT AND TUESDAY. WITH A RIDGE RAPIDLY BUILDING OUT AHEAD OF IT
THE SYSTEM WILL STALL WEST OF THE FORECAST AREA...BUT A STORM
FORCE FRONTAL SYSTEM WILL PROGRESS NORTH AND EAST ACROSS THE
WESTERN ALEUTIANS AND WESTERN BERING SEA. THE FRONT WILL QUICKLY
BEGIN TO ELONGATE AND WEAKEN AS IT STRETCHES OUT ALONG THE WEST
SIDE OF THE RIDGE. THUS...DO NOT EXPECT IT TO MAKE IT ANY FARTHER
EAST THAN AROUND ATKA.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
SHOWER ACTIVITY WILL BE ON A DOWNWARD TREND WEDNESDAY NIGHT INTO
THURSDAY ACROSS MUCH OF SOUTHCENTRAL ALASKA AS THE UPPER TROUGH
LIFTS NORTHWARD TOWARDS THE BROOKS RANGE...WITH THE BULK OF THE
REMAINING PRECIPITATION CONFINED TO EASTERN PRINCE WILLIAM SOUND
AND THE COPPER RIVER BASIN AS A WEAK WAVE MOVES THROUGH THE
EASTERN GULF. MODELS HAVE NOW STARTED TO COME INTO BETTER
AGREEMENT ON HOW THE PATTERN WILL EVOLVE TOWARDS THE WEEKEND...AS
THE DEPARTING TROUGH IS REINFORCED BY SHORTWAVE ENERGY DROPPING
OUT OF THE ARCTIC AND THEN RETROGRADES BACK OVER THE CENTRAL
MAINLAND AS A DEEP CLOSED UPPER LOW. THIS SCENARIO WOULD BRING
GENERALLY COOL AND DRY CONDITIONS TO THE SOUTHERN MAINLAND THROUGH
THE WEEKEND...WITH THE CHANCE FOR A FEW SHOWERS PERSISTING ACROSS
SOUTHCENTRAL AK IN THE PRESENCE OF DEEP CYCLONIC FLOW.

OUT WEST...THE STRONG FRONT POISED TO ENTER THE WESTERN ALEUTIANS
TUESDAY NIGHT WILL BE IMPEDED IN ITS EASTWARD PROGRESS BY HIGH
AMPLITUDE RIDGING DEVELOPING OVER THE EASTERN HALF OF BERING AS
THE MAINLAND TROUGH LIFTS TO THE NORTH DURING THE MIDDLE OF THE
WEEK. THIS WILL YIELD GENERALLY BENIGN CONDITIONS ACROSS THE
BERING FOR THE END OF THE WEEK AS THE FRONT DISSIPATES...WITH THE
BEST CHANCE FOR PRECIPITATION SHIFTING TO THE ALASKA PENINSULA AND
THE EASTERN ALEUTIANS ON THE PERIPHERY OF A DEEP NORTH PACIFIC
SYSTEM WELL TO THE SOUTH.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...HEAVY FREEZING SPRAY...160 179 180 185
 		STORM WARNING...178
 		GALE WARNING...170 172 173 174 175 176 177.

&&

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...SA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SB
LONG TERM...CB