Small Craft Advisory 986, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
514 AM AKST THU DEC 10 2015

.ANALYSIS AND UPPER LEVELS...THE PATTERN REMAINS RATHER QUIET
THIS MORNING...BUT A FEW SUBTLE CHANGES ARE WORTH NOTING. FIRST
OFF...THE SUBTROPICAL JET REMAINS VERY ACTIVE...BUT WELL TO THE
SOUTH OF THE GULF OF AK. THERE IS A 60 KT FINGER OF THE POLAR JET
STREAMING OFF OF THE KAMCHATKA PENINSULA WHICH IS HELPING TO
ENHANCE SOME SHOWERS OVER THE WESTERN ALEUTIANS. THE CUT-OFF LOW
JUST EAST OF KODIAK CONTINUES TO MEANDER NORTH. AS IT DOES SO...IT
IS FINALLY BRINGING ENOUGH MOISTURE UP INTO SOUTH CENTRAL TO
CREATE SOME LIGHT PRECIPITATION. BUT FOR NOW...RADAR IS SHOWING
MOST OF THIS PRECIPITATION HOLDING TO THE COAST AND HIGHER
ELEVATIONS. IT APPEARS THERE IS JUST TOO MUCH DRY AIR IN THE LOW
LEVELS (SEE THE 10/00Z ANCHORAGE SOUNDING) FOR ANYTHING TO MAKE IT
TO THE GROUND ACROSS COOK INLET. THE OTHER STORY WITH THE LOW IS
THE AIR MASS REPLACEMENT. MUCH OF THE MAINLAND HAS BEEN UNDER
COLD HIGH PRESSURE THE PAST SEVERAL DAYS. THIS SYSTEM IS BRINGING
IN WARMER TEMPERATURES AND LOWER PRESSURE. IN FACT...MOST PLACES
HAVE ACTUALLY SEEN THEIR TEMPERATURE RISE DURING THE OVERNIGHT
HOURS. WITH THE DECREASING DISPARITY IN AIR MASSES...THIS IS
LEADING TO A WEAKENING SURFACE GRADIENT. THUS...THE GUSTY OUTFLOW
WINDS ARE ALSO STARTING TO RELAX. ONE OTHER NOTEWORTHY ITEM IS
THAT A CLOSED SURFACE LOW DEVELOPED ALONG THE FRONT OVER THE
EASTERN BERING. WHILE WE DON'T EXPECT THIS LOW ITSELF TO BRING ANY
TERRIBLY SIGNIFICANT WEATHER...IT WILL SERVE AS A COLD AIR FEED
FOR THE PARENT LOW IN THE GULF. THIS COULD HELP TO INTENSIFY THE
GULF LOW A BIT MORE.

&&

.MODEL DISCUSSION...OUT WEST...MODELS REMAIN IN SOLID OVERALL
AGREEMENT. THE OVERNIGHT PACKAGE LEANED ON A BLEND OF THE
GFS/ECMWF. FORECAST CONFIDENCE REMAINS MODERATE TO HIGH OUT THERE
THROUGH SAT MORNING. PLEASE SEE THE EXTENDED DISCUSSION FOR MODEL
DIFFERENCES WITH BERING LOW OVER THE WEEKEND.

IN THE EAST...MODELS ARE IN GOOD AGREEMENT WITH SYNOPTIC SCALE
FEATURES. BUT AS IS OFTEN THE CASE IN A VERTICALLY STACKED
ENVIRONMENT WITH WEAK STEERING FLOW...THEY DIFFER ON SOME OF THE
MORE IMPORTANT DETAILS. THIS MORNING...WITH DRY AIR HOLDING TOUGH
IN THE LOWER LEVELS AROUND COOK INLET...MODELS HAVE BACKED OFF ON
MUCH IF ANY PRECIPITATION CAN MAKE IT TO THE GROUND. FRI INTO
SAT...THEY ARE STARTING TO COME INTO BETTER AGREEMENT IN HANDLING
HOW THE GULF LOW GETS "KICKED" OUT BY A TROUGH FROM THE WEST AND
"EXITS" THROUGH COOK INLET. THERE ARE SOME SLIGHT TIMING AND
STRENGTH DIFFERENCES IN JUST HOW STRONG THE UPPER LEVEL WAVE WILL
BE AS IT COMES THROUGH SOUTH CENTRAL. THE NAM IS THE MOST
AGGRESSIVE AT THIS TIME...BUT THE GFS IS TRENDING STRONGER.
THUS...WE HAVE NUDGED POPS AND QPF HIGHER. SOME IMPORTANT
VARIABLES THAT REMAIN UNCERTAIN AT THIS TIME ARE HOW MUCH
DOWNSLOPING MIGHT PLAY A ROLE AND JUST HOW MUCH COLD AIR GETS
WRAPPED AROUND THE LOW FROM SW AK. SO WHILE SOME SNOW LOOKS MORE
AND MORE CERTAIN...FORECAST CONFIDENCE IN AMOUNTS AND TIMING
REMAINS ON THE LOWER END.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE GULF LOW IS SLOWLY TRACKING WEST AND WILL CURVE NORTHWEST
TOWARD THE BARREN ISLANDS TONIGHT. THIS WILL ALLOW FOR A CONTINUED
WARMING TREND OVER SOUTHCENTRAL INTO FRIDAY. SOME LIGHT SNOW MAY
MAKE IT PAST THE COASTAL RANGES BUT WITH MINIMAL UPPER LEVEL
SUPPORT ACCUMULATIONS WILL ALSO BE MINIMAL.

ATTENTION THEN TURNS TO LATE FRIDAY INTO SATURDAY WITH THE
PROGRESSION OF A SHARPENING UPPER LEVEL TROUGH MOVING NORTH ACROSS
THE NORTH GULF COAST AND COOK INLET REGION FRIDAY NIGHT. THERE
ARE SEVERAL IMPORTANT ASPECTS TO CONSIDER WITH THIS SYSTEM AS IT
APPROACHES THAT WILL PLAY A LARGE ROLE AS TO HOW MUCH SNOW MAY
FALL ACROSS THE ANCHORAGE AREA FRIDAY EVENING THROUGH EARLY
SATURDAY MORNING. THE FIRST ASPECT IS THAT LOW-LEVEL COLD
ADVECTION WILL PRECEDE THE UPPER LEVEL TROUGH AND MOVE NORTH UP
THE INLET A FEW HOURS BEFORE THE UPPER TROUGH ARRIVAL.
ADDITIONALLY...THE AIR MASS PRECEDING THE COLD AIR WILL BE
CHARACTERIZED BY NOT PARTICULARLY STRONG SOUTHEAST FLOW THROUGH
TURNAGAIN ARM ADVECTING A WARM AND (RELATIVELY) UNSTABLE AIR MASS
ACROSS THE ANCHORAGE AREA. THIRD...MODEL CROSS-SECTIONS ALL
CONCEPTUALLY DEPICT SOME DEGREE OF LAYER LIFTING OCCURRING AS THE
COLD AIR MASS MOVING UP THE INLET ENCOUNTERS THE WARM AND MOIST
AIR MASS MOVING THROUGH AND OVER TURNAGAIN ARM FROM PRINCE WILLIAM
SOUND AND THE GULF OF ALASKA. LAYER LIFTING IN THIS MANNER IS A
RELATIVELY INFREQUENT BUT NOT RARE OCCURRENCE...AND SIMILAR
SITUATIONS SUCH AS THIS HAVE PRODUCED VERY SIGNIFICANT TO
INCREDIBLE SNOW RATES WITH MESOSCALE SNOW BANDS MOVING FROM
TURNAGAIN ARM ACROSS THE ANCHORAGE AREA...ESPECIALLY THE HILLSIDE.
MODELS ALL CONTINUE TO TREND TOWARD INCREASING CERTAINTY FOR AT
LEAST A PERIOD OF SNOW DEVELOPING BY LATE FRIDAY EVENING (IF NOT
SOONER)...BUT IT IS TOO EARLY TO SAY AT WHAT INTENSITY AND FOR HOW
LONG ANY SNOW MAY LAST. INTERESTS ACROSS THE ANCHORAGE
MUNICIPALITY WILL WANT TO STAY TUNED FURTHER UPDATES AS THIS
POTENTIAL EVENT APPROACHES.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
GUSTY WINDS ACROSS THE KUSKOKWIM DELTA ARE EXPECTED TO TAPER DOWN
DURING THE AFTERNOON WITH WEAK LOW PRESSURE DEVELOPING NEAR
BRISTOL BAY. ACROSS THE ALASKA PENINSULA TODAY...AN UPPER TROUGH
WILL BE THE FOCUS FOR A MIX OF RAIN AND SNOW THIS MORNING AND THEN
CHANGE OVER TO ALL SNOW BY THE AFTERNOON AS COLDER AIR ADVECTS
FROM THE WEST. SNOW CHANCES INCREASE THIS EVENING INTO FRIDAY
ACROSS THE SOUTHWEST MAINLAND AS THE UPPER TROUGH MOVES A
SHORTWAVE UP FROM THE ALASKA PENINSULA. THE HIGHEST SNOW AMOUNTS
ARE EXPECTED ACROSS BRISTOL BAY WHERE THE BEST LIFT SETS UP WITH
THE APPROACHING SURFACE LOW FROM THE GULF.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
COLD AIR ADVECTION FROM THE WEST WILL KEEP PRECIPITATION ALL SNOW
THROUGH FRIDAY. NORTHERLY FLOW CONTINUES ACROSS THE EASTERN BERING
AND ALEUTIANS THROUGH THE END OF THE WEEK...WITH FLOW ACROSS THE WESTERN
AREAS VARYING IN DIRECTION AS WEAK SURFACE LOWS ENTER FROM KAMCHATKA
AND THE NORTH PACIFIC LATER TODAY.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
IT APPEARS MORE AND MORE LIKELY WITH THE MOST RECENT MODEL RUNS THAT
SOUTHWEST AND SOUTHCENTRAL ALASKA ARE HEADING INTO QUITE AN ACTIVE
WEATHER PATTERN STARTING FRIDAY. WE WILL START ACROSS SOUTHCENTRAL
ALASKA WHERE AN AREA OF LOW PRESSURE SITUATED OVER THE KENAI
PENINSULA BEGINS TO WEAKEN AND PUSH EASTWARD BY SUNDAY...AS A RIDGE
TO THE WEST DEVELOPS AHEAD OF A DEEPENING LOW PUSHING INTO THE
BERING SEA. THIS WILL AMPLIFY THE UPPER LEVEL PATTERN OVER
SOUTHCENTRAL ALASKA BY PUSHING THE UPPER LEVEL TROUGH ACROSS
SOUTHCENTRAL ALASKA (MAINLY THE COOK INLET REGION)...HELPING TO
INCREASE SNOW CHANCES. MOST MODEL PRECIPITATION FIELDS CONTINUE TO
SHOW ONLY LIGHT SNOW ACCUMULATION ACROSS THE REGION. HOWEVER...
THERE REMAINS THE POTENTIAL FOR SLIGHTLY HIGHER SNOW ACCUMULATION
OVER AREAS THAT EXPERIENCE SOME MESOSCALE BANDING AND POCKETS OF
HEAVIER SNOWFALL. CURRENTLY THE MODELS ARE SETTING UP THE HEAVIER
SNOWFALL AMOUNTS ACROSS THE KENAI PENINSULA AND AREAS SURROUNDING
PRINCE WILLIAM SOUND.HOWEVER...WITH THE SLIGHT DIFFERENCES IN THE
MODELS STILL...WE CAN NOT RULE OUT THE POTENTIAL FOR HIGHER SNOW
AMOUNTS FOR ANCHORAGE AND PARTS OF THE MATANUSKA AND SUSITNA
VALLEYS.

OUT WEST...NORTHWESTERLY FLOW AND COLD AIR ADVECTION SHOWERS OVER
THE EASTERN ALEUTIANS AND ALASKA PENINSULA WILL GRADUALLY
DIMINISH...AS THE RIDGING MENTIONED ABOVE STARTS PUSHING INTO THE
AREA TO START OFF THE WEEKEND. THE SOUTHWEST MAINLAND WILL SEE A
VERY BRIEF BREAK FROM PRECIPITATION SATURDAY EVENING AND INTO
SUNDAY MORNING AS THIS RIDGE MOVES OVER THE AREA. A RAPIDLY
EVOLVING NORTH PACIFIC LOW WILL TRACK AN ASSOCIATED FRONT INTO
THE WESTERN ALEUTIANS SATURDAY MORNING...THEN CONTINUE TO SPREAD
WET AND WINDY CONDITIONS NORTH AND EASTWARD ACROSS THE BERING SEA
THROUGH THE WEEKEND. THE FRONT WILL THEN TRACK INTO THE SOUTHWEST
MAINLAND BY SUNDAY NIGHT AND BEGIN TO WEAKEN AS IT TRACKS FURTHER
INTO MAINLAND ALASKA BY THE BEGINNING OF THE WEEK. MODELS HAVE
COME INTO BETTER AGREEMENT WITH THE STRENGTH AND TRACK OF THIS
SYSTEM...BRINGING A STORM FORCE SYSTEM AND ROUGH SEAS TO THE
CENTRAL BERING FROM THE NORTH PACIFIC OVER THE WEEKEND...THEN
TRACKING THE LOW CENTER INTO FAR NORTHEASTERN RUSSIA BY TUESDAY
NEXT WEEK. WE ARE EXPECTING THE STRONGER WINDS TO REMAIN ON THE
WESTERN FLANK OF THE SYSTEM WITH A FAIR AMOUNT OF COLD AIR
ADVECTION HELPING TO MIX GUSTIER WINDS DOWN FROM ALOFT. THESE
GUSTIER WINDS HAVE THE POTENTIAL TO WRAP AROUND THE SOUTHERN FLANK
OF THE SYSTEM AS IT TRACKS TOWARD THE BERING STRAIT. THIS CAN
CREATE A DECENT FETCH OF WESTERLY WINDS OVER THE BERING SEA AND
DIRECTED AT THE SOUTHWESTERN COAST DURING THE BEGINNING OF THE
WEEK. THIS RAISES SOME CONCERN FOR COASTAL FLOODING ALONG THE
BRISTOL BAY AND KUSKOKWIM DELTA COAST. CURRENTLY WE ARE EXPECTING
MINIMAL IMPACTS TO THE COASTAL AREAS WITH MINIMAL AMOUNTS OF ICE
PILING UP...HOWEVER THIS IS SOMETHING WE WILL CONTINUE TO MONITOR
AS THE SYSTEM EVOLVES OVER THE NEXT FEW DAYS.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MSO
SOUTHCENTRAL ALASKA...CC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...TP/MMC