Small Craft Advisory 335, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
444 PM AKDT FRI APR 17 2015

.ANALYSIS AND UPPER LEVELS...

A LOOK AT AFTERNOON SATELLITE IMAGERY REVEALS THE LEFTOVERS OF THE
STORM YESTERDAY OVER SOUTHCENTRAL HAVE NOW PUSHED ACROSS THE
ALASKA RANGE AND INTO THE INTERIOR. BREAKS OF SUN HAVE DEVELOPED
ACROSS THE SOUTHERN MAINLAND AS WEAK HIGH PRESSURE MOVES THROUGH
THE AREA. THE USUAL SOUTHEASTERLY GAP WINDS REMAIN ACTIVE THROUGH
TURNAGAIN ARM...PORTAGE VALLEY...AND ALONG THE COPPER RIVER...WITH
WARMING FROM THE AFTERNOON SUN HELPING TO PULL A BIT OF THE
TURNAGAIN WIND BACK INTO ANCHORAGE THIS AFTERNOON.
MEANWHILE...COLD NORTHERLY FLOW CONTINUES ACROSS MUCH OF THE
BERING SEA WITH THE MOST ENHANCED SNOW SHOWER ACTIVITY CURRENTLY
CONFINED TO THE WESTERN HALF OF THE BERING.

A WEAKENING GALE-FORCE FRONT IS MOVING INTO THE SOUTHWEST GULF
WITH RAIN NOW FALLING ACROSS KODIAK ISLAND. A SIGNIFICANT MIDLEVEL
SHORTWAVE SOUTH OF THE ALASKA PENINSULA IS ROTATING AROUND THE
BASE OF THE LONGWAVE TROUGH AND IS QUICKLY CATCHING UP TO THIS
WEAKENING FRONT. FURTHER WEST...WATER VAPOR IMAGERY SUPPORTS THE
IDEA THAT A STRONG JET STREAK DIVING DOWN THE EAST/FRONT SIDE OF
AN EASTERN RUSSIAN RIDGE IS NOW ROUNDING THE BASE OF THE LONGWAVE
TROUGH. IN RESPONSE...A BRAND NEW LOW IS NOW DEVELOPING SOUTH OF
THE CENTRAL ALEUTIANS. THIS LOW WILL BE THE MAIN DRIVER OF WEATHER
IMPACTS ACROSS THE MAINLAND FOR THE NEXT SEVERAL DAYS.

&&

.MODEL DISCUSSION...

THE MODELS ARE IN VERY GOOD AGREEMENT...ESPECIALLY GIVEN HOW
COMPLEX THE INTERACTIONS OF THE MAJOR FEATURES IN THE ATMOSPHERE WILL
BE OVER THE NEXT 48-72 HOURS. ALL MODELS SHOW THE LOW FORMING
SOUTH OF THE ALEUTIANS MOVING TOWARD THE ALASKA PENINSULA BEFORE
WEAKENING AND ELONGATING TO THE NORTHWEST AND QUICKLY MOVING
INLAND OVER SOUTHWEST ALASKA ON SUNDAY. THERE ARE SOME SUBTLE
TIMING DIFFERENCES WITH THIS TRANSITION INLAND. THE MAIN IMPACT OF
THESE TIMING DIFFERENCES WILL BE HOW LONG SNOW AND BLOWING SNOW
WILL PERSIST OVER THE KUSKOKWIM DELTA COAST AND NUNIVAK ISLAND.
FORECAST CONFIDENCE IS AVERAGE TO SLIGHTLY ABOVE AVERAGE.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
ABUNDANT PRECIPITATION WILL CONTINUE ALONG THE GULF COAST THROUGH
THE WEEKEND AS THE STORM FORCE FRONT CURRENTLY OVER THE GULF MOVES
INLAND TONIGHT AND THEN ANOTHER FRONT APPROACHES THE COAST LATE
SATURDAY NIGHT. PRECIPITATION WILL BEGIN AS A RAIN/SNOW MIX
TONIGHT BEFORE QUICKLY TRANSITIONING TO ALL RAIN ALONG ON SATURDAY
AS WARMER AIR CONTINUES TO MOVE INTO THE AREA IN DEEP SOUTH TO
SOUTHEAST FLOW. RAIN WILL BE HEAVY AT TIMES THROUGH SUNDAY IN
COASTAL AREAS...BUT NO SIGNIFICANT IMPACTS ARE EXPECTED AT THIS
TIME. INLAND AREAS WILL STAY MOSTLY DRY THROUGH SATURDAY AS A
RESULT OF STRONG DOWNSLOPE FLOW ALTHOUGH A FEW SHOWERS WILL REMAIN
POSSIBLE TONIGHT AS A WEAK SHORTWAVE TROUGH MOVES OVERHEAD. THE
APPROACHING SYSTEM WILL ALSO HELP TO GENERATE STRONG GAP WINDS
THROUGH TURNAGAIN ARM AND THE MATANUSKA VALLEY THROUGH MUCH OF THE
WEEKEND AS THE PRESSURE GRADIENT INCREASES...BUT INCREASING DOWN
INLET FLOW WILL GENERALLY KEEP THESE WINDS AWAY FROM THE ANCHORAGE
BOWL THROUGH SATURDAY NIGHT.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (TONIGHT THROUGH SUNDAY)...
THE FRONT ASSOCIATED WITH THE NORTH PACIFIC LOW WILL CAUSE
EASTERLY WINDS OVER THE AREA TO INCREASE THIS EVENING WITH
DEVELOPING RAIN AND SNOW OVER BRISTOL BAY. THIS SHOULD BECOME
MOSTLY SNOW AS IT REACHES THE KUSKOKWIM DELTA LATE TONIGHT. OVER
THE NORTHERN COAST OF THE KUSKOKWIM DELTA...PRECIP TYPE SHOULD
REMAIN AS SNOW THROUGHOUT THE EVENT WITH UP TO 6 INCHES OF
ACCUMULATION POSSIBLE BY LATE SATURDAY EVENING. BECAUSE OF THE
GUSTY NORTHEASTERLY WINDS THAT WILL DEVELOP DURING THIS TIME...A
BLOWING SNOW ADVISORY HAS BEEN ISSUED FOR AREAS ALONG THE COAST
NORTH OF KIPNUK. OVER THE REST OF THE AREA THERE SHOULD BE ENOUGH
WARM AIR TO CAUSE MOST OF THE PRECIPITATION TO SWITCH OVER THE
RAIN BY TOMORROW AFTERNOON...INCLUDING THE BETHEL AREA. ON SUNDAY
THE UPPER LEVEL TROUGH WILL REMAIN JUST OFF THE SOUTHWEST
COAST...WHICH WILL KEEP RAIN AND SNOW IN THE AREA THROUGH THE
WEEKEND...BUT WITH DIMINISHING WINDS THE BLOWING SNOW POTENTIAL
SHOULD END.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (TONIGHT THROUGH SUNDAY)...
THE NORTH PACIFIC LOW CURRENTLY ENTERING THE CHAIN WILL BRING SNOW
AND STRONG NORTHEASTERLY WINDS TO THE DUTCH HARBOR AREA...BECOMING
RAIN OVER THE AKPENN INTO TOMORROW EVENING. HOWEVER WITH THE WARM
SEA SURFACE TEMPERATURES IN THE AREA...ANY SNOW SHOULD BE A WET
SNOW WITH TEMPERATURES IN THE MID TO UPPER 30S. BECAUSE OF THIS BLIZZARD
CONDITIONS ARE NO LONGER POSSIBLE AND THE BLIZZARD WATCH FOR THE
EASTERN ALEUTIANS WAS CANCELLED. FOR THE PRIBILOFS...THE MODELS
ARE NOW IN MUCH BETTER AGREEMENT THAT MOST IF NOT ALL OF THE SNOW
FROM THIS SYSTEM WILL REMAIN JUST OFF TO THE EAST OVER THE
BERING...WHICH ALSO ENDS ANY BLOWING SNOW CONCERNS FOR THIS AREA.
THIS WILL LIKELY CHANGE ON SUNDAY AS A SHORTWAVE MOVING UNDER THE
UPPER LEVEL TROUGH OVER THE EASTERN BERING WILL BRING ACCUMULATING
SNOW TO THE PRIBILOFS FOR THE DAY SUNDAY BEFORE SPREADING INTO THE
ALEUTIANS ON SUNDAY EVENING.

&&

.LONG TERM FORECAST (MONDAY THROUGH FRIDAY)...

MODELS LOOK PRETTY SIMILAR TO YESTERDAY IN REGARDS TO THE POSITION
OF THE JET STREAM AND UPPER LEVEL FEATURES. THE TWO MAJOR CHANGES
OBSERVED IN THE MOST RECENT DATA IS THE SEPARATION OF CENTRAL
PACIFIC FLOW (WHICH DELAYS THE LOW PRESSURE SYSTEM THAT HAD BEEN
ANALYZED TO BE SOUTHWEST OF THE PENINSULA ON WEDNESDAY) AND A
FASTER TREND WITH THE KAMCHATKA SYSTEM (AT LEAST WITH THE PARENT
LOW VS THE LEADING FRONT). THE KAMCHATKA LOW IS EXPECTED TO PUSH
OFF THE KAMCHATKA PENINSULA MONDAY AFTERNOON AND DRIVE A FRONT
ACROSS THE BERING SEA/ALEUTIAN ISLANDS DURING THE WEEK WITH
PERIODS OF GALE FORCE WINDS INITIALLY OVER THE WESTERN BERING
SEA. MODEL CONSENSUS NOW SHOWS THE LOW TRACKING ACROSS THE
ALEUTIAN CHAIN BEFORE DIVING SOUTH INTO THE GULF OF ALASKA. THE
GULF OF ALASKA WILL SEE PRECIPITATION TAPER OFF DURING THE
BEGINNING OF THE WEEK AS A FRONT ALONG THE GULF COAST DISSIPATED
AND AN AREA OF LOW PRESSURE OVER SOUTHWEST ALASKA PUSHES INTO THE
BERING STRAIT. ONE THING THAT WILL BE WATCHED IN THE COMING DAYS
IS THE POTENTIAL FOR A BUILDING PACIFIC NORTH AMERICAN RIDGE WHICH
COULD USHER IN A MUCH WARMER AIR MASS WITH PLACES IN SOUTH CENTRAL
ALASKA WARMING INTO THE LOWER TO MID 50S. THIS IS THE FIRST DAY
THE MODELS HAVE HINTED AT THIS BUILDING RIDGE ON FRIDAY SO DID NOT
GO TOO WARM WITH TEMPERATURES JUST YET.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...BLOWING SNOW ADVISORY 155.
MARINE...STORM WARNING 119 120 130 131.
 GALE WARNING 125 129 132 136 137 139 141 155 160 165 170 171 172
173 174 175 176 179 180 181 185.
 HEAVY FREEZING SPRAY 179.
FIRE WEATHER...NONE.

&&

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