Small Craft Advisory 230, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
453 PM AKDT SAT MAR 14 2015

.ANALYSIS AND UPPER LEVELS...
ALL ATTENTION IS SHIFTING TO A STORM SYSTEM SOUTH OF THE EASTERN
ALEUTIANS AND SOUTHERN ALASKA PENINSULA WHICH HAS BEEN RAPIDLY
DEEPENING IN THE LEFT EXIT REGION OF A 180 KT SUBTROPICAL JET.
IN ADDITION TO THE FAVORABLE JET DYNAMICS THERE IS A POTENT UPPER
LEVEL SHORT-WAVE ALONG WITH STRONG UPPER LEVEL DIFFLUENCE (AND
DIVERGENCE) RIGHT OUT AHEAD OF IT. THESE ARE CONTRIBUTING TO
STRONG UPWARD VERTICAL MOTION AND HEAVY PRECIPITATION AS IS
EVIDENT BY THE COLD CLOUD TOPS AND DEEP MOISTURE SEEN IN SATELLITE
IMAGERY. A DEEP ARCTIC AIR MASS IS IN PLACE OUT AHEAD OF THIS
SYSTEM ACROSS MUCH OF THE BERING SEA...GULF OF ALASKA...AND
MAINLAND ALASKA. A BUILDING RIDGE OUT AHEAD OF THE STORM IS
STARTING TO PUSH THE COLD ARCTIC TROUGH OVER MAINLAND ALASKA
NORTHWARD. HOWEVER...THE PIECES ARE IN PLACE FOR SIGNIFICANT
WINTER WEATHER AS THIS SYSTEM BEGINS TO AFFECT KODIAK ISLAND AND
THE SOUTHERN MAINLAND TONIGHT AND TOMORROW.

&&

.MODEL DISCUSSION...
MODELS HAVE CONVERGED ON A COMMON SOLUTION FOR THE TRACK OF THE
MAIN LOW CENTER...WITH THE STORM STALLING SOUTH OF THE EASTERN
ALEUTIANS THIS AFTERNOON AND EVENING AS THE CENTRAL PRESSURE
BOTTOMS OUT AND THE LOW BECOMES VERTICALLY STACKED. THE NAM IS
JUST A BIT OF AN OUTLIER AS THE LOW WOBBLES AROUND A BIT THE
NEXT COUPLE DAYS. SIGNIFICANT DIFFERENCES ARISE WITH TRACK OF
A STRONG UPPER LEVEL SHORT-WAVE FROM THE NORTHEAST PACIFIC TO
SOUTHCENTRAL ALASKA SUNDAY AFTERNOON THROUGH MONDAY. ALL SOLUTIONS
NOW INDICATE THIS SHORT-WAVE WILL INDUCE DEVELOPMENT OF A SURFACE
LOW ALONG THE FRONTAL BOUNDARY OVER THE GULF SUNDAY NIGHT...BUT
THE SAME TIMING PROBLEMS EXIST AS WITH THE UPPER LEVEL FEATURE.
THESE MODEL DIFFERENCES LOWERS FORECAST CONFIDENCE FOR FRONTAL
POSITION/STRENGTH OVER THE GULF AND MORE IMPORTANTLY PRECIPITATION
FORECAST AS SHORT-WAVE TRACKS ACROSS SOUTHCENTRAL. ASIDE FROM
THESE FEATURES FORECAST CONFIDENCE IS AVERAGE TO ABOVE AVERAGE.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE LAST OF THE GAP WINDS ALONG THE GULF COAST WILL RAPIDLY
DIMINISH EARLY THIS EVENING AS THE UPPER LEVEL TROUGH EXITS TO
THE NORTH AND EAST TAKING WITH IT THE SURFACE LOW WHICH HAS BEEN
SITTING IN THE PRINCE WILLIAM SOUND/NORTHERN GULF REGION. THE
FOCUS THEN SHIFTS TO THE STRONG STORM SYSTEM SOUTH OF THE EASTERN
ALEUTIANS WHICH WILL SEND A FRONTAL SYSTEM NORTHEASTWARD ACROSS
THE GULF TONIGHT THROUGH MONDAY.

AS DISCUSSED IN THE ANALYSIS SECTION ABOVE THE IS A VERY STRONG
SYSTEM...BOTH AT THE SURFACE AND ALOFT. THE KEY ASPECT OF THIS
IMPENDING STORM IS THE DEEP COLD AIR MASS CURRENTLY IN PLACE. AS
PRECIP SPREADS NORTHWARD ACROSS THE GULF OUT AHEAD OF THE FRONTAL
SYSTEM THIS WILL ENSURE PRECIP STARTS OUT AS ALL SNOW...AND IT
WILL BE HEAVY AT TIMES. ALTHOUGH STRONG WINDS WILL ACCOMPANY THE
SNOW IN MANY COASTAL LOCATIONS...SURFACE TEMPERATURES WILL WARM
PRETTY QUICKLY TO AROUND FREEZING...SO IT SEEMS THE LARGER THREAT
IS THE HEAVY PRECIP AND NOT THE BLOWING SNOW. BASED ON THIS...HAVE
ISSUED A WINTER WEATHER ADVISORY FOR SNOW AND BLOWING SNOW FOR
KODIAK ISLAND. AS ALL OF THIS MOVES NORTHWARD THERE IS POTENTIAL
FOR EVEN HEAVIER SNOWFALL OVER THE EASTERN KENAI PENINSULA. LOW
LEVEL FLOW IS INITIALLY DUE EAST OUT AHEAD OF THE FRONT. THE HEAVIEST
SNOW EVENTS FOR THE ALYESKA TO TURNAGAIN PASS CORRIDOR GENERALLY
HAVE MORE OF A SOUTHEAST COMPONENT TO THE LOW LEVEL FLOW.
HOWEVER...IF THE STRONG UPPER LEVEL SHORT-WAVE DISCUSSED ABOVE
HEADS DIRECTLY FOR THE EASTERN KENAI (AS SOME OF THE GUIDANCE
INDICATES) THIS WILL GREATLY ENHANCE SNOWFALL RATES. THERE IS ALSO
A THREAT FOR BLIZZARD CONDITIONS IN THE PORTAGE VALLEY/TURNAGAIN
ARM/TURNAGAIN PASS AREA AS WINDS STRENGTHEN AHEAD OF THE FRONT
SUNDAY NIGHT. WITH UNCERTAINTY IN SOME OF THE KEY FEATURES WILL
LIKELY OPT TO PUT OUT A WINTER STORM WATCH.

THE FORECAST FOR THE REST OF SOUTHCENTRAL POSES ADDITIONAL CHALLENGES.
THERE WILL BE A BRIEF PERIOD OF OVERRUNNING OUT AHEAD OF THE
STORM SYSTEM ON SUNDAY WHICH COULD BRING SOME LIGHT SNOW TO MUCH
OF THE REGION. STRONG SOUTHEAST FLOW WILL THEN DEVELOP ACROSS THE
KENAI AND CHUGACH MOUNTAINS TOMORROW AFTERNOON AND EVENING. THIS
OFTEN RESULTS IN DOWNSLOPE DRYING FROM THE WESTERN KENAI PENINSULA
TO ANCHORAGE AND PARTS OF THE MAT-SU. IN THIS CASE THE DEEP ARCTIC
AIR MASS IN PLACE WILL LIMIT DOWNSLOPING WITH THE STRONGER WINDS
RIDING RIGHT OVER TOP OF THE COLD DOME. THUS...THERE IS A GOOD
CHANCE OF SOME PERIODS OF LIGHT SNOW IN THE LEE OF THE MOUNTAINS.
THE BEST CHANCE FOR PRECIP THROUGH THIS CORRIDOR WILL BE WITH
UPPER TROUGH PASSAGE SOMETIME MONDAY OR MONDAY NIGHT. AT THIS
POINT IT LOOKS LIKE ANY SNOW THAT DOES FALL IN THE POPULATION
CENTERS OVER THE NEXT COUPLE DAYS WILL BE LIGHT WITH LITTLE OR NO
ACCUMULATION. THE BIGGEST IMPACTS FROM THIS SYSTEM WILL MOST
DEFINITELY BE IN THE COASTAL ZONE.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
ONE MORE NIGHT OF COLD AND DRY CONDITIONS IS IN STORE ACROSS THE
SOUTHWEST MAINLAND BEFORE THE FRONT OVER THE SOUTHEASTERN BERING
USHERS IN A WARMER AND WETTER PATTERN BEGINNING ON SUNDAY.
NORTHEAST WINDS HAVE BEGUN TO INCREASE ACROSS THE KUSKOKWIM DELTA
AHEAD OF THE FRONT...LEADING TO THE ISSUANCE OF A WIND CHILL
ADVISORY THROUGH SUNDAY MORNING. LIGHT SNOW WILL THEN MOVE INTO
THE BRISTOL BAY REGION ON SUNDAY MORNING AND SPREAD NORTHWARD
TOWARDS THE KUSKOKWIM DELTA AND VALLEY LATER IN THE DAY. GUSTY
WINDS WILL LEAD TO THE POTENTIAL FOR BLOWING SNOW OVER WESTERN
PORTIONS OF BRISTOL BAY INCLUDING THE DILLINGHAM AREA...HOWEVER
VISIBILITIES ARE EXPECTED TO REMAIN ABOVE ADVISORY CRITERIA GIVEN
THE RELATIVELY LIGHT NATURE OF THE PRECIPITATION AND A LACK OF
TRANSPORTABLE SNOW ON THE GROUND.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE MOST ACTIVE WEATHER REMAINS CENTERED OVER THE SOUTHEASTERN
BERING AS A DEEP NORTH PACIFIC LOW MEANDERS NORTH AND THEN
WESTWARD BETWEEN COLD BAY AND DUTCH HARBOR THROUGH MONDAY...
KEEPING A MIX OF RAIN AND SNOW OVER THE ALASKA PENINSULA AND
EASTERN ALEUTIANS. HOWEVER...THE BIGGEST AREA OF CONCERN CONTINUES
TO BE THE PRIBILOF ISLANDS WHERE SNOW AND STORM FORCE WINDS WILL
PRODUCE BLIZZARD CONDITIONS INTO SUNDAY MORNING. CONDITIONS WILL
IMPROVE ON SUNDAY AS THE HEAVIEST PRECIPITATION MOVES AWAY FROM
THE AREA...BUT STRONG NORTHERLY WINDS WILL KEEP AREAS OF BLOWING
SNOW OVER THE REGION THROUGH MONDAY.

&&

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

MODELS ARE STRUGGLING WITH A CLOSED UPPER LOW THAT REMAINS OVER
THE EASTERN BERING AND AKPEN INTO NEXT WEEKEND. THE MAIN SURFACE
LOW WILL MOVE FROM THE NORTH PACIFIC TO THE EASTERN ALEUTIANS
DURING THE WEEK. ALTHOUGH WAVES WILL BE MOVING AROUND THE
LOW...THE TIMING AND STRENGTH OF THESE FEATURES ARE OF LOW CONFIDENCE.

THE EFFECT OF THESE SYSTEMS WILL BE CONTINUED UNSETTLED WEATHER.
SNOW SHOWERS WILL PERSIST OVER MUCH OF THE BERING...WHILE WARMER
AIR STREAMS INTO SW AK WITH RAIN AND SNOW. AS THE GULF LOWS MOVE
NORTH DURING THE WEEK...THEY WILL BRING RAIN TO THE GULF COAST
WITH SOME SNOW CROSSING COASTAL RANGES AND REACHING INLAND ZONES.

THE FORECAST UTILIZES SOME EC AND THEN REVERTS TO THE ENSEMBLE
BASED WPC GUIDANCE LATER IN THE WEEK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...BLIZZARD WARNING 195 WINTER STORM WATCH 125 WINTER WX
ADVISORY 171 AND WIND CHILL ADVISORY 155.
MARINE...STORMS 119 130 131 139 414. GALES 120 125 129 150 155 160
165 170 175 176 180 181 351 352 411 412 413.
FIRE WEATHER...NONE.

&&

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