Heavy Snow and High Wind in Alaska, November 19 to 20, 2015

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
512 AM AKST THU NOV 19 2015

.ANALYSIS AND UPPER LEVELS...
THE ARCTIC AIR MASS THAT HAS EXTENDED OVER MUCH OF MAINLAND ALASKA
IS FINALLY BEING PUSHED BACK NORTH AS A FRONT FROM A DEEP BERING
LOW IS PUSHING INTO SOUTHWEST ALASKA. THIS SYSTEM FOLLOWS BEHIND A
WEAKER ONE THAT HAS DISSIPATED BUT HAS PRODUCED LINGERING SNOW
AROUND THE SOUTHCENTRAL. THE MAIN LOW IS SUPPORTED ALOFT BY A
STRONG 175 KT JET HELPING TO MOVE IT EAST. IT ALSO HAS A LONG
FETCH OF WARMER AIR BEING PULLED UP FROM THE SOUTH.

&&

.MODEL DISCUSSION...
THE MODELS HAVE A DECENT GRASP OF THE BERING LOW AND ITS IMPACTS
FOR TODAY BUT MOVING OUT IN TIME THEY STRUGGLE WITH THE NEXT LOW
MOVING TOWARD THE ALEUTIANS TONIGHT. THE NAM MOVES THIS NEXT
SYSTEM FASTER THAN THE OTHERS WHILE THE GEM LAGS BEHIND WITH THE
EC AND GFS IN THE MIDDLE. THIS DISCREPANCY LOWERS CONFIDENCE IN
WHEN THE NEXT BATCH OF PRECIPITATION WILL MAKE IT TO MAINLAND
ALASKA SOMETIME ON FRIDAY.

&&

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

STRONG SOUTHERLY FLOW AND WARM ADVECTION IN ADVANCE OF AN
APPROACHING WEATHER FRONT FROM THE WEST WILL SPREAD SNOW ACROSS
MUCH OF SOUTHCENTRAL TODAY. SNOW WILL BE HEAVY AT TIMES ACROSS
MUCH OF THE SUSITNA VALLEY BEGINNING THIS MORNING AND CONTINUING
UNTIL FRONTAL PASSAGE TONIGHT. THERE WILL ALSO BE PERIODS OF
MODERATE SNOW ACROSS THE KENAI PENINSULA INTO THE EVENING HOURS
TODAY. SNOW IS EXPECTED TO BECOME FAIRLY HEAVY TONIGHT OVER THE
VALDEZ ZONE WITH THE ONSET OF MOIST UPSLOPE FLOW. THE DEGREE OF
WARMING AND EXACT PRECIPITATION TYPES CONTINUES TO BE THE MAIN
FORECAST CHALLENGE ACROSS SOUTHCENTRAL. THE CURRENT THINKING IS
WARM ADVECTION AND TURBULENT MIXING ACROSS THE SOUTHERN KENAI PEN
WILL PRODUCE MIXED PRECIPITATION TYPES THIS AFTERNOON WHICH WILL
SPREAD NORTH TO THE ANCHORAGE BOWL LATE THIS EVENING AND MATANUSKA
VALLEY ON FRI. PRECIPITATION ALONG THE NORTH GULF WILL BECOME
MIXED AT TIMES THIS AFTERNOON...THOUGH THE COLD AIR IN THE VALDEZ
ZONE WILL BE SLOW TO SCOUR OUT. SNOW OR MIXED SHOWERS ARE EXPECTED
ACROSS MANY AREAS ON FRI BEFORE THE NEXT SYSTEM MOVES IN FRI
NIGHT.

STRONG GAP FLOWS WILL ACCOMPANY THE FRONTAL SYSTEM TODAY AND
TONIGHT WITH MAINLY OF THE MARINE AREAS IN THE UPPER SMALL CRAFT
TO GALE FORCE RANGE. THERE WILL ALSO LIKELY BE A NARROW BAND OF
STORMS ALONG THE LEADING EDGE OF THE FRONT OVER THE NORTH GULF
WATERS TODAY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
A STRONG FRONT WORKING ITS WAY INTO THE WESTERN COAST OF ALASKA
EARLY THIS MORNING WILL TRACK FURTHER INLAND THROUGHOUT THE DAY
TODAY...BRINGING NOT ONLY A SURGE OF WARM AIR AND MIXED
PRECIPITATION...BUT ALSO BRINGING THE STRONG AND PROLONGED
ONSHORE WINDS THAT ARE LIKELY TO RESULT IN SOME WATER "PILING UP"
ALONG THE COAST. A HIGH SURF ADVISORY REMAINS IN EFFECT FOR BOTH
THE BRISTOL BAY AND DELTA COASTLINES DURING THE DAY TODAY. THE
HEAVIER PRECIPITATION WITH THIS STORM WILL FALL INITIALLY AS
SNOW...BUT TRANSITION TO RAIN OVER BRISTOL BAY AND COASTAL
PORTIONS OF THE DELTA. SNOW WILL REMAIN THE DOMINANT PRECIPITATION
TYPE OVER THE LOW-LYING AREAS OF THE LOWER KUSKOKWIM VALLEY THIS
MORNING...AS THERE SEEMS TO BE A FAIR AMOUNT OF RESIDUAL COLD AIR
TRAPPED IN THESE AREAS. HOWEVER...WITH WITH THE WARMER SOUTHERLY
AIR SURGING INTO THE AREA...THERE IS STILL A CHANCE OF SOME RAIN
TO MIXING IN IN THE AFTERNOON.

COLDER AIR WILL QUICKLY FIND ITS WAY BACK TO THE SOUTHWEST
MAINLAND FRIDAY...BEFORE ANOTHER SYSTEM TRACKS ALONG THE ALASKA
PENINSULA FROM THE NORTH PACIFIC AND SPREADS SNOW BACK INTO THE
BRISTOL BAY AREA LATE FRIDAY AFTERNOON.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
A STRONG STORM FORCE LOW ENTERING THE CENTRAL BERING THIS MORNING
WILL CONTINUE TO SLOWLY TRACK NORTHEASTWARD INTO THE KUSKOKWIM
DELTA COAST TONIGHT...WITH THE ASSOCIATED FRONT ALONG THE
SOUTHWEST COAST TRACKING INLAND BY THIS AFTERNOON. THE STRONGEST
CORE OF STORM FORCE WINDS ARE CURRENTLY IMPACTING THE CENTRAL
ALEUTIANS...AND WILL SLOWLY WEAKEN TO STRONG GALES TO MIN STORM
FORCE AS THE SYSTEM TRACKS FURTHER EASTWARD TODAY. ANOTHER GALE
FORCE LOW WILL TRACK ALONG THE EASTERN ALEUTIANS AND ALASKA
PENINSULA DURING THE DAY FRIDAY. THIS WILL BRING ANOTHER ROUND OF
RAIN AND SNOW TO THE EASTERN HALF OF THE ALEUTIAN CHAIN...ALONG
WITH SOME COLDER AND GUSTIER NORTHWEST WINDS BEHIND THE SYSTEM.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
MODELS HAVE COME INTO BETTER...ALTHOUGH LESS THAN OPTIMAL...AGREEMENT
WITH THE LOW EXPECTED TO MOVE THE NORTHERN GULF OF ALASKA AND INTO
SOUTH CENTRAL ALASKA FRIDAY NIGHT INTO SATURDAY. BROADLY...THE 12Z
MODEL RUNS PLACE A SURFACE LOW FRIDAY NIGHT OVER SOUTH-CENTRAL
ALASKA WITH THE 12Z GFS/NAM PLACING A LOWER 980S MB LOW OVER THE
NORTH CENTRAL GULF COAST THAT MOVES INTO THE COPPER RIVER BASIN
OVERNIGHT FRIDAY. ON THE OTHER HAND...THE ECMWF IS SLOWER WITH A
DEEPER LOW OVER THE BRISTOL BAY REGION THAT MOVES FURTHER INLAND
BEFORE CURVING EASTWARD INTO THE YUKON. THE 18Z RUN OF THE GFS IS
APPROXIMATELY THE SAME STRENGTH AND POSITION AS THE 12Z RUN...SO
ON DAY 3 THE GFS GUIDANCE IS PREFERRED. THUS...USING THE GFS
SOLUTION...SNOW IS EXPECTED ALONG THE HIGHER ELEVATIONS OF PRINCE
WILLIAM SOUND ON FRIDAY EVENING THROUGH SATURDAY MORNING. IN THE
COOK INLET REGION...PRECIPITATION IS EXPECTED TO BEGIN EARLY FRIDAY
EVENING AS A RAIN/SNOW MIX BEFORE RAPIDLY TRANSITIONING TO ALL
SNOW AS THE LOW MOVES EASTWARD AND THE REGION FALLS INTO THE
BACKSIDE OF THE LOW. ACCUMULATING SNOW IS EXPECTED IN THE
ANCHORAGE AREA...BUT DUE TO THE UNCERTAINLY IN THE TRACK OF THE
LOW...IT IS TOO EARLY TO DISCUSS POTENTIAL SNOW AMOUNTS IN THE
COOK INLET REGION AT THIS TIME. AS THIS LOW MOVES INTO THE YUKON
TERRITORY ON SATURDAY...MOST OF SOUTHERN ALASKA WILL STAY IN A
COLD SHOWERY PATTERN AS A RIDGE BUILDS IN OVER THE EASTERN BERING
SEA THROUGH EARLY MONDAY MORNING.

FURTHER WEST...MODELS ARE IN GOOD AGREEMENT IN DEVELOPING A STRONG LOW
IN THE WESTERN BERING SEA NEAR THE COMMANDER ISLANDS LATE SATURDAY
NIGHT. THIS LOW IS EXPECTED TO MOVE INTO THE NORTHWEST BERING SEA
BY TUESDAY WHILE ITS ASSOCIATED FRONT ARCS EASTWARD THROUGH THE
BERING SEA AND WILL BEGIN TO IMPACT SOUTHWEST ALASKA TUESDAY
AFTERNOON. THIS FRONT WILL BRING AT LEAST GALE FORCE WINDS WITH
HIGH SEAS THROUGH MOST OF THE BERING AS IT PROGRESSES EASTWARD.
THIS FRONT IS EXPECTED TO HAVE A DEEP SOUTHERLY CONNECTION TO THE
SUB-TROPICS.

FINALLY...AT THE END OF THE EXTENDED PERIOD...THE REMNANTS OF EX-
TYPHOON IN-FA WILL BEGIN TO ENTER THE NORTHERN PACIFIC OCEAN. AS
WITH MOST FORMER TROPICAL SYSTEMS...MODELS ARE STRUGGLING WITH THE
EXTRA-TROPICAL TRANSITION AND TRACK. DUE TO THE LOW CONFIDENCE IN
THE DETAILS WITH THIS SYSTEM AT DAY 7...LITTLE DETAIL WAS INCLUDED
IN THE FORECAST OVER THE WESTERN BERING SEA FOR THIS FORECAST
PACKAGE.

THE GFS WAS USED FOR MOST OF THE FORECAST AREA ON DAY 3 AS A
MIDDLE GROUND SOLUTION BEFORE TRANSITIONING TO WPC GUIDANCE FOR
DAYS 4-7.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...HIGH WIND WARNING...101.
         WINTER STORM WARNING...145.
         WINTER WEATHER ADVISORY...125 131.
         HIGH SURF ADVISORY...155 161.
MARINE...HEAVY FREEZING SPRAY...128 128 139
         STORM WARNING...131 132 150 155 170 173 175 185
         GALE WARNING...119 120 130 136 137-139 141 160 165 172
                        174 176-181

&&

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...RF/MMC