Heavy Snow and High Wind in Alaska, November 22 to 24, 2015

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
503 PM AKST SUN NOV 22 2015

.ANALYSIS AND UPPER LEVELS...
THE DOMINANT FEATURE ON SATELLITE IMAGERY THIS MORNING IS A
VERY LARGE STORM FORCE LOW CENTERED JUST WEST OF THE WESTERN
TIP OF THE ALEUTIANS. THE LOW HAS FULLY OCCLUDED WITH CENTRAL
PRESSURE ESTIMATED TO BE DROPPING BELOW 970 MB. THIS HAS CAUSED
THE FORWARD MOMENTUM OF THE LOW TO STALL. HOWEVER THE OCCLUDED
FRONT IS SPREADING NORTH AND EAST ACROSS THE BERING SEA AND
ALEUTIANS. OBSERVED WINDS (VIA SATELLITE BASED SCATTEROMETER AND
THE FEW OBSERVATION PLATFORMS OUT THAT WAY) ARE ON THE STRONG SIDE
OF ALL THE GUIDANCE. IN FACT...ADAK REPORTED A SINGLE WIND GUST
TO HURRICANE FORCE (65 KT) JUST PRIOR TO PASSAGE OF THE OCCLUDED
FRONT. THE DEVELOPMENT OF THIS STORM IS LEADING TO MUCH MORE
AMPLIFIED UPPER LEVEL FLOW... WITH A RIDGE BUILDING ACROSS THE
EASTERN BERING AND TOWARD MAINLAND ALASKA. THIS WILL ULTIMATELY
LEAD TO TRANSPORT OF WARMER AND MOISTER AIR TO THE SOUTHERN
MAINLAND.

MEANWHILE...AN UPPER LEVEL SHORT-WAVE TROUGH IS TRACKING FROM
SOUTHWEST ALASKA TO SOUTHCENTRAL ALASKA TODAY. THE AIR MASS WITH
THIS WAVE IS WEAKLY UNSTABLE AND THE CHARACTER OF PRECIPITATION
IS SHOWERY (AS SEEN IN RADAR IMAGERY). THE TROUGH AXIS HAS A WEAK
NEGATIVE TILT TO IT BUT THE BUILDING RIDGE UPSTREAM RIDGE IS
BEGINNING TO OVERTAKE THE NORTHERN END OF THIS SHORT-WAVE. THIS
IS CAUSING IT TO BECOME MORE NEUTRALLY TILTED...WHICH MEANS IT IS
WEAKENING WITH TIME.

&&

.MODEL DISCUSSION...
THE RECENT TRENDS WITH HANDLING OF THE MAJOR STORM SYSTEM
CROSSING THE BERING SEA THE NEXT COUPLE DAYS REMAIN THE SAME...
STRONGER AND SLOWER. BASED ON WIND VERIFICATION (AS MENTIONED
ABOVE) WILL GENERALLY MAKE USE OF THE STRONGEST GUIDANCE...WHICH
IS LARGELY THE GFS MODEL. THE SLOWER EASTWARD PROGRESSION OF THE
OCCLUDED FRONT IS LARGELY REFLECTED WELL IN CURRENT FORECAST FOR
SOUTHWEST ALASKA. HOWEVER...IT NOW APPEARS THE DEEP MOISTURE OUT
AHEAD OF THE FRONT WILL TAKE MORE TIME TO REACH THE WESTERN GULF
TO COOK INLET AND SUSITNA VALLEY CORRIDOR. BASED ON THIS TREND AND
THE MODEL SPREAD IN ARRIVAL OF THE DEEPER MOISTER...FORECAST
CONFIDENCE IN THE PRECIPITATION FORECAST OVER SOUTHCENTRAL HAS
DROPPED TO BELOW AVERAGE. PRIOR TO THAT...THERE ARE ALSO SOME KEY
DIFFERENCES WITH WARM ADVECTION/ISENTROPIC LIFT OUT AHEAD OF THE
FRONT ON MONDAY. THIS CREATES AN ESPECIALLY TRICKY FORECAST FOR
THE ANCHORAGE AND MAT-SU AREAS. WITH A DEEP COLD AIR MASS IN PLACE
IT APPEARS THERE IS ENOUGH OVERRUNNING TO WARRANT LIKELY
POPS...SO WILL GENERALLY TREND TOWARD THE HIGHER POP/QPF
GUIDANCE.

&&

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

A WEAK UPPER LEVEL TROUGH IS CURRENTLY CROSSING SOUTH CENTRAL
ALASKA AND WILL BRING SOME LIGHT SNOW THIS EVENING FOLLOWED BY
SCATTERED SNOW SHOWERS ALONG THE MOUNTAINS OVERNIGHT. SNOW FALL
ACCUMULATIONS FROM BOTH THE INITIAL DISTURBANCE ITSELF AND POST
FRONTAL SHOWERS WILL BE LIGHT WITH ACCUMULATIONS UP TO AN INCH AT
LOWER ELEVATIONS AND UP TO 2 INCHES ALONG WEST FACING SLOPES WHERE
THE POST FRONTAL SHOWERS WILL BE MOST OROGRAPHICALLY FAVORED.

FOLLOWING THIS TROUGH AND THE A SURFACE LOW CROSSING THE GULF
TONIGHT A STRONG UPPER LEVEL RIDGE WILL BUILD OVER THE AREA IN
RESPONSE TO THE VERY DEEP LOW TRACKING INTO THE BERING. DO HAVE
SOME CONCERNS FOR PATCHY FOG AND AREAS OF LOW STRATUS FORMING
OVERNIGHT AND MONDAY MORNING ALONG AND NEAR COOK INLET AND IN THE
MATANUSKA AND SOUTHERN SUSITNA VALLEYS BENEATH THE BUILDING RIDGE.

OVERRUNNING FROM THE STRONG FRONT PUSHING INTO SOUTHWEST ALASKA
WILL BEGIN SPILLING OVER THE ALASKA RANGE AND PUSHING INTO WESTERN
SOUTH CENTRAL ALASKA MONDAY AFTERNOON BRINGING ANOTHER ROUND OF
LIGHT SNOW. SNOW FALL WILL RAPIDLY INCREASE INTENSITY UP THE
SUSITNA VALLEY MONDAY NIGHT AS MID LEVEL SOUTHERLY UP-VALLEY FLOW
STRENGTHENS. ELSEWHERE SNOW WILL MIX OVER TO RAIN AS TEMPERATURES
WARM AND/OR GET DOWNSLOPED OUT AS THE SOUTHEASTERLY GRADIENT
TIGHTENS OVER THE AREA. SOUTHEAST WINDS THROUGH TURNAGAIN ARM AND
OTHER FAVORED GAPS WILL INCREASE RAPIDLY MONDAY AFTERNOON AND
EVENING AND THEN CONTINUE THROUGH TUESDAY. ON TUESDAY SNOW IN THE
SOUTHERN HALF OF THE SUSITNA VALLEY WILL MIX WITH OR CHANGE OVER
TO RAIN...HOWEVER HEAVY SNOW IS EXPECTED TO CONTINUE NORTH OF
TALKEETNA TOWARDS THE PARK WITH INCREASING MOISTURE AND LIFT
CONTINUING TO INCREASE SNOW FALL RATES.

&&

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

THE MAJOR BERING SEA STORM WILL BEGIN TO IMPACT SOUTHWEST ALASKA
THIS EVENING INTO THE OVERNIGHT AS A WARM FRONT BRINGS A VERY
BRIEF PERIOD OF SNOW FOLLOWED BY MUCH WARMER AIR AND A CHANGEOVER
OR MIX WITH RAIN BY MID-MORNING MONDAY. AT THE INITIAL ONSET OF
SNOW ALONG NORTH COASTAL SECTIONS OF THE KUSKOKWIM DELTA...A BRIEF
PERIOD OF NEAR WHITE-OUT CONDITIONS IS POSSIBLE WITH SNOWFALL AND
STRONG WINDS. GIVEN THE BREVITY OF THE POTENTIAL WHITE-OUT
CONDITIONS...A BLIZZARD WARNING IS NOT NECESSARY AT THIS
TIME...BUT WILL STRESS BRIEF AND LOCALIZED LOW VISIBILITIES ALONG
THE NORTHERN COAST IN THE AFTERNOON ZONE FORECAST PACKAGE.
OTHERWISE...THE MAIN IMPACTS WILL BE WITH STRONG SOUTH WINDS AND
RAIN AND/OR RAIN/SNOW MIX EXTENDING WELL INLAND AS THIS FRONT
STALLS OVER THE BRISTOL BAY AND KUSKOKWIM RIVER REGIONS LATE
MONDAY THROUGH MID-WEEK.

THE OTHER MAIN FORECAST CONCERN WITH THIS SYSTEM IS THE POTENTIAL
FOR HIGH SURF CONDITIONS FOR COASTAL COMMUNITIES ALONG THE
KUSKOKWIM DELTA...BEGINNING WITH THE HIGH TIDE CYCLE AROUND
MIDDAY MONDAY. CURRENT GUIDANCE SUGGESTS SEAS REACHING 2 TO 4 FEET
ABOVE MAX ASTRONOMICAL TIDE FOR SEVERAL CONSECUTIVE HIGH TIDE
CYCLES NORTH OF CAPE NEWENHAM. HAVE THEREFORE ISSUED A COASTAL
FLOOD ADVISORY FROM MONDAY MORNING THROUGH NOON TUESDAY TO COVER
THESE TIDE CYCLES.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...

THE POWERFUL LOW APPROACHING THE WESTERN ALEUTIANS WILL REMAIN THE
DOMINANT FEATURE IN THE BERING OVER THE NEXT FEW DAYS AS THE STORM
FORCE FRONT MOVES GRADUALLY EASTWARD. MOST LOCATIONS IN THE BERING
ALONG THE FRONT WILL BRIEFLY FLIRT WITH HITTING HIGH WIND
CONDITIONS...ESPECIALLY THOSE EXPOSED COMMUNITIES EXPOSED TO GAP
WINDS. BEHIND THE FRONT...COLD ADVECTION AND VERY LOW STABILITY
ALONG WITH STRONG WINDS IN THE LOWER LEVELS OF THE ATMOSPHERE MEAN
THAT THERE WILL BE VERY STRONG AND GUSTY WINDS OVER A LARGE
PORTION OF THE REGION FROM THE WESTERN AND CENTRAL ALEUTIANS
TONIGHT AND MOVING NORTHEAST TO THE PRIBILOFS MONDAY BEFORE
CONTINUING TO WEAKEN. WHILE CONFIDENCE IS HIGH THAT WINDS WILL
APPROACH WARNING LEVEL OVER PARTS OF THE WESTERN ALEUTIANS
BETWEEN SHEMYA AND ADAK TONIGHT...WILL NOT BE ISSUING A HIGH WIND
WARNING AT THIS TIME AS THE AFFECTED AREA IS LARGELY UNPOPULATED.
HOWEVER...HURRICANE FORCE WIND WARNINGS WILL BE IN EFFECT FOR
ADJACENT MARINE AREAS THROUGH THE OVERNIGHT HOURS.

&&

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

THE APPROACH OF THE REMNANTS OF IN-FA SIGNALS A MAJOR PATTERN
CHANGE ALONG WITH A GOOD AMOUNT OF UNCERTAINTY...AT LEAST OUT
WEST. IT WILL BEGIN WITH LOW PRESSURE FROM THE NORTH PACIFIC
TAKING AIM AT THE AKPEN AND FINALLY MOVING INLAND LATE THU. THIS
WILL ASSIST IN THE BREAKDOWN OF A WEAK BLOCKING RIDGE OVER THE
CENTRAL ALEUTIANS. THE EC AND GFS HAVE SLIGHTLY DIFFERENT MOVEMENT
WITH THIS FEATURE...BUT WPC UTILIZED ENSEMBLES AND OPERATIONAL
MODELS TO COME UP WITH A SOLUTION WHICH BEGINS WITH A FAIR AMOUNT
OF EC. THIS FORCES THE RESIDUAL RIDGE OUT OF THE WAY..AND ALLOWS A
FASTER EASTWARD PROGRESSION OF THE EX-IN-FA LOW SO IT CROSSES
FROM THE WESTERN ALEUTIANS TUE NIGHT AND REACHES SOUTHWEST AK
EARLY SAT. YET ANOTHER LOW WILL THEN MOVE ALONG THE ALEUTIANS SAT
THROUGH SUN.

IN THE EAST EXPECT WINDY CONDITIONS THROUGH CHANNELED TERRAIN AS
THE FIRST LOW MOVES TO THE SOUTHERN END OF SHELIKOF STRAIT INTO
THE WEEKEND. AS THE WARM AIR MASS ASSOCIATED WITH THE INITIAL LOW
MOVES OVER THE FORECAST AREA ON SOUTHERLY FLOW...RAIN WILL AFFECT
THE COAST AND INLAND THROUGH THE WEEKEND...ALTHOUGH A MIX OF RAIN
AND SNOW WILL STILL BE POSSIBLE ALONG THE ALASKA RANGE. IN THE
WEST EXPECT GALES NEAR THE AKPEN DURING THE WEEKEND...AND THEN
ACROSS THE ALEUTIANS AS THE STRONG EX-TYPHOON MOVES ACROSS THE
ISLANDS AND THE BERING...FOLLOWED BY A SECOND LOW IN ITS WAKE.

CONFIDENCE IS DECENT THAT THIS SCENARIO WILL BE A WIND AND RAIN
MAKER OVER THE BERING AND ALEUTIANS AS WELL AS THE GULF COAST.
WHETHER PRECIPITATION REMAINS LIQUID OVER INTERIOR ZONES REMAINS
UNCERTAIN.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WSW(SNOW) 145, COASTAL FLOOD ADVISORY 155.
MARINE...HURRICANE FORCE 177 178.
         STORMS 170 172 173 174 175 176 179 185 411 412 413 414
         GALES 130 131 132 138 139 150 155 160 165 171.

&&

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