Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 201322
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
422 AM AKST SAT DEC 20 2014

.ANALYSIS AND UPPER LEVELS...

AN ELONGATED TROUGH OF LOW PRESSURE EXTENDING FROM THE KAMCHATKA
PENINSULA TO THE PACIFIC NORTHWEST COAST OF THE CONTINENTAL UNITED
STATES CONTINUES TO AMPLIFY AS IT INTERACTS WITH AN IMPRESSIVE 180+
KT JET STREAK OVER THE NORTH PACIFIC. A COMPLEX LOW RESIDES IN
THIS ELONGATED TROUGH WITH LOW CENTERS SOUTH OF ATKA...SOUTH OF
THE SHUMAGIN ISLANDS AND WEST OF THE ALASKA PANHANDLE. A DEFORMATION
ZONE DEVELOPED OVER DUTCH HARBOR/UNALASKA DUE TO THE PROXIMITY OF
THE LOW SOUTH OF ADAK AND SOUTH OF THE SHUMAGIN ISLANDS AND
CONTINUES NORTHWARD TOWARD THE PRIBILOF ISLANDS. THIS HAS ALLOWED
LIGHT RAIN AND ENHANCED CLOUD COVER TO RESIDE OVER THESE LOCATIONS
DURING THE OVERNIGHT HOURS. THE LOW WEST OF THE ALASKA PANHANDLE IS
TRACKING RAPIDLY NORTHWARD TOWARD THE NORTHERN GULF. THIS LOW
COMBINED WITH THE LOW SOUTH OF THE SHUMAGIN ISLANDS HAS INCREASED
THE PRESSURE GRADIENT ACROSS THE NORTHERN GULF ALLOWING FOR A GALE
FORCE (40 KT) BARRIER JET TO DEVELOP. A SHOWERY REGIME HAS SET UP
OVER THE NORTHERN GULF WITH MINOR INSTABILITY AND WEAK UPPER LEVEL
DISTURBANCES PUSHING ACROSS THE AREA. MUCH OF THE SHOWER ACTIVITY
REMAINED OFFSHORE WITH OFFSHORE FLOW KEEPING THE LOW LEVELS
DRY...FORCING PRECIPITATION TO EVAPORATE PRIOR TO REACHING THE
GROUND. SOUTHWEST ALASKA IS ALSO SEEING OFFSHORE FLOW WHICH HAS
KEPT THEM ON THE DRY SIDE AS WELL.

&&

.MODEL DISCUSSION...

MODELS HAVE CHANGED A GOOD BIT IN THE LAST 24 HOURS. LAST NIGHT
MODELS INDICATED THE LOW WEST OF THE ALASKA PANHANDLE WOULD
DISSIPATE OVER THE NORTHERN GULF AND KEEP MUCH OF SOUTHCENTRAL
ALASKA ON THE DRY SIDE. HOWEVER...THERE IS GREAT MODEL CONSENSUS
REGARDING THIS LOW NOW HOLDING TOGETHER A BIT LONGER AND MOVING
OVER THE KENAI PENINSULA ON SUNDAY AND INTO MONDAY WHEN IT BEGINS
TO ELONGATE AND BECOME MORE OF A BROAD TROUGH VS A CLOSED LOW.
THIS SIGNIFICANTLY IMPACTS THE OVERALL FORECAST FOR SOUTHCENTRAL
ALASKA TO WHERE CHANCES FOR SNOWFALL OVER THE ANCHORAGE AND MAT-SU
VALLEY ARE SIGNIFICANTLY HIGHER IF THE MODEL CONSENSUS PANS OUT.
THE 00Z NAM WAS A BIT ON THE SLOW SIDE AND WAS AN OUTLIER BUT THE
06Z NAM CAME IN ALIGNMENT WITH THE FASTER SOLUTION DEPICTED BY THE
GFS AND ECMWF. THIS JUST CONTINUES TO INCREASE THE CONFIDENCE IN
THE MODELS FOR THIS FORECAST PACKAGE. THE ECMWF WAS UTILIZED
THROUGH THE SHORT TERM IN THE EASTERN DOMAIN WITH A BLEND OF THE
GFS AND ECMWF THROUGH THE MIDTERM AND THE BEGINNING OF THE
EXTENDED GRIDS.

OUT WEST...MODELS ARE IN GOOD AGREEMENT WITH THE LOW SOUTH OF ATKA
MOVING EASTWARD WHILE WEAKENING OVER THE NEXT SEVERAL DAYS BEFORE
IT PHASES WITH THE LOW PRESSURE THAT MOVES OVER THE KENAI
PENINSULA LATE THIS WEEKEND. WEAK RIDGING WILL DEVELOP IN THE
WAKE OF THIS LOW BUT AN AREA OF LOW PRESSURE OVER THE KAMCHATKA
PENINSULA WILL DRIVE A FRONT INTO THE WESTERN BERING SEA BY LATE
SUNDAY. MODELS THEN DIVERGE ON AN AREA OF LOW PRESSURE DEVELOPING
OVER THE WESTERN BERING SEA DURING THE BEGINNING OF THE UPCOMING
WEEK. REGARDLESS...A RAPIDLY DEVELOPING LOW WITH MAX GALE FORCE
WINDS IS EXPECTED AS THE DEVELOPING LOW MOVES INTO THE BERING SEA.
A MIX OF THE 00Z NAM AND ECMWF WAS UTILIZED ON THE WESTERN DOMAIN.

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

A SERIES OF GENERALLY WEAK SURFACE LOWS AND UPPER LEVEL WAVES
ROTATING AROUND THE GULF ARE STAYING PRIMARILY SOUTH OF THE
NORTHERN GULF COAST. THERE IS OCCASIONALLY A BAND OF PRECIP WHICH
MAKES IT NORTHWARD INTO PRINCE WILLIAM SOUND BUT PERSISTENT
OFFSHORE FLOW IS MAINTAINING DRY AIR IN THE LOW LEVELS...SO MUCH
OF THE PRECIPITATION IS EVAPORATING BEFORE IT REACHES THE GROUND.

A STRONGER SURFACE LOW AND UPPER WAVE ARE ABOUT TO MOVE INTO THE
EASTERN GULF AND CHANGE ALL OF THIS. THE LATEST NUMERICAL GUIDANCE
CONTINUES TO TREND TOWARD A STRONGER AND FASTER SOLUTION IN WHICH
THE LOW MAKES IT ALL THE WAY TO PRINCE WILLIAM SOUND REGION
BY SUNDAY MORNING AND THEN BECOMES VERY WEAK AND DIFFUSE AS IT
CROSSES THE KENAI PENINSULA SUNDAY AFTERNOON. THIS IS A STARK
DIFFERENCE FROM PRIOR MODEL SOLUTIONS WHICH STALLED THE LOW OVER
THE GULF PUSHING VERY LITTLE PRECIP ONSHORE. IT IS VERY
SIGNIFICANT THAT THE GUIDANCE WEAKENS THE LOW PRIOR TO REACHING
THE KENAI PENINSULA. THIS MEANS MOISTURE CAN FREELY ADVECT ACROSS
THE MOUNTAINS INTO ANCHORAGE AND THE MAT-SU WITHOUT ANY IMPACT
FROM DOWNSLOPE FLOW THAT WE SO OFTEN SEE IN THESE AREAS.

THE UPPER FLOW ALSO LOOKS VERY FAVORABLE FOR PRECIP...WITH A
SHORT-WAVE ACCOMPANYING THE ARRIVAL OF THE SURFACE LOW. THE UPPER
LOW WHICH IS NEARLY STATIONARY NEAR KODIAK ISLAND THEN OPENS UP
INTO A TROUGH AND LIFTS NORTHWARD TOWARD ANCHORAGE...WHICH COULD
RESULT IN A PROLONGED PERIOD OF VERTICAL MOTION OVER PORTIONS OF
SOUTHCENTRAL. WHEN YOU PUT THIS ALL TOGETHER IT IS LOOKING LIKE
THE BEST CHANCE OF ACCUMULATING SNOW IN QUITE SOME TIME FOR
ANCHORAGE ON UP TO THE MATANUSKA VALLEY BEGINNING SUNDAY AND
POSSIBLY EXTENDING INTO MONDAY.

BASED ON THE EXCELLENT AGREEMENT IN THE NUMERICAL GUIDANCE WILL
UP POPS AREA-WIDE. THERE IS CERTAINLY SOME UNCERTAINTY IN THE
DETAILS OF THE UPPER WAVES MOVING THROUGH...SO WILL START OUT WITH
MINOR SNOW ACCUMULATION AMOUNTS...ALTHOUGH THERE IS POTENTIAL FOR
MORE THAN A COUPLE INCHES OF SNOW.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (SUNDAY AND MONDAY)...

THE REMNANTS OF A WEAKENING LOW POSITIONED OVER KODIAK WILL
CONTINUE TO BRING SCATTERED SHOWERS ALONG THE ALEUTIAN RANGE AND
PARTS OF THE ALASKA PENINSULA THROUGH MONDAY. THE REST OF
SOUTHWEST ALASKA REGION WILL HAVE OFFSHORE FLOW RESULTING IN DRY
CONDITIONS. BY LATE SUNDAY AN UPPER LEVEL RIDGE OF HIGH PRESSURE
WILL BUILD INTO THE EASTERN BERING SEA. THIS SYNOPTIC FEATURE
COMBINED WITH THE COLD AIR FILTERING INTO THE REGION FROM THE
NORTH WILL BRING COOLER TEMPERATURES RESULTING IN BELOW NORMAL
TEMPERATURES THROUGH MONDAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (SUNDAY AND MONDAY)...
THE GALE FORCE LOW LOCATED JUST SOUTH OF ATKA THIS MORNING WILL
SLOWLY WEAKEN AS IT TRACKS EASTWARD THROUGH SUNDAY. THIS WILL
RESULT IN GUSTY NORTHWESTERLY WINDS ON THE BACKSIDE OF THE LOW
ALONG THE WESTERN TO CENTRAL ALEUTIAN CHAIN THROUGH THE WEEKEND.
BY SUNDAY NIGHT A WEATHER FRONT WILL APPROACH THE WESTERN
ALEUTIANS BRINGING GALE FORCE SOUTHERLY WINDS HEADING INTO MONDAY.
THIS WILL BRING WARMER TEMPERATURES TO THE WESTERN ALEUTIANS WITH
PRECIPITATION CHANGING FROM SNOW TO RAIN SHOWERS. THE BERING SEA
WILL CONTINUE TO HAVE NORTHERLY FLOW WITH COLD AIR FILTERING INTO
THE NORTHERN BERING SEA THROUGH MONDAY. THE WESTERN AND CENTRAL
BERING SEA WINDS WILL SHIFT FROM NORTHERLY TO SOUTHERLY BY MONDAY
AS THE WEATHER FRONT TRACKS FROM THE WESTERN ALEUTIANS INTO THE
WESTERN BERING SEA.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
A PERIOD OF WIDESPREAD SNOW LOOKS TO LINGER THROUGH THE DAY ON
MONDAY ACROSS MUCH OF SOUTHCENTRAL ALASKA AS THE TROUGH OVER THE
WESTERN GULF LIFTS NORTHWARD THROUGH THE AREA. THIS WILL THEN
GIVE WAY TO A MORE SHOWERY REGIME FOR THE REMAINDER OF THE WEEK AS
THE LONGWAVE TROUGH OVER THE BERING KEEPS GENERALLY SOUTHWESTERLY
FLOW OVER THE MAINLAND...WITH VARIOUS DISTURBANCES MOVING TOWARDS
THE GULF PERIODICALLY ENHANCING PRECIPITATION POTENTIAL ACROSS THE
REGION.

MEANWHILE...MODELS CONTINUE TO TREND STRONGER WITH A DEEP LOW AND
ASSOCIATED FRONT POISED TO MOVE OVER THE WESTERN ALEUTIANS LATE
MONDAY INTO TUESDAY...BRINGING RAIN AND GALE FORCE WINDS TO A WIDE
SWATH OF THE WESTERN BERING AND POTENTIAL STORM FORCE WINDS TO
COASTAL WATERS OF THE WESTERN ALEUTIANS. RAINY AND WINDY
CONDITIONS WILL THEN REMAIN IN THE FORECAST THROUGH MUCH OF THE
WEEK AS THE FRONT MOVES NORTHWARD THROUGH THE BERING AND
GRADUALLY WEAKENS...WITH GUIDANCE HINTING AT ANOTHER DEEP SYSTEM
POSSIBLY APPROACHING THE WESTERN ALEUTIANS BY THE END OF THE WEEK.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE
MARINE...GALE WARNING 119 130 131 138 176 177 178
 HEAVY FREEZING SPRAY 180 181 185
FIRE WEATHER...NONE

&&

$$

SYNOPSIS AND MODEL DISCUSSION...MMC
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PLD
LONG TERM...CB


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