Blizzard Watch 29, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
500 PM AKST SAT DEC 21 2013

.ANALYSIS AND UPPER LEVELS...

THE PRINCIPAL WAVE FEATURE IN THE UPPER LEVELS IS A LONG WAVE TROUGH
EXTENDING FROM THE NORTHERN BERING SOUTH AND EAST THROUGH THE
SOUTHEASTERN BERING SEA. DOWNSTREAM AN UPPER RIDGE RESIDES FROM THE
EASTERN GULF OF ALASKA EXTENDING NORTH ALONG THE ALCAN BORDER.
BETWEEN THE TWO...SOUTHWEST FLOW WITH A MULTITUDE OF EMBEDDED
DISTURBANCES CONTINUES TO IMPACT MOST OF SOUTHERN ALASKA WITH ACTIVE
WEATHER. LOW AMPLITUDE TRANSITORY RIDGING RESIDES OVER THE SOUTHERN
BERING THIS AFTERNOON...WITH A VERY LARGE 960 MB LOW CURRENTLY 725 NM
SOUTHWEST OF SHEMYA MOVING SLOWLY NORTH AND EAST TOWARD THE GENERAL
DIRECTION OF THE ALEUTIANS.

THE UPPER LEVEL LOW CURRENTLY OVER THE GULF OF ANADYR SPLITS
TONIGHT...PUSHING A POTENT DISTURBANCE EAST ACROSS SOUTHWEST AND
SOUTHCENTRAL ALASKA TONIGHT THROUGH SUN NIGHT. THE RESULTING COLD
ADVECTION FROM THE WEST COMBINED WITH PREFRONTAL DISTURBANCES ALREADY
IN PLACE WILL HELP SQUEEZE OUT AVAILABLE MOISTURE AND GENERATE
ACCUMULATING SNOWS MOST AREAS FOR THE REMAINDER OF THE
WEEKEND...ESPECIALLY ACROSS SOUTHCENTRAL.

OUT WEST...THE STRONG SURFACE LOW CURRENTLY SOUTHWEST OF SHEMYA WILL
MOVE TO 300 NM SOUTH OF SHEMYA BY MONDAY AFTERNOON. HOWEVER...THE
ASSOCIATED GALE FORCE WEATHER FRONT (WITH ISOLATED STORM FORCE WINDS
MAINLY IN THE WESTERN BERING) WILL MOVE NORTH AND EAST ACROSS THE
ALEUTIANS AND BERING SEA THROUGH MON NIGHT.

.MODEL DISCUSSION...

THE EUROPEAN (EC) MODEL CONTINUES TO INITIALIZE THE KEY WEATHER
FEATURES VERY WELL...WITH THE GFS AND ESPECIALLY THE NAM CONTINUING
TO STRUGGLE A BIT...THOUGH MODEL AGREEMENT HAS BECOME MUCH BETTER
OVER THE LAST 12 TO 24 HOURS WITH THE GENERAL SYNOPTIC PATTERN. THE
AFTERNOON FORECAST PACKAGE CONTINUES TO RELY MAINLY ON THE EC AS A
RESULT BUT DOES NOT DISCOUNT THE GFS.

THE PRINCIPAL PROBLEM RESIDES WITH EXACT PLACEMENT OF LOWER TO MIDDLE
LEVEL CIRCULATION CENTERS OVER THE GULF. FORECASTED SNOWFALL TOTALS
WILL RELY HEAVILY ON WHICH SOLUTION IS MORE CORRECT...ESPECIALLY FOR
POPULATED CENTERS OF ANCHORAGE AND THE MAT-SU BOROUGH. IF THE EC AND
UKMET MODELS VERIFY WITH A FARTHER WEST SOLUTION OF THESE
FEATURES...SNOW ACCUMULATIONS ARE LIKELY TO REACH LOW-END ADVISORY
CRITERIA FOR ANCHORAGE AND VICINITY...BUT ARE LIKELY TO REMAIN BELOW
ADVISORY CRITERIA IF THE NAM AND GFS VERIFY WITH A MORE EASTWARD
PLACEMENT OF THESE FEATURES.

.SHORT TERM FORECAST...

SOUTHCENTRAL...

FREEZING RAIN ADVISORIES FOR THE KENAI PENINSULA WERE EXTENDED UNTIL
9 PM THIS EVENING SINCE IT APPEARS AS THOUGH A WARM ABOVE-FREEZING
LAYER STILL EXISTS ABOVE THE COLD SURFACE INVERSION.
OTHERWISE...PRECIPITATION SHOULD REMAIN ALL SNOW EXCEPT FOR NEAR SEA
LEVEL NEAR PRINCE WILLIAM SOUND AND THE GULF COAST. EXPECTING LIGHT
ACCUMULATING SNOWS INTO THIS EVENING...THEN WITH COLD ADVECTION
INCREASING WITH THE AFOREMENTIONED STRONGER DISTURBANCE APPROACHING
FROM THE WEST WE SHOULD START TO SEE A SUBSTANTIAL INCREASE IN SNOW
RATES FROM WEST TO EAST LATE TONIGHT THROUGH SUNDAY MORNING AND
EXTENDING THROUGH THE AFTERNOON. THE CURRENT THINKING IS TO MAINTAIN
CURRENT FORECAST SNOW ACCUMULATIONS MOST AREAS...WHICH APPROACHES BUT
DOES NOT QUITE MAKE IT TO ADVISORY CRITERIA FOR ANCHORAGE AND
VICINITY. AS OF RIGHT NOW...WE DO NOT FORSEE ISSUING ANY WINTER
WEATHER ADVISORIES FOR SNOW WITH THE AFTERNOON PACKAGE. HOWEVER...A
GENERAL 3 TO 6 INCHES CAN BE EXPECTED MOST AREAS THROUGH SUNDAY
NIGHT...ENDING SOONER TOWARD THE WEST.

CLEARING COMMENCES MONDAY WITH RIDGING MOVING OVER THE AREA...WITH
THE NEXT WEATHER SYSTEM APPROACHING FROM THE SOUTH AROUND CHRISTMAS
DAY. OFFSHORE FLOW WILL LIKELY KEEP MOST OF THE PRECIPITATION SNOW
AND CONFINED FAIRLY CLOSE TO THE COAST.

SOUTHWEST ALASKA...

SNOW WILL CONTINUE TONIGHT AND GRADUALLY PUSH EAST AS COLD ADVECTION
INCREASES AS THE UPPER LEVEL TROUGH INCREASES IN SPEED. PARTIAL
CLEARING COMMENCES BEHIND THIS SYSTEM WITH LIGHT SNOW SHOWERS MAINLY
CONFINED TO THE KUSKOKWIM DELTA REGION SUNDAY INTO SUNDAY NIGHT. BY
MONDAY AND MONDAY NIGHT...A STRONG FRONT WILL PUSH NORTH INTO AND
ACROSS SOUTHWEST ALASKA...BRINGING A RETURN TO SNOW AND THE POTENTIAL
FOR MIXED PRECIPITATION TYPES TO COASTAL AREAS.

BERING SEA/ALEUTIANS...

COLD ADVECTION SNOW SHOWERS CONTINUE ACROSS THE AKPEN THIS EVENING
BEFORE DIMINISHING. THE NEXT LARGE-SCALE WEATHER MAKER IS A STRONG
GALE-FORCE WEATHER FRONT WHICH MOVES ACROSS THE WESTERN AND CENTRAL
ALEUTIANS SUNDAY...THEN THE EASTERN ALEUTIANS AND PRIBILOFS AND
POINTS EAST LATE SUNDAY NIGHT THROUGH MON NIGHT. WINDS WILL BE STRONG
AND PRECIPITATION IS LIKELY TO START AS WET SNOW...BUT WILL QUICKLY
CHANGEOVER TO RAIN IN ALL AREAS FAIRLY QUICKLY. AS OF RIGHT NOW IT
APPEARS NO HAZARD PRODUCTS WILL BE NECESSARY FOR THE PUBLIC FORECAST
ZONES OUT WEST.

.LONG TERM FORECAST...

THE EC AND WPC ARE SIMILAR IN BRINGING THE NORTH PACIFIC LOW TO THE
ALEUTIANS AND DEVELOPING A TRIPLE POINT LOW IN THE CENTRAL BERING ON
TUE. THIS WILL KEEP RAIN SHOWERS OVER THE BERING...ALEUTIANS...AND
SOUTHWEST AK ON TUE. AS THE BERING LOW WEAKENS INTO A BROAD AREA OF
LOW PRESSURE...COLDER AIR WILL RETURN TO THE WESTERN ISLANDS WED WITH
SOME SNOW SHOWERS. ON THU THE NEXT NORTH PACIFIC LOW TAKES AIM AT THE
CENTRAL ALEUTIANS...WITH ANOTHER ROUND OF RAIN FOR THE EASTERN
ALEUTIANS AND SOUTHWEST AK...AND SNOW SHOWERS FOR THE WESTERN
ALEUTIANS...PERSISTING INTO THE WEEKEND.

FOR SOUTHEAST OF THE AK RANGE...THE BERING FRONT WILL SPREAD SNOW
CHANGING TO RAIN ALONG THE COAST INTO WED...WITH SOME SNOW SPREADING
ACROSS THE COASTAL MOUNTAINS AND REACHING THE EASTERN FLANK OF THE AK
RANGE. THESE CONDITIONS WILL THEN PERSIST INTO THE WEEKEND.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALES...119 120 131 132 137 138 150 155 160 165 170-178 180
                 185
         STORMS...130
         HVY FRZG SPRY...139 150 160 180 181 185
FIRE WEATHER...NONE.

CC DEC 13