Winter Storm in Alaska, December 24 to 25, 2013

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
326 PM AKST TUE DEC 24 2013

.SHORT TERM...A LOW OVER THE SW GULF WILL MOVE TO THE NE GULF BY
LATE WED AFTERNOON. STRONG OCCLUDED FRONT OVER THE EASTERN GULF
WILL MOVE NE ACROSS THE PANHANDLE TONIGHT AND WED MORNING. ARCTIC
FRONT OVER CENTRAL LYNN CANAL WILL GRADUALLY SHIFT N TO FAR
NORTHERN LYNN CANAL BY LATE WED. USED A BLEND OF THE 12Z ECMWF AND
GEM...AS THEY HAD BEST HANDLE ON MAIN FEATURES THROUGH WED.

 MAIN FORECAST ISSUES WILL BE PRECIP TYPE...AMOUNTS...TEMPS...AND
WINDS. RIGHT NOW PRECIP IS PRETTY MUCH ALL SNOW NORTH OF THE PAJN
AREA. EXPECT THE FAR N TO REMAIN SNOW THROUGH AT LEAST MOST OF WED
MORNING. THE AREA JUST N OF PAJN WILL LIKELY CHANGE TO RAIN BY
MIDNIGHT TONIGHT AS WARMER AIR SLOWLY MOVES IN. THE PAYA AREA
SHOULD SEE THEIR SNOW BECOME MIXED BY MIDNIGHT AS LOW LEVEL FLOW
TURNS FROM ENE TO ESE. THERE MAY BE A FEW WET FLAKES MIXED IN S OF
PAJN BUT WILL NOT MENTION AS THEY SHOULD BE SHORT LIVED WITH
WARMER AIR MOVING IN. TEMPS SHOULD BE STEADY OR SLOWLY RISING
THROUGH THE NIGHT IN MOST PLACES.

 SNOWFALL LOOKS LIKE IT WILL BE HEAVIEST AROUND PAHN. SE FLOW
OVERRUNNING THE ARCTIC AIRMASS IS VERY FAVORABLE FOR HEAVY SNOW
THERE...AND THEY COULD SEE WELL OVER 1 FOOT OF SNOW BY MID-MORNING
WED. PAGY WILL ALSO SEE STRONG OVERRUNNING BUT LOOK TO NOT GET AS
MUCH SNOW AS SETUP IS A LITTLE LESS FAVORABLE...BUT THEY COULD
STILL SEE UP TO 1 FOOT OF SNOW BY LATE WED MORNING. KEPT THE
WINTER STORM WARNINGS GOING IN THOSE AREAS THRU 18Z WED. ALSO KEPT
IN WINTER WEATHER ADVY FOR THE PAJN AREA...MAINLY FOR N OF AUKE
BAY. THINK THAT AREA COULD GET UP TO 6 INCHES OF SNOW BEFORE
CHANGING TO RAIN OVERNIGHT. THE MENDENHALL VALLEY HAS BEEN GOING
BACK AND FORTH BETWEEN RAIN AND SNOW...AND THIS SHOULD LIMIT
ACCUMULATIONS TO 2 INCHES OR LESS BEFORE IT FINALLY BECOMES ALL
RAIN BY LATE EVENING. HOWEVER...IF LOWER LEVEL WINDS DROP OFF WITH
MODERATE PRECIP RATES...COULD SEE IT GO TO ALL SNOW FOR A WHILE
THIS EVENING. CURRENTLY EXPECT THE PAYA AREA TO SEE UP TO 4 INCHES
BEFORE CHANGING TO RAIN LATER TONIGHT...BUT IF THEIR PRECIP RATES
REMAIN HEAVIER THIS EVENING THEY MAY NEED AN ADVY. NORTHERN
GLACIER BAY MAY ALSO GET UP TO 4 INCHES OF SNOW BEFORE CHANGING TO
RAIN LATE THIS EVENING. PAGS ITSELF WILL BE A TRICKY CALL ON
SNOWFALL AS THEY MAY END UP BEING MORE MIXED BUT COULD SEE PERIODS
OF ALL SNOW DURING HEAVIER PRECIP. AT THIS POINT THINK THEY MAY
GET AN INCH EARLY THIS EVENING. HYDER HAS REMAINED IN SW LOW LEVEL
FLOW AND THAT HAS KEPT THEIR TEMPS WARM...SO EVEN WITH SOME
DIMINISHING IN THE WINDS THERE TONIGHT DO NOT THINK THEY WILL GET
MORE THAN MIXED PRECIP...BUT AS USUAL THEY WILL NEED TO BE KEPT AN
EYE ON.

 AS FOR WINDS...STRONGEST WINDS ARE EXPECTED AHEAD OF THE OCCLUDED
FRONT...WITH MAINLY GALES FOR THE FAR EASTERN AND NORTHERN
GULF...BUT CLARANCE STRAIT WILL LIKELY GET TO STORM FORCE BY LATE
EVENING OVER THE NORTHERN END. THE NLY GRADIENTS SHOULD PICK UP
OVER THE FAR NORTHERN INNER CHANNELS THIS EVENING...SO EXPECT
WINDS TO INCREASE TO SCA LEVELS IN LYNN CANAL. MUCH OF THE
REMAINING INNER CHANNELS WILL GET SCA LEVEL E-SE WINDS TONIGHT.
ONCE THE OCCLUDED FRONT PASSES...WINDS SHOULD DROP OFF IN GENERAL
ALTHOUGH THE MORE EASTERN INNER CHANNELS MAY KEEP SCA LEVEL SE
WINDS WED. ALSO...THERE WILL BE A BAND OF STRONGER SW WINDS ON S
SIDE OF LOW MOVING ACROSS THE GULF...SO WILL HAVE GALES RETURN TO
THE OFFSHORE AREA WED MORNING BEFORE THEY DIMINISH LATER IN THE
DAY WITH THE LOW WEAKENING.

 OTHERWISE...PRECIP WILL BE LOCALLY HEAVY TONIGHT AS SYSTEM HAS A
TROPICAL CONNECTION. SHOULD SEE PRECIP DIMINISH AFTER THE OCCLUDED
FRONT MOVES THROUGH THE AREA LATE TONIGHT AND WED MORNING.

 &&

 .LONG TERM...MODELS HAVING DIFFICULTY WITH DEEPS LOWS DEVELOPING
OFF THE KAMCHATKA PENINSULA AND THE EASTWARD MIGRATION OF THESES
SYSTEMS. THERE WAS SHIFT IN MODEL PRESSURE FIELD POSITION TODAY
RESULTING IN SOME SIGNIFICANT CHANGES IN WIND SPEEDS AND
DIRECTIONS FOR THE ENTIRE EXTENDED FORECAST. WITH THE MODEL
INCONSISTENCIES AND RUN TO RUN CHANGES FORECAST CONFIDENCE IS LOW.

 FOR WEDNESDAY EVENING THE REMNANTS OF THE SHORT WAVE IN THE
NORTHERN AK GULF WILL MERGE WITH A DEVELOPING WAVE MOVING ACROSS
THE NORTHERN GULF FROM A PARENT LOW NEAR THE ALEUTIANS. THE WEAK
FRONT ASSOCIATED WITH THE ADVANCING WAVE WILL REACH THE PANHANDLE
COAST THURSDAY. THIS WAVE LOOKS TO BE A BIT FURTHER NORTH AND THUS
THE PRESSURE GRADIENT BETWEEN A RIDGE OVER THE SOUTHERN PANHANDLE,
THE SHORT WAVE EXTENDING OVER THE CENTRAL PANHANDLE AND HIGH
PRESSURE OVER THE YUKON IS WEAKER...RESULTING IN MUCH WEAKER WINDS
FOR THE NORTHERN REGIONS FOR WEDNESDAY INTO WEDNESDAY NIGHT. THE
12Z GFS WAS STRONGEST WITH THE GRADIENT AND ALSO SHOWED A COLD
POOL OF ARCTIC AIR DROPPING INTO THE NORTHERN PANHANDLE...HOWEVER
HAD BETTER INITIALIZATION AND AGREEMENT WITH THE 12Z ECMWF/GEM
WHICH BOTH KEEP THE COLD AIR FURTHER NORTH AND A WEAKER NORTHERLY
PRESSURE GRADIENT. DEEP ONSHORE FLOW ALONG WITH THE FRONT WILL
KEEP PRECIPITATION OVER THE AREA THROUGH THE WEEK. TEMPERATURES
WILL BE A BIT ABOVE NORMAL KEEPING MOST PRECIPITATION AS RAIN.

 HIGH PRESSURE WILL ADVANCE OVER THE AK GULF THROUGH THE WEEK
FOLLOWED BY NEXT ADVANCING LOW AND FRONT REACHING THE PANHANDLE
FOR THE WEEKEND. WITH THE LOW MOVING IN FROM A SOUTHERLY TRACK
LOOKING TO KEEP THE WARMER MILD AIR OVER THE REGION. WINDS OVER
THE AK GULF WILL INCREASE TO NEAR GALES WITH THIS SYSTEM FOR THE
WEEKEND BUT SOME DIFFERENCES ON DEVELOPMENT OF A TRIPLE POINT
FEATURE IN THE NORTHERN GULF WITH A BARRIER JET FORMATION. MODELS
CONTINUE TO ADVANCE THESE SYSTEMS EASTWARD INTO THE AREA AND
SHOULD KEEP PRECIPITATION IN THE FORECAST.

 HAD GOOD AGREEMENT BETWEEN THE 12Z ECMWF/GEM WITH NAM SHOWING A
SIMILAR PATTERN. GFS BECAME AN OUTLIER EARLY AND WAS SHOWING MUCH
LOWER RUN TO RUN CONSISTENCY SO USED THE ECMWF/GEM BLEND WITH
INHERITED GRIDS THROUGH 12Z SATURDAY. THIS DID RESULT IN A CHANGE
TO THE FORECAST AND THE EARLIER MODEL RUNS. NEW WPC LEANED AWAY
FROM A GENERAL ENSEMBLE BLEND AND WENT WITH THE TREND OF USING
ECMWF FOR MAIN GUIDANCE. USED A BLEND WITH NEW WPC AND PREVIOUS
FORECAST GRIDS TO EASE TRANSITION WITH MAINLY INCREASED WINDS FOR
THE WEEKEND.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...STRONG WIND UNTIL MIDNIGHT AKST TONIGHT FOR AKZ026-027.
         WINTER WEATHER ADVISORY UNTIL MIDNIGHT AKST TONIGHT FOR AKZ025.
         STRONG WIND UNTIL 3 AM AKST WEDNESDAY FOR AKZ028.
         STRONG WIND FROM 9 PM THIS EVENING TO 3 AM AKST WEDNESDAY FOR
     AKZ029.
         WINTER STORM WARNING UNTIL 9 AM AKST WEDNESDAY FOR AKZ018-019.
MARINE...STORM WARNING FOR PKZ036.
         GALE WARNING FOR PKZ035-041>043-051-052.
         SMALL CRAFT ADVISORY FOR PKZ012-013-021-022-031>034-053.

&&

RWT/PRB