Small Craft Advisory 964, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
414 PM AKST THU DEC 3 2015

.ANALYSIS...
UPPER LEVELS TODAY SHOW A CLOSED LOW OVER THE ALEUTIANS AND
NORTHERN PACIFIC AND A NEGATIVELY TILTED LONGWAVE TROUGH OVER THE
EASTERN BERING. AT THE SURFACE...THIS IS REFLECTED IN A LOW
COMPLEX MOVING SOUTH OF THE BERING AND A CLOSED LOW SOUTH OF THE
GULF OF ALASKA MOVING EASTWARD OUT OF THE FORECAST AREA. THE LOW
SOUTH OF THE GULF HAS A TROUGH EXTENDING NORTHWARD TOWARDS PRINCE
WILLIAM SOUND...WHICH IS EXPECTED TO DEVELOP INTO A LOW OVER
SOUTHCENTRAL AS AN UPPER LEVEL TROUGH MOVES IN FROM THE WEST
PROVIDING SOME ADDITIONAL INSTABILITY. WEST OF THE MAINLAND...THE
ALEUTIANS AND BERING CONTINUE TO BE AFFECTED BY SEVERAL TROUGHS
ASSOCIATED WITH THE MAIN LOW COMPLEX. HOWEVER WITH A 175 KNOT JET
MOVING INTO THE BASE OF THE TROUGH...EXPECT FUTURE LOW DEVELOPMENT
TO OCCUR SOUTH OF THE ALEUTIANS IN THE NORTHERN PACIFIC.

.MODEL DISCUSSION...
MODELS ARE IN GOOD AGREEMENT ON THE PARENT LOWS...HOWEVER THEY ARE
HAVING MUCH MORE DIFFICULTY RESOLVING THE SMALLER FEATURES. NAM
PICKED UP THE DEVELOPMENT OF THE LOW IN SOUTHCENTRAL
EARLIEST...AND HAS BEEN MOST CONSISTENT WITH TIMING AND RESOLVING
THE SMALLER SCALE FEATURES. THIS IS A DYNAMIC SYSTEM SO LOCAL
EFFECTS WILL BE KEY IN DETERMINING SNOW AMOUNTS AND TIMING...SO
THE NAM IS A LOGICAL CHOICE FOR THE EAST SIDE. IN THE WEST...NAM
AND ECMWF ARE IN BETTER AGREEMENT ON DEVELOPMENT OF THE TROUGHS
OFF OF THE MAIN LOW COMPLEX WHILE GFS WAS THE OUTLIER WITH THE
STRONGER SOLUTION. NAM/EC SOLUTION WAS THE CHOICE FOR THE WEST AS
WELL. IN THE SHORT TERM...FORECAST CONFIDENCE IS AVERAGE ON TIMING
AND IMPACTS GIVEN THE MODEL DISAGREEMENT.

&&

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

BANDS OF SNOW WILL DEVELOP IMMEDIATELY IN ASSOCIATION WITH AN
UPPER TROUGH CURRENTLY APPROACHING SOUTHCENTRAL ALASKA THIS
EVENING THROUGH TONIGHT. A LOW-STABILITY ENVIRONMENT ALONG WITH
PLENTIFUL GULF MOISTURE AND GOOD ISENTROPIC LIFT WILL PROMOTE THE
DEVELOPMENT OF BANDS OF SNOW. BECAUSE OF THE BANDING NATURE OF THE
SNOW...ACCUMULATIONS FROM HOMER NORTH THROUGH THE MATANUSKA VALLEY
WILL LIKELY BE HIGHLY VARIABLE.

FROM PRINCE WILLIAM SOUND NORTH THROUGH THE COPPER RIVER
BASIN...SNOW IS EXPECTED TO BE HEAVIEST ALONG THE RICHARDSON
HIGHWAY FROM VALDEZ NORTH TO THE ALASKA RANGE AS THE FORCING
WITH THE UPPER TROUGH WILL BE MORE FOCUSED ALONG THIS CORRIDOR.
TOWARD CORDOVA...RAIN IS LIKELY TO MIX WITH THE SNOW THUS LIMITING
ACCUMULATIONS IN THIS AREA.

ANOTHER MUCH WEAKER UPPER WAVE WILL FOLLOW DURING THE DAY LATE
FRIDAY AND A THIRD ONE SATURDAY WILL KEEP A CHANCE OF SHOWERS IN
THE FORECAST FOR MOST OF THE REGION...WITH GREATER AMOUNTS FROM
KODIAK TO WESTERN PRINCE WILLIAM SOUND.

&&

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

THE LINGERING TROUGH OVER SOUTHWEST ALASKA HAS FINALLY PUSHED
INTO THE INTERIOR OF ALASKA WHICH HAS ALLOWED THE ISOLATED SNOW
SHOWERS THAT WERE OBSERVED THIS MORNING TO DISSIPATE. WITH THE
PASSAGE OF THE TROUGH...PRECIPITATION CHANCES HAVE TAPERED OFF TO
ZERO PERCENT AND WILL LIKELY REMAIN THAT WAY THROUGH THE FIRST
HALF OF THE WEEKEND. HOWEVER...A WEAK DISTURBANCE ROTATING AROUND
A BROAD UPPER LEVEL LOW IN THE BERING SEA WILL BRING SNOW SHOWERS
TO COASTAL LOCATIONS ON SATURDAY. PATCHY FOG IS EXPECTED TO RE-
DEVELOP ALONG THE COAST OF THE KUSKOKWIM DELTA WITH A DECENT
INVERSION EXPECTED TO STRENGTHEN (AS SEEN ON THE PABE RAOB). FOG
SHOULD NOT BE AN ISSUE INLAND AS A DECENT SFC GRADIENT SHOULD
PRODUCE ENOUGH WIND TO MIX THE LOWER LEVELS OF A NON-SATURATED
ENVIRONMENT. TEMPERATURES WILL REMAIN BELOW NORMAL THROUGH
SATURDAY.

&&

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

THE ORIENTATION OF THE UPPER LEVEL JET WILL KEEP THE STORM TRACK
SOUTH OF THE ALEUTIAN ISLANDS FOR QUITE SOME TIME. ANY STORMS THAT
DEVELOP IN THE NORTH PACIFIC OR KAMCHATKA PENINSULA WILL EITHER
DROP SOUTH INTO THE NORTH PACIFIC OR RESIDE IN THE NORTH PACIFIC
AS THE CURRENT LOW IMPACTING THE ALEUTIANS IS DOING. THE UPPER
LEVELS OVER THE BERING SEA ARE CURRENTLY QUITE MESSY WITH LOTS OF
FRAGMENTS OF ENERGY SCATTERED THROUGHOUT THE REGION. THE MOST
PROMINENT FEATURE IS A DEFORMATION BAND EXTENDING ACROSS THE
PRIBILOF ISLANDS. THIS IS THE RESULT OF ENERGY TRAPPED BETWEEN THE
DEPARTING LOW INTO THE NORTH PACIFIC AND A WEAKER LOW OVER THE
BERING STRAIT. THIS WILL LEAD TO STEADY SNOW SHOWERS THROUGH MUCH
OF TONIGHT WITH MINOR SNOW ACCUMULATIONS. OTHERWISE...A MIX OF
RAIN AND SNOW WILL CONTINUE UNDER A CONDITIONALLY UNSTABLE
ATMOSPHERE THROUGH THE SHORT TERM.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7...SUN THROUGH THU)...THE
LONG TERM PATTERN WILL BE DOMINATED BY A FAST MOVING NORTH PACIFIC
ZONAL JET KEEPING MUCH OF THE ACTIVE WEATHER SOUTH OF THE
MAINLAND. MODELS NOTORIOUSLY HAVE A HARD TIME HANDLING THESE TYPES OF
PATTERNS. THEY STRUGGLE WITH WHERE EACH INDIVIDUAL LOW
DEVELOPS...WHICH ONE BECOMES THE DOMINANT LOW...AND HOW STRONG
THAT DOMINANT LOW WILL BE. THUS...THERE IS LOW TO MODERATE
FORECAST CONFIDENCE DURING THIS TIME PERIOD. TOWARD THE END OF THE
WEEKEND...A FEW REMNANT FRONTAL BOUNDARIES WILL BE WORKING THEIR
WAY FROM EAST TO WEST ACROSS THE GULF. ANY ONE OF THESE COULD
BRING A BRIEF SHOT OF SNOW TO THE GULF COAST AND SOUTHCENTRAL. BY
MONDAY...A MORE DEFINED LOW WILL WORK FROM SOUTH TO NORTH IN THE
EASTERN GULF AND COULD BE ANOTHER SNOW MAKER...ESPECIALLY ALONG
THE GULF COAST NEAR CORDOVA AND VALDEZ. MEANWHILE OVER THE
BERING...A BROAD TROUGH WILL KEEP THE ENTIRE AREA VULNERABLE TO
SHOWERS. BUT WITHOUT ANY ESPECIALLY STRONG FORCING...WE DON'T
EXPECT MUCH IN THE WAY OF ACCUMULATIONS AND WINDS SHOULD REMAIN
RELATIVELY LIGHT.

BY TUE INTO WED...MODELS ARE SHOWING A MUCH STRONGER CONSOLIDATED
LOW TAKING SHAPE OVER THE NORTH PACIFIC AND MOVING INTO THE GULF
OF ALASKA. THERE HAS BEEN A GREAT DEAL OF UNCERTAINTY AMONG THE
MODELS AND THEIR ENSEMBLE MEMBERS IN DEALING WITH THIS FEATURE.
FOR NOW WE HAVE BROUGHT THE LOWEST CENTRAL PRESSURE DOWN INTO THE
LOW 950 MB RANGE. THIS WOULD MEAN AT LEAST SOME GALE FORCE WINDS
AND HEAVY PRECIPITATION FOR THE MARINERS BEFORE IT TRACKS TOWARD
SE AK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...HEAVY FREEZING SPRAY 160 180 181.

&&

SYNOPSIS AND MODEL DISCUSSION...LF
SOUTHCENTRAL ALASKA...CC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MC
LONG TERM...MO