Small Craft Advisory 55, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
100 PM AKST THU JAN 17 2013

.ANALYSIS AND UPPER LEVELS...
THERE IS A LONGWAVE TROUGH THAT IS ENCOMPASSING MUCH OF MAINLAND
ALASKA THIS MORNING. A WEAKENING UPPER LOW IN THE BASE OF THIS TROUGH
IS COMBINING WITH CYCLONIC SURFACE FLOW AROUND A WEAK NORTH GULF LOW
AND CONTINUING COLD AIR ADVECTION TO PRODUCE WIDESPREAD SNOWFALL
ACROSS SOUTHCENTRAL ALASKA. ALL THREE OF THE AFOREMENTIONED
CONTRIBUTING FACTORS SHOULD BE DIMINISHING LATE THIS
AFTERNOON...LEADING TO THE END OF THE SNOW EVENT. A FAIRLY NARROW
RIDGE OF HIGH PRESSURE IS CREATING CLEAR AND COLD CONDITIONS ACROSS
THE SOUTHWEST MAINLAND...AND ALLOWING DEEP-LAYER SOUTHERLY FLOW TO
IMPACTING THE EASTERN HALF OF THE ALEUTIANS. OUT WEST...A WEAKENING
LOW WEST OF SHEMYA IS DRIFTING TOWARD THE KAMCHATKA PENINSULA. AN
ASSOCIATED FRONT HAS STALLED ACROSS THE CENTRAL BERING SEA AND IS
WEAKENING AS IT IS SHEERED APART.

.MODEL DISCUSSION...
THE MODELS ARE IN ABOVE-AVERAGE SYNOPTIC AGREEMENT OVER THE NEXT
SEVERAL DAYS ACROSS THE REGION. THE RIDGE OVER THE SOUTHWEST MAINLAND
WILL CONTINUE TO INCH TO THE NORTH THROUGH THE START OF THE
WEEKEND...ALLOWING SEVERAL LOW PRESSURE CENTERS TO MAKE SLOW AND
STEADY EASTWARD PROGRESS ACROSS THE BERING/ALEUTIAN REGION. AS THIS
HAPPENS...A WEATHER FRONT WILL MOVE UP INTO THE GULF OF ALASKA FROM
THE SOUTH AND IMPACT THE SOUTHERN MAINLAND COAST STARTING LATE FRIDAY
NIGHT. FURTHER WEST...THE MODELS ALL DEVELOP A LOW OVER THE NORTH
PACIFIC THAT APPROACHES THE ALASKA PENINSULA BY SATURDAY. AT THIS
SAME TIME...A SEPARATE LOW WILL MOVE ZONALLY SOUTH OF THE ALEUTIAN
CHAIN. THESE TWO LOWS COMBINE INTO ONE LARGE AND COMPLEX LOW CENTER
OVER THE CENTRAL ALEUTIANS BY SUNDAY. THERE ARE SOME EXPECTED
DISCREPANCIES WITH THE TIMING OF THE MERGER OF THESE TWO LOWS...BUT
THESE WILL HAVE FAIRLY MINOR OVERALL IMPACT ON THE FORECAST AT THIS
POINT. THE ECMWF IS THE PREFERRED MODEL FOR THIS EVOLUTION DUE TO
SUPERIOR RUN-TO-RUN CONSISTENCY.

.SHORT TERM FORECAST...
SOUTHCENTRAL MAINLAND...ONGOING SNOW EVENT WILL WIND DOWN THIS
EVENING AS UPPER LOW WEAKENS AND THE COLD AIR ADVECTION COMES TO AN
END. CLOUDS WILL DIMINISH OVERNIGHT...WHICH GIVEN RECENT SNOWFALL
WILL PROBABLY SET THE STAGE FOR SOME AREAS OF FOG TO DEVELOP OVER THE
UPPER COOK INLET REGION. CLOUDS WILL START TO OVERTAKE THE REGION
FROM SOUTH TO NORTH DURING THE DAY ON FRIDAY...WITH THE
AFOREMENTIONED FRONT MOVING IN LATE TOMORROW NIGHT. THE OVERALL
ATMOSPHERIC FLOW WILL BE MORE OUT OF THE SOUTHEAST WITH THIS SYSTEM
THAN THE STORM LAST WEEKEND (WHICH WAS MORE SOUTHWESTERLY)...WHICH
WILL LEAD TO A MORE TYPICAL UPSLOPE/DOWNSLOPE PATTERN ACROSS THE
REGION. TEMPERATURES NOW LOOK LIKE PRECIPITATION MAY EVEN STAY
ALL-SNOW AT THE LOWER ELEVATIONS ALONG THE PRINCE WILLIAM SOUND COAST
FOR MUCH OF THE WEEKEND...INCREASING THE LIKELIHOOD OF SIGNIFICANT
ACCUMULATIONS.

SOUTHWEST MAINLAND...PARTLY SUNNY AND VERY COLD CONDITIONS WILL
CONTINUE THROUGH THE END OF THE WEEK AS FAIRLY LIGHT OFFSHORE FLOW
CONTINUES. CLOUDS MOVE IN LATE FRIDAY NIGHT...WITH AREAS OF SNOW
DEVELOPING BY SATURDAY MORNING...MAINLY ALONG THE COASTS. THE WHOLE
AREAS SHOULD SEE THE COLD OFFSHORE FLOW CONTINUE THROUGH THE WEEKEND
AS LOW PRESSURE CENTERS STAY WILL TO THE SOUTH AND WEST.

BERING SEA AND ALEUTIANS...WEAKENING STATIONARY FRONT ACROSS THE
CENTRAL BERING SEA IS STILL FOSTERING A FAIRLY TIGHT PRESSURE
GRADIENT THIS AFTERNOON. STRONG GALE-FORCE WINDS OVER THE WESTERN
BERING WILL GRADUALLY DIMINISH AS THE PARENT LOW PULLS OFF TO THE
WEST OVERNIGHT. CLOUDS AND SNOW SHOWERS OVER THE EASTERN HALF OF THE
REGION WILL GRADUALLY TURN INTO STEADIER WIDESPREAD RAIN AND SNOW BY
FRIDAY EVENING AS A FAIRLY WEAK NORTH PACIFIC LOW DEVELOPS AND MOVES
NORTHWARD. UNSETTLED WEATHER WILL CONTINUE THROUGH THE WEEKEND AS
LARGE COMPLEX LOW SETS UP OVER THE CENTRAL ALEUTIANS.

.LONG TERM FORECAST...
THERE IS STILL CONSIDERABLE UNCERTAINTY WITH THE EXTENDED FORECAST
PERIODS...BUT THE OVERALL IS TOWARD A MORE ZONAL OVERALL PATTERN
DEVELOPING BY THE MIDDLE OF NEXT WEEK. THIS WOULD LEAD TO A MORE
PROGRESSIVE WEATHER PATTERN WITH ALTERNATING LOW AND HIGH PRESSURE
CENTERS MOVING THROUGH THE PATTERN AND AFFECTING MOST LOCATIONS.
AFTER A WARM-UP THIS WEEKEND (BUT STILL LESS THAN LAST WEEKEND)
TEMPERATURES DROP BACK TOWARD SEASONAL NORMS AT MOST LOCATIONS NEXT
WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE
MARINE...GALE 130 132 138 140 150 155 160 165 170 179 180 185
         HEAVY FREEZING SPRAY 121 130 140 150 179 180 185
FIRE WEATHER...NONE

AMD JAN 13