Small Craft Advisory 72, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1130 AM AKST TUE JAN 22 2013

.ANALYSIS AND UPPER LEVELS...THE UPPER LEVEL WAVE PATTERN HAS
CONTINUED ITS EASTWARD PROGRESSION THE PAST 24 HOURS...PUSHING
THE RIDGE (WHICH HAD BEEN OVER THE EAST PACIFIC) INLAND ACROSS
WESTERN NORTH AMERICA. THE LONG-WAVE TROUGH NOW ENCOMPASSES ALL
BUT THE NORTHEASTERN PORTION OF MAINLAND ALASKA...EXTENDING FROM
ASIA ACROSS THE ENTIRE NORTH PACIFIC. SATELLITE IMAGERY SHOWS TWO
LARGE LOWS IN THE FORECAST AREA...ONE MODERATE STRENGTH VERTICALLY
STACKED LOW TRACKING NORTHWARD INTO THE GULF OF ALASKA AND A MUCH
DEEPER AND MATURE LOW PASSING SOUTH OF THE ALEUTIANS. UPPER WAVES
AND ASSOCIATED MOISTURE ARE FLOWING NORTHWESTWARD FROM THE GULF
LOW ACROSS THE SOUTHERN MAINLAND TO THE NORTHERN BERING. THE BULK
OF PRECIPITATION IS FALLING ALONG THE COASTAL ZONE THANKS TO LOW
LEVEL EAST TO NORTHEAST FLOW. QUITE A BIT OF WARM AIR HAS ADVECTED
OVER THE COASTAL RANGE AS SEEN IN THE 12Z ANCHORAGE SOUNDING. THIS
PRODUCED SOME ISOLATED AREAS OF FREEZING RAIN IN THE ANCHORAGE
AREAS EARLY THIS MORNING. THE LOW SOUTH OF THE ALEUTIANS HAS
BOTTOMED OUT AND WILL BEGIN TO FILL THIS AFTERNOON. WEAK RIDGING
OVER THE CENTRAL BERING HAS HELPED SLOW THE PROGRESSION OF THE
LOW AND IS CAUSING IT TO TAKE MORE OF AN EASTERLY TRACK.

.MODEL DISCUSSION...MODELS ARE IN BETTER AGREEMENT WITH LOW POSITIONS
THAN THEY HAVE BEEN OVER THE PAST COUPLE DAYS. SOME LARGER
DIFFERENCES DO EVOLVE ON THURSDAY AS COMPLEX SERIES OF LOWS ROTATE
AROUND EACH OTHER ACROSS THE NORTHEAST PACIFIC AND GULF. THE ENTIRE
COMPLEX WILL BE SHIFTING EASTWARD TOWARD SOUTHEAST ALASKA AND
CANADA...SO THESE DIFFERENCES REALLY ONLY AFFECT THE WIND FORECAST
FOR THE GULF OFFSHORE WATERS. WILL SIDE WITH THE ECMWF FOR THE TIME
BEING. ONE FEATURE WHICH WILL LIKELY HAVE MUCH GREATER IMPACT ON
THE FORECAST IS A TROUGH DROPPING DOWN OUT OF THE ARCTIC ACROSS
MAINLAND ALASKA THURSDAY AND FRIDAY. MODELS ALL SHOW THE SAME GENERAL
IDEA BUT DIFFER ON ORIENTATION AND AMPLITUDE AS THIS TROUGH REACHES
SOUTHCENTRAL. THERE IS POTENTIAL FOR ANCHORAGE AND THE MAT VALLEY
TO SEE SOME SNOW OUT OF THIS...BUT WILL HAVE TO WAIT TO SEE HOW
SUCCESSIVE MODEL RUNS HANDLE THE TROUGH.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...FLOW WILL BACK TOWARD THE NORTH OUT AHEAD OF
THE LOW MOVING INTO THE GULF THIS AFTERNOON WHICH WILL CAUSE PRECIP
TO DIMINISH OR TAPER OFF ALONG THE NORTH GULF COAST AND PRINCE
WILLIAM SOUND AREAS. HOWEVER...THE REMNANTS OF THE OCCLUDED FRONT
ALONG WITH AN UPPER LEVEL SHORT-WAVE WILL ROTATE NORTHWESTWARD INTO
THE COPPER BASIN AND PRINCE WILLIAM SOUND OVERNIGHT TONIGHT THROUGH
WEDNESDAY MORNING...BRINGING ANOTHER ROUND OF PRECIP. SOME OF THE
MOISTURE ASSOCIATED WITH THIS MAY MAKE IT OVER THE MOUNTAINS TO
ANCHORAGE AND THE MAT-SU...BUT ANYTHING THAT DOES WILL BE LIGHT
AND SHORT-LIVED BECAUSE THE SHORT-WAVE WILL SHEAR APART WITH TIME.
THE LOW WILL WEAKEN AS IT MOVES TO NEAR KODIAK ISLAND LATE WEDNESDAY
AND MAINTAIN AREAS OF LIGHT PRECIP OR SHOWERS ALONG THE GULF COAST.
A PATTERN CHANGE WILL THEN ENSUE AS WE HEAD THROUGH THURSDAY AND
FRIDAY AS THE LOW COMPLEX OVER THE GULF AND NORTHEAST PACIFIC EXITS
TO THE EAST AND A TROUGH DROPS DOWN FROM THE NORTH BRINGING ARCTIC
AIR BACK TO SOUTHCENTRAL ALASKA. AS MENTION DIN THE MODEL DISCUSSION
ABOVE...THERE IS POTENTIAL FOR SNOW AS THE TROUGH DROPS DOWN...BUT
IT IS TOO EARLY TO SAY WHICH AREAS WILL GET SNOW AND HOW MUCH WILL
FALL. THE HIGHER PROBABILITY ASPECT OF THE FORECAST IS THAT STRONG
GAP WINDS WILL DEVELOP ACROSS SOUTHCENTRAL AND THE GULF AS ARCTIC
AIR ARRIVES FRIDAY AND FRIDAY NIGHT.

SOUTHWEST ALASKA...WEAK UPPER LEVEL WAVES WILL CONTINUE TO TRANSIT
SOUTHWEST ALASKA THIS AFTERNOON THROUGH THIS EVENING PRODUCING AREAS
OF PRECIP. THINGS WILL THEN BEGIN TO DRY OUT AS FLOW BACKS TOWARD
THE NORTH OUT AHEAD OF LOW MOVING ACROSS THE GULF OF ALASKA. THERE
MIGHT STILL BE A FEW LINGERING SHOWERS IN THE BRISTOL BAY AREA INTO
WEDNESDAY WITH THE LOW MOVING TO THE WESTERN GULF...BUT ANYTHING
THAT FALLS WILL BE SHIRT-LIVED. MUCH COLDER AND DRIER AIR IS IN
STORE FOR THE END OF THE WEEK AS A TROUGH DROPS DOWN FROM THE
ARCTIC.

BERING SEA/ALEUTIANS...THE STRONG LOW CROSSING THE NORTH PACIFIC
THIS MORNING WILL JUST GRAZE THE THE CENTRAL ALEUTIANS WITH A
LITTLE WIND. ANOTHER LOW CURRENTLY OVER THE WESTERN PACIFIC WILL
RAPIDLY DEEPEN TONIGHT AND WEDNESDAY AND THEN SLOWLY WEAKEN AS IT
MOVES TO THE WESTERN ALEUTIANS LATER THIS WEEK. THE RIDGE BUILDING
OUT AHEAD OF IT WILL CAUSE THE LOW TO INITIALLY CURVE NORTHWARD
AND THEN STALL RIGHT OVER THE WESTERN ALEUTIANS AS WE HEAD INTO
THE WEEKEND. THE EASTERN BERING WILL REMAIN RELATIVELY QUIET THE
NEXT FEW DAYS WITH JUST A FEW WEAK FEATURES MOVING THROUGH THE AREA.

.LONG TERM FORECAST...MEDIUM RANGES MODELS HAVE TAKEN A TURN FOR
THE WORSE...WITH EXTREMELY POOR RUN-TO-RUN CONSISTENCY AND A HUGE
SPREAD IN SOLUTIONS AMONG THE VARIOUS INDIVIDUAL MODELS AND
ENSEMBLES. THE KEY FEATURE IS A LOW MOVING NORTHWARD OUT OF THE
NORTH-CENTRAL PACIFIC NEXT WEEKEND AND TOWARD THE EASTERN
BERING/ALASKA PENINSULA. SOME SOLUTIONS ULTIMATELY TRACK THE LOW
INTO THE GULF WHILE OTHER BRING IT INTO THE CENTRAL BERING. BASED
ON THIS...FORECAST CONFIDENCE FOR NEXT WEEKEND INTO EARLY NEXT
WEEK IS LOW.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING 120 130 138 140 172 176.
FIRE WEATHER...NONE.

SEB JAN 13