Small Craft Advisory 1046, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
120 PM AST TUE DEC 11 2007

.ANALYSIS AND UPPER LEVELS...LONG-WAVE TROUGH IS CENTERED FROM
EASTERN ASIA TO THE BERING SEA WITH WEAK RIDGING ALONG WEST COAST
OF NORTH AMERICA. THE JET IS WELL SOUTH OF ALASKA AS IT COMES OUT
OF ASIA BUT CURVES NORTHWARD TO JUST SOUTH OF THE GULF. THIS IS
SERVING TO DRIVE STORMS ALONG THE ALEUTIANS AND THEN INTO THE GULF.

THERE ARE TWO STORMS OF INTEREST THIS MORNING. THE FIRST IS A LOW
OVER THE NORTHERN GULF. THE TROUGH AXIS (IN THE UPPER AND LOWER
LEVELS) EXTENDS FROM THIS LOW BACK WESTWARD TO JUST SOUTH OF
ANCHORAGE...SO THAT MOISTURE IS BEING TRANSPORTED FROM EASTERN
PRINCE WILLIAM SOUND AND THE COPPER BASIN WESTWARD TO ANCHORAGE
AND THE MAT-SU. MEANWHILE...ANOTHER UPPER TROUGH IS APPROACHING
FROM THE SOUTHWEST...PROVIDING ADDITIONAL LIFT. COLD ADVECTION
IS ALSO OCCURRING IN THE LOW LEVELS AND HAS CHANGED WHAT WAS
MAINLY RAIN THIS MORNING IN THE ANCHORAGE/MAT VALLEY TO ALL SNOW.

THE SECOND SYSTEM IS A LARGE MATURE LOW SOUTH OF THE EASTERN
ALEUTIANS. IT IS TRACKING EASTWARD AFFECTING THE EASTERN ALEUTIANS
WITH THE TYPICAL KIND OF WEATHER FOR THIS YEAR...RAIN AND SNOW AND
SOME GALE FORCE WINDS.

.MODEL DISCUSSION...MODELS DONT HAVE A VERY GOOD HANDLE ON THE
DETAILS OF WHAT IS OCCURRING OVER SOUTHCENTRAL...BUT SYNOPTICALLY
LOOK GOOD. WILL HAVE USE PATTERN RECOGNITION/CONCEPTUAL MODELS TO
FORECAST SNOWFALL OVER SOUTHCENTRAL TONIGHT AS UPPER TROUGH PUSHES
ACROSS THE REGION. MODELS DO HAVE A GOOD HANDLE ON ALEUTIAN LOW AND
ARE ALL TRENDING TOWARD A COMMON SOLUTION OF DEVELOPING A SECONDARY
LOW SOUTH OF KODIAK ISLAND WEDNESDAY MORNING IN RESPONSE TO AN UPPER
LEVEL SHORT-WAVE EMBEDDED IN THE UPPER FLOW. THIS LOW BECOMES THE
DOMINANT LOW AND TRACKS INTO THE GULF. HAVE CONTINUED TO STICK VERY
CLOSE TO THE GFS SOLUTION WITH THIS LOW DUE TO GOOD RUN-TO-RUN
CONSISTENCY. THERE ARE DISCREPANCIES ONCE LOW REACHES THE GULF...BUT
ALL SHOW A DISSIPATING LOW AS ANOTHER LOW COMES UP FROM THE PACIFIC.
MODELS ARE ALSO CONVERGING ON A SOLUTION WITH NEXT LOW OUT WEST
THURSDAY AND FRIDAY...ALL KEEPING THE THE LOW SOUTH OF THE ALEUTIAN
CHAIN. GFS HAS JUMPED MUCH FARTHER SOUTHWARD...IN LINE WITH THE ECMWF
AND IS SUPPORTED BY THE ENSEMBLE MEAN. BECAUSE CURRENT FORECAST WAS
BASED OFF A MUCH FARTHER NORTH SOLUTION WILL MAKE A JUMP TO THE NEW
GFS SOLUTION.

.SHORT TERM FORECAST...

SOUTHCENTRAL...EASTERLY TRANSPORT OF MOISTURE WILL CUT OFF OVER
ANCHORAGE THIS AFTERNOON...BUT UPPER TROUGH WILL NOT MOVE THROUGH
UNTIL LATER THIS EVENING. SO SNOW WILL DIMINISH THIS AFTERNOON...
BUT FLURRIES OR A FEW SHOWERS MAY CONTINUE INTO THE EVENING HOURS.
MEANWHILE SNOW WILL SPREAD TO NORTHERN PORTIONS OF
SOUTHCENTRAL...INCLUDING ALONG THE ALASKA RANGE. SNOW WILL TAPER OFF
INT THESE AREAS AS WE HEAD INTO WEDNESDAY MORNING. PARTIAL CLEARING
BEHIND THE TROUGH COMBINED WITH REMNANT LOW LEVEL MOISTURE WILL
LIKELY RESULT IN THE FORMATION OF AREAS OF LOW CLOUDS AND FOG
TONIGHT. THE NEXT STORM SYSTEM LOOKS LIKE IT WILL NOT MAKE IT AS
FAR NORTH...SO MAIN EFFECTS WILL BE FOR KODIAK ISLAND AND THE GULF
COAST WHILE INLAND AREAS WILL LIKELY JUST SEE SOME CLOUDS.

BERING SEA/ALEUTIANS/SOUTHWEST ALASKA...THE LOW SOUTH OF THE
ALEUTIANS WILL SPREAD RAIN AND SNOW TO THE AKPEN AND PORTIONS OF
BRISTOL BAY TONIGHT INTO WEDNESDAY. THE NEXT SYSTEM LOOKS LIKE IT
WILL JUST SKIM THE ALEUTIANS LATE THURSDAY AND FRIDAY. OTHERWISE...
CALM CONDITIONS ACROSS THE REGION WITH INCREASING COLD ADVECTION
AS WE HEAD THROUGH THE WEEK AND RIDGE BEGINS TO BUILD OVER THE
WESTERN BERING AND PACIFIC.

.LONG TERM FORECAST...GFS 500MB 5-WAVE IS CONSISTENT IN DEPICTING
RIDGE BUILDING OVER BERING AND TROUGH SETTING UP OVER GULF/WESTERN
NORTH AMERICA. THUS...EXPECT COLD AND DRY CONDITIONS FOR MAINLAND
ALASKA THROUGH THE EXTENDED PERIOD WHILE MAJOR STORMS ARE HELD OVER
THE BERING SEA/ALEUTIANS.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALES 155 165 170 172.
FIRE WEATHER...NONE.

SEB DEC 07