Small Craft Advisory 910, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
135 PM AKST THU NOV 20 2008

.ANALYSIS AND UPPER LEVELS...GULF/NE PACIFIC LOW COMPLEX HAS NOW
MERGED INTO A SINGLE CENTER OVER THE SOUTHERN GULF. REMNANTS OF
THE OCCLUDED FRONT FROM THIS LOW ARE PUSHING NORTHWEST ACROSS
WESTERN PORTIONS OF SOUTHCENTRAL. RADAR/SATELLITE IMAGERY SHOW A
BAND OF SNOW FROM KODIAK ISLAND NORTHWARD TO THE KENAI PENINSULA
AND THE ANCHORAGE/MAT-SU REGION. SNOW IS GENERALLY LIGHT...BUT RADAR
ECHOES INDICATE GREATER INTENSITY SNOW ON THE WESTERN KENAI
PENINSULA. MOISTURE ON THE EAST SIDE OF THE TROUGH/UPPER LOW IS
STREAMING NORTHWESTWARD ALONG THE FAR EASTERN GULF AND INTO THE
COPPER BASIN...BRINGING SOME LIGHT SNOW TO EASTERN PORTIONS OF THE
BASIN. WINDS ACROSS THE GULF AND SOUTHCENTRAL ARE SLOWLY BEGINNING
TO DIMINISH AS THE LOW DRIFTS SOUTHEASTWARD.

AN UPPER LEVEL LOW IS DROPPING DOWN INTO SOUTHWEST ALASKA THIS
MORNING...BRINGING LOTS OF ARCTIC AIR WITH IT. COLD ADVECTION
INDUCED WINDS AREA AFFECTING SOME LOCATIONS...BUT A DEEP SURFACE
BASED INVERSION ACROSS INTERIOR SOUTHWEST ALASKA IS KEEPING THE
STRONGER WINDS ALOFT FROM MIXING DOWN TO THE SURFACE.

A LARGE COLD CORE LOW HAS MOVED OUT OF EASTERN ASIA INTO THE SEA
OF OKHOTSK. A STRONG JET WITH ESTIMATED MAX SPEED AROUND 180KT
EXTENDS FROM JAPAN NORTHEASTWARD TO THE WESTERN ALEUTIANS. THIS
JET IS DRIVING A SURFACE FRONT EASTWARD ACROSS THE BERING AND
ALEUTIANS...AND GRADUALLY ERODING THE RIDGE OVER THE EASTERN BERING.

A SURFACE LOW HAS FORMED ALONG THE FRONT IN THE LEFT EXIT REGION
OF THE UPPER JET AND LOCATION OF MAXIMUM UPPER LEVEL DIFFLUENCE.
THIS LOW IS TRACKING ACROSS THE WESTERN ALEUTIANS INTO THE BERING
SEA AND WILL CONTINUE TO DEEPEN UNDER SUCH FAVORABLE CONDITIONS.

.MODEL DISCUSSION...MODELS INITIALIZED WELL WITH SYNOPTIC
POSITIONS...BUT SOLUTIONS QUICKLY DIVERGE...ESPECIALLY WITH THE
BERING SEA SYSTEM. CONTINUE TO PREFER THE GFS IN THE GULF BECAUSE
IT HAS PERFORMED BETTER THAN THE NAM THE PAST COUPLE DAYS. OUT
WEST...LIKE THE GENERAL IDEA OF MULTIPLE TRIPLE POINT DEVELOPMENT
AS THE STRONG UPPER JET DRIVES EASTWARD ALONG THE ALEUTIANS FRIDAY.
MODEL SOLUTIONS ARE GROUPED IN THE SAME GENERAL AREA.
HOWEVER...BEYOND 48 HOURS SOLUTIONS DIVERGE WITH EVOLUTION OF
THESE TRIPLE POINT LOWS AS WELL AS ENTRANCE OF NEXT MAJOR SYSTEM
OUT WEST. THUS...CHANGES TO FORECAST WILL BE CONFINED TO THE FIRST
48 TO 60 HOURS.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...WINDS WILL DIMINISH TONIGHT ACROSS THE REGION
AS THE GULF LOW TRACKS EASTWARD. MOST OF THE PRECIPITATION WILL
ALSO END AS THE LOW MOVES AWAY. HOWEVER...THE TROUGH DROPPING DOWN
THROUGH SOUTHWEST ALASKA WILL SWING NORTHEASTWARD AND MOVES ACROSS
SOUTHCENTRAL LATE TONIGHT THROUGH FRIDAY NIGHT. IT LOOKS LIKE THERE
WILL BE ENOUGH LEFT OVER MOISTURE IN PLACE TO GENERATE A LOCAL AREAS
OF SNOW...ESPECIALLY ALONG THE MOUNTAINS. IF ACCUMULATION OCCURS IT
WILL BE VERY MINOR. A TRIPLE POINT LOW ASSOCIATED WITH THE BERING
SEA SYSTEM WILL MOVE INTO THE GULF FRIDAY NIGHT AND NORTHWARD TOWARD
SOUTHCENTRAL SATURDAY. ALTHOUGH THE DETAILS OF THE TRACK OF THIS LOW
ARE IN QUESTION...IT LOOKS LIKE A GOOD CHANCE OF SNOW FOR MOST AREAS.
THIS SYSTEM WONT HANG AROUND LONG AS THE PATTERN BECOMES VERY
PROGRESSIVE. SO IT WILL LIKELY EXIT SUNDAY AS A NEW SYSTEM COMES
INTO THE PICTURE.

SOUTHWEST ALASKA...COLD ADVECTION WINDS WILL CONTINUE THIS EVENING
AND THEN DIMINISH OVERNIGHT INTO FRIDAY AS THE UPPER TROUGH BEGINS
TO SWING NORTHEASTWARD...ENDING MOST OF THE COLD ADVECTION. THE
BERING SEA SYSTEMS WILL PASS MAINLY TO THE SOUTH...IMPACTING ONLY
SOUTHERN PORTIONS OF BRISTOL BAY AS WELL AS THE ALASKA PENINSULA.
AREAS TO THE NORTH WILL REMAIN COLD AND DRY THROUGH AT LEAST
SATURDAY. ANOTHER BERING STORM SYSTEM WILL APPROACH SOUTHWEST
ALASKA SUNDAY. WITH LARGE MODEL DISAGREEMENTS DETAILS ARE NOT
CLEAR AT THIS TIME.

BERING SEA ALEUTIANS...STORM FORCE WINDS WILL DEVELOP ALONG THE
ALEUTIANS AND BERING SEA TONIGHT AS A LOW DEEPENS OVER THE CENTRAL
BERING. TRIPLE POINT LOWS WILL QUICKLY DEVELOP TO THE EAST ON FRIDAY
AS THE JET MOVES EASTWARD. THUS THE PARENT LOW WILL QUICKLY WEAKEN
FRIDAY AND FRIDAY NIGHT. THE PATTERN WILL CONTINUE TO PROGRESS
THROUGH THE WEEKEND WITH ANOTHER STRONG STORM MOVING TO THE ALEUTIANS
SATURDAY AND LIKELY REPLAYING A SIMILAR SCENARIO WITH TRIPLE POINT
DEVELOPMENT IN A VERY DYNAMIC ENVIRONMENT.

.LONG TERM FORECAST...A VERY PROGRESSIVE HEMISPHERIC PATTERN WITH A
STRONG SUB-TROPICAL JET WILL LEAD TO A VERY STORMY EXTENDED PERIOD
WITH RAPIDLY CHANGING CONDITIONS ACROSS THE FORECAST AREA. THIS WILL
MAKE IT VERY DIFFICULT TO FORECAST THE DETAILS OF THESE STORMS VERY
FAR OUT IN TIME.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...STORM WARNING...179 185.
         GALE WARNING...130 132 150 155 165 170 171 172 175 176 180.
         HEAVY FREEZING SPRAY...160 180 185.
FIRE WEATHER...NONE.

SEB NOV 08