Small Craft Advisory 625, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
130 PM AKDT THU SEP 1 2011

.ANALYSIS AND UPPER LEVELS...
UPPER LEVEL PATTERN IS AMPLIFYING AS A TROUGH OVER THE EASTERN
BERING DIGS SOUTHWARD TOWARD THE ALEUTIANS. WITH A NEUTRAL TILT
PRECIPITATION ALONG THE FRONT EDGE OF THE TROUGH IS MAKING VERY
LITTLE EASTWARD PROGRESSION ACROSS SOUTHWEST ALASKA. THIS TROUGH
IS BEGINNING TO INFLUENCE A SHORT-WAVE TROUGH TO THE SOUTH OVER THE
NORTH-CENTRAL PACIFIC. EXPECT THESE TROUGHS TO PHASE AND INITIATE
DEVELOPMENT OF THE PACIFIC SYSTEM....WHICH WILL MOVE INTO THE GULF
AND BECOME THE PRIMARY WEATHER PRODUCER THE NEXT COUPLE DAYS.
RIDGING AHEAD OF THESE SYSTEMS IS PROVIDING FOR DRY CONDITIONS
ACROSS SOUTHCENTRAL TODAY...BUT ALSO PROMOTED FOG FORMATION THIS
MORNING FOR THE MAT-SU AND COASTAL LOCATIONS. FOG HAS MOSTLY
DISSIPATED BUT LOW CLOUDS CONTINUE TO HANG AROUND. AT THE SURFACE
THERE ARE TWO AREAS OF LOW PRESSURE...ONE IN THE EASTERN BERING
WHICH IS WEAKENING AND ANOTHER SOUTH OF THE ALASKA PENINSULA WHICH
IS INTENSIFYING AS IT MOVES TOWARD THE GULF.

.MODEL DISCUSSION...
THE MORNING MODEL GUIDANCE ALL HANDLE THE UPPER LEVEL PATTERN WELL
FOR THE IMMEDIATE SHORT TERM. THERE ARE SOME DIFFERENCES ON DEPTH OF
THE SURFACE LOW BUT CONSENSUS IS THAT IT WILL TRACK JUST TO THE EAST
OF KODIAK ISLAND BY FRIDAY AFTERNOON. AT THIS JUNCTURE THE LOW WILL
SLOWLY FILL IN THE GULF OF ALASKA WHILE A NEW SYSTEM TAKES SHAPE IN
THE BERING. AS A VERY STRONG JET COMES INTO PLAY FROM KAMCHATKA INTO
THE ALEUTIANS A NEW CLOSED UPPER LOW WILL TAKE SHAPE OVER BRISTOL
BAY. SLIGHT DIFFERENCES IN THE TIMING OF THIS FEATURE ARISE BETWEEN
THE GLOBAL MODELS. LARGER DIFFERENCES EXIST WITH HOW THE SURFACE LOW
PRESSURE IS HANDLED WITH THIS SYSTEM IN BOTH TIMING AND INTENSITY.
THE MAIN THEME HERE THOUGH IS A SIGNIFICANT LOW WORKING INTO THE
GULF EARLY NEXT WEEK.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...A SHORT-WAVE RIDGE WILL MOVE OVERHEAD OF
SOUTHCENTRAL THIS AFTERNOON AND EVENING AHEAD OF MAJOR STORM
TRACKING INTO THE GULF. THEREFORE...ONCE FOG AND STRATUS DISSIPATE
PARTLY CLOUDY TO MOSTLY SUNNY SKIES WILL DOMINATE FOR THE REMAINDER
OF THE DAY. CONDITIONS WILL QUICKLY DETERIORATE OVER KODIAK ISLAND
AND THE GULF TONIGHT AS AN INTENSIFYING PACIFIC LOW MOVES INTO THE
WESTERN GULF. A BARRIER JET WILL DEVELOP AS THE OCCLUDED FRONT
APPROACHES THE COAST LATE FRIDAY MORNING. AN UPPER LEVEL SHORT-WAVE
WILL ROTATE UP THE EAST SIDE OF THE LOW CENTER AND OVER THE PRINCE
WILLIAM SOUND REGION FRIDAY EVENING WHICH WILL HELP PUSH THE
OCCLUDED FRONT ONSHORE. AS THIS OCCURS STRONGER WINDS WILL BRIEFLY
MOVE ONSHORE LATE FRIDAY AFTERNOON THROUGH EARLY FRIDAY EVENING
FROM CORDOVA TO WHITTIER TO PORTAGE VALLEY AND TURNAGAIN ARM.
THE BULK OF RAIN WILL FALL ALONG THE COAST...BUT AS THE UPPER
SHORTWAVE ROTATES WESTWARD OVER TOP OF THE LOW CENTER AND DISSIPATING
OCCLUSION MOVES ONSHORE EXPECT MOISTURE AND LIFT TO SPREAD INLAND.
IN ALL LIKELIHOOD IT WILL BE A BRIEF PERIOD OF RAIN...PERHAPS MORE
CHARACTERISTIC OF SHOWERS. THE LOW WILL SLOWLY FILL OVER THE CENTRAL
GULF FRIDAY NIGHT AND SATURDAY WITH SHOWERS CONTINUING MAINLY ALONG
THE COAST. WHILE THE UPPER LOW WILL THEN MOVE OFF TO THE EAST THE
SURFACE LOW WILL ACTUALLY PULLED WESTWARD AS A SHORT-WAVE MOVES IN
FROM THE BERING SEA. THIS WILL RESULT IN A GOOD SHOT OF RAIN OVER
THE SOUTHERN HALF OF SOUTHCENTRAL SUNDAY AND SUNDAY NIGHT...ALTHOUGH
THE DETAILS OF THIS ARE STILL IN QUESTION.

SOUTHWEST ALASKA...THE UPPER TROUGH OVER THE EASTERN BERING WILL
CONTINUE TO DIG SOUTHWARD THIS AFTERNOON AND EVENING AND PICK UP THE
DEVELOPING PACIFIC SYSTEM. WITH THAT DOWNSTREAM LOW BECOMING DOMINANT
THE SURFACE LOW OVER THE EASTERN BERING WILL DISSIPATE AND THE FRONT
WILL STALL AND WEAKEN OVER COASTAL SOUTHWEST ALASKA. THUS...THE BULK
OF RAIN WILL REMAIN WEST OF A KING SALMON TO SLEETMUTE LINE THIS
AFTERNOON AND EVENING. AS THIS AREA OF RAIN DISSIPATE OVERNIGHT INTO
FRIDAY MORNING MOISTURE FROM THE GULF WILL PUSH WESTWARD...BRINGING
A CHANCE OF RAIN ALONG THE ALASKA AND ALEUTIAN RANGES FRIDAY. THE
PATTERN WILL REMAIN PROGRESSIVE WITH ANOTHER TROUGH DIGGING DOWN
FROM THE ARCTIC SATURDAY AND SUNDAY BRINGING ANOTHER SHOT OF RAIN
TO SOUTHWEST ALASKA.

BERING SEA/ALEUTIANS...MORE UNSTABLE CONDITIONS BEHIND THE EASTERN
BERING LOW WILL HELP PRODUCE SOME BREAKS IN THE CLOUDS FROM THE
CENTRAL TO EASTERN BERING AS THE SYSTEM EXITS TONIGHT AND FRIDAY.
HOWEVER...A BUILDING RIDGE OVER THE WESTERN BERING ALONG WITH WARM
ADVECTION WILL BE CONDUCIVE TO FOG AND STRATUS FORMATION..ESPECIALLY
ALONG THE WESTERN ALEUTIANS. THE NEXT TROUGH DIGGING OUT OF SIBERIA
WILL RAPIDLY FLATTEN THIS RIDGE OUT FRIDAY AND FRIDAY NIGHT WITH
A SURFACE FRONT QUICKLY CROSSING THE BERING. AS THE SHORT-WAVE
CONTINUES TO DIG THE LONG-WAVE TROUGH WILL BECOME HIGHLY AMPLIFIED
AS WE HEAD THROUGH THE WEEKEND. A SURFACE LOW WILL TAKE A SIMILAR
TRACK TO THE CURRENT ONE ON SUNDAY. DECENT COLD ADVECTION ON THE
BACK SIDE WILL PRODUCE GUSTY WINDS AND SHOWERS.

.LONG TERM FORECAST...THE DAY THROUGH 7 FORECAST PERIOD WILL BE
DOMINATED BY A DEEP UPPER LOW CENTERED SOUTH OF KODIAK ISLAND
WITHIN A HIGHLY AMPLIFIED LONG-WAVE PATTERN. IT IS DIFFICULT FOR
MODELS TO RESOLVE INDIVIDUAL WAVES MOVING AROUND THIS LOW...BUT
BOTTOM LINE IS EXPECT PERIODS OF RAIN ACROSS THE SOUTHERN MAINLAND.
ON THE BACK SIDE...FREEZING LEVELS WILL DROP SIGNIFICANTLY AS COLD
AIR DROPS DOWN FROM THE ARCTIC AND PRODUCE THE TYPICAL GUSTY WINDS
AND SHOWERS ACROSS PORTIONS OF THE BERING SEA AND ALEUTIANS.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...STORM WARNING 120.
         GALE WARNING 125 130 132 138 140.
FIRE WEATHER...NONE.

ML/SEB SEP 11