Small Craft Advisory 632, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1230 PM AKDT SAT SEP 3 2011

.ANALYSIS AND UPPER LEVELS...A HIGH AMPLITUDE TROUGH IS CENTERED
OVER THE GULF AND SOUTHCENTRAL WITH A DISSIPATING LOW NEARLY
STATIONARY OVER THE NORTHWEST GULF. THE BULK OF RAIN IS CONFINED
TO COASTAL AREAS...ALTHOUGH WITH WEAK WAVES EMBEDDED IN UPPER
FLOW DO EXPECT AREAS OF LIGHT RAIN OR SHOWERS TO DEVELOP INLAND
THIS AFTERNOON. WATER VAPOR IMAGERY HOWS A SHARP SHORT-WAVE RAPIDLY
DIGGING DOWN THE BACK SIDE OF THE TROUGH WITH THE HELP OF A STRONG
SUBTROPICAL JET (WHICH MODELS INDICATE TO BE AROUND 160 KTS). THERE
IS A WEAK SURFACE LOW WITH THIS DROPPING SOUTHEASTWARD OFF THE
SOUTHWEST ALASKA COAST. ALSO OF NOTE IS AN AREA OF DEEP MOISTURE
(WITH 200%+ OF NORMAL PRECIPITABLE WATER) FROM THE WEST PACIFIC
TROPICS NORTHEASTWARD TO KAMCHATKA PENINSULA AND THEN EASTWARD
ACROSS THE ALEUTIANS.

.MODEL DISCUSSION...MODELS ARE IN VERY GOOD AGREEMENT WITH EVOLUTION
OF MAJOR LOW AND FORECAST CONFIDENCE IS ABOVE AVERAGE. THE ONE
NOTABLE DIFFERENCE IS HANDLING OF THE SHORT-AVE CURRENTLY DIGGING
SOUTHWARD ACROSS THE BERING SEA. THE NAM IS THE BIGGEST OUTLIER
IN SWINGING THIS NORTHWARD ACROSS THE ANCHORAGE AREA SUNDAY NIGHT
WHILE OTHER MODELS KEEP IT SOMEWHERE FARTHER SOUTH. THIS AFFECTS
THE POSITION OF A WEAKENING FRONT OVER THE NORTH GULF COAST WITH
THE NAM BRINGING STRONG WINDS AHEAD OF THIS FRONT INTO PRINCE
WILLIAM SOUND. WILL IGNORE THE NAM SOLUTION FOR NOW.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THE UPPER LOW OVER THE GULF WILL OPEN UP INTO
A TROUGH TONIGHT AND EXIT TO THE NORTHEAST LEAVING THE SURFACE LOW
TO DISSIPATE EAST OF SEWARD. RAIN ALONG THE COAST WILL BE RE-INFORCED
AS BERING SHORT-WAVE APPROACHES LATE TONIGHT AND SUNDAY. INLAND
AREAS WILL CONTINUE TO EXPERIENCE VERY WEAK LIFT ASSOCIATED WITH
UPPER WAVES...SO A CHANCE OF LIGHT RAIN OR SHOWERS WILL CONTINUE.
A RAPIDLY INTENSIFYING LOW WILL SOUTH OF THE ALASKA PENINSULA SUNDAY
AFTERNOON WILL MOVE INTO THE WESTERN GULF BY MONDAY AFTERNOON. THE
INFLUENCE OF THIS LOW WILL BEGIN TO BE FELT SUNDAY NIGHT ACROSS THE
GULF WATERS AND WINDS WILL RAPIDLY INCREASE DURING THE DAY MONDAY.
WIDESPREAD STORM FORCE WINDS ARE EXPECTED ALONG THE GULF COAST AS
THE CENTRAL PRESSURE OF THIS LOW DROPS INTO THE 950S. THERE MIGHT
BE A BRIEF BREAK IN CLOUDS OVER INTERIOR SOUTHCENTRAL AS LOW LEVEL
FLOW SHIFTS TO OFFSHORE AHEAD OF THE DEVELOPING LOW...BUT THIS WILL
BE SHORT-LIVED AS UPPER WAVES MOVING AROUND THE LOW WORK THERE WAY
UP TO SOUTHCENTRAL BY TUESDAY.

SOUTHWEST ALASKA...EFFECTS OF THE EASTERN BERING LOW WILL BE
LIMITED TO COASTAL SOUTHWEST ALASKA WITH UPPER TROUGH DIGGING
SOUTH-SOUTHEASTWARD. HOWEVER...A VERY WEAK UPPER WAVE CURRENTLY
OVERHEAD OF SOUTHWEST ALASKA WILL CONTINUE TO PRODUCE SHOWERS
OR LIGHT RAIN INLAND THROUGH TONIGHT. ALL THE ACTION WILL SHIFT
SOUTHWARD LATE SUNDAY THROUGH MONDAY WITH DEVELOPMENT OF MAJOR LOW.
BRISTOL BAY WILL SEE SOME MOISTURE COMING OVER-TOP OF THE ALEUTIAN
RANGE. OTHERWISE...FLOW OVER SOUTHWEST ALASKA WILL SHIFT TO OFFSHORE
AND DRY CONDITIONS WILL PREVAIL.

BERING SEA/ALEUTIANS...A SHORT-WAVE COMING OUT OF ASIA IS ONE OF
THE KEYS TO DEVELOPMENT OF MAJOR LOW SOUTH OF THE ALASKA PENINSULA.
UPPER JET WILL DRIVE THIS SHORT-WAVE RAPIDLY EASTWARD TONIGHT AND
SUNDAY. AS MAJOR LOW THEN DEVELOPS STRONG NORTHERLY FLOW AND COLD
ADVECTION WILL ENSUE ACROSS THE BERING SEA AND ALEUTIANS. THIS
FALL-TYPE PATTERN WILL LEAD TO VERY GUSTY WINDS ALONG WITH SHOWERS.

.LONG TERM FORECAST...A HIGH AMPLITUDE TROUGH WILL PERSIST FROM
MAINLAND ALASKA TO THE NORTHEAST PACIFIC DURING THE DAY 4 THROUGH 7
TIME PERIOD. THE VERY DEEP SURFACE LOW WILL VERY GRADUALLY WEAKEN
SOMEWHERE NEAR THE ALASKA PENINSULA AS WE HEAD THROUGH NEXT WEEK.
GREATEST IMPACT WILL REMAIN ALONG THE COAST...BUT UPPER LEVEL WAVES
TRAVELING AROUND THE LOW CENTER WILL PROVIDE A CHANCE OF RAIN INLAND
AS WELL. THE PICTURE BECOMES A LITTLE FUZZIER AT THE END OF NEXT WEEK
AND INTO THE WEEKEND AS MORE ENERGY MOVING OUT OF ASIA TRIES TO MOVE
INTO THE BACK SIDE OF THE WEAKENING TROUGH.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING 120 170 172 175 176.
FIRE WEATHER...NONE.

SEB SEP 11