Small Craft Advisory 35, NWS Anchorage, AK

AREA FORECAST DISCUSSION...CORRECTED
SOUTHCENTRAL AND SOUTHWEST ALASKA
142 PM AKST WED JAN 13 2010

CORRECTED TYPO

.ANALYSIS AND UPPER LEVELS...
THE PRIMARY WEATHER STORY OF THE DAY IS A DEVELOPING 955 MB LOW
PRESSURE SYSTEM APPROACHING THE WESTERN ALEUTIANS...WITH SNOW AND
STRONG WINDS CURRENTLY MOVING INTO THE FAR WESTERN ALEUTIANS.
AN IMPRESSIVE 210 KT JET TO THE IMMEDIATE SOUTH AND WEST OF THIS
SYSTEM WILL AID IN STRENGTHENING THIS LOW AS IT APPROACHES THE
ALEUTIAN CHAIN.

A WEAKENING CLOSED UPPER LEVEL LOW CONTINUES TO SPIN OUT ITS FINAL
DAYS SOUTH OF THE ALASKA PENINSULA. THIS LOW IS EMBEDDED IN A BROAD
UPPER LEVEL TROUGH OF LOW PRESSURE AND ASSOCIATED JET THAT DIVES
SOUTHWARD TO THE HAWAIIAN ISLAND CHAIN. A SERIES OF WEAK SURFACE
LOW PRESSURE SYSTEMS AND EMBEDDED SHORTWAVES RESIDE UNDER THIS
LOW...BRINGING SNOW TO THE COPPER RIVER BASIN AND PRINCE WILLIAM
SOUND. DESPITE WARM AIR ADVECTION IN THE UPPER LEVELS AS SEEN IN
AREA SOUNDINGS...A SHALLOW COLD POOL REMAINS STAGNATED ACROSS MUCH
OF CENTRAL ALASKA. THIS...COMBINED WITH THE LOW IN THE GULF...WILL
CONTINUE TO BRING LOCALIZED GUSTY WINDS THROUGH MOUNTAIN GAPS IN
SOUTH CENTRAL ALASKA THROUGH THIS AFTERNOON. HOWEVER...THIS SURFACE
COLD POOL WILL BECOME SHALLOWER TONIGHT AS LOW LEVEL FLOW BECOMES
MORE SOUTHERLY AND ADVECTS WARMER AIR INLAND...BRINGING AN END TO
THESE GAP WINDS.

ELSEWHERE ACROSS ALASKA...A SECOND UPPER LEVEL TROUGH OF LOW
PRESSURE ORIGINATING FROM THE ARCTIC OCEAN IS SLOWLY PROPAGATING
SOUTHEASTWARD ACROSS NORTHERN ALASKA WHILE A NEARLY STATIONARY
HIGH PRESSURE SYSTEM CONTINUES TO DISSIPATE IN THE NORTH BERING
SEA.

.MODEL DISCUSSION...
THE MODELS ARE OVERALL IN GOOD AGREEMENT IN THE SHORT TERM WITH
REGARDS TO THE TRACK OF THE GULF LOW...BUT ARE STILL HAVING PROBLEMS
DEPICTING THE SMALLER MESOSCALE FEATURES OF THIS STORM. THIS IS
EVIDENT ON THE INFRARED SATELLITE IMAGERY...WHICH CLEARLY DENOTES A
BARBELL LOW CENTERED 350 MILES SOUTHEAST OF THE ALASKA PENINSULA. THE
NAM DOES THE BEST JOB OF RESOLVING THIS FEATURE WHILE THE GFS MISSES
IT COMPLETELY. THE GFS IS ALSO THE MOST AGGRESSIVE IN THE DEVELOPMENT
AND LONGEVITY OF THE SHORTWAVES ROTATING AROUND THE GULF LOW. THIS
LOW SHOULD BE ABSORBED BY THE APPROACHING ALEUTIAN ISLAND STORM AND
SHOULD BE ALL BUT A MEMORY BY FRIDAY NIGHT.

THE DEVELOPING LOW OUT WEST OF THE ALEUTIANS IS ALSO BEING HANDLED
WELL BY THE MODELS IN THE SHORT TERM...CENTERING THE SYSTEM 300 MILES
SOUTH OF SHEMYA MID DAY THURSDAY. THE ONE FEATURE THE MODELS ARE
DISAGREEING ON IS THE STRENGTH OF THE FRONT OUT AHEAD OF THE
LOW...WITH THE GFS ONCE AGAIN BEING THE MOST AGGRESSIVE IN THIS
RESPECT. THE GFS IS ALSO THE MOST AGGRESSIVE IN THE DEVELOPMENT OF
A TRIPLE POINT LOW SOUTH OF COLD BAY ON FRIDAY. THE LATEST EUROPEAN
MODEL RUN NOW ALSO DEVELOPS THIS FEATURE 12 HOURS LATER THAN THE
GFS..WITH THE NAM NOW BEING THE ODD MAN OUT. THE MODELS DIVERGE
SIGNIFICANTLY PAST SATURDAY...WITH TWO POSSIBLE SCENARIOS TAKING
SHAPE. ONE FAVORED BY THE GFS STRENGTHENS THE NEWLY DEVELOPED TRIPLE
POINT LOW AND HAS THIS LOW ABSORBING THE MAIN LOW TO THE
WEST...WHEREAS THE EUROPEAN WRAPS THE NEW LOW INTO THE PRIMARY LOW.
THE TRACK AND STRUCTURE OF THE STORM IN THE LONG TERM WILL DEPEND
GREATLY ON WHICH SCENARIO PLAYS OUT. AS A RESULT OF THIS
DISCREPANCY...CONFIDENCE IN THE MODELS IN THE LONG TERM IS VERY LOW.

MINIMAL CHANGES WERE MADE TO THE FORECAST...WITH THE NAM MODEL
PREFERRED IN THE SHORT TERM.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THE MAIN UPPER WAVE PRODUCING SNOW FROM
PRINCE WILLIAM SOUND TO THE COPPER BASIN TODAY WILL MOVE OFF TO
THE NORTHEAST TONIGHT INTO THE YUKON. SO PRECIP WILL DIMINISH...BUT
WON'T END COMPLETELY AS LOW LEVEL MOISTURE AND UPSLOPE FLOW
CONTINUE TO PRODUCE LIGHT PRECIP ALONG THE COASTAL RANGE AND
WRANGELL MOUNTAINS/ALASKA RANGE. MEANWHILE...A WEAK LOW TRACKING
TOWARD KODIAK ISLAND WILL PRODUCE RAIN AND SNOW DOWN THERE.
MODELS SHOW A FAIRLY CONCENTRIC VORT-MAX TRACKING FROM THE PACIFIC
NORTHWARD TO SOUTHCENTRAL THURSDAY AFTERNOON...BUT DISAGREE ON
THE CHARACTERISTICS OF THIS UPPER LEVEL FEATURE ONCE IT REACHES
SOUTHCENTRAL. THE BULK OF THE PRECIP WILL FALL IN THE PRINCE
WILLIAM SOUND REGION ONCE AGAIN...BUT WITH WIDESPREAD SOUTHERLY
FLOW IN THE LOW TO MID LEVELS IT WON'T TAKE MUCH LIFT TO PRODUCE
LIGHT SNOW ELSEWHERE. THEREFORE...WILL BUMP UP POPS AND PUT IN
SOME VERY LIGHT SNOW ACCUMULATION. AS THIS FEATURE EXITS THURSDAY
NIGHT THE UPPER LOW WHICH HAS BEEN NEARLY STATIONARY SOUTH OF
KODIAK ISLAND WILL OPEN UP INTO A TROUGH AND HEAD NORTH TOWARD
SOUTHCENTRAL. SO CHANCE OF PRECIP WILL CONTINUE ON
FRIDAY...ESPECIALLY ALONG THE COAST. UNCERTAINTY GROWS SIGNIFICANTLY
BEYOND THAT AS THE TROUGH ASSOCIATED WITH MAJOR ALEUTIAN LOW DIGS
EASTWARD AND ENERGY BEGINS TO EJECT OUT OF THE TROUGH NORTHWARD
ACROSS THE GULF. MODELS ARE NOT YET ABLE TO RESOLVE DETAILS OF
THESE FEATURES...BUT UNDER THIS PATTERN PRECIP WILL BE LIKELY
ALONG THE COAST AND CHANCES OF PRECIP INLAND WILL INCREASE BY
THE WEEKEND AS THESE FEATURES MOVE TO SOUTHCENTRAL.

SOUTHWEST ALASKA...A FEW SNOW SHOWERS WILL PERSIST RIGHT ALONG
THE ALASKA PENINSULA AND ALEUTIAN RANGE DUE TO ITS POSITION
UNDERNEATH THE UPPER TROUGH AXIS. THE REST OF SOUTHWEST ALASKA
WILL REMAIN DRY THROUGH THE END OF THE WEEK. THE SURFACE PRESSURE
GRADIENT WILL SLACKEN TONIGHT...SO WINDS SHOULD FINALLY DIE DOWN A
BIT OVER THE KUSKOKWIM DELTA. HOWEVER...THE GRADIENT WILL TIGHTEN
RIGHT BACK UP THURSDAY AND FRIDAY AS ALEUTIAN SYSTEM PROGRESSES
EASTWARD. AN UPPER TROUGH DROPPING OUT OF THE ARCTIC WILL ALSO
BRING ANOTHER ROUND OF COLD AIR ADVECTION BEGINNING FRIDAY...WHICH
WILL CONTRIBUTE TO STRONGER WINDS.

BERING SEA/ALEUTIANS...THE MAJOR LOW APPROACHING THE ALEUTIANS
IS STILL ON TRACK AND LITTLE CHANGE WILL BE MADE TO THE CURRENT
FORECAST. THERE IS STILL SOME POSSIBILITY OF BLIZZARD CONDITIONS
ALONG THE ALEUTIANS OUT AHEAD OF THE FRONT. TEMPERATURES OUT
THERE TEND TO JUMP AS WIND INCREASE...DUE TO FLOW OFF THE WARMER
WATERS...BUT IN THIS CASE TEMPS ARE STARTING OUT QUITE COLD AND
FLOW AHEAD OF THE FRONT WILL BE EASTERLY WHICH MEANS THERE IS NOT
WARM ADVECTION FROM THE SOUTH. FOR NOW HAVE HIT THIS AS HARD AS
POSSIBLE WITHOUT ISSUING A BLIZZARD WARNING...BUT WILL NEED TO
CONTINUE TO RE-EVALUATE THIS. PRECIP WILL MIX WITH OR CHANGE TO
RAIN AS THE FRONT REACHES THE ALEUTIANS ON THURSDAY. THIS
STORM...WHICH IS EXPECTED TO BOTTOM OUT IN THE 940S...WILL AFFECT
THE ENTIRE BERING SEA REGION WITH INCREASING PRESSURE GRADIENT
AND RESULTANT STRONG WINDS.

.LONG TERM FORECAST...THE ALEUTIAN TROUGH WILL MERGE WITH THE
TROUGH DROPPING DOWN OUT OF THE ARCTIC SOMETIME THIS
WEEKEND...SHIFTING ALL OF THE ACTIVITY TOWARD THE GULF AND
SOUTHERN MAINLAND. MEANWHILE...ARCTIC AIR WILL PLUNGE SOUTHWARD
ACROSS THE BERING SEA...SETTING UP A LONG DURATION OF SNOW SHOWERS
AND GUSTY WINDS.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...STORM WARNING...172 175 176.
         GALE WARNING...127 128 170 179 185.
         HEAVY FREEZING SPRAY...160.
FIRE WEATHER...NONE.

KS/SEB JAN 10