Small Craft Advisory 769, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
145 PM AKDT WED OCT 2 2013

.ANALYSIS AND UPPER LEVELS...
THE MAIN FEATURE ACROSS THE DOMAIN REMAINS THE MATURE VERTICALLY
STACKED LOW THAT HAS MOVED THROUGH THE BERING AND IS NOW SOUTH OF THE
EASTERN ALEUTIANS. A DEEP TROPICAL MOISTURE PLUME FEEDING INTO THE
SYSTEM REACHES SOUTHWARD PAST 30N LATITUDE WITH A 140+ KT JET
TRANSPORTING MOISTURE NORTHWARD. THE UPPER/SURFACE CIRCULATION HAS
LOST JET SUPPORT...AND HAS BEGUN TO WEAKEN. THE FIRST SIGNS OF
EXPECTED TRIPLE POINT DEVELOPMENT ARE EVIDENT ON WATER VAPOR IMAGERY
AS DEEP MOISTURE ASSOCIATED WITH THE JET IS BEGINNING TO BE PULLED
BACK WESTWARD AS A SUPPORTING SHORTWAVE AMPLIFIES. GUSTY WINDS BEHIND
THE LOW ARE AFFECTING THE ALEUTIAN ISLANDS...THOUGH A LACK OF
APPRECIABLE COLD AIR ADVECTION BEHIND THE SYSTEM IS LIMITING HOW MUCH
WIND IS MIXING DOWN TO THE SURFACE FROM ALOFT. AHEAD OF THE MAIN
SYSTEM...AN UPPER TROUGH/WEAK UPPER LOW THAT FORMED OVER THE
SOUTHWEST MAINLAND HAS PROVIDED ENOUGH LIFT TO KICK OFF WIDESPREAD
SHOWER ACTIVITY ACROSS THE NORTHERN GULF AND MUCH OF SOUTHCENTRAL
ALASKA. THESE SHOWERS WILL DISSIPATE AND MOVE INLAND THROUGH THE
AFTERNOON HOURS AS THE UPPER TROUGH WEAKENS AND DRIFTS NORTHWARD.

.MODEL DISCUSSION...
THE MAIN ISSUE THE MODELS CONTINUE TO STRUGGLE WITH IS THE STRENGTH
AND PLACEMENT OF THE SECONDARY LOW EXPECTED TO DEVELOP AND INTENSIFY
IN THE SOUTHERN GULF OVERNIGHT TONIGHT. THE ECMWF HAS BEEN
CONSISTENTLY DEVELOPING A LOW IN THE 980-985 RANGE THAT ROTATES
WESTWARD TOWARD KODIAK ISLAND DURING THE DAY THURSDAY BEFORE TURNING
EASTWARD AGAIN LATE THURSDAY NIGHT AND RAPIDLY WEAKENING. THE GFS AND
GEM HAVE GENERALLY TRENDED TOWARD A SOLUTION LIKE THE ECMWF...AND THE
18Z NAM HAS EVEN FINALLY GOTTEN ITS ACT TOGETHER WITH A LOW TRACK
THAT RESEMBLES THE EMERGING CONSENSUS. INCREASING CONFIDENCE IN THE
LOW TRACK ALSO INCREASES CONFIDENCE IN THE DEVELOPMENT OF WHAT IS
SHAPING UP TO BE AN IMPRESSIVE COASTAL JET ALONG THE NORTH GULF COAST
AND EASTERN KENAI PENINSULA DURING THE DAY THURSDAY...WITH SIMILARLY
STRONG WINDS FROM LOWER COOK INLET THROUGH SHELIKOF STRAIT.

.SHORT TERM FORECAST...
SOUTHCENTRAL...SHOWER ACTIVITY AND WIDESPREAD LOW CLOUDINESS WILL
GRADUALLY DIMINISH THROUGH THE EARLY EVENING HOURS AS SOUTHEASTERLY
FLOW INCREASES AHEAD OF THE APPROACHING STORM AND THE UPPER TROUGH
CURRENTLY OVER THE AREA WEAKENS AND SLOWLY LIFTS NORTHWARD. THE VAST
MAJORITY OF RAIN ASSOCIATED WITH THE DEVELOPING LOW WILL FALL OVER
COASTAL AREAS OF THE MAINLAND AS WELL AS KODIAK ISLAND. DOWNSLOPING
WILL LARGELY PREVAIL OVER THE INTERIOR AREAS...THOUGH WITH A TROPICAL
MOISTURE-LADEN ENVIRONMENT SOME LIGHT RAIN IS EXPECTED TO FALL EVEN
OVER THESE TRADITIONALLY DRIER AREAS. HOWEVER...THE MOST WIDESPREAD
IMPACT WILL BE STRONG NORTHEASTERLY WINDS IN MOST COASTAL AREAS. BY
FRIDAY...THE LOW WILL BE WEAKENING SIGNIFICANTLY AND MOVING INTO THE
EASTERN GULF. THIS WILL BRING A DRYING AND CLEARING TREND TO THE AREA
THROUGH THE START OF THE WEEKEND.

SOUTHWEST...THE FRONT PUSHED AS FAR NORTH AS THE SOUTHERN ALASKA
PENINSULA BEFORE STALLING. THE FRONT WILL PIVOT IN PLACE TONIGHT AS
THE STORMS ENERGY SHIFTS RAPIDLY INTO THE GULF. THIS WILL SET MOST OF
THE SOUTHWEST MAINLAND UP WITH CONTINUED DRY NORTHERLY/OFFSHORE FLOW
THROUGH THE END OF THE WEEK. BY SATURDAY MORNING...A MASSIVE BUT
FAIRLY WEAK FRONT WILL CROSS THE BERING AND REACH THE SOUTHWEST
COAST. THIS IS LIKELY TO BRING WIDESPREAD RAINFALL TO COASTAL AND
INLAND SECTIONS ALIKE...AND USHER IN A NEW WEATHER PATTERN
CHARACTERIZED BY MOIST ONSHORE FLOW WITH MULTIPLE PERIODS OF
WIDESPREAD RAINFALL.

BERING SEA AND ALEUTIANS...THE EASTERN HALF OF THE ALEUTIANS WILL
CONTINUE TO EXPERIENCE GUSTY NORTHERLY WINDS EVEN AS THE STORM
TRANSITIONS FULLY INTO THE GULF ON FRIDAY. THESE WINDS WILL THEN
DECREASE RAPIDLY FRIDAY AFTERNOON/EVENING AS THE PRESSURE GRADIENTS
ACROSS THE REGION DECREASE DRAMATICALLY. THE LARGE (BUT FAIRLY WEAK)
AFOREMENTIONED FRONT WILL MOVE INTO THE WESTERN ALEUTIANS DURING THE
DAY FRIDAY WITH RAIN AND SOUTHERLY WINDS...RACING ACROSS THE BERING
TO THE MAINLAND COAST BY MIDDAY SATURDAY. THIS LOW WILL STALL OVER
THE NORTHERN BERING...AND EFFECTIVELY BECOME AN ANCHOR LOW FOR A NEW
LONGWAVE TROUGH AS WE HEAD INTO THE START OF NEXT WEEK.

.LONG TERM FORECAST...THERE IS A FAIRLY LARGE SPREAD IN MODEL
SOLUTIONS DURING THE DAY 4 THROUGH 7 PERIOD...LEADING TO BELOW
AVERAGE CONFIDENCE OVERALL. HOWEVER...THERE IS SOME COMMON GROUND
IN THE LARGER SCALE EVOLUTION WHICH ALLOWS FOR IDENTIFICATION OF
THE TYPE OF SYSTEMS OR GENERAL WEATHER WHICH CAN BE EXPECTED. FIRST
OF ALL...THE FRONT WHICH REACHES THE SOUTHWEST MAINLAND SATURDAY WILL
CONTINUE TO MARCH EASTWARD THIS WEEKEND LIKELY BRINGING WIDESPREAD
RAIN TO SOUTHCENTRAL ALASKA SATURDAY NIGHT INTO SUNDAY. THE PARENT
LOW WILL WEAKEN SOMEWHERE BACK OVER THE NORTHERN BERING SEA.

BEYOND THIS TIME FRAME EXPECT AMPLIFICATION OF THE BERING SEA TROUGH
AS ENERGY DIGS SOUTHEASTWARD OUT OF ASIA INTO THE NORTH PACIFIC.
MODEL DIFFERENCES AND RUN-TO-RUN INCONSISTENCIES ARISE WITH REGARD
TO THE AMPLITUDE AND SPEED OF THIS TROUGH AS WELL AS THE LOCATION
AND EXTENT OF CYCLOGENESIS AT THE SURFACE. BASED ON THE STRENGTH OF
THE SUBTROPICAL JET AND CONSIDERING THE LIKELY ZONE FOR CYCLOGENESIS
WOULD EXPECT A STORM SYSTEM TO DEVELOP AND MOVE TO SOMEWHERE ALONG
THE SOUTHERN MAINLAND OF ALASKA TUESDAY/WEDNESDAY TIME FRAME. THERE
WILL BE SOME CONNECTION TO TROPICAL MOISTURE...SO WIDESPREAD HEAVY
RAIN WOULD BE A LIKELIHOOD. IN SUMMARY...ALTHOUGH THE DETAILS MAY
BE FUZZY AT THIS POINT...EVERYTHING IS POINTING TOWARD A WET PATTERN
FOR THE SUNDAY THROUGH WEDNESDAY TIME FRAME.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE
MARINE...STORM 119 120
         GALE 130 131 132 138 139 150 165 170 172 173 174
FIRE WEATHER...NONE

AMD/SEB OCT 13