Small Craft Advisory 705, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
454 PM AKDT TUE SEP 8 2015

.ANALYSIS AND UPPER LEVELS...
DYNAMIC UPPER LEVEL FLOW WILL LEAD TO VERY ACTIVE WEATHER OVER THE
NEXT COUPLE DAYS. AN UPPER LEVEL LOW HAS FORMED JUST EAST OF THE
PRIBILOF ISLANDS AT THE CENTER OF A LONG-WAVE TROUGH CENTERED OVER
THE BERING SEA. A POLAR JET IS DIVING SOUTHWARD ACROSS THE WEST-
CENTRAL BERING SEA CAUSING A SHORT-WAVE NEAR THE WESTERN ALEUTIANS
TO DIG SOUTHEASTWARD ACROSS THE NORTH CENTRAL PACIFIC. THIS IN
TURN IS CAUSING A STRONG SUBTROPICAL JET (130KT+) CROSSING THE
NORTH PACIFIC TO BEGIN TO BUCKLE. THESE ARE TEXTBOOK CONDITIONS
FOR SURFACE CYCLOGENESIS WHICH IS LIKELY ALREADY UNDER WAY. THE
DEVELOPMENT AND INTENSIFICATION OF THIS STORM SYSTEM WILL BE THE
FOCUS FOR HIGH IMPACT WEATHER OVER THE NEXT COUPLE DAYS.

IN THE MEANTIME...THE LARGER SCALE TROUGH IS PROGRESSING NORTHEASTWARD
FROM THE EASTERN BERING INTO THE SOUTHERN MAINLAND OF ALASKA. A
SERIES OF SHORT-WAVES ROTATING INTO A VERY MOIST WEAKLY UNSTABLE
ENVIRONMENT IS LEADING TO AREAS OF SHOWERS OR RAIN ACROSS SOUTHWEST
AND SOUTHCENTRAL ALASKA. ON THE BACK SIDE OF THE UPPER TROUGH OVER
THE CENTRAL BERING...WHAT HAD BEEN A STATIONARY FRONT IS NOW
HEADED SOUTHWARD AS A WEAK ARCTIC FRONT. RAIN IS FOCUSED OUT AHEAD
OF THE ARCTIC FRONT WITH SHOWERS AND GUSTY NORTHERLY WINDS MOVING
IN BEHIND IT.

&&

.MODEL DISCUSSION...
THE BIGGEST FORECAST CHALLENGE PRIOR TO DEVELOPMENT OF THE STRONG
SURFACE LOW IS TIMING AND LOCATION OF PRECIP OVER THE SOUTHERN
MAINLAND AS A SERIES OF UPPER LEVEL WAVES ROTATE THROUGH THE
REGION. FORTUNATELY...VERY CLOSE SPACING OF WAVES MEANS PERIODS
OF RAIN WILL WORK FOR MOST PLACES. THE AVIATION TERMINAL FORECASTS
ARE MUCH MORE DIFFICULT WITH LARGE VARIATIONS IN CEILING AND
VISIBILITY AS EACH WAVE MOVES THROUGH...SO CONFIDENCE IS LOW IN
THAT REGARD.

THE GENERAL FORECAST LOOKS TO BE ON TRACK WITH THE RAPIDLY
DEEPENING LOW TONIGHT AND WEDNESDAY. ALL SOLUTIONS HAVE NOW LOCKED
ON TO THE LOW FULLY OCCLUDING (AND BOTTOMING OUT) AROUND 980 MB
NEAR THE SOUTH END OF SHELIKOF STRAIT WEDNESDAY EVENING. MINOR
DIFFERENCES STILL REMAIN BEYOND THIS POINT WITH TRACK OF THE LOW
NORTHWARD. THE RUN-TO-RUN TREND IN THE MODELS IS FOR A MORE
WESTWARD SOLUTION...WITH THE MAIN LOW CENTER CROSSING THE ALASKA
PENINSULA INTO INTERIOR BRISTOL BAY EARLY THURSDAY MORNING. THE
NAM IS THE NOTABLE OUTLIER TO THIS IDEA IN KEEPING THE MAIN LOW
CENTER IN THE KODIAK ISLAND/BARREN ISLANDS AREA. THE INHERITED
FORECAST LARGELY CAPTURED THE BIGGER PICTURE IMPACTS OF THIS
SYSTEM. CHANGES TODAY WILL LARGELY CONSIST OF FINE-TUNING OF THE
MANY DETAILS OF THE WINDS WITH THIS SYSTEM. THE STRENGTH OF GAP
FLOW...PARTICULARLY VERY CLOSE TO THE LOW CENTER IS VERY SENSITIVE
TO SMALL CHANGES IN LOW TRACK. THUS...THIS FINE-TUNING PROCESS
WILL BE ONGOING AS THE LOW GETS CLOSER AND WE ARE ABLE TO REALLY
LOCK IN ON THE EXACT TRACK. BASED ON THIS REASONING...OVERALL
FORECAST CONFIDENCE IS MODERATE TO HIGH...WHILE CONFIDENCE IS MUCH
LOWER WITH FORECAST DETAILS NEAR THE LOW CENTER.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
WIDESPREAD RAIN WILL CONTINUE ACROSS MUCH OF SOUTHCENTRAL AK
THROUGH TONIGHT AS SEVERAL WEAK DISTURBANCES MOVE THROUGH THE
ALREADY MOIST ENVIRONMENT IN PLACE IN BROAD SOUTHWEST FLOW.
ATTENTION THEN TURNS TO THE DEVELOPING LOW SOUTH OF THE AKPEN AS
IT APPROACHES THE SOUTHERN END OF KODIAK ISLAND WEDNESDAY EVENING
AND THEN MOVES INTO INTERIOR PORTIONS OF SOUTHWEST AK BY THURSDAY.
AN ASSOCIATED FRONT WILL MOVE THROUGH THE GULF ON WEDNESDAY AND
BRING POTENTIALLY HEAVY RAIN ALONG WITH GALE FORCE WINDS TO AREAS
ALONG THE NORTH GULF COAST THROUGH WEDNESDAY NIGHT. MEANWHILE...RAIN
WILL TAPER OFF TO SCATTERED SHOWERS BY WEDNESDAY MORNING OVER
INLAND LOCATIONS AS CROSS BARRIER FLOW INCREASES IN ADVANCE OF
THE FRONT...WITH WINDS PICKING UP THROUGH TURNAGAIN ARM AND OTHER
FAVORED GAPS AS THE PRESSURE GRADIENT BEGINS TO TIGHTEN. RAIN WILL
CONTINUE THROUGH THURSDAY IN ONSHORE FLOW ALONG THE COAST AND WILL
ALSO REDEVELOP OVER INLAND LOCATIONS AS THE SURFACE LOW MOVES
INLAND AND UPPER LEVEL FLOW SHIFTS BACK TO THE SOUTHWEST. THIS
WILL ALSO ALLOW FOR A SOUTHERLY PUSH OF WINDS ALONG THE GULF COAST
AND THROUGH THE COOK INLET REGION AS PRESSURES BEGIN TO RISE
BEHIND THE DEPARTING SYSTEM.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
A COUPLE OF WEAK WEATHER SYSTEMS WILL CROSS SOUTHWEST ALASKA
TONIGHT. THESE WILL CONTINUE TO PRODUCE SHOWERS THROUGH TONIGHT. A
MORE SIGNIFICANT WEATHER SYSTEM WILL SPIN UP SOUTH OF THE ALASKA
PENINSULA WEDNESDAY MORNING. THE LEADING FRONT FOR THIS SYSTEM
WILL MOVE INTO THE AREA FROM THE SOUTHEAST ON WEDNESDAY. THE LOW
ITSELF WILL SLIDE SLOWLY NORTH-NORTHEAST AND BE NEAR KING SALMON
THURSDAY MORNING. A "WRAP AROUND" OCCLUDED FRONT WILL DEVELOP ON
THE WEST SIDE OF THE LOW BY WEDNESDAY AFTERNOON...APPROXIMATELY
SITUATED FROM BRISTOL BAY THROUGH THE ALASKA PENINSULA. THIS
OCCLUSION WILL BREAK APART ON THURSDAY AS THE MAIN LOW MOVES NORTH
INTO THE MAINLAND. THIS MAIN LOW WILL CONTINUE TO DRIFT SLOWLY
NORTH AND WEAKEN INTO THURSDAY EVENING.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE WEAK LOW OVER THE EASTERN BERING SEA WILL SLIDE SOUTH AND
WEAKEN TONIGHT. THE RESPONSIBLE UPPER TROUGH WILL CROSS THE
EASTERN ALEUTIANS BY WEDNESDAY AFTERNOON. FLOW ALOFT OVER THE
BERING AND ALEUTIANS WILL BE GENERALLY FROM THE NORTH WEDNESDAY
AFTERNOON THROUGH THURSDAY AFTERNOON. WITH THE LOW MOVING INTO
SOUTHWEST ALASKA AND A RIDGE OVER THE WESTERN BERING SEA WINDS
WILL GENERALLY BE FROM THE NORTH AND BREEZY.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
TROPICAL SYSTEMS KILO AND JIMENA ARE BIG QUESTION MARKS FOR THIS
FORECAST. TYPHOON KILO WILL GO EXTRA-TROPICAL AND JOIN THE ARCTIC
JET LATER THIS WEEK...THEN WORK ITS WAY INTO THE WESTERN BERING
SEA BY THE END OF THE WEEKEND THEN EASTWARD INTO MAINLAND ALASKA
EARLY NEXT WEEK. NUMERICAL MODELS ARE STRUGGLING WITH THE DETAILS
ON THIS. IT IS ALSO POSSIBLE THAT MOISTURE FROM TROPICAL STORM
JIMENA WILL JOIN THE "PARTY" AS THE LEADING FRONT REACHES WESTERN
ALASKA...PROBABLY LATE MONDAY OR SOMETIME EARLY TUESDAY. BEST BET
IS TO USE THE ENSEMBLE FORECASTS. WPC PREFERS THE ECMWF ENSEMBLE
AND WENT VERY STRONGLY IN THAT DIRECTION. MODELS SHOULD CATCH ON
OVER THE NEXT DAY OR TWO...AND THEN MORE DETAIL CAN BE ADDED TO
THE FORECAST.

MEANWHILE...EARLIER IN THE FORECAST THE UPPER LOW OVER NORTHERN
ALASKA WILL SLIDE GRADUALLY NORTHWARD OVER THE WEEKEND. THIS WILL
ALLOW THE FLOW ALOFT TO FLATTEN (BECOME MORE WESTERLY AND LESS
CYCLONIC). THE RESULT WILL BE A BRIEF BREAK FROM THE WET/WINDY
WEATHER IN THE SHORT TERM PORTION OF THE FORECAST.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE 119 125 130 131 132 150 155 160 165
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RLF
LONG TERM...BL