Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 301333
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
533 AM AKDT TUE SEP 30 2014

ANALYSIS AND UPPER LEVELS...WATER VAPOR SATELLITE IMAGERY THIS
MORNING INDICATED THE ARCTIC TROUGH IN SOUTHERN ALASKA HAS CLOSED
OFF INTO A WEAK UPPER LEVEL CIRCULATION...WITH ASSOCIATED SURFACE
CIRCULATION MOVING QUICKLY INTO THE NORTHERN PANHANDLE. AS
EXPECTED...STRONG GAP FLOW AROUND PRINCE WILLIAM SOUND PEAKED
EARLIER THIS MORNING...WITH OBSERVATIONS SHOWING A STEADY DECLINE
IN THOSE WINDS AS THE TROUGH PUSHES OFF FURTHER TO THE EAST.
MEANWHILE...HIGH PRESSURE CONTINUES TO SLOWLY BUILD INTO FROM THE
WEST.

FURTHER WEST...AN UPPER RIDGE PERSISTS OVER THE NORTHERN BERING
SEA...WITH A VERTICALLY STACKED LOW NEARLY STATIONARY WELL SOUTH
OF THE ALEUTIANS. LIGHT RAIN AND GUSTY WINDS ON THE FRINGES OF
THIS SYSTEM ARE BEING OBSERVED ACROSS THE EASTERN HALF OF THE
ALEUTIAN CHAIN.

&&

MODEL DISCUSSION...THE MODELS REMAIN IN GOOD SYNOPTIC AGREEMENT AT
BOTH THE SURFACE AND UPPER LEVELS THROUGHOUT MUCH OF THE SHORT-
TERM (84 HOUR) FORECAST PERIOD. HOWEVER...DIFFERENCES DO ARISE
EARLY THURSDAY MORNING SOUTH OF THE GULF WITH THE HANDLING OF THE
NEXT LOW COMPLEX. THE MODELS GENERALLY SHOW THE REMNANTS OF AN OLD
TROPICAL SYSTEM (KAMMURI) WITH AMPLE MOISTURE BECOMING ABSORBED BY
THE LOW CURRENTLY SOUTH OF THE CENTRAL ALEUTIANS. AS EXPECTED THE
MODELS CONTINUE TO STRUGGLE WITH THIS COMPLEX SYSTEM AS IT TRACKS
SLOWLY NORTHEASTWARD TOWARD THE SOUTHERN GULF. THE FORECAST
CHALLENGE WITH THIS SYSTEM WILL BE IF AND WHERE A TRIPLE POINT
CIRCULATION FORMS ON THURSDAY...AND HOW DEEP/STRONG IT WILL GET.
WITH THIS BEING AT THE END OF THE SHORT TERM AND HAVING SUCH MODEL
SPREAD AT THAT TIME FRAME...FORECAST CONFIDENCE REMAINS LOW
REGARDING THE KEY DETAILS OF THIS EVOLUTION. THE GEM GLOBAL WAS
THE PREFERRED SOLUTION OVERALL WITH THIS SYSTEM AND IS FAIRLY
CLOSE TO THE GFS ENSEMBLE MEAN AND WPC SOLUTIONS.

PRIOR TO 60 HOURS...THE NAM WAS MAINLY UTILIZED OVER THE EAST
DOMAIN TO CONTINUE CAPTURING THE ONGOING GAP WINDS. THE ECMWF
SOLUTION WAS PREFERRED OVER THE WESTERN DOMAIN FOR THE MORNING
PACKAGE FOR SUPERIOR RUN-TO-RUN CONSISTENCY.

&&

.SHORT TERM FORECAST...
SOUTHCENTRAL ALASKA...SUBSIDENCE BEHIND THE DEPARTING UPPER LEVEL
WAVE COUPLED WITH TIGHT PRESSURE GRADIENTS LED TO GUSTY
CONDITIONS AND STRONG GAP FLOW OVER PRINCE WILLIAM SOUND AND PARTS
OF SOUTHCENTRAL OVERNIGHT. WINDS HAVE NOW PEAKED AT MANY
LOCATIONS...AND WILL GRADUALLY DIMINISH THROUGHOUT THE DAY TODAY
AS THE UPPER LEVEL TROUGH CONTINUES TO MOVE AWAY. PRECIPITATION
ASSOCIATED WITH THIS FEATURE IS LIMITED TO THE FAR EASTERN COPPER
RIVER BASIN...AND EVEN THIS WILL RAPIDLY DIMINISH BY LATER THIS
MORNING AS COLDER AND DRIER AIR MOVES IN FROM THE WEST.

SOUTHWEST ALASKA...CONDITIONS WILL BE RELATIVELY DRY ACROSS
THE SOUTHWEST MAINLAND OVER THE NEXT COUPLE OF DAYS AS HIGH
PRESSURE REMAINS OVER THE AREA. AN ONSHORE COMPONENT OF THE WINDS
ACROSS THE KUSKOKWIM DELTA WILL KEEP THE POTENTIAL FOR A LITTLE
FOG AROUND THE AREA THROUGH THIS EVENING. HOWEVER...INCREASINGLY
OFFSHORE FLOW BY LATE THIS AFTERNOON WILL LIKELY HAMPER ANY FOG
FORMATION/ADVECTION OVER THE MAJORITY OF THE AREA OVER THE NEXT
DAY OR SO. MEANWHILE...BRISK (BUT MOSTLY DRY) EASTERLY FLOW
ASSOCIATED WITH A LOW COMPLEX OVER THE NORTH PACIFIC WILL CONTINUE
ACROSS THE AK PEN THROUGH WED.

BERING SEA/ALEUTIANS...RAIN AND WIND WILL CONTINUE TO JUST BARELY
BRUSH THE ALEUTIAN ISLANDS AS THE LOW CONTINUES TO SLOWLY TRACK
TO THE EAST. OTHERWISE HIGH PRESSURE WILL BUILD OVER THE CENTRAL
BERING SEA AND CONTINUE TO STRENGTHEN THROUGH MID
WEEK...CONTINUING THE SPLIT-FLOW BLOCKING PATTERN AND PROVIDING
VERY QUIET CONDITIONS FOR MOST OF THE REGION.

&&

.LONG TERM FORECAST (FRI OCT 3 - MON OCT 6)...

ON FRIDAY...A HIGHLY AMPLIFIED PATTERN WILL BE DEVELOPING ACROSS
THE REGION...WITH A BLOCKING RIDGE ACROSS THE CENTRAL BERING
EXTENDING INTO SIBERIA...AND A LARGE TROUGH ACROSS THE MAINLAND.
ARCTIC AIR WILL ACCOMPANY THE TROUGH...WITH STEADILY DROPPING
TEMPERATURES HEADING INTO THE WEEKEND. THE PATTERN WILL GENERALLY
BE DRY...BUT THERE IS THE THREAT FOR SNOW AND SNOW SHOWERS ACROSS
MOST OF SOUTHCENTRAL BY THE WEEKEND...ESPECIALLY ACROSS THE COPPER
BASIN. THERE REMAINS UNCERTAINTY WITH A LOW MOVING THROUGH THE
EASTERN GULF OF ALASKA...SO A CONSERVATIVE MIDDLE GROUND APPROACH
WAS TAKEN WITH THE PRECIPITATION FORECAST. BY MONDAY...UNCERTAINTY
IN THE PATTERN GROWS IN MAGNITUDE QUICKLY AS THE TRACK OF
NORTHWEST PACIFIC CYCLONES (POSSIBLY INCLUDING RECURVING REMNANTS
OF PACIFIC TROPICAL CYCLONES) MOVING INTO THE BERING BECOMES
HIGHLY ERRATIC...THUS RESULTING IN SYNOPTIC SCALE DISAGREEMENT
AMONG THE VARIOUS GLOBAL NUMERICAL MODELS. WITH THAT SAID...THE
COLDEST TEMPS OF THE FALL CAN BE EXPECTED BY THIS WEEKEND.


&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING...119 127 150 155 172 174.
FIRE WEATHER...NONE.

&&

$$

TP SEP 14


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