Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 282155
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
155 PM AKDT THU AUG 28 2014

.ANALYSIS AND UPPER LEVELS...
LARGE UPPER LEVEL LOW COMPILEX OVER ALASKA...ARCTIC...AND NORTH
PACIFIC. THERE ARE CURRENTLY THREE CLOSED LOWS TO MENTION. ONE IS
OVER THE ARCTIC...ONE OVER THE GULF OF ALASKA AND ONE JUST WEST OF
SHEMYA. THESE ALL HAVE ASSOCIATED SURFACE LOWS AND LOTS OF
MOISTURE. IN ADDITION...THERE IS A MODERATE SHORT WAVE TROUGH
SWINGING AROUND THE ARCTIC LOW INTO SOUTHWEST ALASKA TODAY. THIS
IS SPARKING SOME CONVECTION...NO LIGHTNING YET. THE LOW OVER THE
GULF OF ALASKA IS BRINGING MODERATE WINDS TO THE COAST AND THROUGH
TURNAGAIN ARM. ANCHORAGE AREA IS SEEING A TYPICAL DOWNSLOPE
PRECIPITATION PATTERN...BUT HAVE SOME SPRINKLES DUE TO HOW MOIST
THE AIR MASS HAS BEEN.

&&

.MODEL DISCUSSION...
THERE IS GREAT AGREEMENT WITHIN THE MODEL GUIDANCE OVER THE STATE
OF ALASKA. THE BIGGER QUESTION COMES AT THE UPPER LEVELS TOWARD
SATURDAY AND SUNDAY WITH A STRONGER SHORT WAVE TROUGH COMING OUT
OF THE NORTH ACROSS THE BERING SEA. THE QUESTION WITH THIS
FEATURE IS THE DEVELOPMENT OF A 500MB CLOSED LOW OR NOT. ECMWF
DOES NOT CLOSE A LOW AND KEEPS THE FLOW AROUND THE LARGE UPPER LOW
OVER THE MAINLAND QUICKER. WHEREAS...THE NCEP MODELS CLOSE A LOW
IN THE SOUTHERN PORTION OF THE BERING SEA. WE ARE LEANING TOWARD
THIS SOLUTION AT THE MOMENT.

ESSENTIALLY...THERE IS A LARGE UPPER LEVEL LOW OVER THE ARCTIC TO
MOVE LITTLE OVER THE NEXT FEW DAYS EXPECT TO DRIFT CLOSER TO THE
ALASKAN NORTH COAST. A SERIES OF SHORT WAVE TROUGHS WILL PUSH
THROUGH THE STATE THROUGH THE WEEKEND KEEPING THE AREA A BIT BELOW
NORMAL FOR TEMPERATURES. THIS FEATURE IS HANDLED WELL BY ALL
GUIDANCE. THE UPPER AND ASSOCIATED SURFACE LOW IN THE GULF OF
ALASKA TO SWING EAST TONIGHT AND TOMORROW IS HANDLED THE SAME BY
ALL MODELS AS WELL. IN RELATION...THE UPPER AND ASSOCIATED SURFACE
LOW OVER THE WESTERN ALEUTIANS CURRENTLY TO SWING INTO THE BERING
TONIGHT AND TOMORROW AND THEN PUSHED ALONG WITH A STRONG JET
THROUGH THE AKPEN AND SOUTH OF THE GULF OF ALASKA SATURDAY AND
SUNDAY. THE NEXT SWING OF A SHORT WAVE TROUGH AROUND THIS LOW IS
THE DEBATABLE ONE AS PREVIOUSLY MENTIONED.

GUIDANCE IS IN
GREAT AGREEMENT WITH THE EVOLUTION OF SYNOPTIC FEATURES IN THE
SHORT TERM. NOW THAT THE INCOMING GULF SYSTEM HAS BROUGHT THE
NORTHERN STREAM TO OUR SOUTH...A STRONG COLD CORE LOW OVER THE
ARCTIC OCEAN IS ANCHORING THE LONGWAVE PATTERN. THE CLOSED OFF
KAMCHATKA LOW PRESSURE WILL DRIFT EAST AND PHASE WITH THE NORTHERN
STREAM AS IT WEAKENS. THERE ARE VERY SLIGHT POSITIONAL DIFFERENCES
AS THE SYSTEM PHASES INTO AN OPEN WAVE BUT FOR THE MOST PART
GUIDANCE SEEMS TO HAVE A GOOD HANDLE ON THE PATTERN. THE WEST
DOMAIN FAVORED THE CANADIAN REGIONAL FOLLOWED BY A GFS/ECMWF BLEND
TOWARD THE END OF THE SHORT TERM. THE EASTERN DOMAIN FAVORED THE
NAM/CANADIAN REGIONAL TRANSITIONING TO THE ECMWF.

&&

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...
THE UPPER LEVEL LOW AND ASSOCIATED SURFACE LOW IN THE GULF OF
ALASKA IS PUSHING A LOT OF PRECIPITATION TO THE COASTAL LOCATIONS
WITH MODERATE BARRIER JET WINDS AND GAP WINDS ALONG TURNAGAIN ARM.
WINDS FROM THIS LOW WILL DIMINISH TONIGHT INTO TOMORROW AS THE
FRONT WEAKENS ALONG THE COAST. THE PRECIPITATION IS NOT GOING TO
DIMINISH THOUGH AS THE UPPER TROUGH SWINGING THROUGH SOUTHWEST
ALASKA ENHANCES THE LIFT FROM WEST TO EAST TONIGHT AND TOMORROW.
THE PRECIPITATION CAN BE MODERATE TO HEAVY AT TIMES. IN
ADDITION...THE COOLER AIR ARRIVES FRIDAY. WE ANTICIPATE SOME SNOW
TO BE AROUND HIGHER ELEVATIONS...ABOVE 2500 FEET...IN THE NORTHERN
COPPER RIVER BASIN...NORTH OF GULKANA...LATE FRIDAY NIGHT AND
SATURDAY MORNING AS THE TROUGH PUSHES THROUGH THE AREA. THE TROUGH
PUSHING THROUGH KEEPS A LOW IN THE NORTHERN GULF THROUGH SATURDAY.
WITH THIS SWITCH IN PRESSURE GRADIENT CHANGING ON FRIDAY WITH THE
TROUGH PASSAGE...NORTH AND WEST GAP WINDS WILL BECOME ENHANCED TO
MODERATE LEVELS. THIS IS ESPECIALLY TRUE FOR WHITTIER GLACIER WIND
GUSTING ABOVE 30 MPH POSSIBLY FRIDAY NIGHT AND SATURDAY AND
POSSIBLY INTO SUNDAY. PRECIPITATION WILL BE CONFINED TO EAST OF
THE CHUGACH SATURDAY AND DWINDLE FROM THERE SATURDAY NIGHT AS THE
LOW IN THE GULF WEAKENS AND THE TROUGH MOVES EAST OF THE AREA.

SOUTHWEST ALASKA...
A SHORT WAVE TROUGH WITH SOME MODERATE LIFT AND INSTABILITY IS
PRODUCING A GOOD AMOUNT OF CONVECTIVE SHOWERS OVER SOUTHWEST
ALASKA TODAY. THERE WILL BE ENOUGH BREAKS OF SUN TODAY TO INITIATE
SOME THUNDERSTORMS THROUGH THE KUSKOKWIM VALLEY. WITH HIGH
PRESSURE AT THE SURFACE MOVING OVER THE ST LAWRENCE ISLAND AREA
TONIGHT INTO FRIDAY AND A LOW IN THE GULF...LIGHT TO MODERATE WEST
TO NORTHWEST WINDS TO REMAIN OVER SOUTHWEST ALASKA TONIGHT AND
TOMORROW. THE HIGH MOVES CLOSER TO THE COAST FRIDAY NIGHT AND
SATURDAY FOR AN OFFSHORE FLOW FOR SOUTHWEST ALASKA. AS SKIES
CLEAR...COLDER AIR THAT WAS USHERED IN BY THE PASSAGE OF THE
TROUGH TONIGHT WILL DROP OVERNIGHT TEMPS TO NEAR 10 DEGREES BELOW
NORMAL INTO TO 20S AND 30S. THE NEXT SHORT WAVE TROUGH MOVING
AROUND THE LARGE UPPER LOW SWINGS INTO THE AREA SUNDAY AFTERNOON
WITH AN INCREASE IN RAIN AND SOUTHWEST WINDS TO THE KUSKOKWIM
DELTA.

BERING SEA/ALEUTIANS...
THE LOW PRESSURE WITH THE STACKED UPPER LEVEL LOW OVER THE
WESTERN ALEUTIANS WILL QUICKLY TRANSITION INTO THE BERING SEA
TONIGHT RIDING A STRONG JET THAT SITUATES ALONG THE ALEUTIAN
CHAIN. AN ASSOCIATED FRONT WILL WEAKEN AS THE LOW MOVES QUICKLY.
THE LOW BECOMES MORE BROAD AS IT MOVES INTO THE CENTRAL BERING AS
THE JET ENERGY STAYS TO THE SOUTH. THE LOW SPLITS A BIT WITH A
PIECE REMAINING IN THE CENTRAL BERING THROUGH SATURDAY AS THE MAIN
PORTION OF THE LOW GETS SWEPT IN THE BROAD NORTHWEST FLOW FROM THE
MAIN UPPER LOW IN THE ARCTIC. THE WEAK LOW IN THE THAT REMAINS
ALSO IS UNDER THE INFLUENCE OF THE BROAD NORTHWEST FLOW ALOFT AND
SWINGS SOUTH ACROSS THE CHAIN SATURDAY NIGHT INTO SUNDAY.

&&

.LONG TERM FORECAST...
A BROAD UPPER LEVEL LOW CENTERED OVER THE CHUKCHI SEA WILL BE THE
PRIMARY WEATHER MAKER FOR THE MAINLAND FROM THIS WEEKEND TROUGH
THE MIDDLE OF NEXT WEEK. A SERIES OF WAVES FROM THIS LOW WILL
BEGIN TO DRAG SOME OF THE COLDEST AIR OF THE SEASON INTO THE
SOUTHERN MAINLAND FOR LATE THIS WEEKEND. THIS UPPER LOW WILL THEN
SLOWLY DESCEND TOWARDS THE CENTRAL INTERIOR BY THE MIDDLE OF NEXT
WEEK...KEEPING MUCH OF THE SOUTH MAINLAND GENERALLY CLOUDY AND
COOL WITH A CHANCE OF RAIN. OVER THE BERING...THE PATTERN WILL
BECOME MUCH LESS ACTIVE AND PROGRESSIVE LATE THIS WEEKEND AS
RIDGING SLOWLY BUILDS IN...WHICH NOW APPEARS AS IF IT WILL HOLD
INTO THE MIDDLE OF NEXT WEEK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...SPS FOR COLD WEEKEND.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

$$

JN AUG 14


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