Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 191304
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
504 AM AKDT FRI SEP 19 2014

.ANALYSIS AND UPPER LEVELS...
A MULTI-CENTERED LOW PRESSURE SYSTEM ALOFT SITS OVER THE SOUTHERN
MAINLAND/NORTHERN GULF THIS MORNING. THE LONGER LIVED CENTER OVER
THE SOUTHWEST MAINLAND IS WHAT IS LEFT OVER FROM THE BIG BERING
STORM LAST WEEK AND HAS BEEN ANCHORING THE LONGWAVE PATTERN SINCE
THEN. THE SECOND CENTER IS A POTENT BUT MATURE SHORTWAVE IN THE
NORTHERN GULF. THE INTERACTION OF THESE TWO SYSTEMS IS RESPONSIBLE
FOR ACTIVE WEATHER THE PAST COUPLE DAYS AND WILL CONTINUE FOR THE
NEXT COUPLE AS WELL. AS A WHOLE THIS TROUGH IS VERY HIGH AMPLITUDE
STRETCHING FROM THE BROOKS RANGE SOUTHWARD WITH THE BASE OF THE
TROUGH HITTING 35N LATITUDE. THE JET STREAM EXHIBITS A SPLIT FLOW
PATTERN WITH THE EAST ASIAN JET BEING THE PREDOMINANT DRIVER OF
THE PATTERN THOUGH THE NORTHERN STREAM IS HELPING AMPLIFY THE
CURRENT TROUGH.  FURTHER OUT WEST...A COUPLE FEATURES ARE KEEPING
THE PATTERN FROM BECOMING TOO PROGRESSIVE. AN UPPER LEVEL RIDGE IS
PARKED OVER THE WESTERN/CENTRAL BERING SEA WHILE THE EAST ASIAN
JET IS DEVELOPING A KAMCHAKTA SYSTEM.

FOCUSING ON THE SOUTHCENTRAL...THE UPPER LOW LOOKS TO BE WEAKENING
WHILE THE DEFORMATION ZONE BETWEEN THE TWO UPPER SYSTEMS STRETCHES
FROM THE WESTERN SUSITNA VALLEY THROUGH EASTERN KACHEMAK BAY AND
INTO THE CENTRAL GULF. EMBEDDED DISTURBANCES POLEWARD OF THE GULF
LOW ARE CONTINUING TO FORCE PRECIPITATION OVER THE NORTH GULF
COAST ALONG WITH MECHANICAL LIFT FROM UPSLOPE FLOW.

&&

.MODEL DISCUSSION...
GUIDANCE IN BETTER AGREEMENT TODAY IN HOW THE GULF SYSTEM IS
HANDLED AS IT WEAKENS AND MOVES INLAND. THE GENERAL CONSENSUS IS
THE CLOSED CIRCULATION WILL TURN BACK INTO AN OPEN WAVE WITH
RESPECT TO THE SOUTHWEST CENTER AS IT DEAMPLIFIES. THERE ARE SMALL
DIFFERENCES IN DETAILS IN HOW A NEW DEFORMATION ZONE SETS UP AS
THE UPPER LOW DRIFTS NORTHWEST...THE CURRENT INCARNATION HAD
PRETTY SHARP EDGES ON EITHER SIDE OF THE ZONE SO THE EXACT
LOCATION AND NATURE WILL HAVE IMPLICATIONS ON COVERAGE AND AMOUNT
OF PRECIPITATION. THERE IS GOOD AGREEMENT WITH WIDESPREAD RAIN AS
THE TROUGH SWINGS OVERHEAD TOMORROW NIGHT. NO PARTICULAR SOLUTION
WAS FAVORED IN THE SHORT TERM WITH GUIDANCE ALL ON THE SAME PAGE
BUT THE ECMWF WAS FAVORED WITH A SYSTEM MOVING INTO THE EASTERN
GULF ON SUNDAY AS WELL AS FOR THE KAMCHATKA LOW PRESSURE OUT WEST.

&&

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THE FORECAST TODAY WILL CENTER AROUND
ANOTHER DEFORMATION ZONE SETTING UP OVER THE AREA AND THEN AN
UPPER TROUGH PASSAGE. THE PREVIOUS DEFORMATION IS WEAKENING OVER
THE WESTERN KENAI WHILE AN UPPER DISTURBANCE IS FORCING RAIN OVER
THE MAT-SU AND ANCHORAGE BOWL. STEADY RAIN CAN BE EXPECTED FROM
NINILCHIK NORTHWARD THROUGH THE ANCHORAGE BOWL. RAIN WILL BE MORE
INTERMITTENT IN THE MATANUSKA VALLEY. THE NORTH GULF COAST WILL
SEE A TRANSITION FROM SHOWERY PRECIPITATION BACK TO STEADIER RAIN
IN THE AFTERNOON. AS THE SURFACE LOW PRESSURE MOVES INLAND...AN
AREA OF STOUT PRESSURE RISES MOVES UP THE CENTRAL GULF. THESE
RISES WILL BRING SOME GUSTY WINDS TO CORDOVA...VALDEZ...AND THE
COPPER RIVER IN THE AFTERNOON THEN INTO THE KNIK RIVER VALLEY
TOWARD THE EVENING. THE UPPER TROUGH WILL KEEP RAIN GOING FROM THE
NORTHERN KENAI THROUGH THE SUSITNA VALLEY DURING THE EVENING AND
OVERNIGHT BEFORE WANING TOWARD MORNING. THE AFOREMENTIONED
PRESSURERISES WILL BRING GUSTY SOUTHEASTERLY WINDS THROUGHOUT
THE BOWL LATE THIS EVENING INTO SATURDAY MORNING. SATURDAY SHOULD
BEGIN A DRYING TREND WITH SHOWERS HANGING AROUND THE GULF COAST.
COOLER AIR WILL FILTER IN OVER THE WEEKEND AS A STRONG SYSTEM
BEGINS TO TAKE SHAPE IN THE EASTERN GULF.

SOUTHWEST ALASKA...THE TWO BIGGEST CHANGES FOR THE REGION COMING
OVER THE NEXT COUPLE OF DAYS WILL BE AN END TO THE WET WEATHER AND
COOLING TEMPERATURES FOR THE MAINLAND. THE UPPER LEVEL THAT HAS
LINGERED OVER THE AREA FOR THE PAST SEVERAL DAYS IS NOW POSITIONED
NEAR BETHEL WHILE IT CONTINUES TO WEAKEN. THE LOW WILL DRIFT
SLOWLY SOUTHWARD TOWARDS THE BASE OF THE LONGWAVE TROUGH IN THE
GULF TODAY THROUGH SATURDAY. IT WILL BE REPLACED BY A SHARP RIDGE
THAT IS CURRENTLY TRACKING ACROSS THE BERING SEA. UNTIL THIS RIDGE
ARRIVES LATE SATURDAY THOUGH...WIDESPREAD SHOWERS WILL CONTINUE
TO DOMINATE THE FORECAST. GUSTY NORTH TO NORTHWEST WINDS WILL
ALSO PERSIST...ESPECIALLY FROM THE ALASKA PENINSULA THROUGH THE
WESTERN CAPES.
DRYING WILL START IN EARNEST EARLY SUNDAY. THE RIDGE ORIENTATION
WILL MAKE CONDITIONS FOR MODEST OFFSHORE FLOW FOR THE MAINLAND.
WHILE THERE WILL CERTAINLY BE ABUNDANT MOISTURE LEFT IN THE WAKE
OF ALL THIS RAIN...FOG DEVELOPMENT MAY BE LIMITED BY THESE DRY
OFFSHORE WINDS. THE DRIER AIRMASS AND NORTHEAST FLOW WILL HELP TO
USHER IN SOME COLDER TEMPERATURES SUNDAY MORNING AND ESPECIALLY
EARLY NEXT WEEK.

BERING SEA/ALEUTIANS...


&&

.LONG TERM FORECAST...

BEGINNING THE EXTENDED PERIOD ON MONDAY...A STOUT UPPER LEVEL
BLOCKING HIGH WILL EXIST OVER THE BERING...WITH A TROUGH OVER THE
GULF OF ALASKA AND NORTH PACIFIC. ALONG THE POLEWARD JET EXTENDING
INTO THE EASTERN GULF OF ALASKA...A STRONG LOW WILL BE IN THE
PROCESS OF TRACKING INTO THE COPPER BASIN. EXACTLY HOW FAR WEST
THE PRECIPITATION SHIELD STRETCHES REMAINS UNCERTAIN...BUT
LOCATIONS EAST OF THE CHUGACH AND TALKEETNA MOUNTAINS WILL LIKELY
SEE PERIODS OF RAIN INTO TUESDAY. WESTERN ALASKA WEST OF THE
ALASKA RANGE WILL BE DRY AS OFFSHORE FLOW INCREASES IN RESPONSE TO
THE LOW ACROSS THE GULF. LATE TUESDAY THROUGH THURSDAY WILL
GRADUALLY DRY OUT ACROSS THE COPPER BASIN...WITH LARGELY DRY
CONDITIONS DEVELOPING ACROSS MOST OF SOUTHERN ALASKA AS THE STORM
TRACK SHIFTS SOUTH ACROSS THE NORTH PACIFIC.
&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE 178.
FIRE WEATHER...NONE.

&&

$$

MTL/MO/JA SEP 14


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