Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 282147
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
147 PM AKDT SUN SEP 28 2014

.ANALYSIS AND UPPER LEVELS...LARGE SCALE SPLIT FLOW CONTINUES OVER
THE ALASKA REGION. IN THE SOUTHERN STREAM A COUPLE OF MATURE
VERTICALLY STACKED LOWS ARE AFFECTING A SMALL PORTION OF THE
FORECAST AREA. ONE OVER THE SOUTHERN GULF OF ALASKA WITH TROUGH
EXTENDING NORTHWARD TO THE ALASKA/CANADA BORDER IS EXITING TO THE
EAST WITH JUST A FEW SHOWERS LEFT IN THE EASTERN COPPER RIVER
BASIN. ANOTHER STRONGER LOW SOUTH OF THE ALEUTIANS IS TRACKING
VERY SLOWLY TO THE EAST. SATELLITE SCATTEROMETER DATA CONTINUES TO
SHOW MINIMUM GALE FORCE WINDS ALONG THE OCCLUDED FRONT SOUTH OF
THE ALEUTIAN CHAIN. WATER VAPOR IMAGERY SHOWS A STRONG SHORT- WAVE
MOVING INTO THE BASE OF THE TROUGH RESULTING IN AMPLIFICATION.
THIS WILL EVENTUALLY CAUSE THE LOW TRACK TO CURVE TOWARD THE SOUTH
AWAY FROM THE ALEUTIANS.

MEANWHILE...THE FOCUS FOR THE SHORT-TERM FORECAST IS THE NORTHERN
STREAM TROUGH WHICH IS DIGGING OUT OF THE ARCTIC AND INTO WESTERN
ALASKA THIS MORNING.

&&

.MODEL DISCUSSION...MODELS REMAIN IN GOOD SYNOPTIC AGREEMENT AND
FORECAST CONFIDENCE IS GENERALLY HIGH. THERE ARE SOME MINOR
DIFFERENCES IN ORIENTATION/AMPLITUDE OF DIGGING TROUGH AS IT MOVES
THROUGH SOUTHCENTRAL INTO THE NORTHERN GULF. NAM IS THE OUTLIER
WITH A MUCH MORE ELONGATED AND POSITIVELY TILTED WAVE. THE TREND
IN THE OTHER SOLUTIONS IS TOWARD A SLIGHTLY LESS CONCENTRATED/AMPLIFIED
WAVE. THIS WILL SLOW THE DEVELOPMENT OF UPPER LOW AND CAUSE THE
SURFACE LOW TO DEVELOP FARTHER EAST. THE IMPLICATIONS FOR THE
FORECAST ARE A BIT WEAKER SURFACE PRESSURE GRADIENTS IN THE PRINCE
WILLIAM SOUND AREA (MARGINALLY WEAKER WINDS) AND LESS PRECIP IN
THE COPPER RIVER BASIN. WILL FINE TUNE THE WINDS BASED ON THESE
MODEL TRENDS AND BACK OFF ON QPF OVER THE COPPER RIVER BASIN.

SIGNIFICANT MODEL DIFFERENCES ARISE OVER THE NORTH PACIFIC TUE
NIGHT AND WED WITH HANDLING OF THE REMNANTS OF TROPICAL STORM 17W.
FORTUNATELY THIS ONLY AFFECTS THE FORECAST FOR THE FAR WESTERN
ALEUTIANS. WILL USE A FAIRLY BROAD-BRUSH AVERAGE OF THE GFS/ECMWF
FOR NOW.

&&

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...EXPECT A LITTLE BIT OF WIND JUST ABOUT
EVERYWHERE MONDAY INTO MONDAY NIGHT AS ARCTIC TROUGH DROPS
SOUTHWARD ACROSS SOUTHCENTRAL WITH THE HIGHEST WINDS THROUGH AND
NEAR MOUNTAIN GAPS. AS IS TYPICALLY THE CASE THE STRONGEST WINDS
BY FAR WILL BE ALONG PRINCE WILLIAM SOUND AND THE NORTH GULF
COAST. BASED ON THE TREND MENTIONED IN THE MODEL DISCUSSION ABOVE
CONFIDENCE IS NOW MUCH GREATER THAT WINDS WILL REMAIN SUB-WARNING
LEVEL THROUGHOUT THIS EVENT. CURRENT FORECAST HAS A PRETTY GOOD
HANDLE ON THE WINDS SO WILL JUST BE FINE-TUNING TIMING AND SPEEDS
FOR THE AFTERNOON PACKAGE.

WITH A LESS CONCENTRATED SHORT-WAVE CHANCE OF PRECIP IS A LITTLE
HIGHER OVER SOUTHCENTRAL AS THE TROUGH MOVES THROUGH...ESPECIALLY
FROM THE TALKEETNA MOUNTAINS INTO THE COPPER RIVER BASIN. TROUGH
WILL STILL BE BARRELING RIGHT ALONG A FAST CLIP SO ANY SNOW THAT
DOES FALL WILL BE VERY BRIEF. THE ONE EXCEPTION IS THE EASTERN
COPPER RIVER BASIN WHICH STILL HAS A SHOT AT SEEING SOME MINOR
SNOW ACCUMULATION MONDAY NIGHT. THIS EVENT WILL WRAP UP QUICKLY
TUESDAY NIGHT AS THE UPPER TROUGH EXITS TO THE EAST AND PRESSURE
GRADIENTS QUICKLY SLACKEN WITH HIGH PRESSURE BUILDING IN. MID WEEK
WILL SEE A RETURN TO CLEAR AND DRY WEATHER IN WHAT IS TURNING OUT
TO BE ONE OF THE NICEST STRETCHES OF WEATHER IN YEARS FOR LATE
SEPTEMBER/EARLY OCTOBER TIME FRAME.

SOUTHWEST ALASKA...THERE ARE A FEW SHOWERS OUT AHEAD OF THE UPPER
TROUGH FROM THE KUSKOKWIM VALLEY INTO THE FAR WESTERN KUSKOKWIM
VALLEY. AS TROUGH DIGS SOUTHEASTWARD ACROSS INTERIOR SOUTHWEST
ALASKA TONIGHT EXPECT SHOWERS TO BECOME A LITTLE MORE NUMEROUS
OVER THE KUSKOKWIM VALLEY. HIGH PRESSURE WILL RAPIDLY BUILD IN
BEHIND THIS TROUGH MONDAY AND PROVIDE DRY AND SEASONABLY COOL
TEMPERATURES THROUGH MUCH OF THE WEEK.

BERING SEA/ALEUTIANS...RAIN AND WIND WILL HOLD ON ALONG THE
CENTRAL ALEUTIANS UNTIL THE LOW TO THE SOUTH FINALLY DROPS
SOUTHWARD MONDAY NIGHT. OTHERWISE HIGH PRESSURE WILL BUILD OVER
THE HEART OF THE BERING SEA AND CONTINUE TO STRENGTHEN THROUGH MID
WEEK...SETTING UP A BLOCKING PATTERN AND PROVIDING VERY QUIET
CONDITIONS FOR MOST OF THE REGION. THE ONE EXCEPTION WILL BE THE
WESTERN ALEUTIANS ON THE UPSTREAM SIDE OF THE RIDGE WHICH WILL
SEE IMPACTS FROM A SYSTEM PASSING TO THE WEST MID WEEK.

&&

.LONG TERM FORECAST (THU OCT 2ND - SUN OCT 5)...

ON THURSDAY...A CLOSED BLOCKING HIGH OVER THE BERING WITH A RIDGE
AXIS EXTENDING INTO EASTERN SIBERIA WILL FORCE A CROSS POLAR FLOW
PATTERN TO DEVELOP...WITH A STRENGTHENING ARCTIC TROUGH DEVELOPING
OVER THE MAINLAND. DISTURBANCES IN NORTH TO NORTHWEST FLOW WILL
TRACK INTO THE AREA...WITH PROGRESSIVELY COLDER AIR PUSHING INTO
THE REGION. 850 HPA TEMPERATURES (APPROXIMATELY 5000 FEET) WILL
BE APPROACHING -4 TO -8 CELSIUS BY SUNDAY...WHICH WILL EVENTUALLY
BRING THE COLDEST TEMPERATURES OF THE YEAR TO SOUTHERN ALASKA.
SOME UNCERTAINTY DEVELOPS LATE FRIDAY AND SATURDAY WITH THE
DEVELOPMENT OF A COLD UPPER LEVEL VORTEX OVER SOUTHERN ALASKA AND
THE NORTHERN GULF...WITH THE ECMWF AND CANADIAN GDPS MUCH MORE
BULLISH WITH THE SCENARIO THAN THE PROGRESSIVE GFS. SUCH A
SOLUTION WOULD INCREASE SNOW CHANCES...ESPECIALLY ACROSS THE
COPPER BASIN. THE FORECAST WILL TAKE A MIDDLE GROUND APPROACH AND
WILL CONTINUE TO MENTION SNOW CHANCES ACROSS PORTIONS OF
SOUTHCENTRAL BY THE WEEKEND.

- AHSENMACHER

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING...119 121 127 129 130 131 174 175 176 177.
FIRE WEATHER...NONE.

&&

$$

SEB/AHSENMACHER SEP 14


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