Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 291326 AAA
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ANCHORAGE AK
526 AM AKDT MON SEP 29 2014

.ANALYSIS AND UPPER LEVELS...LARGE SCALE SPLIT FLOW PERSISTS OVER
THE ALASKA REGION. A VERTICALLY STACKED LOW SOUTH OF THE CENTRAL
ALEUTIANS IS THE SHORT-TERM FEATURE IN THE SOUTHERN
STREAM...PRODUCING SCATTERED SHOWERS AND GUSTY WINDS OVER THE
CENTRAL ALEUTIANS AS IT CONTINUES TO TRACK SOUTHEASTWARD. THE MAIN
FOCUS FOR THE SHORT-TERM FORECAST IS THE NORTHERN STREAM TROUGH
CURRENTLY MOVING THROUGH WESTERN ALASKA. CURRENT WATER VAPOR
IMAGERY SHOWS THIS STRONG TROUGH RAPIDLY DIGGING THROUGH THE
WESTERN MAINLAND AND EDGING INTO THE WESTERN PORTION OF
SOUTHCENTRAL...BEING THE MAIN WEATHER PRODUCER OVER THE NEXT 36
HOURS FOR SOUTHCENTRAL ALASKA.

&&

.MODEL DISCUSSION...MODELS REMAIN IN GOOD SYNOPTIC AGREEMENT AND
FORECAST CONFIDENCE IS GENERALLY HIGH. THERE ARE SOME MINOR
DIFFERENCES IN THE POSITIONING AND PROGRESSION OF THE UPPER LOW
FORMING OVER THE NORTHERN GULF TONIGHT...WITH THE NAM BEING A
SLIGHT OUTLIER WITH A FASTER EASTWARD MOVEMENT OF THE AMPLIFYING
UPPER TROUGH. CONFIDENCE CONTINUES TO INCREASE THAT THE NEW
SURFACE LOW WILL DEVELOP NEAR CAPE SUCKLING THIS AFTERNOON AND
QUICKLY FOLLOW THE UPPER LOW OFF TO THE EAST. PRESSURE GRADIENTS
WILL QUICKLY TIGHTEN ALONG THE NORTH GULF COAST AS THIS LOW
DEVELOPS...COINCIDING WITH THE ARRIVAL OF COLD DENSE AIR FROM
INTERIOR ALASKA.

MEANWHILE...THE REMNANTS OF A TROPICAL SYSTEM CURRENTLY EAST OF
JAPAN ARE STILL EXPECTED TO GET ABSORBED BY THE LARGE VERTICALLY
STACKED LOW SOUTH OF THE ALEUTIANS BY TUESDAY NIGHT. NOT
SURPRISINGLY THE MODELS CONTINUE TO STRUGGLE WITH THE EXACT
DETAILS OF THIS INFUSION OF MOISTURE AND ENERGY...BUT FORTUNATELY
THE ENTIRE COMPLEX WILL REMAIN LARGELY SOUTH OF THE FORECAST AREA.
A BROAD-BRUSHED SOLUTION OF THE GFS AND ECMWF GLOBAL SOLUTIONS IS
PREFERRED.

&&

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...WINDS WILL BEGIN TO PICK UP OVER MUCH OF
SOUTHCENTRAL TODAY AS THE ARCTIC TROUGH DROPS SOUTHEASTWARD
THROUGH THE AREA...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. WHILE
WINDS WILL BE VERY STRONG FOR THIS EARLY IN THE
SEASON...CONDITIONS ARE STILL EXPECTED TO REMAIN BELOW WARNING
LEVELS.

CLOUDS WILL DEVELOP LATER THIS MORNING AS STRONG DYNAMIC LIFT
ASSOCIATED WITH THE TROUGH ENTERS THE AREA...BUT BEST CHANCES FOR
PRECIPITATION TO ACTUALLY REACH THE GROUND REMAIN LARGELY FROM THE
MATANUSKA VALLEY NORTHWARD. FURTHER EAST...AS THE SURFACE LOW
BEGINS TO FORM AND PUMP MOISTURE OVER THE COASTAL MOUNTAINS...MORE
WIDESPREAD PRECIPITATION WILL DEVELOP THIS AFTERNOON. PRECIP WILL
NOT LAST OVERLY LONG AS THE BEST LIFT QUICKLY RACES EASTWARD
TONIGHT. HOWEVER...WHAT DOES FALL WILL BE IN THE FORM OF SNOW AT
HIGHER ELEVATIONS OVER THE EASTERN COPPER RIVER BASIN WITH SOME
ACCUMULATIONS LIKELY. AS THE UPPER LEVEL TROUGH EXITS TO THE EAST
TOMORROW AFTERNOON...WINDS WILL BEGIN TO DIMINISH AS THE PRESSURE
GRADIENT QUICKLY LOOSENS UP OVER THE AREA. HIGH PRESSURE WILL THEN
BEGIN TO BUILD IN FROM THE WEST...BRINGING CLEAR AND DRY
CONDITIONS BACK TO THE REGION BY MIDWEEK.

SOUTHWEST ALASKA...A FEW SHOWERS ALONG THE BASE OF THE UPPER LEVEL
TROUGH ARE EXPECTED TO LAST THROUGHOUT THE DAY AND TAPER OFF
TONIGHT...AS THE UPPER TROUGH TRACKS FURTHER EAST. 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
FURTHER INTO THE PACIFIC LATER TONIGHT. OTHERWISE HIGH PRESSURE
WILL BUILD OVER THE CENTRAL 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.


&&

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

&&

$$

TP SEP 14



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