Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 270120
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
420 PM AKST THU FEB 26 2015

.ANALYSIS AND UPPER LEVELS...

THE SOUTHERN MAINLAND REMAINS UNDER A FAIRLY ROBUST RIDGE OF HIGH
PRESSURE THAT EXTENDS NORTHWARD FROM THE PACIFIC. HIGH CIRRUS
CLOUDS ARE SPILLING-OVER THE RIDGE AXIS INTO SOUTHWEST AND
SOUTHCENTRAL...BUT ARE RUNNING INTO STRONG SUBSIDENCE FROM THE
RIDGE THAT IS SIGNIFICANTLY DRYING OUT MUCH OF THE LOWER
ATMOSPHERE (AS EVIDENCED BY 12Z MORNING SOUNDINGS). THE ONLY FLY
IN THE OINTMENT IS THE FACT THAT IT IS THIS VERY SUBSIDENCE THAT
IS CREATING A STRONG LOWER-TROPOSPHERIC INVERSION THAT IS HELPING
TO GENERATE FOG AROUND MUCH OF THE AREA...ESPECIALLY NEAR MOISTURE
SOURCES SUCH AS KNIK ARM.

MEANWHILE...A WEAK AND DISORGANIZED LOW PRESSURE SYSTEM WELL SOUTH
OF THE ALASKA PENINSULA IS MOVING NORTHWARD AND WILL BRING SOME
MORE ACTIVE WEATHER TO SOUTHERN ALASKA DURING THE DAY TOMORROW AND
TOMORROW NIGHT. AN UNRELATED GALE-FORCE FRONT HAS STALLED OVER THE
CENTRAL ALEUTIANS AND CONTINUES TO WEAKEN AS THE PARENT LOW HAS
NOW MOVED NORTHWARD INTO THE RUSSIAN FAR EAST.

&&

.MODEL DISCUSSION...

WITH A SLOWLY-CHANGING LONGWAVE PATTERN IT IS NO SURPRISE THAT THE
MODELS ARE IN OUTSTANDING OVERALL AGREEMENT ON THE PATTERN EVOLUTION
OVER THE NEXT FEW DAYS. MODELS DEPICT A SHORTWAVE TROUGH MOVING
EASTWARD THROUGH ALL OF SOUTHERN ALASKA DURING THE DAY
SATURDAY/SATURDAY NIGHT...BUT THERE ARE SUBTLE DIFFERENCES WITH
HOW FAR SOUTH THE MAIN AREA OF DYNAMIC LIFT WILL SPREAD...OR
WHETHER IT WILL STAY CONFINED LARGELY NORTH OF THE ALASKA RANGE
AND INTO THE INTERIOR. THE ECMWF IS LEADING THE WAY WITH THE LESS-
AMPLIFIED CONFIGURATION THAT KEEPS THE STRONGEST DYNAMICS NORTH OF
OUR FORECAST AREA. THE GFS IS THE STRONGEST VOICE IN THE OPPOSITE
DIRECTION...AND WOULD BRING MORE SIGNIFICANT LIFT TO THE COOK
INLET AREA. WILL PLAN TO STICK CLOSE TO INHERITED FORECAST FOR
TODAY...WHICH DOES BRING A GOOD CHANCE OF SNOW TO THE ANCHORAGE
AREA SAT AFTERNOON/EVENING...FOR A FEW MORE MODEL RUNS TO ALLOW
TIME FOR A MORE COHERENT TREND TO DEVELOP AMONG AVAILABLE
GUIDANCE.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...

GAP WINDS WILL CONTINUE TO DECREASE TONIGHT AS THE TROUGH ALONG
THE NORTH GULF COAST WEAKENS. THE INCREASING SUBSIDENCE AHEAD OF
THE SHORTWAVE RIDGING WILL HELP TO STEEPEN INVERSIONS EVEN MORE
ACROSS THE AREA TONIGHT...WHICH WILL LIKELY LEAD TO EVEN MORE
ROBUST FOG PRODUCTION/DEVELOPMENT TONIGHT AROUND ANCHORAGE AND
UPPER COOK INLET AS WELL AS IN THE COPPER RIVER BASIN. CLOUDS MOVE
IN FROM THE SOUTHWEST FRIDAY AFTERNOON AS THE REMNANTS OF THE
DISORGANIZED LOW SOUTH OF THE AKPEN MOVES INTO THE AREA. MODELS
ARE CURRENTLY ALL IN GOOD AGREEMENT THAT THIS MIDLEVEL FEATURE
WILL MOVE THROUGH THE AREA COMPLETELY DRY...THOUGH THE POSSIBILITY
OF GOOD LOW-LEVEL CONVERGENCE IN WESTERN TURNAGAIN ARM AS WELL AS
RAPIDLY COOLING 850 MB TEMPERATURES IN SOUTHWESTERLY FLOW WOULD
TYPICALLY LEAD TO AT LEAST LOCALIZED PRECIPITATION PRODUCTION.
THUS WILL INTRODUCE SOME LOW POPS TO UPPER COOK INLET AS THESE
FEATURES MOVE THROUGH THE AREA FRI EVENING AND OVERNIGHT INTO
EARLY SATURDAY MORNING.

THE NEXT AND BEST CHANCE FOR SNOW COMES IN FROM THE WEST SATURDAY
AFTERNOON...DRIVEN BY THE SHORTWAVE TROUGH MENTIONED IN THE MODEL
DISCUSSION ABOVE. WILL KEEP LIKELY POPS ADVERTISED FOR
ANCHORAGE...GIVEN THE FACT THAT WHATEVER LIFT DOES COME THROUGH
THE AREA (WHETHER WEAK OR STRONG) WILL BE RELATIVELY CONSOLIDATED
AND SHOULD SUPPORT AT LEAST A SHORT PERIOD OF SNOWFALL TO MUCH OF
THE AREA BEFORE HIGH PRESSURE BUILDS BACK IN AGAIN ON SUNDAY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...

A RIDGE OF HIGH PRESSURE ALOFT WILL GIVE WAY TO A NORTH PACIFIC
LOW LATE FRIDAY AFTERNOON INTO FRIDAY NIGHT. CLOUDS AND MOISTURE
WILL STREAM IN FROM THE SOUTH...WITH RAIN AND SNOW DEVELOPING
ACROSS BRISTOL BAY BEFORE RAPIDLY SPREADING NORTHWARD. ALL SNOW IS
EXPECTED ACROSS THE KUSKOKWIM DELTA AND LOWER KUSKOKWIM
VALLEY...WITH RAIN ALONG THE ALASKA PENINSULA.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...

A SLOW MOVING GALE FORCE FRONT WILL REMAIN OVER THE CENTRAL
ALEUTIANS BEFORE WEAKENING AND MOVING EAST. COLDER AIR WILL FILTER
INTO THE CENTRAL BERING SEA ALONG WITH WEST NORTHWEST WINDS. RAIN
AND SNOW SHOWERS WILL MOVE INTO THE PRIBILOF ISLANDS AND EASTERN
ALEUTIANS AS THE FRONT MOVES EAST...AND A WEAK NORTH PACIFIC LOW
MOVES TOWARD THE ALASKA PENINSULA. A NEAR STORM FORCE LOW WILL
MOVE INTO THE FAR WESTERN ALEUTIANS LATE FRIDAY NIGHT.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
BY SATURDAY EVENING THE FIRST TROUGH IN THIS WESTERLY PROGRESSIVE
PATTERN WILL HAVE CLEARED MOST OF THE SOUTHCENTRAL TO THE EAST.
BEHIND THIS DISTURBANCE...RIDGING WILL BUILD OVERHEAD AS CUT-OFF
LOW PHASES WITH YET ANOTHER WESTERLY TROUGH IN THE NORTHERN
PACIFIC. CONDITIONS WILL REMAIN DRY UNTIL THE FRONT ASSOCIATED
WITH THIS SYSTEM MOVES INTO THE NORTHERN GULF EARLY TO MID
WEEK. GUIDANCE IS UNSURE ABOUT THE TIMING AND DETAILS OF THE PHASE
BUT THE FRONTAL PROGRESSION IS PRETTY HIGH CONFIDENCE FOLLOWED BY
ANOTHER WESTERLY TROUGH PASSAGE. THE PATTERN REMAINS PROGRESSIVE
CONTINUING SLIGHTLY ABOVE NORMAL TEMPERATURES.

&&

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

&&

$$



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