Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 161303
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
403 AM AKST TUE DEC 16 2014

.ANALYSIS AND UPPER LEVELS...
A LONG-WAVE TROUGH IS CENTERED OVER THE BERING SEA AND NORTH
PACIFIC WITH A SEMI-PERMANENT DOWNSTREAM RIDGE OVER WESTERN
CANADA. A SPOKE OF THIS RIDGE EXTENDS WESTWARD INTO NORTHERN
ALASKA. THUS...THE MAIN STORM TRACK IS EASTWARD ACROSS THE NORTH
PACIFIC THEN NORTHWARD UP THE WEST WIDE OF THE RIDGE INTO MAINLAND
ALASKA.

THERE IS A HOST OF GENERALLY WEAK FEATURES EMBEDDED WITHIN THE
UPPER FLOW WITH FAIRLY BENIGN CONDITIONS ACROSS ALASKA FOR THIS
TIME OF YEAR. THE ONE EXCEPTION IS A FAIRLY COMPACT GALE FORCE LOW
WHICH IS TRACKING NORTHWARD ACROSS THE CENTRAL GULF AND HEADED
TOWARD SOUTHCENTRAL ALASKA. OUT AHEAD OF THIS LOW STRONG EASTERLY
FLOW ACROSS THE KENAI AND CHUGACH MOUNTAINS HAS LARGELY MAINTAINED
DRY CONDITIONS FROM THE WESTERN KENAI PENINSULA UP TO ANCHORAGE
AND THE MAT-SU...WITH THE EXCEPTION OF A FEW BRIEF PERIODS OF RAIN
OR FREEZING RAIN AS UPPER WAVES MOVE THROUGH AND TEMPORARILY
WEAKEN THE FLOW ACROSS THE MOUNTAINS. THE APPROACHING LOW COULD
PROVIDE ANOTHER ONE OF THOSE BRIEF SPILL-OVERS OF PRECIP. THE
CHALLENGE IS IN DETERMINING WHETHER THERE WILL BE ENOUGH
PRECIPITATION TO WARRANT A FREEZING RAIN ADVISORY FOR ANCHORAGE OR
THE SUSITNA VALLEY (WHERE SURFACE TEMPERATURES REMAIN AT OR BELOW
FREEZING). DISSIPATION OF THE CROSS MOUNTAIN FLOW WILL ALSO ALLOW
LOW LEVEL TEMPERATURES TO COOL...BUT IN THE ABSENCE OF COLD AIR
ADVECTION IT WILL TAKE QUITE A WHILE TO COOL THE ANCHORAGE
SOUNDING ENOUGH FOR ALL SNOW.

THE SECOND IMPORTANT PIECE TO THE SOUTHCENTRAL ALASKA FORECAST IS
A TROUGH WHICH EXTENDS FROM NORTHWEST TO SOUTHEAST ALONG THE
SOUTHWEST ALASKA COAST AND INTO THE WESTERN GULF. THIS TROUGH IS
LIFTING NORTHWARD AND BRINGING WITH IT SOME MARGINALLY COLDER
AIR. THIS WEAK COLD ADVECTION SHOULD BE ENOUGH TO TRANSITION
PRECIP BACK TO MOSTLY SNOW ACROSS THE SOUTHERN MAINLAND OF ALASKA
OVER THE NEXT 12 TO 18 HOURS.

&&

.MODEL DISCUSSION...
WITH THE EXCEPTION OF THE GFS SOLUTION...MODELS HAVE COME INTO MUCH
BETTER AGREEMENT WITH THE TRACK AND STRENGTH OF THE LOW OVER THE
GULF. THE BIGGER QUESTION IS NOW THE DEPICTION OF THE TROUGH
MOVING IN BEHIND THE LOW. PRIOR SOLUTIONS INDICATED A NICE CLEAN
TROUGH AXIS WITH LOW LEVEL SOUTHWESTERLY FLOW (FAVORABLE FOR
PRECIP IN ANCHORAGE AND THE MAT-SU). THE MODELS NOW DEVELOP A WAVE
OVER THE CENTRAL GULF WHICH CHANGES THE ORIENTATION OF THE TROUGH
AS IT MOVES THROUGH SOUTHCENTRAL AND RESULTS IN LOW LEVEL SOUTHERLY
(OR EVEN SOUTHEASTERLY) FLOW. ALTHOUGH THERE ARE DIFFERENCES IN
THE DETAILS...WILL BUY INTO THIS NEW DEPICTION. THIS CREATES A
MUCH MORE CHALLENGING QPF FORECAST...SO FORECAST CONFIDENCE FOR
TUESDAY AFTERNOON THROUGH TUESDAY NIGHT IN THE POPULATION CENTERS
OF SOUTHCENTRAL IS NOW BELOW AVERAGE.

ELSEWHERE...MODELS ARE IN GOOD AGREEMENT WITH LARGE SCALE FEATURES
THROUGH WEDNESDAY AND FORECAST CONFIDENCE IS GENERALLY ABOVE
AVERAGE.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
FREEZING RAIN WILL REMAIN A THREAT TO ANCHORAGE AND THE SUSITNA
VALLEY THROUGH EARLY THIS MORNING AS A WEAK UPPER LEVEL WAVE MOVES
THROUGH. RIGHT NOW DO NOT THINK PRECIP WILL BE WIDESPREAD ENOUGH TO
WARRANT A FREEZING RAIN ADVISORY...BUT IT IS A VERY TOUGH CALL. AS
THE GULF LOW REACHES THE KENAI PENINSULA THIS AFTERNOON THE STRONG
CROSS MOUNTAIN FLOW WILL COME TO AN END AND THE COLUMN WILL BEGIN
TO COOL. THUS...BY THE TIME PRECIP ARRIVES AHEAD OF THE UPPER TROUGH
THIS EVENING EXPECT PRECIP TO BE MAINLY IN THE FORM OF SNOW. THERE
WILL BE AN AREA OF STRONGER LIFT AHEAD OF AN AMPLIFYING WAVE ALONG
THE TROUGH...WHICH WILL PRODUCE LOCALLY HEAVIER SNOWFALL. THE
PRINCE WILLIAM SOUND AREA SEEMS TO BE BEST POSITIONED TO SEE A
BRIEF PERIOD OF HEAVIER PRECIP...BUT THERE IS A POSSIBILITY SOME
OF THIS MAY MAKE IT INTO ANCHORAGE AND THE MAT VALLEY. FOR NOW
WILL STICK WITH MINOR SNOW ACCUMULATION...WHICH IS WHAT MOST OF
THE REGION WILL SEE AS THIS FAIRLY WEAK TROUGH MOVES THROUGH.

THE MODELS ARE NOW RESOLVING ANOTHER WEAK UPPER TROUGH APPROACHING
SOUTHCENTRAL ON WEDNESDAY. THIS COULD PROLONG SNOW IN SOME
AREAS...THOUGH IT WILL BE LIGHT. LOW LEVEL FLOW WILL BEGIN TO
SHIFT AROUND TO THE NORTH LATE WEDNESDAY AS A LARGE AND DEEP
PACIFIC LOW APPROACHES THE GULF. THIS WILL LEAD TO A DRYING AND
COOLING TREND.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
A SERIES OF WEAK UPPER LEVEL TROUGHS WILL MOVE THROUGH SOUTHWEST
ALASKA THE NEXT COUPLE DAYS BRINGING NO MORE THAN A FEW SHOWERS.
AIR MASS HAS BEEN COOLING OVERNIGHT WITH THE 12Z SOUNDING FROM
KING SALMON ALMOST ALL BELOW FREEZING...SO PRECIP TYPE WILL BE
PRIMARILY SNOW. WITH BREAKS IN THE CLOUDS IN BETWEEN SHORT-WAVES
CONDITIONS ARE FAVORABLE FOR FOG.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE ALEUTIANS AND SOUTHERN ALASKA PENINSULA WILL BE ON THE
NORTHERN PERIPHERY OF SYSTEMS TRACKING ACROSS THE NORTH PACIFIC
AND ON THE SOUTHERN PERIPHERY OF A NEARLY STATIONARY LOW OVER THE
NORTH-CENTRAL BERING SEA. THUS...EXPECT CONTINUED VERY BENIGN
WINTER-TIME CONDITIONS.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
THE LARGE SCALE PATTERN WILL REMAIN ESSENTIALLY THE SAME OVER THE
NEXT WEEK WITH PROGRESSIVE FLOW ACROSS THE NORTH PACIFIC AND A
DOWNSTREAM BLOCK FORCING STORMS NORTHWARD INTO THE GULF WHERE
THEY WILL WEAKEN. THIS MEANS WARMER AIR WILL ONCE AGAIN MOVE INTO
THE SOUTHERN MAINLAND OF ALASKA...AND FREEZING RAIN WILL REMAIN A
POSSIBILITY WITH EACH OF THESE SYSTEMS. MODELS ARE ALL INDICATING
AMPLIFICATION OF THE FLOW OVER THE NORTH PACIFIC AND BERING SEA
EARLY NEXT WEEK AS A DEEP TROUGH MOVES OUT OF ASIA. THIS WOULD
BRING A RETURN TO MORE WINTER-LIKE CONDITIONS ACROSS THE BERING
AND ALEUTIANS.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...STORM WARNING...119.
         GALE WARNING...125 179 185.
         HEAVY FREEZING SPRAY WARNING...185.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SEB
LONG TERM...SEB



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