Area Forecast Discussion
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000
FXAK67 PAJK 201415
AFDAJK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
515 AM AKST SAT DEC 20 2014

.SHORT TERM...CYCLONIC FLOW OVER THE GULF CONTINUES THROUGH
SATURDAY WITH WAVES OF VORTICITY AND AN UPPER LEVEL TROUGH PASSING
ACROSS THE REGION. THESE ARE WORKING TO DEVELOP A SURFACE LOW
WHICH WILL TRACK TO THE NORTH CENTRAL GULF BY TONIGHT. GALE FORCE
WINDS ARE ACCOMPANYING THIS LOW ON THE NE SIDE AND ARE CAUSING GUSTY
WINDS TO THE OUTER COASTAL AREAS AND OUT OF INTERIOR PASSES.

WINDS THROUGH THE CENTRAL INSIDE WATERS WILL SHIFT TO THE SOUTH
LATE THIS AFTERNOON AS THE LOW AND THE FRONT PROGRESS NORTHWARD.
LATE TONIGHT WINDS WILL DIMINISH WITH THE WEAKENING PRESSURE
GRADIENT AND COULD EVEN SHIFT BACK TO THE NORTH AS YET ANOTHER
WAVE DEVELOPS IN THE CYCLONIC FLOW AND MOVES INTO THE SOUTHERN
OUTSIDE WATERS. THIS WILL DEPEND ON HOW STRONG THE NEXT WAVE ENDS
UP BEING...THE LAST COUPLE HAVE BEEN STRONGER WITH EACH NEW MODEL
RUN SO IF THAT TREND CONTINUES STRONGER WINDS AND A DIRECTION
SHIFT MAY NEED TO BE CONSIDERED.

TODAY`S LOW DOES HAVE SOME WARMER TEMPERATURES WITH IT...BUT THAT WAA
IS WEAK AND LIMITED TO THE SOUTHERN OUTER AREAS. OTHERWISE THERE
DOESN`T LOOK TO BE MUCH OF A DIURNAL TEMPERATURE FLUCTUATION
TODAY. THE NORTHERN INNER CHANNELS WILL CONTINUE TO HAVE NORTH
WINDS WHICH WILL KEEP TEMPERATURES COLD ENOUGH FOR SNOW. THIS
EXTENDS AS FAR SOUTH AS THE MENDENHALL VALLEY...HOWEVER THE LOW IS
ALSO CAUSING AN OFFSHORE COMPONENT TO THE WINDS OVER THESE AREAS
WHICH IS LIMITING THE ONSET OF PRECIPITATION AND QPF SO NOT
EXPECTING MUCH ACCUMULATION (UP TO AN INCH TODAY AND ANOTHER INCH
TONIGHT).

AT THE OTHER END OF THE SPECTRUM...THE PERSISTENT SOUTHERLY ONSHORE
FLOW OVER THE SOUTHERN ZONES ALONG WITH THE WEAK WAA HAS KEPT
TEMPERATURES ABOVE 40F DEGREES OVERNIGHT FROM SITKA SOUTH (WITH
EXCEPTION OF HYDER). THESE VALUES WILL LIKELY BE NEAR THE HIGH FOR
TODAY AS NOT EXPECTING MUCH VARIATION WITH OVERCAST SKIES AND
RAIN.

00Z AND 06Z MODEL RUNS HAD A STRONGER/MORE DEVELOPED LOW FOR TODAY
AND THE NAM12 INITIALIZED FAIRLY WELL IN THAT RESPECT...THUS WAS
USED FOR BASIS OF UPDATES. FORECAST CONFIDENCE WITH POP OVER THE
NORTH IS BELOW NORMAL. MOST MODELS HAVE THE PRECIP OVER THE
NORTHERN ARES ALREADY THIS MORNING...BUT THE OFFSHORE FLOW HAS
RESULTED IN VIRGA THUS FAR. OTHERWISE FORECAST CONFIDENCE IS
AVERAGE.

.LONG TERM...A WEAKENING LOW SHOULD MOVE INTO THE SE GULF SUN
MORNING AND CROSS THRU CENTRAL SE AK SUN AFTERNOON. A LOW WILL
REMAIN OVER THE WRN GULF SUN-MON. ANOTHER LOW WILL MOVE NE INTO
THE SE GULF MON NIGHT OR TUE...BUT MODELS DIFFER ON STRENGTH AND
TRACK SOMEWHAT. THEY WERE ALL STRONGER THAN PREVIOUS FORECAST
HOWEVER. PATTERN CHANGE SHOULD BEGIN LATE IN THE WEEK AND CONTINUE
INTO THE WEEKEND AS A LARGE UPPER RIDGE BUILDS INTO THE AREA...BUT
THERE ARE CRITICAL MODEL DIFFERENCES ON WHERE THE RIDGE SETS
UP...WITH THE ECMWF SHOWING IT FURTHER W THAN THE GFS. ENDED UP
USING A BLEND OF THE 00Z GFS AND ECMWF FOR SUN-MON...THEN
BASICALLY USED LATEST WPC FOR MON NIGHT ONWARD.

THE LOW FOR SUN APPEARS TO BE STRONG ENOUGH TO DIMINISH PRECIP FOR
A TIME OVER THE NRN PANHANDLE SUN MORNING. DOES LOOK LIKE SOME
PRECIP WILL MOVE BACK INTO THE PAJN AREA SUN AFTERNOON THOUGH AS
LOW TRIES TO DRIFT A LITTLE MORE TO THE N AS IT CROSSES SE AK. THE
FAR NRN AREAS WILL HAVE LOWEST PRECIP POTENTIAL SUN. ONCE LOW IS
INLAND...A DEEP LAYER ONSHORE FLOW AND COOLER AIR ALOFT WILL MEAN
SHOWERS FOR THE AREA SUN NIGHT AND MON MORNING. THINK THEY WILL BE
MOSTLY RAIN SHOWERS BUT THERE COULD BE SOME SNOW SHOWERS OVER THE
FAR NRN AREA.

STRONGER LOW SHOULD SPREAD PRECIP NE INTO THE SE GULF MON...THEN
LIKELY INTO THE SRN 2/3RDS OF SE AK MON NIGHT. THE MODEL
DIFFERENCES MAKE PLACEMENT OF THE NRN EDGE OF THE PRECIP TRICKY
THOUGH...SO LOWER FORECAST CONFIDENCE FOR THE NRN THIRD. THE GFS
KEEPS THE LOW OVER THE GULF...WHILE THE ECMWF AND NAM TAKE IT
ACROSS SE AK TUE AFTERNOON AND EVENING. THIS DIFFERENCE WILL
AFFECT PRECIP POTENTIAL AND TYPE OVER THE NRN THIRD OF THE AREA
FOR TUE-TUE NIGHT. THE WIND FORECAST IS EVEN MORE TRICKY AS THE
GFS WOULD SUGGEST STRONG TO POTENTIALLY HIGH WINDS FOR ALL BUT THE
FAR NRN PANHANDLE...WHILE THE ECMWF WOULD LIMIT STRONG WIND
THREAT TO THE SRN AREA...WITH MINIMAL THREAT FOR HIGH WINDS. USING
WPC TRENDED WINDS STRONGER FOR MON NIGHT AND TUE SOME. STILL
PLENTY OF TIME FOR LATER SHIFTS TO HONE IN ON THIS SYSTEMS
TRACK AND STRENGTH.

BY MIDWEEK...MAIN UPPER TROF WILL BE WEAKENING ACROSS THE AREA.
WILL HAVE SOME SMALLER SYSTEMS TO DEAL WITH FOR WED-FRI...AND
MODELS DIFFER ON THESE. WILL KEEP IN THE MIDDLE GROUND POPS FOR
THE AREA FOR WED-FRI AT THIS POINT. COULD BE MORE SNOW IN THE AREA
AS SOME SLIGHTLY COOLER AIR WILL BE OVER THE PANHANDLE BY
WED...WITH BEST THREAT FOR ANY SNOW OVER THE NRN THIRD.

BY NEXT WEEKEND THE LARGE SCALE RIDGE SHOULD BE DOMINATING THE
AREA BUT THE DIFFERENT PLACEMENT BY THE MODELS MAKES FORECAST
TRICKY. THE ECMWF WOULD MEAN MUCH COLDER AIR WITH TEMPS BELOW
AVERAGE...WHILE THE GFS WOULD MEAN ONLY A LITTLE COLDER AIR WITH
MORE NORMAL TEMPS. STAY TUNED...

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...STRONG WIND UNTIL NOON AKST TODAY FOR AKZ025.
         STRONG WIND THROUGH THIS AFTERNOON FOR AKZ018.
         STRONG WIND UNTIL 9 AM AKST THIS MORNING FOR AKZ028.
MARINE...GALE WARNING FOR PKZ012-022-035-036-041>043-051-052.
         SMALL CRAFT ADVISORY FOR PKZ011-013-021-031>034-053.

&&

$$

FERRIN/RWT

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