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FXAK67 PAJK 171449
AFDAJK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
549 AM AKST WED DEC 17 2014

.SHORT TERM...MINOR UPPER WAVE AND ASSOCIATED SURFACE LOW PRESSURE
LOCATED OVER THE SERN GULF WILL CONTINUE TO DISSIPATE THROUGH THIS
AFTERNOON AS THEY APPROACH THE SRN COAST OF THE PANHANDLE. COLD
CLOUD TOPS ASSOCIATED WITH THE UPPER DISTURBANCE WILL CORRESPOND
TO A SLIGHT INCREASE IN SHOWER ACTIVITY OVER THE SRN HALF OF THE
PANHANDLE THIS AFTERNOON...AND THEN DISSIPATE AS THE WAVE EXITS
INTO BRITISH COLUMBIA.

ATTENTION THEN TURNS TO A STRONG DEEP-LAYER CYCLONE THAT WILL
BECOME POSITIONED JUST S OF THE GULF BY THE END OF THE SHORT TERM
PERIOD /12-15Z THU/. AN OCCLUDED FRONT ASSOCIATED WITH THIS
CYCLONE IS FORECAST TO HAVE ALMOST COMPLETELY WRAPPED AROUND ITS
PARENT LOW...WITH IT/S ESEWD BRANCH APPROACHING THE SERN GULF THU
MORNING. A TIGHT PRESSURE GRADIENT N OF THE FRONT...AND ESELY 925
MB WINDS EXCEEDING 50 KT...WILL FAVOR GALE FORCE WINDS OVER THE
OUTER WATERS BY LATE WED NIGHT. HIGH END SMALL CRAFT GAP WINDS
WILL BEGIN TO DEVELOP IN CROSS SOUND BY THIS POINT AS WELL...AND
30 KT SELY WINDS IN CLARENCE STRAIT ALSO APPEAR PROBABLE. THE
STRONGER 925 MB WINDS ARE ALSO FORECAST TO CLIP LAND ZONE 27 BY
12Z THU...AND MAY SUPPORT GUSTS APPROACHING STRONG WIND CRITERIA.

SHORTWAVE RIDGING WILL OCCUR OVER THE PANHANDLE DOWNSTREAM FROM
THE OFFSHORE CYCLONE. THIS WILL YIELD WEAK LARGE-SCALE ASCENT OVER
THE REGION...THOUGH MODEST 850 MB WAA AHEAD OF THE APPROACHING
FRONT MAY SUPPORT LIGHT PRECIPITATION OVER THE REGION DURING WED
NIGHT...HIGHEST POP/S LOCATED OVER THE SRN PANHANDLE CLOSEST TO
THE FRONT. ONLY EXCEPTION WILL BE IN THE YAKUTAT AREA LATE
TONIGHT...WHERE OFFSHORE BOUNDARY LAYER WINDS WILL PRODUCE AN
ENVIRONMENT TOO DRY FOR PRECIPITATION. OFFSHORE/NLY PRESSURE
GRADIENT LOCATED FROM JUNEAU/GUSTAVUS N TO SKAGWAY IS FORECAST TO
BRING A COOLER LOW-LEVEL AIRMASS SWD INTO THE NRN HALF OF THE
PANHANDLE. THIS WILL FAVOR TEMPERATURES CLOSER TO FREEZING...AND
THUS INCREASE THE CHANCE FOR LIGHT SNOW. HOWEVER...ASCENT WILL BE
VERY WEAK...AND THE ENVIRONMENT WILL NOT BE PARTICULARLY MOIST.
THUS...LITTLE OR NO SNOW ACCUMULATION IS EXPECTED.

.LONG TERM...STORM FORCE FRONT POISED TO MOVE MOVE INTO THE
SOUTHEASTERN GULF LIFTING NORTHWEST THROUGH THURSDAY EVENING. THUS
WE HAVE ISSUED A HIGH WIND WATCH FOR THURSDAY MORNING THROUGH
THURSDAY AFTERNOON FOR WESTERN PRINCE OF WALES ISLAND FOR GUSTS TO
AROUND 60 MPH. GIVEN 65 MPH JET AT 925 MB, THIS WAS NOT A TOUGH
CALL. STRONG WINDS UP TO 50 MPH SHOULD ALSO SPREAD INTO THE
SOUTHERN INSIDE CHANNELS THURSDAY MORNING AND BY MID-DAY FOR
SOUTHEASTERN BARANOF ISLAND. METLAKATLA AND ANNETTE ISLAND WILL
PROBABLY SEE MORE WIND THAN KETCHIKAN WITH THE FRONTAL ORIENTATION
AND TRACK WITHIN ZONE 28. MEANWHILE TO THE NORTH GALE FORCE OUTFLOW
WINDS WILL AFFLICT CROSS SOUND AND NORTHERN LYNN CANAL WITH SOLID
SMALL CRAFTS IN MANY CHANNELS ELSEWHERE. WHILE THE GRADIENT WOULD
SUPPORT OUTFLOW WINDS THROUGH DOWNTOWN JUNEAU, THE SETUP IS
MARGINAL WITH ONLY A MINOR WEAK DIRECTIONAL SHIFT AT THE CRITICAL
LEVEL. BUT GIVEN NEW HIGH RES GUIDANCE SUGGESTING SOME SURFACE
WINDS IN THE VICINITY OF SALISBURY RIDGE WE ALLOWED 15 TO 25 MPH
FOR DOWNTOWN JUNEAU.

THERE IS AN INTERESTING ROLE REVERSAL LOOKING AT THE TWO MOST
USEFUL SOLUTIONS FOR MODELING THIS FRONT. THE NAM, OFTEN USING THE
OFFSHORE WINDS DEVELOPING AHEAD OF SUCH A FRONT, IS GENERALLY
DRIER IN CASES SUCH AS THIS ONE. WHILE THE ECMWF OFTEN SPREADS THE
PRECIPITATION A LITTLE TOO QUICKLY, NOT ALLOWING THE TIME FOR THE
COLUMN TO THE NORTH TO MOISTEN ENOUGH FOR VIRGA TO REACH THE
SURFACE. THE NAM AND THE HIGH RES WERE STRONGER THAN THE ECMWF
REGARDING THE ADVANCEMENT OF PRECIPITATION INTO JUNEAU. GIVEN OFFSHORE
GRADIENT AT THE MID-LEVELS THAT WOULD NATURALLY BE EXPECTED
AHEAD OF A FRONT, THERE SHOULD BE A LEVEL OF ATMOSPHERIC COOLING AS
THE PRECIPITATION FALLS AND EVAPORATES THROUGH THE AIR. SO A
RAIN/SNOW MIX LIKELY FOR MUCH OF JUNEAU AT LEAST BY LATE THURSDAY
AFTERNOON. WE DID EXTEND THE SNOW THROUGH THURSDAY NIGHT FOR THIS
ZONE. AS GRADIENTS AND 925 MB WINDS DO NOT LOOK AS FULL-PROOF ON
THE SOUTHERLIES TURNING THINGS TO ALL RAIN AS QUICKLY AS GUIDANCE
WAS SUGGESTING YESTERDAY. THEREFORE, CONFIDENCE FALLING ON WHEN
EXACTLY A COMPLETE CHANGEOVER TO RAIN WILL OCCUR. IT WOULD BE AN
EASIER CALL IF ONLY A LOW LEVEL JET WOULD CLEARLY MANIFEST ITSELF,
BUT MODELS AT BEST ONLY TEASING WITH A SLIGHT SUGGESTION.
NEVERTHELESS WE DID PULL BACK ON POPS JUST A TAD AS THE FRONT
PULLS WESTWARD AND SOME WEAK RIDGING DEVELOPS AHEAD OF THE NEXT
SYSTEM THAT IS LOOKING A BIT MORE LIKELY FOR SATURDAY. SNOW MAY
NEED TO BE MIXED IN WITH THE RAIN AS LATE AS FRIDAY MORNING FOR
JUNEAU. STAY TUNED. LIKEWISE GUSTAVUS WILL BE A TRICKY CALL AT
TIMES FOR RAIN/SNOW LINE.

YET TWO MORE WAVES LOOK LIKELIER TO GRAZE THE SOUTHERN PANHANDLE
FRIDAY AND SATURDAY, AND THUS KEEPING RAIN IN THE FORECAST--WITH
SNOW LIKELY AROUND LYNN CANAL. LOOKING INTO EARLY NEXT
WEEK...MODELS SEEM TO BE LOWERING TEMPERATURES FOR THE AREA.
LOOKING FOR SUCCESSIVE SIGNALS ON THIS BEFORE WE BEGIN NUDGING IN
THAT DIRECTION.

USED NAM AND ECWMF FOR GUIDANCE ON THURSDAY/THURSDAY NIGHT. FEW
CHANGES BEYOND. FORECAST CONFIDENCE GOOD FOR WINDS
THURSDAY/THURSDAY NIGHT WITH MODEL SPREAD LOWERING CONFIDENCE FOR
NEXT TWO SYSTEMS FRIDAY INTO SATURDAY. CONFIDENCE FALLING FOR
RAIN/SNOW LINE THURSDAY NIGHT/FRIDAY.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...HIGH WIND WATCH FROM THURSDAY MORNING THROUGH THURSDAY AFTERNOON
     FOR AKZ027.
         STRONG WIND FROM THURSDAY MORNING THROUGH THURSDAY AFTERNOON FOR
     AKZ023.
         STRONG WIND FROM THURSDAY MORNING THROUGH THURSDAY AFTERNOON FOR
     AKZ028.
MARINE...GALE WARNING FOR PKZ041>043-051-052.
         SMALL CRAFT ADVISORY FOR PKZ012-013-021-022-031>036.

&&

$$

GARNER/JWA

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