Storm Warning 20, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
524 AM AKDT WED SEP 9 2009

.SHORT TERM...THE 12Z SATELLITE AND RADAR REVEAL A RAPIDLY
SHEARING APART MID LEVEL TROUGH LOCATED ACROSS THE CENTRAL
PANHANDLE WITH THE ASSOCIATED SURFACE LOW 50 NM SOUTHWEST OF
SITKA. THIS SURFACE LOW AND ASSOCIATED MID LEVEL TROUGH WILL BE
THE MAIN FORCING MECHANISMS FOR THE CONTINUED PERIODS OF RAIN
ACROSS THE PANHANDLE TODAY. WHILE THE SYSTEM AS A WHOLE WILL
WEAKEN THROUGH THE DAY AND MOVE INTO THE NORTHERN GULF...THE
850 MB WINDS WILL BECOME MORE SOUTH TO SOUTHWESTERLY DURING THE
DAY INCREASING THE ONSHORE FLOW ESPECIALLY NORTH OF FREDERICK
SOUND AND SO EXPECT CONTINUED LIGHT TO MODERATE RAINFALL THROUGH
THE DAY. BLENDED INHERITED GRIDS TOWARD A 00Z UKMET/GFS SOLUTION
THIS MORNING AS THE 00Z UKMET INITIALIZED THE DEVELOPING STORM
FORCE LOW IN THE NORTHEASTERN PACIFIC THE BEST AND THIS LOW WILL
MOVE INTO THE GULF AT THE BEGINNING OF THE LONG TERM. THE 00Z GFS
HAD THE BEST REPRESENTATION OF THE SHEARING APART MID LEVEL
TROUGH CURRENTLY OVER THE PANHANDLE. FORECAST CONFIDENCE IS HIGH
THROUGH THE SHORT TERM AS 00Z MODEL AGREEMENT WAS GOOD ON THE
WEAKENING SYSTEM TODAY AND THE DEVELOPMENT OF THE STORM FORCE LOW
IN THE NORTHEASTERN PACIFIC TONIGHT.

 FOR TODAY PERIODS OF RAIN WILL CONTINUE ACROSS THE PANHANDLE
WITH THE HEAVIEST RAINFALL AMOUNTS NORTH OF FREDERICK SOUND WHERE
THE ONSHORE FLOW STRENGTH AND DEPTH WILL INCREASE THROUGH THE DAY
WITH THE INCREASED SOUTHERLY 850 MB WINDS. WINDS THIS MORNING
WILL REMAIN BELOW 20 KNOTS ACROSS THE GULF AND INNER CHANNELS AS
THE SURFACE PRESSURE GRADIENT DECREASES THROUGH THE DAY.
NORTHERLY WINDS ACROSS THE INNER CHANNELS WILL BECOME SOUTHERLY
INTO THE AFTERNOON AS A WEAK SURFACE RIDGE BUILDS ALONG THE
EASTERN PANHANDLE.

 FOR TONIGHT ALL EYES WILL BE ON THE DEVELOPING STORM FORCE LOW
IN THE NORTHEASTERN PACIFIC. A WEAK MID LEVEL RIDGE WILL MOVE
OVER THE PANHANDLE TONIGHT CAUSING THE RAINFALL TO BECOME MORE
SCATTERED EXCEPT ACROSS THE EASTERN INNER CHANNELS WHERE THE
850 MB FLOW WILL REMAIN SOUTHERLY NEAR 25 KNOTS. THE FRONTAL
BOUNDARY ASSOCIATED WITH THE STORM FORCE LOW IN THE NORTHEASTERN
PACIFIC WILL MOVE INTO THE EASTERN GULF BY LATE TONIGHT BRINGING
GALE FORCE WINDS TO 40 KNOTS. THE 850 MB WINDS WILL BECOME MORE
SOUTHEASTERLY THROUGH THE NIGHT ACROSS THE SOUTHERN PANHANDLE AS
THE LLJ DEVELOPS AND WINDS WILL INCREASE TO 60 KNOTS BY EARLY
THURSDAY MORNING AT 850 MB. WIND ADVY GUSTS WILL BEGIN ALONG THE
SOUTHERN OUTER COAST SHORTLY BEFORE HAND OFF WITH LONG TERM.
ELSEWHERE ACROSS THE PANHANDLE EAST-WEST ORIENTED CHANNEL WINDS
WILL INCREASE TO SMCR BY LATE TONIGHT WITH THE RAPIDLY INCREASING
SURFACE PRESSURE GRADIENT IN THE EASTERN GULF AND BREEZY
CONDITIONS WILL DEVELOP NEAR BARANOF...KUIU AND CHICHAGOF
ISLANDS.

.LONG TERM...STRONG LOW WILL CONTINUE N INTO THE SW GULF
THU...THEN WEAKEN AND DRIFT N TO THE FAR N-CENTRAL GULF BY SAT.
THE 00Z UKMET WAS THE PREFERRED MODEL FOR THIS SYSTEM. THE
ASSOCIATED COMPLEX FRONTAL SYSTEM WILL MOVE TO THE SRN OUTER COAST
OF SE AK BY 00Z FRI...THEN LIFT NE ACROSS THE AREA THU NIGHT. THIS
WILL BRING STRONG WINDS AND LOCALLY HEAVY RAINS TO MUCH OF THE
AREA THU INTO THU EVENING. HAVE ISSUED WIND ADVYS FOR ALL BUT THE
FAR NRN INNER CHANNEL ZONES AND PAYA AREA. NORMALLY DO NOT LIKE TO
ISSUE 3RD/4TH PERIOD ADVYS BUT GIVEN THE SIZE AND STRENGTH OF THE
SYSTEM...CONFIDENCE IN ADVY LEVEL WINDS IS VERY HIGH. WITH 850
WINDS OF 50-70KT EXPECTED AHEAD OF THE FRONT...SFC WINDS GUSTING
TO AT LEAST 40-50 MPH SHOULD OCCUR FOR A 6-12 HR PERIOD PRIOR TO
FROPA. AS LOW TRACKS TOWARD THE N-CENTRAL GULF FRI AND SECOND WRAP
OF OCCLUDED FRONT MOVES ONSHORE...STRONGER WINDS SHOULD ARRIVE IN THE
PAYA AREA AND IT IS POSSIBLE THAT AN ADVY WILL BE NEEDED THEN.

THE LOW SHOULD WEAKEN OVER THE FAR NRN GULF FRI NIGHT THEN LIKELY
DISSIPATE BY LATE SAT. NEXT SYSTEM WILL BE A DIGGING TROF THAT
WILL LIKELY TRIGGER ANOTHER LOW DEVELOPMENT OVER THE NE PAC. IT IS
LOOKING MORE LIKELY THAT AS THIS TROF DIGS AND TURNS NEGATIVELY
TILTED...OFFSHORE FLOW WILL DEVELOP IN LOWER LEVELS SUN OR MON AND
DIMINISH PRECIP THREAT. MODELS HAVE BEEN ALL OVER THE PLACE ON
THIS SYSTEM THOUGH SO WILL KEEP CHANCE POPS AT LEAST FOR SUN INTO
MIDWEEK. ENDED UP USING THE HPC GRIDS FOR THIS SYSTEM AS A
COMPROMISE SOLUTION HERE. ALL IN ALL...LOOKS LIKE FALL HAS ARRIVED
IN FULL FORCE TO THE AREA.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WIND ADVISORY FROM 6 AM TO 6 PM AKDT THURSDAY FOR AKZ026-028-029.
         WIND ADVISORY FROM 3 AM TO 3 PM AKDT THURSDAY FOR AKZ027.
         WIND ADVISORY FROM 9 AM TO 6 PM AKDT THURSDAY FOR AKZ024.
         WIND ADVISORY FROM NOON TO 9 PM AKDT THURSDAY FOR AKZ020-025.
         WIND ADVISORY FROM 9 AM TO 9 PM AKDT THURSDAY FOR AKZ021-022.
         WIND ADVISORY FROM 6 AM TO 3 PM AKDT THURSDAY FOR AKZ023.
MARINE...GALE WARNING FOR PKZ033-036-041>043-051.
         SMALL CRAFT ADVISORY FOR PKZ021-022-031-032-034-035-052.

PSS/RWT