Small Craft Advisory 448, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
232 PM AKDT TUE AUG 7 2012

.SHORT TERM...THE LOW PRESSURE SYSTEM IN THE CENTRAL GULF WILL
WEAKEN THROUGH THE EVENING. THE ASSOC FRONT HAS SHIFTED SLIGHTLY
TODAY GIVING THE INNER PANHANDLE A BIT OF A BREAK FROM THE RAIN
WITH JUST PERIODS TODAY. THE UPPER LEVEL LOW WILL DRIFT TO THE
SOUTHEAST THIS EVENING AND ALLOW FOR A SLIGHT OFFSHORE FLOW OVER
THE NORTHERN PANHANDLE...LIMITING RAINFALL AMOUNTS EVEN FURTHER.
AT THE SAME TIME ONSHORE FLOW WILL INCREASE OVER THE SOUTH AND
BRING INCREASED CHANCES OF RAIN INTO THOSE AREAS.

 AT THE SURFACE A WAVE ALONG THE FRONTAL BAND WILL DEVELOP INTO A
NEW LOW CENTER AND MOVE NORTH INTO THE SOUTHEASTERN GULF EARLY
THIS EVENING. THE NEW LOW WILL HELP TO ENHANCE OFFSHORE FLOW OVER
THE NORTH AND INCREASE CHANCES OF RAIN OVER THE SOUTH AS THE
FRONT DEVELOPS MORE NEAR THE LOW CENTER. THE LOW WILL CONTINUE
NORTH ALONG THE EASTERN GULF THROUGH TONIGHT THEN WEAKEN IN THE
NORTH CENTRAL GULF THROUGH WEDNESDAY. THE FRONTAL BAND WILL MOVE
NORTH WITH IT AND BRING RAIN BACK TO THE CENTRAL PANHANDLE
WEDNESDAY AFTERNOON. THE UPPER LEVEL LOW WILL START TO TURN BACK
TO THE NORTHEAST AND BE JUST OFF THE CAPE DECISION AREA WEDNESDAY
AFTERNOON AS WELL. THIS ALONG WITH THE SURFACE FRONT WILL ENHANCE
RAINFALL FURTHER ALONG THE CENTRAL COASTAL AREAS FOR THE SECOND
HALF OF WEDNESDAY.

 AS THE LOW DEVELOPS OVER THE SOUTHEAST GULF THE PRESSURE
GRADIENT BETWEEN IT AND A RIDGE OF HIGH PRESSURE OVER HECATE
STRAIT WILL CAUSE WINDS TO INCREASE TO 25KT OVER THE OUTSIDE
WATERS. THE RIDGE WILL BUILD INTO THE PANHANDLE FROM THE SOUTH AS
THE LOW MOVES NORTH WEDNESDAY AND INCREASE THE SOUTHERLY PRESSURE
GRADIENT THROUGH THE INSIDE WATERS. HAVE GONE WITH A SMALL CRAFT
ADVISORY FOR CLARENCE STRAIT LATER TONIGHT THROUGH TOMORROW. ALSO
INCREASED WINDS THROUGH ICY STRAIT AND CROSS SOUND AS THE LOW IN
THE GULF INCREASES THE EASTERLY WINDS THROUGH THERE WITH 25KT
EXPECTED NEAR THE OCEAN ENTRANCE. NORTHERN LYNN CANAL WILL GO
FROM CALM OR LIGHT NORTH WINDS TO SOUTHERLY 25KT NEAR CHILKOOT
AND TAIYA INLETS BY LATE AFTERNOON DUE TO THE BUILDING RIDGE.

 USED MAINLY THE NAM12 AND GFS FOR UPDATES TODAY. BIGGEST
FORECAST CHALLENGE WAS TIMING OF RAINFALL. FORECAST CONFIDENCE IS
AVERAGE.

 &&

 .LONG TERM...SOMEWHAT DAMP BEGINNING TO THE EXTENDED PERIOD AS
THE DEVELOPING LOW CURRENTLY WEST OF HAIDA GWAII MOVES NORTH INTO
THE NORTH CENTRAL GULF BY THURS TRAILING A FRONT INTO THE
PANHANDLE. SOME SMALL DIFFERENCES SHOWING UP IN THE MODELS WITH
REGARD TO THE LOW TRACK ONCE IT GETS INTO THE GULF THAT WILL HAVE
AN EFFECT ON WIND FORECASTS IN THE GULF ON THURS. THE GFS APPEARS
TO BE THE OUTLIER IN THIS INSTANCE TRACKING THE LOW INTO THE
CENTRAL GULF. THE NAM IS ALSO SOMEWHAT OF AN OUTLIER BUT IN A
DIFFERENT DIRECTION PREFERRING A LOW IN THE NORTHEAST GULF ON
THURS. THE CONSENSUS IS CENTERED AROUND THE UKMET, ECMWF, AND THE
CANADIAN GEM MODELS SENDING THE LOW UP TO JUST SOUTH OF CAPE
SUCKING. PREFERRED THE CONSENSUS AS IT LOOKED TO BE FAIRLY
CONSISTENT WITH THE PREVIOUS FORECAST AND PREVIOUS MODELS RUNS.
AS FOR THE REST OF THE FORECAST AREA, WED NIGHT INTO THURS NIGHT
WILL BE WET AS FIRST THE FRONT BRINGS RAIN TO THE AREA ON WED
NIGHT FOLLOWED BY A FEW SHORT WAVE TROUGHS FOR THURS AFTERNOON
INTO THURS NIGHT.

 BEHIND THEM SURFACE RIDGE REBUILDS OVER THE AREA THURS NIGHT
INTO FRI AND WE GET A BRIEF BREAK FROM THE RAIN AND MAYBE SOME
BREAKS IN THE CLOUD COVER. SAT STARTS TO SEE MORE CLOUDS COME
BACK INTO THE PICTURE AND A CHANCE OF RAIN RETURN FOR THE OUTER
COAST AS ANOTHER LOW MOVES INTO THE SOUTH CENTRAL GULF. THIS WILL
NOT LAST AS THE LOW IS LOSING STEAM FAST AND IS GONE BY SUN NIGHT
DUE TO IT LOSING WHAT LITTLE JET SUPPORT IT HAD.

 BY MON, SURFACE RIDGE WILL ONCE AGAIN REBUILD OVER THE GULF BUT
THIS TIME IS HAS UPPER LEVEL SUPPORT AS THERE IS A BUILDING RIDGE
AT 500 MB AS WELL RATHER THAN THE THROUGH THAT IS CURRENTLY
OVERHEAD. THUS THE RIDGE WILL HAVE SOME STAYING POWER AS THAT IS
THE DOMINATE FEATURE THROUGH THE REST OF THE EXTENDED PERIOD.
BECAUSE OF IT, THE WEATHER FORECAST IS LOOKING TO BE DRY FOR THE
MOST PART FOR THE FIRST HALF OF NEXT WEEK. MODEL CONSENSUS IS
DECENT ON THIS BUT THERE IS SOME CONTENTION ON HOW LONG IT WILL
LAST. DECIDED ON USING A BLEND OF HPC/GFS/ECMWF WITH A GREATER
WEIGHT APPLIED TO THE HPC FORECAST TO EVEN OUT THE DIFFERENCES.
FORECAST CONFIDENCES IS A LITTLE ABOVE AVERAGE FOR THE PERIOD.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...SMALL CRAFT ADVISORY FOR PKZ012-022-036-041>043.

&&

HANNON/EAL