Small Craft Advisory 468, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
539 AM AKDT MON AUG 27 2012

.SHORT TERM...RIDGE ACROSS SOUTHEAST GULF AND SOUTHERN HALF OF
SOUTHEAST ALASKA WILL CONTINUE TO SLIDE OFF TO THE SOUTHEAST TODAY
AND SHOULD BE OUT OF THE FORECAST AREA BY THIS EVENING. A SHORT
WAVE TROUGH ASSOCIATED WITH THE NORTHERN BRANCH OF THE JET STREAM
WILL MOVE OVER THE NORTHEAST GULF TODAY AND DIG SOUTHEAST ACROSS
SOUTHEAST ALASKA TONIGHT. A 1004 MB SURFACE LOW NEAR PRINCE
WILLIAM SOUND WILL MOVE INTO THE NORTHEAST GULF BY THIS EVENING
THEN START TO OPEN UP INTO A TROUGH AS IT DROPS SOUTHWARD THROUGH
SOUTHEAST ALASKA TONIGHT.

 CLOUDS WILL BE ON THE INCREASE FOR ALL AREAS TODAY NOT ALREADY
OVERCAST. RAIN WILL SPREAD TO YAKUTAT THIS MORNING, ACROSS THE
NORTHERN PANHANDLE BY LATE THIS AFTERNOON AND OVER SOUTHERN ZONES
TONIGHT. INITIAL PRECIPITATION SHOULD BE LIGHT BUT EXPECTED TO
INCREASE TO MODERATE RAIN ALONG WIND FAVORED TERRAIN TONIGHT AS
JET STREAK PUSHES ACROSS THE AREA. 6 HOUR AMOUNTS PROGGED TO REACH
ONE HALF INCH - AND IN SOME PLACES ONE INCH IN 12 HOURS - FOR PAYA
AREA THIS EVENING AND ALONG THE COAST MOUNTAINS LATER TONIGHT.
EXTENSIVE MARINE LAYER OVER GULF WITH ONSHORE FLOW AND LEFTOVER OF
A FRONT WRAPPING NORTHWARD WILL ALSO BE CONTRIBUTING FACTORS.
MARINE PUSH COULD BRING CIGS DOWN TO LOW MVFR AND EVEN SOME IFR
FOR COASTAL AREAS

 SOUTH OF THE RIDGE AXIS NORTHWEST WINDS HAVE BEEN LOCALLY
REACHING SMCR LEVELS IN CROSS SOUND AND SOUTHEAST GULF. SHOULD SEE
NW WIND PICK UP ACROSS SOUTHERN INNER CHANNELS LATER TODAY. NORTH
OF THE RIDGE AXIS WILL SEE INCREASING SOUTHERLY WINDS. WITH A LOW
MOVING INTO THE SOUTHERN YUKON TODAY, THE PRESSURE GRADIENT IN
LYNN CANAL INCREASES BY THIS AFTERNOON TO SUPPORT 30 KT. ONCE
LOW/DEVELOPING TROUGH START TO PUSH SOUTH TONIGHT THIS WILL WEAKEN
AND COULD TURN TO LIGHT NORTHERLY IN THE FAR NORTH LATE TONIGHT.
OVER THE GULF HAVE A GENERALLY WESTERLY AROUND 20 KT THAT GAINS A
SOUTHERLY COMPONENT TONIGHT WITH LOW IN NORTHEAST GULF BUT GOES
BACK TO WESTERLY AFTER TROUGH PASSAGE AND EVEN INCREASES TO SMCR
IN OFFSHORE WATERS.

 GOOD MODEL AGREEMENT THROUGH THE SHORT TERM. USED THE NAM FOR
PRESSURE AND WIND BUT ALSO INCORPORATED IN THE GFS AND EC FOR MASS
FIELDS.

 &&

 .LONG TERM...LOW PRESSURE SYSTEM IN THE NORTHERN AK GULF WILL
CONTINUE TO TRACK TO THE SOUTHEAST. AS THIS LOW WEAKENS TO AN OPEN
WAVE TROUGH PRECIPITATION OVER THE AK PANHANDLE WILL BEGIN TO
DIMINISH FROM NORTH TO SOUTH THROUGH THE DAY AND TRANSITION TO
PERIODIC SHOWERS. MEANWHILE A HIGH PRESSURE SYSTEM IN THE NORTH
PACIFIC WILL BEGIN TO BUILD INTO THE AK GULF. AS THE RIDGE DEVELOP
WILL SEE A CLEARING TREND FOR THE PANHANDLE WHICH SHOULD LAST FROM
WEDNESDAY INTO THURSDAY. THE EXCEPTION BEING THE AK GULF AND
COASTAL AREAS WHICH WILL MOST LIKELY SEE MARINE LAYER DEVELOPING
UNDER THE STABLE HIGH PRESSURE AIR MASS. MEANWHILE A LOW PRESSURE
SYSTEM IN THE BERING SEA WILL MOVE OVER THE AK INTERIOR WITH SHORT
WAVE EXTENDING INTO THE NORTHWESTERN AK GULF BY THURSDAY. STILL
SOME QUESTION IF THE SURFACE FEATURE IN THE GULF WILL DEVELOP INTO
A CLOSED LOW OR REMAIN AN OPEN WAVE TROUGH. WITH WAY EXPECT
ANOTHER ROUND OF PRECIPITATION TO MOVE IN FROM THE WEST AND
DEVELOP FROM NORTH TO SOUTH THROUGH MEMORIAL DAY WEEKEND. IN
ADDITION TO THE STRENGTH OF THE SURFACE FEATURE MODELS STILL HAVE
SOME TIMING ISSUES ON THIS ADVANCING WAVE. FOR THE WEEKEND SYSTEM
THERE ARE ALSO INDICATIONS OF 6 HOUR QPF VALUES GREATER THEN ONE
HALF INCH DUE TO POSSIBLE TROPICAL MOISTURE CONNECTION.

 WINDS IN THE AK GULF WILL DIMINISH TUESDAY AS THE LOW WEAKENS.
NOT EXPECTING WINDS TO INCREASE SIGNIFICANTLY IN THE GULF UNTIL
THE WEEKEND. WINDS OVER THE INNER CHANNELS WILL SHIFT TO THE NORTH
THROUGH WEDNESDAY WITH WINDS OVER THE NORTHERN INNER CHANNELS
PICKING UP TO SMALL CRAFT LEVELS FROM PRESSURE GRADIENT RESULTING
FROM HIGH IN THE YUKON WITH A LOW OVER THE SOUTHERN AK PANHANDLE.
WINDS WILL SHIFT BACK TO THE SOUTH AS THE YUKON HIGH DRIFTS SOUTH
AND IS REPLACED BY A LOW FROM THE AK INTERIOR.

 MODEL DIFFERENCES WILL FAIRLY MINOR FOR THE START OF THE LONG
RANGE FORECAST. LIKED THE GFS INITIALIZATION BUT MAINLY THOUGH THE
GFS HAS SHOWN THE BEST RUN TO RUN CONSISTENCY. MODEL DIFFERENCES
INCREASE BY THE WEEKEND WITH THE DEPICTION OF THE DEVELOPING SHORT
WAVE. THE ECMWF IS SHOWING FASTER DEVELOPMENT OF THIS FEATURE
COMPARED TO THE GFS WITH THE ECMWF MOVING THE LOW INTO THE
PANHANDLE A GOOD 12 HOURS AHEAD OF THE GFS. HOWEVER THE ECMWF HAS
BEEN SHOWING GREATER RUN TO RUN VARIATIONS. KEPT WITH A GFS BLEND
THROUGH DAY 3...THEN KEPT WITH INHERITED HPC GRIDS. AS THE 00Z GFS
WAA VERY CLOSE TO PREVIOUS FORECAST CHANGES THROUGH OUT THE ENTIRE
LONG RANGE FORECAST WERE MINOR. CONFIDENCE IS ABOVE AVERAGE WITH
RESPECT TO SYNOPTIC PATTERN THROUGH THURSDAY...MAIN QUESTION
DURING THIS TIME WILL BE DEVELOPMENT OF THE MARINE LAYER WHICH IS
ALWAYS TRICKY. LOWER CONFIDENCE GOING INTO THE WEEKEND DUE TO
TIMING OF FEATURES.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...SMALL CRAFT ADVISORY FOR PKZ012-036-041-042-052.

&&

MM/PRB