Small Craft Advisory 555, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
309 PM AKDT THU SEP 27 2012

.SHORT TERM...A 970 MB LOW IN THE CENTRAL GULF WILL SLOWLY FILL AS
IT DRIFTS N TO REACH WESTERN PRINCE WILLIAM SOUND TONIGHT. THE
FIRST FRONT HAS MOVED SE TO DIXON ENTRANCE BUT HAS GONE
QUASI STATIONARY. ISOLATED THUNDERSTORMS IN THE EAST GULF WILL
CONTINUE TO MOVE IN THROUGH OCEAN ENTRANCES UNTIL LATE THU NIGHT.
A RIDGE OF HIGH PRESSURE WILL REMAIN ALONG THE PANHANDLE THROUGH
FRIDAY.

 MODEL LOCK ON LARGE-SCALE FEATURES HAS BEEN CONSISTENT AND HAS
HAD A USEFUL CONSENSUS. UPPER LEVEL: NICE FIT OF AIRCRAFT WIND
VALUES TO SEVERAL MODELS WITH FIRST FRONTAL BAND. BUT FOLLOWING
AIR MASSES ARE ASSOCIATED WITH TOUGHER CHOICES TO FIND WHAT ARE
REPRESENTATIVE DATA: SEVERAL OF THE FEATURES ARE FAST MOVING AND
RAPIDLY EVOLVING. FIRST, THE VAST AREAL COVERAGE OF FAST MOVING
CONVECTION WILL INCLUDE IMPORTANT TRANSIENT DETAILS THAT THE
MODELS CANNOT TRACK EASILY. SECOND, THE FOLLOWING SECOND WRAP
FRONTAL BAND IS REVEALED IN THE GOES IMAGES AS HAVING RAPIDLY
EVOLVING SECOND WRAP. UP TO 4 DISTINCT WRAPS CAN BE IDENTIFIED ON
THE GOES IMAGES.

 &&

 .LONG TERM... COMPLEX WEATHER PATTERN THROUGH THE LONG RANGE
WITH EXTRA TROPICAL TRANSITION OF STORMS FROM THE WESTERN PACIFIC
INTO THE NORTHERN LATITUDES. MODELS HAVE NOT BEEN HANDLING THIS
TRANSITION WELL SHOWING LARGE VARIATIONS AND RUN TO RUN
INCONSISTENCIES. BY THE START OF NEXT WEEK LOOKING AT A SWITCH
FROM THE GULF LOW PRESSURE SYSTEMS THAT HAVE BEEN PROVIDING WET
CONDITIONS TO A BUILDING HIGH PRESSURE RIDGE AND POSSIBLE CALM
AND DRY PERIOD.

 MID AND UPPER LEVELS: JET STREAM AXIS ORIENTATED SOUTHWEST TO
NORTHEAST LOCATED IN THE SOUTHERN AK GULF AND REACHING THE
SOUTHERN PANHANDLE. THIS JET STEAK WILL BE QUASI STATIONARY
THROUGH MONDAY. WEAK RIDGE OF HIGH PRESSURE OVER THE PANHANDLE
WILL BEGIN TO MOVE EAST THROUGH SUNDAY AS A CLOSED LOW PRESSURE
SYSTEM FROM THE ARCTIC WILL MOVE NEAR BRISTOL BAY SATURDAY THEN
DROP INTO THE NORTHWESTERN AK GULF SUNDAY. LOW PRESSURE TROUGH
EXTENDING FROM THE 500 MB PARENT LOW IN THE NORTH PACIFIC ROTATES
INTO THE AK GULF MONDAY. THE PARENT LOW WILL TRACK EASTWARD
ACROSS THE NORTHERN AK GULF THROUGH MONDAY THE DROP SOUTHWARD
OVER THE PANHANDLE AS AN OPEN WAVE TROUGH. A HIGH PRESSURE RIDGE
WILL FOLLOW THE LOW MOVING IN FROM THE WEST AND BECOMING THE
DOMINANT FEATURE IN THE AK GULF THROUGH THE WEEK WITH FLOW
BECOMING MERIDIONAL.

 SURFACE: REMNANTS OF LOW PRESSURE SYSTEM OVER PRINCE WILLIAM
SOUND WILL REFORM AS 500 MB LOW MOVES IN FROM BRISTOL BAY ON
SUNDAY. SHOWER ACTIVITY OVER THE PANHANDLE WILL CONTINUE THROUGH
SATURDAY THEN BEGIN TO DIMINISH THROUGH SUNDAY. A SECOND SURFACE
LOW UNDER THE 500 MB TROUGH IN THE PACIFIC WILL TRACK INTO THE
SOUTHEASTERN GULF NEARING HAIDA GWAII SUNDAY. AS THIS LOW IS
REMNANTS OF TROPICAL SYSTEMS FROM THE WESTERN PACIFIC MODELS ARE
HAVING A DIFFICULT TIME NAILING DOWN DETAILS WITH
STRENGTH...POSITION AND TRAILING SURFACE LOW. LOOKING FOR SHOWER
ACTIVITY TO CONTINUE THROUGH MONDAY WITH HIGHEST POPS OVER THE
SOUTHERN PANHANDLE. WINDS WILL DROP THROUGH FRIDAY EVENING INTO
SATURDAY. WAVE HEIGHTS WHILE SUBSIDING WILL REMAIN HIGH IN THE
GULF DUE THE LONG PERIODS OF SOUTHWEST SWELL.

 AS HIGH PRESSURE RIDGE BUILDS OVER THE AK GULF WILL BEGIN TO SEE
DRYING PERIOD WITH LIGHTER WINDS. HOWEVER WITH THE STABLE AIR
MASS UNDER THE HIGH PRESSURE RIDGE COULD SEE MARINE LAYER
FORMATION. WHILE MODELS WERE IN FAIR AGREEMENT WITH THE HIGH
PRESSURE RIDGE THERE ARE SIGNIFICANT DIFFERENCES WITH SHORT WAVES
TOT HE WEST AND TIMING OF THESE SHORTWAVES BREAKING DOWN THE
HIGH. INCREASED CLOUD COVER AND POPS FOR NEXT WEEK DUE TO THESE
INCONSISTENCIES AND POSSIBILITY OF MARINE LAYER FORMATION. COLDER
AIR MASS MOVES IN THROUGH THE WEEK SO EXPECTING HIGH TO BE IN THE
LOW 50S TO UPPER 40S WITH OVERNIGHT LOWS IN THE MID 30S
POSSIBLE...ESPECIALLY FROM RADIATIONAL COOLING IF THERE ARE CLEAR
SKIES.

 NAM/ECMWF WERE IN GOOD AGREEMENT THROUGH SATURDAY. THE ECMWF WAS
SHOWING BIT BETTER INITIALIZATION COMPARED TO THE NAM...MAINLY AT
UPPER LEVELS. GFS WAS CLOSE BUT SAW DIFFERENCES IN LOCATION OF
SHORTWAVE MOVING OVER THE PANHANDLE FRIDAY NIGHT INTO SATURDAY
WITH THE GFS BIT FURTHER SOUTH COMPARED TO THE ECMWF/NAM. BY
SUNDAY WHILE MODELS WERE IN AGREEMENT WITH THE LOW IN THE
NORTHERN GULF THERE ARE LARGE DIFFERENCES WITH THE STRING OF LOWS
MOVING IN FROM THE NORTH PACIFIC. IN ADDITION TO MODEL
DIFFERENCES RUN TO RUN INCONSISTENCIES HAVE CONTINUED. USED A
BLEND OF 12Z NAM AND ECMWF WITH INHERITED GRIDS THROUGH DAY
3 THEN TRANSITIONED TO AN ECMWF/GFS BLEND THEN TRANSITIONING TO
NEW HPC. AS USE OF ENSEMBLES WAS PREFERRED OVER OPERATIONAL RUNS
BLENDED IN MORE HPC FOR WIND AND POP. FORECAST CONFIDENCE BEGINS
ABOUT AVERAGE BUT THEN DROOPS BELOW AVERAGE BY SUNDAY THROUGH THE
REMAINDER OF THE LONG RANGE.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...STRONG WIND UNTIL 1 AM AKDT FRIDAY FOR AKZ018.
MARINE...GALE WARNING FOR PKZ012-013-022-051-052.
         SMALL CRAFT ADVISORY FOR PKZ011-021-031>036-041>043-053.

&&

JBT/PRB