Small Craft Advisory 640, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
346 PM AKDT THU NOV 1 2012

.SHORT TERM...EXPLOSIVE CYCLOGENESIS HAS GENERATED A 955 MB LOW
ABOUT 500 NM S OF SEWARD. THE ASSOCIATED STRONG OCCLUDED FRONT
EXTENDING FROM NEAR THE LOW THAT IS MOVING TOWARD THE GULF  FROM
THE SOUTH. 12Z FRI THIS FRONTAL BOUNDARY WILL BE IN AN ARC 300 NM
S OF PADQ 300 NM S OF CORDOVA AND 300 NM SW OF KETCHIKAN. THE DRY
OUTFLOW FROM NW CANADA WILL DRIVE THE DEWPOINTS WELL BELOW
FREEZING LATE TONIGHT AND PERMIT THE PRECIPITATION TO BEGIN AS
SNOW OR MIXED RAIN AND SNOW DESPITE THE STRONG  WARM ADVECTION
THAT ALL THE MODELS SHOW FOR HIGHER LEVELS. EXCEPT FOR HIGHER
ELEVATIONS IN THE NORTHERN PANHANDLE, THE PRECIPITATION WILL
RAPIDLY CHANGE TO ALL RAIN BY NOON. CONFIDENCE IS AVERAGE.

THE LARGER-SCALE PRESSURE NORTH PRESSURE GRADIENT IS TO FIRST
WILL OCCUR DUE TO THE LARGE HIGH PRESSURE FIELD IN NW CANADA. THE
LARGER-SCALE HIGH PRESSURE FIELD WILL BE WEAKENING BUT WITHIN THE
PANHANDLE WILL BE REPLACED BY THE STRONG PRESSURE GRADIENTS
ASSOCIATED WITH THE STRONG FRONTAL BOUNDARY DURING 08Z-15Z.

NOTE THE HIGH PRESSURE FIELD IS ASSOCIATED WITH A COOLER
CONTINENTAL AIRMASS, BUT A RIDGE ALOFT OVER THE BROOKS RANGE IS
SUPPORTING THE HIGH, THE DENSITY IS NOT AS HIGH AS IT WOULD BE
WITH TRUE ARCTIC AIR. THE AIRMASS IS DRY ENOUGH TO AID IN THE NRN
HALF OF THE PANHANDLE HAVING INITIAL SNOW OR MIXED RAIN AND SNOW
BEFORE THE RAIN. LATENT HEAT ASSOCIATED WITH SUBLIMATION IS WHAT
WILL AID THE SNOWFALL EVEN WITH TEMPERATURES ABOVE FREEZING.

MODELS: USEFUL MODEL CONSENSUS TODAY AMONG EC/GFS/NAM/UKMET. DID
USE THE GEM FOR RH ISSUES TONIGHT AND EARLY FRI.

&&

.LONG TERM...MADE NOTABLE CHANGES TO THE LONG RANGE PERIOD
ESPECIALLY IN THE SUNDAY AND MONDAY TIME FRAME AS MODEL AGREEMENT
ON A STRONGER BAROCLINIC WAVE HAS INCREASED. AT THE BEGINNING OF
THE LONG RANGE PERIOD ON FRIDAY NIGHT THE AFOREMENTIONED SURFACE
LOW IN THE GULF WILL CONTINUE TO DRIFT NORTH AND SLOWLY WEAKEN.
MULTIPLE FRONTAL BANDS ASSOCIATED WITH THIS SYSTEM WILL BRING
PERIODS OF RAIN TO THE ENTIRE PANHANDLE THROUGH LATE SATURDAY.

THE FORECAST BECOMES RATHER DIFFICULT LATER SUNDAY INTO MONDAY AS
AN INTENSIFYING BAROCLINIC WAVE ROUNDING THE BASE OF THE BROAD
TROUGH MOVES FROM THE NORTHERN PACIFIC INTO THE SOUTHEAST GULF.
THE 12Z EC CAME MORE IN LINE WITH THE STRONGER AND SLOWER GFS
SOLUTION. BOTH MODELS ARE INDICATING A SUB 970MB LOW MAKING
LANDFALL IN THE VICINITY OF CAPE DECISION LATE SUNDAY. THE
STRONGER SOLUTION HAS SUPPORT FROM THE UPPER LEVEL PATTERN WITH A
DUAL JET STRUCTURE AIDING IN THE RAPID DEVELOPMENT OF THIS LOW. AS
A RESULT...HAVE INCREASED WINDS TO STORM FORCE ACROSS THE
SOUTHEAST GULF AND CLARENCE STRAIT ON LATE SUNDAY NIGHT INTO EARLY
MONDAY. AS THE SYSTEM QUICKLY WEAKENS AND PUSHES INTO BRITISH
COLUMBIA WILL SEE A RIDGE POP UP OVER THE PANHANDLE WITH STRONG
PRESSURE RISES WHICH WILL PROVIDE A SOUTHERLY PUSH AND SECOND
BURST OF WINDS EARLY MONDAY. AM CONCERNED THAT WINDS COULD REACH
HIGH WIND CRITERIA ACROSS THE SOUTHERN PANHANDLE AND A WATCH MAY
BE NEEDED IN FUTURE FORECASTS FOR LATE SUNDAY NIGHT INTO MONDAY.
EXPECTING MOST SNOWFALL TO BE CONFINED TO THE HIGHER ELEVATIONS
THROUGH MONDAY WITH THE HAINES AND KLONDIKE PICKING UP A FEW
ADDITIONAL INCHES OVER THE WEEKEND.

BEHIND THIS SYSTEM CONTINUED ONSHORE FLOW WILL KEEP PERIODS OF
RAIN OR SHOWER ACTIVITY IN THE FORECAST THROUGH THE MIDDLE PART OF
THE WEEK. HIGH PRESSURE BUILDING INTO THE INTERIOR AND YUKON WILL
SHIFT THE PARENT LOW OVER THE NORTHERN GULF TO THE SOUTHEAST ON
TUESDAY...REACHING THE SOUTHEAST GULF LATE WEDNESDAY. THIS WILL
RESULT IN PRECIPITATION DIMINISHING FROM NORTHWEST TO SOUTHEAST IN
THE MIDDLE PART OF THE WEEK. AS THE STRONG HIGH BUILDS IN THE
LATTER PORTION OF THE WEEK STRONG NORTHERLY WINDS ASSOCIATED WITH
CAA WILL USHER A COLD AND DRY AIRMASS INTO THE REGION.

USED A BLEND OF THE EC/GFS FOR ITS SIMILAR HANDLING OF THE LOW
MOVING INTO THE SOUTHEAST GULF LATE SUNDAY AND MONDAY BEFORE
TRANSITIONING TO HPC ON 12Z MONDAY. FORECASTER CONFIDENCE HAS
INCREASED SLIGHTLY FOR THIS WEEKEND BUT REMAINS AVERAGE.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...STRONG WIND FROM LATE TONIGHT THROUGH FRIDAY MORNING FOR AKZ023-
     027.
         STRONG WIND FRIDAY MORNING FOR AKZ022.
         STRONG WIND FROM LATE TONIGHT THROUGH FRIDAY AFTERNOON FOR
     AKZ028.
         STRONG WIND FROM FRIDAY MORNING THROUGH FRIDAY AFTERNOON FOR
     AKZ018.
MARINE...GALE WARNING FOR PKZ011>013-021-022-031-033>036-041>043-051-052.
         SMALL CRAFT ADVISORY FOR PKZ032-053.

&&

JBT/TPS