Small Craft Advisory 584, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
340 PM AKDT THU OCT 11 2012

.SHORT TERM...THE ENDING OF OUR LONG DRY PERIOD IS OCCURRING IN
A SOUTH TO NORTH SEQUENCE. THE RAINFALL WILL CONTINUE SOUTH OF
FREDERICK SOUND TONIGHT, AS CLOUD COVER INCREASES TO THE NORTH.
THE RAIN WILL BEGIN TO SPREAD NORTH OVER THE REMAINDER OF THE
PANHANDLE FRIDAY BUT WILL BE MINIMAL FROM NORTH OF A GUSTAVUS TO
JUNEAU LINE DUE TO THE DRYING INFLUENCE OF DRY NORTHEAST WINDS.
SNOW IS POSSIBLE FOR HIGHER ELEVATIONS OF THE KLONDIKE DESPITE
TEMPERATURES ABOVE FREEZING AS LATENT HEAT LOSS FROM THE SNOW IN
DRY AIR CAN HELP IT REACH THE SURFACE.

 CONFIDENCE IN THIS SHORT RANGE PACKAGE IS AVERAGE.

 GOES WV AND IR SHOW THE LARGE SCALE CONFLUENCE OF FLOW ALOFT
FROM THE NORTHWEST AND THE SOUTHWEST. THE BRANCH FROM THE
NORTHWEST IS DRY AND THE BRANCH FROM THE SOUTHWEST IS ASSOCIATED
WITH AN OCCLUSION ABOUT 300 NM S OF A SITKA TO KETCHIKAN LINE AT
THIS TIME.

 THAT SOUTHWEST FLOW BRANCH HAS A GOOD CONNECTION TO HIGH WATER
VAPOR CONTENT FROM LOWER LATITUDES AND RAIN HEAVY AT TIMES WILL
OCCUR IN THE SOUTHERN HALF OF THE PANHANDLE BEGINNING TONIGHT.

 THE AIRMASS COMING FROM THE NORTHWEST (NORTH IN THE BOUNDARY
LAYER) IS COOLER AND DRIER THAN OUR MARINE LAYER. THE DESCENT OF
THIS AIRMASS INTO SOUTHEAST ALASKA HAS RESULTED IN ADIABATIC
WARMING OF THIS DEEP LAYER, WHICH WIDENS THE DEW POINT
DEPRESSION. TRACE AMOUNTS OF PRECIPITATION CAN OCCUR WHEN THIS
AIR MASS SLIDES UNDERNEATH AND LIFTS OUR MARINE LAYER. JUNEAU GOT
A TRACE OF PRECIPITATION BUT THESE AMOUNTS WILL REMAIN MINIMAL
NORTH OF FREDERICK SOUND UNTIL FRIDAY AFTERNOON WHEN MEASURABLE
PRECIPITATION SPREADS UP TO A GUSTAVUS TO JUNEAU LINE.

 AN AGEOSTROPHIC ENHANCEMENT SHOULD BE CONTRIBUTING TO THE N
WINDS IN THE NORTHERN PANHANDLE. AN ENTRANCE REGION TO STRONGER
UPPER LEVEL FLOW OVER THE NORTHERN PANHANDLE AND TO THE EAST WILL
GENERATE AN AGEOSTROPHIC CIRCULATION. FOR A VERTICAL CROSS
SECTION PLANE ORIENTED N-S THIS AGEOSTROPHIC FLOW WILL BE
COUNTER-CLOCKWISE AND THEREFORE FROM THE NORTH IF INDEPENDENT OF
OTHER ACCELERATIONS. THIS NORTH DIRECTION OF THE LOWER LEVEL FLOW
ALSO IS GENERALLY IN PHASE WITH N WINDS IN THE DEEP FRICTION
LAYER IN THE NRN PANHANDLE.

.LONG TERM...FORECAST BECOMES SOMEWHAT TRICKY OVER THE WEEKEND AS
FAR AS THE DETAILS GO. MODELS GENERALLY AGREE THAT A COMPLEX LOW
PRESSURE AREA WILL MOVE N INTO THE CENTRAL GULF SAT NIGHT AND SUN.
THEY DIFFER ON SMALLER SIZED WAVES MOVING AROUND THE MAIN LOW
HOWEVER...AND THESE WILL BE IMPORTANT FOR POTENTIALLY STRONGER
WINDS IF THEY MOVE INTO THE EASTERN GULF. ENDED UP BASING THE FRI
NIGHT THROUGH SUN EVENING PERIODS ON THE 12Z ECMWF...MAINLY FOR
ITS HANDLING OF THE MAIN LOW AND A FRONTAL SYSTEM MOVING IN FOR
FRI NIGHT...BUT BECAUSE THE ECMWF HAD ONE OF THE STRONGER WAVES
AND FURTHER EAST WITH IT FOR LATE SAT NIGHT AND SUN...LIMITED WINDS
TO MIN GALES WITH THIS FEATURE. IF THE 12Z ECMWF IS RIGHT WINDS
OVER THE EASTERN GULF WILL NEED TO BE BOOSTED TO HIGHER END GALE
OR POSSIBLY MIN STORM LATER SAT NIGHT INTO SUN. DID NOT CHANGE
FORECAST AFTER SUN NIGHT THOUGH...AND WILL EXPLAIN THAT BELOW.

SO...FOR FRI NIGHT INTO SAT...A COUPLE OF OCCLUDED FRONTS WILL BE
LIFTING NWD THRU THE PANHANDLE. AIRMASS OUT AHEAD OF THE FIRST FRONT
OVER THE NORTH IS FAIRLY COOL AND DRY...AND THIS MAY ALLOW SOME
SNOW TO REACH FAIRLY LOW LEVELS IN THE PAHN/PAGY AREAS DEPENDING
ON PRECIP RATES FRI EVENING. EVAPORATIONAL COOLING ITSELF WILL
LIKELY ALLOW SNOW AS LOW AS 600 FEET...AND IF A BURST OF HEAVIER
PRECIP OCCURS IN THE EXPECTED LIGHT WINDS OVER THE FAR N...SNOW
COULD REACH SEA LEVEL FRI EVENING. FIRST OCCLUDED FRONT WILL MOVE
N OF THE AREA LATE FRI NIGHT BUT SECOND WEAK OCCLUDED FRONT WILL
MOVE ONSHORE FRI EVENING AND BE RIGHT BEHIND THE FIRST FRONT
MOVING PAST THE NORTH LATE FRI NIGHT. POPS WILL BE HIGH OVER THE
AREA FRI NIGHT WITH A SLIGHT DROP OFF SAT BUT STILL ON THE HIGH
SIDE MOST PLACES.

FOR SAT NIGHT INTO SUN NIGHT...WILL HAVE STRONGER OCCLUDED FRONT
MOVING INTO THE AREA WITH POTENTIAL LOW PRESSURE WAVES MOVING N
THROUGH THE EASTERN GULF AS WELL. POPS WERE KEPT HIGH FOR THIS
TIME FRAME FOR MOST OF THE AREA...WITH ONLY THE PAGY AREA SEEING
LOWER THAN LIKELY POPS DURING THIS PERIOD. WILL LIKELY HAVE SOME
WIND HEADLINES SOMETIME DURING THIS PERIOD...WITH SEVERITY
DEPENDENT ON STRENGTH AND TRACK OF ANY LOW PRESSURE WAVES.

AS FOR MON ONWARD...REASON FOR LEAVING IT AS IS WAS NOT LIKING
LATEST HPC AT THIS POINT...DUE TO IT USING A HEAVY DOSE OF THE 12Z
GFS DURING THE WEEK. MODELS SUGGEST THAT THERE WILL BE SOME
SHORTWAVE FEATURES MOVING NE TOWARD THE SE GULF OR JUST S OF THE
AREA...BUT THEY DIFFER ON TIMING AND TRACK OF ANY SFC LOWS THAT
DEVELOP AS A RESULT OF THESE FEATURES. GFS HAS STRONGEST WITH
LIFTING A LOW NE INTO THE SE GULF AND PANHANDLE AREA TUE NIGHT
INTO WED...WHICH WAS NOT WELL SUPPORTED BY THE OTHER MODELS OR THE
GFS ENSEMBLE MEAN. THAT BEING SAID...IF THE 00Z MODEL RUNS TREND
TOWARD THE 12Z GFS IDEA ON THIS SYSTEM...THEN HPC WILL BE
REASONABLE TO USE FOR THE TUE NIGHT-WED TIME FRAME. HPC DID LOOK
REASONABLE FOR OTHER PERIODS HOWEVER.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING FOR PKZ012-022.
         SMALL CRAFT ADVISORY FOR PKZ011-013-021-031>036-041>043-051-053.
         SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ052.

&&

JBT/RWT