Small Craft Advisory 561, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
321 PM AKDT SUN SEP 30 2012

.SHORT TERM...SATELLITE IMAGERY THIS AFTERNOON SHOWING A
WEAKENING LOW PRESSURE SYSTEM IN THE NORTHEAST GULF WITH
ASSOCIATED FRONT MOVING OVER THE NORTHWESTERN PANHANDLE. THIS
FRONT WILL CONTINUE SHOWER ACTIVITY OVER YAKUTAT THROUGH THE
EVENING. THE FRONT WILL QUICKLY DISSIPATE AS IT MOVES EASTWARD
OVER THE NORTHERN PANHANDLE. EXPECT ONLY ISOLATED SHOWER ACTIVITY
FOR AREAS NORTH OF FREDERICK SOUND FROM THIS WEAKENING
FRONT...WITH MAJORITY OF PRECIPITATION FROM OROGRAPHIC LIFT NEAR
MOUNTAINS. TO THE SOUTH A BAROCLINIC LEAF TRACKING TO THE
NORTHEAST WITH AN INITIAL WEAK LOW NEAR HAIDA GWAII FOLLOWED BY A
DEVELOPING LOW IN THE NORTHEAST PACIFIC. THE FIRST LOW WILL
DISSIPATE THIS EVENING WHILE THE SECOND DEEPENS TO AROUND 1006 MB
AS IT TRACKS TOWARD DIXON ENTRANCE. PRECIPITATION WILL CONTINUE
TONIGHT INTO EARLY MONDAY MORNING FOR THE SOUTHERN PANHANDLE BUT
NOT RULING OUT POSSIBILITY OF SHOWERS MOVING FURTHER NORTH OF
FREDERICK SOUND TONIGHT. CLOUD COVER FOR THE CENTRAL AND NORTHERN
PANHANDLE WILL BE VARIABLE WITH INCREASING CLEAR BREAKS TONIGHT
AND WITH A GENERAL CLEARING TREND FOR MONDAY. A HIGH PRESSURE
RIDGE WILL BUILD IN THE AK GULF MONDAY WITH THE RIDGE AXIS MOVING
OVER THE EASTERN AK GULF MONDAY AFTERNOON.

 WINDS IN THE GULF WILL INCREASE TO NORTHWESTERLY SMALL CRAFT
LEVELS MONDAY WITH GRADIENT AND GEOSTROPHIC FLOW BETWEEN THE HIGH
PRESSURE RIDGE TO THE WEST AND THE LOW PRESSURE SYSTEM MOVING
OVER BRITISH COLUMBIA. POSSIBILITY OF WINDS NEARING GALE FORCE
FOR A SHORT PERIOD ALONG THE SOUTHERN COASTAL WATERS LATE MONDAY
MORNING. AS THE LOW MOVES OVER THE PANHANDLE WINDS OVER THE
SOUTHERN INNER CHANNELS WILL INCREASE TO SMALL CRAFT WITH BREEZY
CONDITIONS OVER THE SOUTHERN PANHANDLE. FOR THE NORTHERN
PANHANDLE GRADIENT WILL SHIFT FROM THE SOUTH TO NORTH EARLY
SUNDAY EVENING. WEAK SEA BREEZE DUE TO PARTIAL CLEARING THIS
AFTERNOON LOOKS TO DELAY THE WIND SHIFT THIS EVENING FOR NORTHERN
AREAS.

 MODELS WERE IN FAIR AGREEMENT AND FELL MORE IN LINE REGARDING
THE DEVELOPING NORTH PACIFIC LOW COMPARED TO PREVIOUS RUNS. MAIN
DIFFERENCES ARE EXTENT OF PRECIP OVER THE NORTHERN PANHANDLE. 12Z
NAM/ 00Z ECMWF SHOWED THE BEST AGREEMENT WITH THE ECMWF BETTER
INITIALIZED AT UPPER LEVELS. NAM SHOWED THE STRONGEST WINDS IN
THE AK GULF MONDAY WITH 850 MB WINDS UP TO 50 KTS. BLENDED
INHERITED GRIDS WITH 12Z NAM THEN A NAM/ECMWF BLEND. FORECAST
CONFIDENCE IS ABOVE AVERAGE WITH RESPECT TO SYNOPTIC PATTERN WITH
MAIN UNCERTAINTY REGARDING EXTENT OF SHOWERS NORTH OF FREDERICK
SOUND AND AMOUNT OF CLEARING THROUGH MONDAY.

 &&

 .LONG TERM...MUCH BETTER MODEL AGREEMENT THIS MORNING WITH
RESPECT TO A LOW & FRONT THAT RIDES UP/THROUGH A BLOCKING RIDGE
ON WEDNESDAY. GFS ENSEMBLE MEMBER STANDARD DEVIATIONS STILL OVER 6
MB FOR THIS FEATURE BUT RUN TO RUN CONSISTENCY IN THE GFS
OVERNIGHT HAS BEEN VERY GOOD. ECMWF RUN TO RUN CONSISTENCY
OVERNIGHT NOT NEARLY AS GOOD, BUT BASIC ORIENTATION OF BLOCKING
PATTERN ELEMENTS BOTH AT THE SURFACE AND ALOFT FROM ALASKA DOWN TO
THE LOWER 48 IN ECMWF CONFORM MUCH BETTER TO CLASSIC
METEOROLOGICAL DESCRIPTIONS. GFS CONTINUES TO BE MORE AGGRESSIVE
ON THE STRENGTH OF THE WEDNESDAY LOW AS IT COMES OVER/THROUGH THE
BLOCKING RIDGE AND WHILE THE CENTRAL PRESSURE IS NOT ANYTHING
OVERLY REMARKABLE, THIS DOES GIVE A LITTLE MORE CREDENCE TO THE
WEAKER ECMWF SOLUTION. BY SATURDAY MORNING, SOME DIFFERENCES BEGIN
TO EMERGE WITH GFS WASHING OUT THE BLOCKING HIGH WHILE ECMWF KEEPS
IT IN PLACE AND STRONG THROUGH NEXT WEEKEND. MODELS HAVE A WELL
DOCUMENTED BIAS FOR BREAKING DOWN BLOCKING HIGHS FAR TOO QUICKLY,
SO PREFERENCE TODAY IS FOR THE ECMWF.

 AS THE BLOCKING HIGH SETS UP, THE SOUTHERN HALF OF THE PANHANDLE
WILL SEE AT LEAST A WEAK OFFSHORE GRADIENT IN THE LOWER LEVELS,
SO WILL CONTINUE THE PARTIAL CLEARING FROM THE SHORT TERM THROUGH
TUESDAY AFTERNOON. THIS WILL ALSO RESULT IN COLDER OVERNIGHT TEMPS
THAN WE HAVE SEEN SO FAR. HAVE OVERNIGHT LOWS IN THE YAKUTAT AREA
BELOW FREEZING, AT FREEZING FOR GUSTAVUS AND JUNEAU, AND NEAR
FREEZING FOR HOONAH, HAINES, AND SKAGWAY MONDAY NIGHT.

 A FRONT ASSOCIATED WITH THE LOW FORECAST TO CLIMB OVER OR COME
THROUGH THE BLOCKING RIDGE WILL ENTER THE EASTERN GULF TUESDAY
NIGHT AND SPREAD GALE FORCE SOUTHEASTERLIES AHEAD OF IT. POST
FRONTAL WINDS WILL FALL OFF RAPIDLY FROM NORTH TO SOUTH ALONG THE
OUTER COAST AND BE BELOW SMALL CRAFT LEVELS BY WEDNESDAY
AFTERNOON. ASSOCIATED PRECIPITATION EXPECTED TO BE ALL RAIN AND
WILL MAKE IT ALL THE WAY DOWN TO THE SOUTHERN ZONES BEFORE
DIMINISHING BELOW CATEGORICAL POPS.

 BLOCKING HIGH IS EXPECTED TO RE-ESTABLISH SOON AFTERWARDS AND A
ANOTHER FRONT LATE IN THE WEEK IS NOT EXPECTED TO GET MUCH PAST
ICY BAY WITH PRECIP AS A RESULT. SECOND HALF OF THE WEEK LOOKING
LIKE IT WILL BE MOSTLY DRY WITH SOME MORE PARTIAL CLEARING OF THE
SKIES TO THE SOUTH AN ALONG THE COAST MOUNTAINS BY NEXT WEEKEND.

 BLENDED NAM AND ECMWF FOR MONDAY NIGHT AND TUESDAY. STRAIGHT
ECMWF LATE TUESDAY NIGHT THROUGH WEDNESDAY NIGHT. EQUAL PARTS GFS
AND ECMWF THROUGH FRIDAY NIGHT. EC/HPC BLEND THEREAFTER HEAVILY
WEIGHTED TOWARDS EC.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...SMALL CRAFT ADVISORY FOR PKZ035-036-041>043-051-052.

&&

PRB/FRITSCH