Small Craft Advisory 746, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
620 AM AKST WED DEC 5 2012

.SHORT TERM...SATELLITE IMAGERY STILL SHOWS A COLD POOL OF AIR
OVER MOST OF THE GULF OF ALASKA WITH SOME HEAVY SHOWERS
OFFSHORE...SOME MAKING IT INTO THE PANHANDLE OVERNIGHT. GUSTAVUS
VISIBILITY DROPPED TO THREE QUARTERS OF A MILE EARLY THIS MORNING
PRESUMABLY IN ONE HEAVY SNOW SHOWER. OTHERS ARE CERTAIN TO
FOLLOW. SNOW SHOWERS WILL BECOME MORE PREVALENT OVER THE
AREA...SPREADING INLAND TODAY AS THE UPPER LEVEL FLOW BECOMES
EVEN MORE SOUTHERLY THIS AFTERNOON AND EVENTUALLY SOUTHWESTERLY
WEDNESDAY NIGHT. IN ADDITION MORE COLD AIR WILL ADVECT INTO THE
PANHANDLE TONIGHT STEEPENING LAPSE RATES AND INCREASING
INSTABILITY. HAVE INCREASED SNOW ACCUMULATIONS TONIGHT A BIT TO
ACCOUNT FOR THIS TREND. SOMEWHAT CONCERNED TONIGHT ABOUT
ACCUMULATIONS...THAT SOME SHOWERS MAY BE HEAVY ENOUGH TO YIELD
ADVISORY LEVEL AMOUNTS...BUT THINK FOR NOW A GOOD INCREASE IN
TREND IS SUFFICIENT TO GET THE WORD OUT. THINK SCATTERED WORDING
WILL BE FINE FOR LYNN CANAL AREA...BUT EVEN THEY WILL BE ON
INCREASE TONIGHT. SITKA MAY NOT HAVE SO MUCH PRECIPITATION
AMOUNTS TODAY...AS THEY MAY BE RAIN-SHADOWED A LITTLE. HOWEVER,
STILL THINK RAIN/SNOW SHOWER WORDING IS APPROPRIATE...AS SHOWER
IMPLIES INTERMITTENT. OVER THE ZONE THERE WILL BE PLENTY OF
SHOWERS INTO TONIGHT. ALSO ELECTED TO GO CATEGORICAL FOR JUNEAU
AS WELL AS DYNAMICS WILL INCREASE INTO THIS AFTERNOON. OVERALL
ARCTIC FRONT STILL HOLDING AROUND SUMNER STRAIT BUT RAIN WILL
SNEAK INTO THE GULF SIDE OF BARANOF THIS AFTERNOON. AND SNOW
LEVELS SHOULD FALL TO SEA LEVEL OVERNIGHT FOR MOST ALL THE
PANHANDLE.

 THE MAIN SURFACE LOW HAS NOW MOVED WEST INTO THE NORTH CENTRAL
GULF WITH ITS ASSOCIATED OCCLUDED FRONT MOVING INTO THE PANHANDLE
TONIGHT. HIGH PRESSURE NORTH IN THE CANADIAN YUKON ALSO HAS
WEAKENED SLIGHTLY AND THUS PRESSURE GRADIENTS WILL RELAX SOMEWHAT
TODAY BUT HOLD THROUGH TONIGHT. THIS WILL GIVE THE INSIDE
PASSAGES A BREAK FROM WHAT HAS BECOME A VERY WINDY PATTERN THAT
HAS SETTLED OVER US FOR DAYS. NORTHERN LYNN HAS SETTLED DOWN TO
WEAK GALE AND WILL REMAIN SUCH INTO THURSDAY MORNING. IN
TURN...SKAGWAY WILL STILL EXPERIENCE STRONG WINDS THROUGH
NOON...SETTLING DOWN SOMEWHAT THIS AFTERNOON. THE GALE AT CROSS
SOUND WILL RELAX THIS EVENING. FREEZING SPRAY WILL CONTINUE TO
PLAGUE NORTHERN LYNN THROUGH TONIGHT AT LEAST.

 MADE SOME MINOR ADJUSTMENTS TO TEMPERATURES...RAISED YAKUTAT FOR
TODAY AS TEMPERATURES HAD RISEN OVERNIGHT...LIKELY FROM
THICKENING CLOUDS. ALSO LOWERED WHITE PASS A LITTLE. IN ADDITION
LOWERED TEMPERATURES OVER STEPHENS PASSAGE.

 MODEL OF CHOICE WAS A BLEND OF GFS AND CANADIAN GEM...ALTHOUGH
DID EMPLOY A HODGEPODGE OF GFS, NAM, GEM, AND ECMWF FOR POP
FIELDS. NAM QPFS WERE HIGH COMPARED TO OTHER MODELS...BUT WANTED
TO BEEF UP THE AMOUNTS A LITTLE FOR FEAR OF THE POWER OF THESE
CONVECTIVE SHOWER REGIMES...ESPECIALLY FOR TONIGHT. FORECAST
CONFIDENCE IS AVERAGE FOR AMOUNTS OF SNOW. ABOVE AVERAGE FOR
TREND LINE THROUGH TONIGHT. WIND FORECAST IS AVERAGE.

.LONG TERM...SYNOPTIC PATTERN CHANGE UNDER WAY NOW WELL UPSTREAM
WILL BEGIN TO AFFECT SOUTHEAST ALASKA BY THURSDAY MORNING. FIRST
UP WILL BE A DEVELOPING UPPER LEVEL WAVE THAT RIDES THE JET
STREAM EAST AND INTO THE SOUTHEAST GULF ON THURSDAY. WEAK SURFACE
REFLECTION AND SURFACE TROUGH WILL GENERATE SNOW OVER MOST OF THE
PANHANDLE SOUTH OF BERNER'S BAY. AT THIS SAME TIME, A DEVELOPING
LOW WILL BE PUSHING A VERY WELL DEVELOPED AND VERY STRONG FRONT
INTO THE GULF FROM THE WEST. THE LOW ITSELF WILL BE FULLY
OCCLUDED BEFORE THE FRONT ARRIVES ALONG THE OUTER COAST ON
SATURDAY AFTERNOON. STRONG WARM SECTOR WILL MOVE OVER SOUTHEAST
ALASKA SATURDAY NIGHT BUT LOW LEVEL COLD AIR CURRENTLY IN PLACE
SHOULD RESULT IN SNOW FOR MOST OF THE PANHANDLE THROUGH SATURDAY
NIGHT.

 A THIRD, FAST MOVING LOW, ALSO WITH A WELL DEVELOPED FRONTAL
STRUCTURE, WILL AFFECT THE PANHANDLE TUESDAY NIGHT. WARM SECTOR
ON THIS FRONT SHOULD PUSH THE RAIN-SNOW LINE AS FAR NORTH AS
BERNER'S BAY BY WEDNESDAY MORNING, BUT COLD AIR ON THE BACK SIDE
OF THIS LOW WILL DRIVE THE SNOW LINE WELL TO THE SOUTH AGAIN AS
THE ARCTIC FRONT ONCE AGAIN PRESSES DOWN FROM THE YUKON. TOO FAR
OUT IN THE FUTURE TO SAY WHETHER OR NOT ANOTHER ARCTIC OUTBREAK
WILL SET UP.

 NUMERICAL MODELS NOT DEALING WELL AT ALL WITH THE CHANGING
PATTERN. BLENDED GEM, GFS, ECMWF, AND HPC FOR PRESSURE AND WIND
AS A RESULT. USED PRIMARILY NAM AND ECMWF FOR POP AND QPF. TEMPS
AND SNOW LEVEL ADJUSTED WITH GFS. FORECAST CONFIDENCE IS BELOW
AVERAGE.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...STRONG WIND UNTIL NOON AKST TODAY FOR AKZ018.
MARINE...HEAVY FREEZING SPRAY WARNING FOR PKZ012.
         HEAVY FREEZING SPRAY WARNING FOR PKZ012.
         GALE WARNING FOR PKZ012-022.
         SMALL CRAFT ADVISORY FOR PKZ011-013-021-031>036-041-043-051-052.
         SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ042.

&&

JWA/FRITSCH