Small Craft Advisory 674, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
347 PM AKST TUE NOV 13 2012

.SHORT TERM...AN UPPER LEVEL VORTICITY MAX HAS CAUSED SNOW SHOWERS
FOR MUCH OF THE DAY IN THE JUNEAU AREA. ABOUT 3 INCHES WAS
MEASURED AT THE FORECAST OFFICE. THE SNOW WILL BECOME MORE
SCATTERED THROUGH THE EVENING WITH UP TO ANOTHER HALF INCH OF
ACCUMULATION. PRECIP FROM THE NEXT FRONT AND INCREASING ONSHORE
FLOW WILL CAUSE SNOWFALL TO INCREASE AGAIN OVERNIGHT FOR THE
INLAND AREAS OF THE JUNEAU ZONE. HAVE ISSUED A WINTER WEATHER
ADVISORY WITH UP TO 5 INCHES OF ACCUMULATION EXPECTED THROUGH THE
MORNING BEFORE CHANGING OVER TO MODERATE TO HEAVY RAIN. IMPACTS
FROM THIS EVENT WILL BE FELT MORE DUE TO THE SNOW ALREADY RECEIVED
TODAY AND WILL MAKE TRAVEL AND SNOWFALL REMOVAL MORE DIFFICULT.
GREATEST SNOWFALL RATES ARE EXPECTED AROUND PEAK COMMUTE TIME WITH
VISIBILITY REDUCED TO 1 MILE OR LESS AT TIMES.

 THE APPROACHING FRONT WILL CAUSE WINDS TO INCREASE 25-30 KT OVER
THE OUTSIDE WATERS TONIGHT. SNOW WITH THE FRONT WILL MOVE INTO THE
YAKUTAT AREA AROUND MIDNIGHT THEN INCREASE IN INTENSITY THROUGH
WEDNESDAY MORNING...BECOMING HEAVY AT TIMES. THE GREATEST LIFT
WITH THE FRONT ALIGNS WELL WITH THE BEST SNOW GROWTH ZONE IN THE
MORNING...SO EXPECT THE LARGEST DENDRITES TO OCCUR THEN. THEREFORE
HAVE ISSUED AN ADVISORY FOR YAKUTAT AS WELL WITH 2 TO 6 INCHES OF
ACCUMULATION TOTAL. ALONG THE COAST THERE WILL BE MORE OF A
MIX...LIMITING ACCUMULATIONS THERE. BY THE AFTERNOON SNOW WILL
CHANGE OVER TO ALL RAIN AND WINDS WILL INCREASE OUT OF THE
SOUTHEAST WITH THE FRONT.

 HAINES AND SKAGWAY WILL SEE SOME MORE SCATTERED SNOW SHOWERS
TONIGHT WITH 1 TO 2 INCHES OF ACCUMULATION THROUGH TOMORROW
MORNING. WINDS ARE EXPECTED TO FLIP TO THE SOUTH AND INCREASE
15-20 MPH THOUGH SO THIS WILL CAUSE THE SNOW TO CHANGE OVER TO
RAIN AND LIMIT ACCUMULATIONS. OUT THE HIGHWAYS HOWEVER THERE COULD
BE A BIT MORE ACCUMULATION INTO THE AFTERNOON...WITH UP TO 3
INCHES AT WHITE PASS.

 ELSEWHERE PRECIP WILL FALL AS RAIN AND BECOME HEAVY AT TIMES AS
THE FRONT PASSES. WINDS WILL INCREASE TO SMALL CRAFT LEVEL THROUGH
ALL OF THE INSIDE WATERS BY THE AFTERNOON AND TO GALE FORCE FOR
CLARENCE STRAIT NEAR LINCOLN ROCK.

 USED MAINLY THE NAM12 FOR UPDATES TODAY. BIGGEST FORECAST
CHALLENGE WAS QPF AND SNOWFALL AMOUNTS WITH TEMPERATURES HOVERING
AROUND THE FREEZING POINT. FORECAST CONFIDENCE IS AVERAGE.

 &&

 .LONG TERM...FOR THE START OF THE LONG RANGE WARM FRONT OVER THE
PANHANDLE WILL WEAKEN AS IT CONTINUES TO TRACK EASTWARD INTO
BRITISH COLUMBIA. TEMPERATURES WILL RETURN TO A MORE DIURNAL TREND
AND DIMINISH OVERNIGHT WEDNESDAY. WHILE MOST OF THE PANHANDLE WILL
STILL SEE RAIN AREAS NORTH OF ICY STRAIT WILL BEGIN TO MIX WITH
SNOW. FOR NOW NOT EXPECTING SNOW ACCUMULATION MUCH MORE THAN AN
INCH TO TWO INCHES AT HIGHER ELEVATIONS.

 THE 968 MB BERING SEA LOW ASSOCIATED WITH THE WARM FRONT WILL
TRACK OVER THE ALEUTIANS MOVING JUST SOUTHWEST OF KODIAK BY EARLY
THURSDAY MORNING WITH LOW CENTER AROUND 990 MB BY 18Z THURSDAY.
THE LOW WILL TRACK TO THE SOUTHEAST DEEPENING TO AROUND 983 MB
THEN BECOME QUASI STATIONARY IN THE SOUTHEAST GULF BY FRIDAY.
WINDS ALONG THE GULF COAST WILL INCREASE TO GALE FORCE THURSDAY
NIGHT INTO FRIDAY WITH SEAS BUILDING TO 15 TO 18 FT. AS THE LOW
MOVES IN AN ARCTIC HIGH WILL BUILD OVER THE YUKON THROUGH FRIDAY.
THE TIGHTENING PRESSURE GRADIENT ALONG WITH PASSING FRONTS WILL
INCREASE NORTHERLY WINDS TO GALE FORCE OVER THE INNER CHANNELS.
WITH INDICATION OF THE LOW WEAKENING AND DRIFTING SOUTH THROUGH
THE WEEKEND WINDS WILL BEGIN TO DIMINISH SUNDAY INTO MONDAY.

 PRECIPITATION TYPE FOR THURSDAY INTO FRIDAY IS A BIT TRICKY. WITH
THE COLD FRONTAL BOUNDARY MOVING AND THE ARCTIC AIR FLOW FROM THE
YUKON EXPECTING TEMPERATURES FOR THE NORTHERN PANHANDLE TO DROP TO
THE 20S TO TO 30S WITH LOWS IN THE TEENS. WARMER TEMPERATURES FOR
THE SOUTHERN PANHANDLE WITH HIGHS IN THE UPPER 30S AND LOW IN THE
LOW 30S. THIS WOULD INDICATE MORE SNOW THAN RAIN BUT 850 MB TEMPS
FOR THE CENTRAL PANHANDLE ONLY DROP TO -4C FRIDAY WITH 1000-500MB
THICKNESS ONLY DOWN TO 5275 TO 5300. THESE VALUES INDICATE A MIXED
PRECIP WITH MORE SNOW UNDER HEAVIER PRECIP BANDS. BY THE WEEKEND
IF THE GALE FORCE LOW DOES DROP SOUTH WILL SEE A DRYING TREND FROM
NORTH TO SOUTH JUST NORTH OF FREDERICK SOUND. BUT MODEL RUNS HAVE
BEEN INCONSISTENT WITH THE LOW TRACK.

 FOR THE START OF THE LONG RANGE MODELS WERE IN GOOD AGREEMENT.
NAM BECOME MORE OF AN OUTLIER BY THURSDAY WITH THE POSITION OF THE
GULF LOW BIT TOO FAR SOUTH WHILE UKMET WAS A BIT TOO WEAK. GFS AND
ECMWF WERE IN GOOD AGREEMENT WITH THE TRACK AND STRENGTH OF THE
LOW THROUGH 06Z SATURDAY. DEPENDING ON WHILE MODEL RUN YOU CHECK
BY THEN THE LOW WILL SLOWLY WEAKEN AND TRACK SOUTH NEARING HAIDA
GWAII BY LATE SATURDAY...WEAKEN BUT THEN REFORM AND MOVE BACK INTO
THE SOUTHEASTERN GULF SUNDAY...OR REFORM AS A BROAD AREA OF LOW
PRESSURE IN THE CENTRAL GULF. WHILE THE ECMWF HAD BEEN MORE
CONSISTENT RUN TO RUN THE GFS NOW TAKES THAT TITLE. USED A BLEND
OF 12Z ECMWF/GFS THROUGH DAY 4 THEN TRANSITIONED TO A BLEND OF NEW
HPC/GFS/ECMWF AND INHERITED GRIDS THROUGH DAY 8. WHILE GENERAL
SYNOPTIC PATTERN IS BEEN CONSISTENT THE POSITION AND STRENGTH OF
THE LOW THROUGH THE WEEKEND WILL HAVE A LARGE IMPACT ON NORTHERLY
EXTENT OF PRECIP...AND STRENGTH AND DIRECTION OF WINDS IN THE GULF
AND INNER CHANNELS. FORECAST CONFIDENCE IS ABOVE AVERAGE FOR THE
START OF THE LONG TERM BUT DROPS BELOW AVERAGE BY THE WEEKEND.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WINTER WEATHER ADVISORY FROM 3 AM TO NOON AKST WEDNESDAY FOR
     AKZ025.
         WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO NOON AKST
     WEDNESDAY FOR AKZ017.
MARINE...GALE WARNING FOR PKZ036.
         SMALL CRAFT ADVISORY FOR PKZ012-013-021-022-031>035-041>043-051-
     052.

&&

HANNON/PRB