Small Craft Advisory 540, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
215 PM AKDT SAT SEP 22 2012

.SHORT TERM...A NW-SE ORIENTED OCCLUDED FRONT OVER THE CENTRAL
GULF WILL GO ALOFT WITH UPPER PART OF THE FRONT MOVING ACROSS SE
AK TONIGHT AND SUN MORNING. LOWER PART OF THE FRONT WILL REACH
THE CENTRAL OUTER COAST SUN AFTERNOON. GENERALLY STAYED WITH THE
CURRENT FORECAST AS FAR AS TIMING OF THE FRONT MOVING THRU
INCLUDING THE FRONTAL SEPARATION ALOFT.

 MAIN FORECAST ISSUES WILL BE TIMING OF PRECIP WITH FRONT...HOW
MUCH PRECIP WILL DIMINISH AFTER THE UPPER PART OF THE FRONT MOVES
BY...AND WINDS. RIGHT NOW LEADING EDGE OF PRECIP IS JUST REACHING
THE NE GULF COAST NEAR PAYA...AND IS ABOUT 120 NM WSW OF PASI
BASED ON RADAR. AM EXPECTING THE RAIN TO ARRIVE IN THE PASI/PAEL
AREAS AROUND THE TIME THE NEW FORECAST PACKAGE GOES OUT...AROUND
00-01Z. IT SHOULD GET TO THE SRN COAST AND CENTRAL INNER CHANNELS
BY 06Z...THEN REACH THE SRN INNER CHANNELS BY 09Z...EXCEPT NOT
UNTIL AROUND 15Z SUN NEAR HYDER. THE PRECIP BAND IS FAIRLY NARROW
BUT WILL LIKELY GET BROADER AS UPPER PART OF FRONT RACES OUT
AHEAD OF THE LOWER PART. THE MORE INTENSE PART OF THE PRECIP BAND
WILL GET NARROWER HOWEVER AS UPPER PART OF FRONT GOES FURTHER
ALOFT WITH TIME AND LIFT ALONG THIS FEATURE WEAKENS. RAINFALL
COULD BE LOCALLY HEAVY FOR SEVERAL HOURS SO WILL MENTION
THIS...MAINLY OVER THE SRN HALF...WHERE FRONT WILL BE A LITTLE
SLOWER TO MOVE THRU  TONIGHT.

 PRECIP WILL LIKELY DIMINISH SOMEWHAT BEHIND UPPER FRONT PASSAGE
BUT SOME PRECIP SHOULD REMAIN ALONG AND AHEAD OF THE LOWER PART
OF THE FRONT. PRECIP WILL LIKELY BECOME MORE SHOWERY WITH TIME
EVEN AHEAD OF THE SFC FRONT AS COLDER AIR ALOFT RUNS OUT AHEAD OF
IT...SO WILL HAVE TRANSITION TO SHOWERS FOR ALL BUT THE FAR SERN
AREA BY SUN AFTERNOON.

 AS FOR WINDS...AM KEEPING GALES GOING OVER THE NERN GULF
TONIGHT. WILL KEEP SCA FOR THE MORE WRN AND SRN INNER CHANNELS AS
WELL. THE WINDS WILL DROP OFF SOMEWHAT ONCE UPPER PART OF THE
FRONT MOVES PAST THE AREA...BUT DO NOT EXPECT WIND SHIFT UNTIL
LOWER PART OF FRONT MOVES PAST. EXCEPTION TO THE DIMINISHING
TREND WILL BE IN LYNN CANAL WHERE WINDS WILL LIKELY INCREASE TO
SCA LEVELS BY SUN AFTERNOON ESPECIALLY N.

 OTHERWISE...ENDED UP BUMPING TEMPS UP A BIT TONIGHT DUE TO
THICKER  CLOUD COVER MOVING IN. THE FOG THAT HAS BEEN PLAGUING
THE AREA THE PAST SEVERAL MORNINGS SHOULD NOT BE AN ISSUE TONIGHT
DUE TO PLENTY OF LOW LEVEL MIXING.

 &&

 .LONG TERM...ACTIVE PATTERN REMAINS THE NORM FOR THE LONG RANGE
PERIOD BUT BEFORE THAT WE WILL HAVE A BRIEF PERIOD OF RELATIVELY
DECENT WEATHER ON MON AND TUES. THE DECENT WEATHER STARTS OUT ON
MONDAY WITH A BUILDING RIDGE OVER THE EASTERN GULF AND PANHANDLE.
SOME SHOWERS WILL STILL BE AROUND THE AREA SUN NIGHT AND MON
PARTICULARLY IN THE NORTH FROM THE PREVIOUS FRONT AND THE FACT
THAT THE RIDGE WILL STILL BE SOMEWHAT FLAT. RIDGE WILL SHARPEN UP
MON NIGHT AND TUES WHICH WILL SHUT OFF THE SHOWERS IN MOST
PLACES. AREAS WEST OF GLACIER BAY MAY STILL SEE SHOWERS INTO TUES
AS THE RIDGE AXIS SETS UP TO THEIR EAST LEAVING ONSHORE FLOW TO
THE WEST. OTHER AREAS OF THE PANHANDLE MAY SEE AREAS OF FOG
DEVELOP AGAIN MON AND TUES MORNING AS THE BREAKS IN THE CLOUDS
ALLOW FOR RADIATIONAL FOG TO FORM OVERNIGHT.

 INTO TUES AFTERNOON AND NIGHT, OUR NEXT FRONT ADVANCES ON THE
PANHANDLE. THIS ONE IS LOOKING TO BE ABOUT THE SAME STRENGTH RAIN
WISE AS THE WEEKEND SYSTEM BUT WILL BE MORE OF A WIND GENERATOR
AS A WEAK TRIPLE POINT LOW FORMS ON THE FRONT IN THE CENTRAL GULF
TUES. THAT LOW WILL TRACK TOWARD PRINCE WILLIAM SOUND BY TUES
NIGHT. THE WINDS IN THE GULF WILL BE REACHING GALE FORCE FOR MOST
AREAS TUES AFTERNOON AND NIGHT WITH THE POSSIBILITY OF GUSTY
WINDS FOR THE OUTER COAST TUES NIGHT AND THE SOUTHERN PANHANDLE
EARLY WED. LONG RANGE MODELS STILL HAVE SOME DIFFERENCES ON THIS
FRONT MAINLY WITH THE TIMING THROUGH THE AREA. GFS IS THE FASTEST
HAVING THE PRECIP REACH THE OUTER COAST AS SOON AS TUES
AFTERNOON. THE NAM IS MUCH SLOWER WAITING TILL LATE EVENING TO
GET THE PRECIP TO THE COAST. THE ECMWF IS SOMEWHERE IN THE MIDDLE
ON TIMING. ELECTED TO GO WITH THE MIDDLE GROUND ON TIMING BUT
STILL LIKED THE WIND SPEEDS DEPICTED IN THE GFS SO WENT WITH A
BLEND OF ECMWF AND GFS MON NIGHT INTO TUES NIGHT PERIOD.

 FOLLOWING THIS, SUPPORT STILL CONTINUES FOR A RATHER STRONG
STORM FOR WED NIGHT INTO THURS IN THE GULF AS A STRONG SHORTWAVE
ROUNDS THE BASE OF THE ALASKA PENINSULA LOW. ODDLY ENOUGH THE
ECMWF IS NOW THE MOST CONSISTENT WITH THIS LOW AS COMPARED TO
YESTERDAY WHERE THE GFS WAS THE MOST CONSISTENT. BOTH THE CURRENT
AND PREVIOUS RUNS OF THE ECMWF HAVE SHOWN A 957 MB LOW TRACKING
THROUGH THE WESTERN AND CENTRAL GULF WED NIGHT INTO THURS. THE
OVERNIGHT RUNS OF THE GFS HOWEVER WERE GOING VERY WEAK AND
FARTHER EAST WITH THE FEATURE THEN YESTERDAY. THEN CAME THE
CURRENT 12Z GFS RUN THAT FEATURES A RETURN OF THE STRONGER LOW AT
960 MB AND A FURTHER WEST TRACK. OTHER SUPPORT CONTINUES FROM THE
GEM GLOBAL, LONG RANGE UK, GFS ENSEMBLE MEAN, AND THE ECMWF
ENSEMBLE MEAN WHICH DISAGREE ON WHERE EXACTLY AND HOW STRONG IT
WILL BE BUT GENERALLY ALL AGREE ON A STRONG LOW TRACKING THROUGH
THE WESTERN OR CENTRAL GULF WED NIGHT INTO THURS WITH A SLIGHTLY
FASTER PROGRESSION THAN WHAT WAS SUGGESTED YESTERDAY. TODAY'S HPC
GUIDANCE WENT WITH A MIDDLE GROUND APPROACH TO THIS SYSTEM TODAY
WITH A TRACK MORE IN LINE WITH THE ECMWF BUT WITH A STRENGTH MORE
ALONG THE LINES OF THE GFS SO WENT IN THAT DIRECTION. WILL STILL
KEEP AN EYE ON WHAT THE NEXT MODEL RUNS SUGGEST AS THIS SYSTEM
STILL LOOKS LIKE IT MAY BRING SOME STRONG WINDS TO THE AREA FOR
THURS.

 AFTER THIS THE FORECAST QUIETS DOWN FRI INTO THE WEEKEND AS
GENERAL SW FLOW AND SHOWERS BECOME THE ORDER OF THE DAY. FORECAST
CONFIDENCE IS AVERAGE THROUGH THE PERIOD ESPECIALLY FOR THE
SYSTEM ON THURS.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING FOR PKZ042-043-051-052.
         SMALL CRAFT ADVISORY FOR PKZ012-022-036-041-053.

&&

RWT/EAL