Small Craft Advisory 329, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
540 AM AKDT FRI MAY 18 2012

.SHORT TERM...A HIGH PRESSURE RIDGE OVER THE ERN GULF WILL DRIFT
ONSHORE THROUGH TONIGHT. A WEAK OCCLUDED FRONT WILL MOVE N AND
EXTEND FROM KODIAK ISLAND TO THE S-CENTRAL GULF BY LATE TONIGHT.
USED THE 00Z ECMWF AS IT HAD BEST HANDLE ON WEATHER FEATURES AS
OF 06Z LAST EVENING.

 MAIN FORECAST ISSUES WILL BE PRECIP POTENTIAL AND WINDS. RIGHT
NOW...A MARINE LAYER IS BEING PUSHED ONSHORE BY THE RIDGE OVER
THE GULF. SOME INSTABILITY WILL BE PRESENT LATER THIS MORNING AND
AFTERNOON IN THE LOWER LEVELS. AM EXPECTING SOME SHOWER ACTIVITY
TO DEVELOP MAINLY OVER THE MOUNTAINS AT FIRST LATER THIS MORNING
THEN THEY MAY SPREAD TO THE VALLEYS THIS AFTERNOON...AND THIS
WILL BE MAINLY ALONG THE COAST MOUNTAIN AREA AND THE PAJN-PAEL
CORRIDOR AS WELL AS THE NE GULF COAST. THE SHOWERS WILL REACH
THEIR PEAK COVERAGE DURING THE MID-LATE AFTERNOON AT PEAK HEATING
TIME...THEN ONCE BOUNDARY LAYER STARTS COOLING THIS EVENING THE
SHOWERS SHOULD DIMINISH. WILL STILL BE SOME LEFT OVER MARINE SC
CLOUDS AFTER THE SHOWERS DIMINISH. WENT FOR SCATTERED COVERAGE AT
THE MAX AS THERE IS NO DECENT FORCING MECHANISM ALOFT TO
CONCENTRATE THE SHOWERS IN A LINE TODAY INTO THE EVENING. THEN
THE PRECIP THREAT WILL SHIFT TO  THE FRONT MOVING IN TONIGHT. BY
LATE TONIGHT STILL EXPECT MUCH IF NOT ALL THE PRECIP TO REMAIN
OVER THE GULF...BUT MAY HAVE A FEW SHOWERS DEVELOP OUT AHEAD OF
THE MAIN FRONTAL PRECIP AS LOW LEVEL CONVERGENCE INCREASES AND A
WEAK VORT LOBE MOVES IN. THEY WILL BE MOVING NEARLY PARALLEL TO
THE OUTER COAST HOWEVER IN SE LOW LEVEL FLOW SO THE BEST THREAT
FOR THESE SHOWERS TO REACH LAND WILL BE OVER THE PAYA AREA.

 AS FOR WINDS...GENERALLY FLOW IS ON THE LIGHT SIDE ALTHOUGH A
FEW SPOTS HAVE AROUND 15 KT WINDS RIGHT NOW...MAINLY IN NRN LYNN
CANAL AND SOME OF THE MORE E-W CHANNELS.AS SFC RIDGE PUSHES
ONSHORE THE GENERAL WIND FIELD WILL WEAKEN BUT LOCALIZED
INCREASES ARE LIKELY OVER NRN LYNN CANAL AREA DUE TO LEE SIDE
TROFFING NE OF PAGY. WINDS WILL INCREASE IN GENERAL TONIGHT AS
THE FRONT MOVES CLOSER...WITH A BAND OF SCA LEVEL WINDS AHEAD OF
THE FRONT REACHING THE SW OFFSHORE WATERS THIS AFTERNOON...THEN
SPREADING N TO THE NRN GULF COASTAL AREA BY LATE TONIGHT.

 &&

 .LONG TERM...EXTENDED RANGE STARTS OUT WITH A WEAK FRONT JUST
OFFSHORE OF THE PANHANDLE. A WEAK WAVE ALONG IT WILL BE HEADING
NORTH AND HITTING THE AREAS NEAR AND WEST OF YAKUTAT SAT WITH
RAIN AND SOME BREEZY WINDS. THE REST OF THE FRONT SOUTH OF THE
WAVE WILL BE RATHER WEAK AND FORECAST MODELS ARE HAVING TROUBLE
DETERMINING WHAT TO DO WITH THE NEXT WEAK WAVE THAT WILL BE
COMING UP FROM THE SOUTH ALONG THE FRONT ON SUN AFTERNOON. EC AND
NAM ARE SUGGESTING A STRONGER VORTICITY CENTER ASSOCIATED WITH
THE WAVE WHILE GEM AND GFS ARE NOTICEABLY WEAKER. AS SUCH THE
DIFFERENCES IN POPS RANGE FROM SLIGHT CHANCE POPS IN THE SOUTHERN
PANHANDLE FROM GFS AND GEM TO LIKELY AND EVEN CATEGORICAL FROM
THE EC AND NAM. WENT FOR A MORE MIDDLE OF THE ROAD APPROACH BUT
TENDED JUST A BIT MORE TOWARD THE WETTER SCENARIO SO 50 TO
60 POPS ARE DOMINATE IN THE SOUTHERN PANHANDLE SUN AFTERNOON.

 THE FOLLOWING WAVE ON MON HAS MORE AGREEMENT WITH IT AMONG THE
MODELS, BUT TIMING ISSUES STILL EXIST WITH THE GFS STILL HOLDING
BACK ON SPREADING THE PRECIP OVER THE PANHANDLE AS FAST AS THE
EC. PART OF THIS DRY BIAS MAY BE FROM THE GREATER EASTERLY
COMPONENT TO THE WINDS IN THE LOW AND MID LEVELS SHOWN IN THE GFS
WHERE THE EC EXHIBITS MORE SE WINDS. IN ANY CASE, HIGHEST POPS
ARE PRESENT IN THE SOUTHERN PANHANDLE MON THROUGH TUES NIGHT AS
THE E TO SE FLOW CONTINUES TO SEND MOISTURE INTO THE AREA IN THE
FORM OF ONE OR TWO MORE WEAK WAVES.

 FROM WED ONWARD A DRYING SPELL IS IN THE WORKS AS A UPPER LOW
THAT WAS OVER THE CENTRAL ALEUTIANS AT THE START OF THE PERIOD
MOVES EASTWARD ACROSS THE SOUTHERN GULF, EVENTUALLY ENDING UP
NEARLY STATIONARY NEAR HAIDA GWAII ON WED AND THURS BEFORE
RAPIDLY HEADING SE LATE WEEK. HOW FAST THIS HAPPENS IS UP FOR
DEBATE WITH THE EC GOING FOR A MUCH FASTER SOLUTION THAN THE GFS.
A RIDGE EVENTUALLY REPLACES THE LOW IN THE GULF BY NEXT WEEKEND
WITH GENERALLY DRY CONDITIONS.

 NUMERICAL MODELS WERE HAVING SOME ISSUES WITH THE SMALL WEAK
FEATURES THAT WILL BE AFFECTING THE PANHANDLE IN THE FIRST HALF
OF THE LONG RANGE PERIOD. GFS GENERALLY EXHIBITED A DRYER
SCENARIO WITH MORE EASTERLY FLOW THEN THAT OF THE EC WHICH HAD
MORE SE FLOW. IN THE MID RANGE GFS SEEMED TO HAVE BETTER
CONSISTENCY, BUT BOTH EC AND GFS BECOME QUESTIONABLE BY MID WEEK.
DECIDED ON MAINLY GFS WITH SOME EC FOR MID RANGE UPDATES AND
LEANED MORE ON THE HPC FOR EXTENDED RANGE UPDATES. MAIN CHANGES
WERE A SLIGHT SPEED UP OF THE THE FRONT HEADING INTO YAKUTAT SAT
AND SOME SLIGHT INCREASED POPS IN SOME PLACES IN THE SOUTH FOR
LATE WEEKEND AND EARLY WEEK PERIOD. FORECAST CONFIDENCE IS ABOUT
AVERAGE.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...SMALL CRAFT ADVISORY FOR PKZ051-052.

&&

RWT/EAL