Small Craft Advisory 457, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
556 AM AKDT WED AUG 22 2012

.SHORT TERM...AN UPPER LEVEL LOW AND THE ASSOC WEAKENING SURFACE
LOW OFF OF THE SITKA AREA IS CAUSING OVERCAST SKIES AND SCATTERED
SHOWERS ACROSS THE PANHANDLE TODAY. THE UPPER LEVEL LOW WILL MOVE
ESE ACROSS THE SOUTHERN PANHANDLE THROUGH AFTERNOON AND BE NEAR
THE HYDER AREA OVERNIGHT. THE SURFACE LOW WILL WEAKEN TO A TROUGH
AS IT PASSES AND A RIDGE WILL BUILD OVER THE GULF BEHIND IT.

 AS THE LOW MOVES EAST CLOUDS WILL DECREASE OVER THE NORTHERN
GULF AND YAKUTAT AREA WITH BREAKS OF SUN INTO THE AFTERNOON.
DECREASING CLOUDS TONIGHT WILL ALLOW FOR GOOD RADIATIONAL COOLING
WITH TEMPERATURES FALLING INTO THE MID 40S. AS A RESULT HAVE
ADDED AREAS OF FOG LATE TONIGHT FOR YAKUTAT AND YAKUTAT BAY.
ELSEWHERE RAIN SHOWERS WILL STILL BE AROUND...BUT ON THE
DECREASING TREND OVER THE NORTHERN AREAS. ANYWHERE THAT THE
CLOUDS THIN OUT SOME MORE WILL SEE PATCHY FOG DEVELOPING...BUT
HAVE LEFT THAT OUT OF THE FORECAST FOR NOW. TEMPERATURES WILL
RANGE FROM THE LOW TO UPPER 50S TODAY DUE TO THE CLOUD COVER AND
PRECIP.

 ANOTHER LOW CENTER WILL MOVE INTO THE YUKON LATER THIS AFTERNOON
AND EVENING. THIS WILL INCREASE THE SOUTHERLY PRESSURE GRADIENT
WITH THE BUILDING RIDGE AND CAUSE WINDS OF 20-25KT THROUGH
NORTHERN LYNN CANAL. HAVE ISSUED A SMALL CRAFT ADVISORY UNTIL
ABOUT MIDNIGHT...STRONGEST WINDS LOOK TO BE FURTHER NORTH NEAR
TAIYA INLET.

 MAIN FORECAST CHALLENGE TODAY WAS DETERMINING WHERE SHOWERS WERE
MOST LIKELY TO FALL. MODELS HAVE DIFFERED ON THIS OVER THE PAST
FEW DAYS. LATEST RUNS HAVE THE SHOWERS WRAPPING AROUND THE UPPER
LEVEL LOW FROM THE EAST AND FALLING ALONG THE COASTAL MOUNTAINS
AND NORTHERN AREAS. PREVIOUSLY IT LOOKED AS THOUGH THE SOUTHERN
AREAS WOULD SEE GREATER AMOUNTS SINCE THE FLOW WOULD BE COMING
FROM THE OCEAN. HAVE LEANED MORE WITH THE LATEST RUNS BUT HAVE
NOT DISCOUNTED THE FACT THAT THE UPPER LEVEL LOW WILL BE PASSING
OVER THE SOUTH...SO HAVE GONE WITH CATEGORICAL SHOWERS OVER THE
NORTH CENTRAL AREAS THAT HAVE SEEN MEASURABLE RAIN ALREADY THIS
MORNING AND SCATTERED SHOWERS ELSEWHERE...SINCE THEY WILL BE MORE
INTERMITTENT THROUGH THE DAY. ANOTHER FORECAST CHALLENGE WAS HOW
STRONG THE SOUTHERLY PRESSURE GRADIENT WILL BE THROUGH LYNN CANAL
TONIGHT. THE FINER RESOLUTION MODELS INCLUDING THE
NAM12/ECMWF/WRF HAD IT THE STRONGEST WHILE THE GFS WAS WEAKER AND
ORIENTED AT MORE OF AN ANGLE WITH THE CANAL. BECAUSE THE GFS
INITIALIZED WELL...AS DID THE OTHERS...IT WAS NOT DISCOUNTED AND
LIMITED SPEEDS SLIGHTLY TO ACCOUNT FOR THE WEAKER SOLUTION.
FORECASTER CONFIDENCE IS AVERAGE.

 &&

 .LONG TERM...LONG RANGE MODELS SEEM TO HAVE COME TO SOMEWHAT OF
A CONSENSUS OVERNIGHT ON THE FORECAST FOR THE WEEKEND.
DIFFERENCES STILL REMAIN HOWEVER. CURRENT SOLUTIONS STILL HAVE
THE UPPER LOW SLIDING SOUTHEAST THROUGH THE GULF FRI AND FRI
NIGHT. THE SURFACE REFLECTION ROUGHLY FOLLOWS THE SAME PATH WITH
MOST MAJOR MODELS ALSO BUILDING A RIDGE BEHIND THE LOW ON SUNDAY.
THE MAIN DIFFERENCES OCCUR WITH HOW FAR INLAND THE ASSOCIATED
FRONT GETS. THE GFS, ECMWF, AND TO A CERTAIN DEGREE THE UKMET ARE
SUGGESTING THAT THE FRONTAL BANDS WILL FALL SHORT OF MAKING
LANDFALL ON THE PANHANDLE OVER THE WEEKEND DUE TO DECENT MID
LEVEL DIVERGENCE OVER THE AREA. THE NAM, SREF, AND THE CANADIAN
GEM ON THE OTHER HAND RIDING ON MORE LOW LEVEL ENERGY ARE ALL
BRINGING THE PRECIP INTO THE PANHANDLE IN VARYING DEGREES. WITH
THE WIDE ASSORTMENT, BUT MORE AGREEMENT BETWEEN THE MODELS THAT
KEPT THE PRECIP OFFSHORE, I DECIDED TO GO WITH MOSTLY CHANCE POPS
OVER THE PANHANDLE WITH LIKELY POPS JUST BRUSHING THE OUTER COAST
AND HIGHER POPS JUST OFFSHORE.

 WINDS ASSOCIATED WITH THE FRONT WILL NOT BE TOO MUCH OF A
PROBLEM AS THE STRONGEST WINDS WILL MOSTLY REMAIN OFFSHORE.
HIGHEST WINDS ARE FORECAST TO BE AROUND 30 KT PARTICULARLY NEAR
CAPE SUCKLING DUE TO WEAK BARRIER JET SETTING UP THERE AGAIN.

 FOLLOWING THE LOW A RIDGE BEGINS TO BUILD INTO THE GULF ON
SUNDAY AND SLOWLY SLIDES SOUTH THROUGH EARLY NEXT WEEK. THIS
COMBINED WITH THE EASTERLY FLOW NORTH OF THE RETREATING LOW MAY
ALLOW SOME BREAKS TO DEVELOP IN THE CLOUD COVER, MAINLY OVER THE
SOUTHERN HALF OF THE AREA ON SUN. IT DOES NOT LAST AS THE RIDGE
DRIFTS SOUTHEASTWARD AND ANOTHER LOW DEVELOPS IN THE NORTHERN
GULF BY TUES BRINGING ANOTHER CHANCE OF RAIN TO THE AREA MAINLY
IN THE NORTH. MODEL AGREEMENT IS NOT VERY GOOD ON THIS FEATURE SO
DECIDED TO STICK MAINLY WITH HPC GUIDANCE TO AVERAGE OUT THE
DIFFERENCES. FORECAST CONFIDENCE IS A LITTE ABOVE AVERAGE FOR THE
WEEKEND AND ABOUT AVERAGE AFTER THAT.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...SMALL CRAFT ADVISORY FOR PKZ012.

&&

HANNON/EAL