Weather Service Report


460 
FXAK67 PAJK 211257
AFDAJK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
457 AM AKDT TUE APR 21 2015

.SHORT TERM...UPPER-LEVEL SHORTWAVE TROUGH LOCATED IMMEDIATELY S
OF THE ALEUTIANS IS ROUNDING THE BASE OF AN UPPER LOW POSITIONED
OVER THE NRN BERING SEA THIS MORNING. BY 12Z WED...THE SHORTWAVE
IS FORECAST TO PROGRESS TO A LOCATION NEAR THE SWRN GULF.
MEANWHILE...A SHARP SHORTWAVE TROUGH WILL MOVE ACROSS THE
PANHANDLE THIS AFTERNOON...WHILE A RIDGE AMPLIFIES OVER THE
CNTRL/ERN GULF. THIS RIDGE WILL THEN CROSS SERN AK TUE NIGHT. AT
THE SURFACE...A RIDGE OF HIGH PRESSURE BUILDING OVER THE GULF WILL
SHIFT E TO THE PANHANDLE TUE NIGHT CAUSING WLY WINDS TO WEAKEN
SIGNIFICANTLY. BEFORE WEAKENING...SMALL CRAFT WINDS ARE EXPECTED
TO OCCUR DURING THE AFTERNOON OVER MARINE ZONES 41 AND 42...AS
WELL AS WITH A TIP JET SPREADING ACROSS SRN CHATHAM AND SUMNER
STRAITS. FARTHER W...A SURFACE CYCLONE AND ASSOCIATED GALE FORCE
FRONT WILL APPROACH THE SWRN-SCNTRL GULF WITH IT/S PARENT UPPER
WAVE LATE TONIGHT.

SHOWERS...LOCALLY HEAVY...CONTINUE TO SPREAD ACROSS MARINE ZONES
51 AND 52...AND LAND ZONE 17...THIS MORNING. THESE SHOWERS ARE
OCCURRING WITHIN AN ENVIRONMENT CHARACTERIZED BY MUCAPE VALUES
RANGING FROM 100-500 J/KG...AND A BOUNDARY LAYER TEMPERATURE
PROFILE THAT IS MARGINALLY SUPPORTIVE OF ISOLATED
THUNDERSTORM/LIGHTNING OCCURRENCE INVOF BOTH THE SAINT ELIAS
MOUNTAINS...AS WELL AS MINOR UPPER IMPULSES MOVING ACROSS THE
REGION. THIS MARGINAL THUNDERSTORM ENVIRONMENT IS FORECAST TO
DIMINISH BY EVENING AS UPPER-LEVEL SUBSIDENCE SPREADS ACROSS THE
AREA WITH THE APPROACHING RIDGE. 

IN ADDITION...WITH 850 MB TEMPERATURES NEAR -8 C...SNOW HAS
OCCASIONALLY MIXED IN WITH THE HEAVIER SHOWERS OVER YAKUTAT...AND
THIS POTENTIAL WILL CONTINUE THROUGH MIDDAY. ELSEWHERE OVER THE
CNTRL/SRN PANHANDLE...SHOWERS WILL BECOME PROGRESSIVELY MORE
ISOLATED AS THE DAY PROGRESSES...AND SHOULD DISSIPATE COMPLETELY
BY TONIGHT AS THE UPPER RIDGE MOVES OVER THE AREA. IN
ADDITION...SKIES MAY PARTIALLY CLEAR TONIGHT...AND GIVEN THE
SUBSIDENT/DRYING MIDLEVEL AIRMASS DEVELOPING OVER THE REGION...IN
ADDITION TO WEAKENING SURFACE WINDS AND RECENT GROUND WETTING
RAIN...FOG WILL BE POSSIBLE FOR MANY LOCATIONS.

NO CHANGE WAS MADE TO PRESSURE GRIDS. WIND EDITS WERE MINOR. POP
AND QPF WERE UPDATED WITH A BLEND OF COARSE AND HI-RES MODEL
GUIDANCE.

.LONG TERM...THE UPPER LOW ALOFT DESCRIBED ABOVE WILL AMBLE
EASTWARD TO THE CENTRAL OUTER GULF BY WEDNESDAY AFTERNOON. THE
ECMWF HAS TAKEN THE LEAD ALONG WITH THE GEM IN SLIDING THE FEATURE
SOUTHEAST TOWARD VANCOUVER ISLAND BY THURSDAY EVENING. THE GFS HAS
BEEN THE MOST CONSERVATIVE WITH SOUTHEASTWARD PROGRESS BUT
ARGUABLY HAS SPED UP IN RECENT RUNS. MEANWHILE THE NAM TAKES A
MIDDLE GROUND, BUT THE 06Z RUN TRENDED MORE WITH THE ECMWF. WHY IS
THIS IMPORTANT? BECAUSE A SUPPORTED SURFACE LOW UNDERNEATH WITH A
MORE SOUTHEASTERLY TRACK WILL KEEP ITS STRONGEST WINDS WELL
OFFSHORE AND THE DEVELOPMENT OF MORE OFFSHORE WINDS AS THE LOW
PASSES TO OUR SOUTH WILL KEEP THE PANHANDLE OVERALL DRYER THAN IF
THE LOW TOOK A MORE NORTHERLY TRACK.

THEREFORE, THIS MORNING WE CONTINUE THE TREND OF DRYING OUT THE
NORTHERN AND CENTRAL PANHANDLE THROUGH THE EARLY WEEKEND AND
PUSHING THE STRONGEST WINDS TO THE SOUTH. THE CHANCE OF GUSTINESS
ACROSS THE COAST CONTINUES TO DECREASE, BUT THERE MAY BE SOME
BREEZY CONDITIONS OVER PRINCE OF WALES ISLAND WEDNESDAY NIGHT.
EASTERLY GALES OVER 310, 41, AND 42 WEDNESDAY WILL FADE TO 30 KT
BY LATE WEDNESDAY NIGHT AS THE SURFACE LOW WEAKENS AND BEGINS
DROPPING SOUTHEAST. LIKEWISE, WE LOWERED WIND SPEEDS ACROSS THE
CENTRAL AND SOUTHERN PANHANDLE. HOWEVER, EXPECT SMALL CRAFT
NORTHERLIES TO DEVELOP OVER LYNN CANAL WEDNESDAY NIGHT INTO
THURSDAY MORNING WITH A TIGHTENING PRESSURE GRADIENT. AT ONE
POINT, THERE APPEARED TO BE A PERIOD FOR THE POTENTIAL OF SOME
MOUNTAIN WAVE ACTIVITY OVER DOWNTOWN JUNEAU WEDNESDAY
AFTERNOON/EVENING. BUT THIS APPEARS TO HAVE LESSENED AS CROSS
BARRIER FLOW IS UNIMPRESSIVE. NEWEST HIGH RES MODELS BARELY GIVING
THIS NOTICE WITH MAYBE 15 TO 25 KT AT MOST. ELECTED TO TAKE OUT THE
NORTHEAST WINDS AND GO WITH A MORE STABLE BUT LIGHT NORTHWEST FLOW
DOWN GASTINEAU CHANNEL. FINALLY WITH A MORE SOUTHERLY TRACK OF THE
LOW, RAIN IS ONLY LIKELY OVER THE FAR SOUTH WEDNESDAY NIGHT.
AMOUNTS SHOULD BE WELL LESS THAN AN INCH.

OWING TO ITS MORE NORTHERLY POSITION OF THE SOUTHEASTWARD MOVING
LOW, THE GFS HINTS AT SOME MOISTURE ARRIVING AT OUR BACKDOOR FROM
THE SOUTHEAST ON FRIDAY. AS NOTED ABOVE, THE GFS STILL SEEMS THE
OUTLIER, BUT IF THE LOW STALLS AROUND HAIDA GWAII AS THE GFS
INDICATES, AS OPPOSED TO VANCOUVER ISLAND, EXPECT A LITTLE MORE
CLOUDINESS AND SOME SCATTERED POPS FOR HYDER AND OTHER
SOUTHERN/CENTRAL COMMUNITIES.

THE LOW ALONG THE BC COAST EVENTUALLY DIES TOWARD THE WEEKEND AS A
BERING SEA SYSTEM MOVES INTO THE GULF AND CAUSES A MORE
ANTICYCLONIC RIDGE TO MOVE IN OVER THE COAST OF NORTH AMERICA.
THERE ARE HINTS OF A DEVELOPING WAVE ALONG THE NEW LOW'S SOUTHERN
FLANK THAT MAY BRING SOME WIND AND RAIN FOR THE SOUTHERN PANHANDLE
LATE IN THE WEEKEND. THIS WILL BE THE NEXT ATTENTION MAKER. 

USED ECWMF/GEM FOR UPDATES THROUGH THURSDAY NIGHT. KEPT WPC
GUIDANCE FOR LATER ON AS MODEL VOLATILITY STILL GREAT TOWARD THE
WEEKEND'S WAVE. FORECAST CONFIDENCE IMPROVING FOR MAINLY DRY WEEK
ACROSS THE NORTHERN AND CENTRAL PANHANDLE. SOME SUNSHINE WILL 
PENETRATE THROUGH BREAKS IN CLOUDS OVER MUCH OF THE WEEK...WHERE
MOSTLY CLOUDY NOTED IN THE PUBLIC FORECAST.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...SMALL CRAFT ADVISORY FOR PKZ033-035-041>043-051-052. 

&&

$$

GARNER/ADKINS

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