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FXAK67 PAJK 011333
AFDAJK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
533 AM AKDT WED APR 1 2015

.SHORT TERM...A VORTICITY SPINNER OFFSHORE OF CHICHAGOF ISLAND,
IS ENHANCING SHOWER ACTIVITY ACROSS SOUTHEAST ALASKA THIS MORNING.
AS OF 12Z, THE WEAK UPPER LOW IS ALREADY MOVING ONSHORE AND AS IT
DOES SHOWERS WILL REMAIN NUMEROUS THROUGH THE DAY ACROSS THE
NORTH WITH SOME ENHANCEMENT ALONG WEST-FACING SLOPES OF THE
CHUGACH AND COAST MOUNTAINS. DEPENDING ON THE SPEED OF THIS
FEATURE, LATE AFTERNOON SHOWERS MAY BEGIN TO DISSIPATE AND THERE
MAY BE A BRIEF PERIOD OF SOME CLEARING BEFORE THE CIRRUS SHIELD
FROM THE NEXT SYSTEM GREYS SKIES ONCE AGAIN THIS EVENING. DUE TO
A RIDGE BUILDING OVER THE EASTERN GULF, THIS UPPER LEVEL FEATURE
AND ITS WEAK SURFACE REFLECTION WILL WEAKEN THROUGH THE DAY AS IT
CANNOT COPE WITH THE MORE ANTICYCLONIC FLOW ALOFT.

AS A PARENT LOW IN THE FAR SOUTHWEST GULF TRACKS EASTWARD THROUGH
THURSDAY, ITS ASSOCIATED FRONT WILL HELP PROGRESS THE RIDGE OVER
THE EASTERN GULF EASTWARD INTO CANADA BY LATE TONIGHT. THE FRONT
WILL SPREAD RAIN INTO THE GULF TODAY, REACHING THE SOUTHEAST
ALASKA COAST TONIGHT WITH AMOUNTS GENERALLY LESS THAN A HALF INCH.
LOCALLY...AMOUNTS MAY STACK UP TO THREE QUARTERS INCH ACROSS
SOUTHERN BARANOF ISLAND. LIGHT RAIN WILL EXTEND DEEPER INLAND
DRIVEN BY THE 700 MB ONSHORE FLOW AS THE WARMER AND MORE MOIST
MARINE AIR MASS GENTLY RIDES OVER THE SLIGHTLY COOLER AIR MASS
OVER THE PANHANDLE. MODELS ARE ADVECTING OMEGA FIELDS ONSHORE AS
EARLY AS THIS EVENING SUPPORTING THE FORWARD ADVANCE OF THE PRECIP
SHIELD WELL EARLIER THAN FRONTAL ARRIVAL, BUT SOME INEVITABLE
OFFSHORE FLOW AT LOWER LEVELS WILL HELP LIMIT QPF ACROSS THE
INTERIOR ZONES WITH THE FRONT`S ORIENTATION.

THE FRONT WILL RAISE WIND SPEEDS TO 30 KT ACROSS THE 310 ZONE
TODAY, AND AS IT NEARS SHORE, A SOUTHEAST/EAST GALE FORCE BARRIER
JET WILL DEVELOP ALONG THE OUTSIDE WATERS TONIGHT. LOOKING INSIDE,
SOME ACCENTUATION OF SOUTHERLIES WILL EVOLVE THIS MORNING AS THE
WEAK CIRCULATION IN THE NORTHEAST GULF MOVES INTO THE NORTHERN
PANHANDLE BUT THESE WILL GENERALLY SLACKEN IN THE AFTERNOON.
HOWEVER, AS WEAK RIDGING SPRINGS NORTH ACROSS THE REGION BEHIND
THE SYSTEM, SOUTHERLIES WILL RE-ENHANCE THIS AFTERNOON THROUGH
LYNN CANAL AND SKAGWAY BEFORE GRADIENTS FLIP WINDS TO GENERALLY
LIGHT NORTHERLY IN ANTICIPATION OF THE FRONT TONIGHT. SOME WEAK
OUTFLOW WINDS ARE POSSIBLE DOWNTOWN JUNEAU LATE TONIGHT, AS WINDS
ALOFT BRIEFLY FORM A CRITICAL LEVEL BUT THE SETUP IS LESS THAN
IDEAL AS CROSS BARRIER FLOW NOT TOO STRONG, SO CONCERN IS MINIMAL.
WIND SPEEDS WILL STRENGTHEN TONIGHT OVER THE SOUTHERN AREAS AS A
LOW LEVEL SOUTHERLY JET IMPOSES SOUTHERLIES OVER CHATHAM STRAIT
AND SOUTHERN PARTS OF STEPHENS PASSAGE, EVEN IN VIOLATION OF A
SLIGHT NORTHERLY PRESSURE GRADIENT, ESPECIALLY LATE. LEFT 30 KT
FOR CLARENCE STRAIT AFTER MIDNIGHT TONIGHT, BUT A MINIMAL GALE IS
POSSIBLE WITH 6 MB OF GRADIENT BETWEEN SITKA AND KETCHIKAN. DID
NOT PLACE STRONG WIND HEADLINES FOR 27 AND 28 LATE TONIGHT, BUT DO
THINK 30 TO 35 MPH IS LIKELY FOR THESE AREAS, SO DEFINITELY EXPECT
BREEZY CONDITIONS ALONG WITH MODERATE BANDS OF RAIN AT TIMES.

BROAD-BRUSHED MODEL DIFFERENCES WITH THE FRONT AND ANY WAVES ALONG
IT BY USING A NAM/ECMWF/GFS BLEND. ONE CONCERN IS THAT THE ECMWF
STILL QUICKER WITH ALL TRANSITING FEATURES. 06Z NAM HAS SPED
THINGS UP AS WELL. THE FORECAST MAY BE A LITTLE TOO WET LATE THIS
AFTERNOON, EARLY THIS EVENING. IF OUTDOOR ACTIVITIES NEED TO BE
DONE, THIS MAY BE THE TIME TO DO THEM.

.LONG TERM...VERTICALLY STACKED LOW IN THE AK GULF THURSDAY WILL
BEGIN TO TRACK TO THE SOUTHEAST FRIDAY NIGHT INTO FRIDAY WHILE THE
ASSOCIATED WEATHER FRONT CROSSES OVER THE PANHANDLE. WINDS OVER
THE COASTAL WATERS WILL DROP OFF SHARPLY THURSDAY MORNING WITH
PANHANDLE AND INNER CHANNEL WINDS PICKING UP THEN DIMINISHING
LATER IN THE DAY AS THE FRONT MOVES OVER THE REGION. GALES FORCE
WINDS ALONG THE COASTAL WATERS WILL DROP OFF WITH BARRIER JET
REAMING A FEW HOURS LONGER IN THE NORTHERN AK GULF. 850 MB WINDS
IN THE 40 TO 45 KT RANGE ALONG THE FRONT AND WITH THE RELATIVELY
WEAK LOW CENTER NOT EXPECTING OVERLAND WINDS TO GET MUCH ABOVE 25
MPH WITH INNER CHANNEL MARINE WINDS JUT IN THE MAX SMALL CRAFT
LEVELS. NEW MODEL RUNS HAVE THE LOW CENTER FURTHER TO THE SOUTH
INITIALLY ON THURSDAY WHICH WOULD INDICATE LOWER POPS FOR THE
NORTHER COASTAL REGIONS, HOWEVER WHILE THE WAS FAIR AGREEMENT
BETWEEN THE ECMWF AND GFS FOR PRESSURE AND WINDS THERE IS MORE
SPREAD WITH POP FOR THAT REGION. DECIDED TO GO WITH RAIN LIKELY
FOR THE YAKUTAT REGION. EXPECT CATEGORICAL POPS FOR THE REMAINDER
OF THE PANHANDLE WITH PRECIP DIMINISHING FROM NORTH TO SOUTH AS
THE LOW PROGRESSES TOWARDS THE N PACIFIC. MOVING INTO THE WEEKEND
EXPECTING DRIER WEATHER FOR MOST OF THE REGION WITH BUILDING HIGH
PRESSURE, WITH THE EXCEPTION OF TWO POTENTIAL SHORT WAVES, ONE
HITTING THE YUKON AND FAR NORTHERN PANHANDLE SATURDAY EVENING AND
A SECOND WAVE IN ADVANCE OF NEXT WEEKS MORE DEFINED FEATURE SUNDAY
EVENING. LOW CONFIDENCE ON THE YUKON WAVE EFFECTING THE NORTHERN
PANHANDLE. THE NEXT LOW SHOULD TRACK FROM THE BERING SEA TO THE
SOUTHEAST, REFORMING IN THE WESTERN AK GULF SUNDAY. THIS IS
FOLLOWED BY A STRONGER, BROADER LOW FOLLOWING A SIMILAR TRACK
REACHING THE AK GULF TUESDAY INTO TUESDAY NIGHT. TEMPERATURES
BEGIN TO SHOW A LARGER DIURNAL RANGE BY THE WEEKEND DUE TO SOME
CLEARING SKIES AND OVERNIGHT RADIATIONAL COOLING IN ADDITION TO
NORTHWESTERLY FLOW WITH THE BUILDING RIDGE MOVING DOWN COLDER AIR
FROM THE INTERIOR. OVERNIGHT LOWS MAY BE LOW ENOUGH FOR A MIX OF
SNOW FOR THE NORTHERN THIRD OF THE PANHANDLE AND AT HIGHER
ELEVATIONS WITH THESE SHORTWAVES. HOWEVER THIS COLDER AIR WILL BE
TIMED WITH DRIER AIR SO NOT EXPECTING ANY SIGNIFICANT SNOW.
DAYTIME HIGHS WILL BE IN THE 40S SO ANY DAY TIME PRECIP WILL FALL
AS RAIN.

MODELS WERE IN FAIR AGREEMENT INITIALLY, BUT THESE NEW RUNS DID
SHIFT THE LOW FURTHER TO THE SOUTH. BY THE WEEKEND MODEL SPREAD
INCREASES SIGNIFICANTLY. USED A BLEND OF ECMWF/GEM/GFS WHICH DID
SHIFT POSITION OF THE INITIAL LOW BUT OTHERWISE CHANGES TO THE
FORECAST WERE MINIMAL THROUGH THE WEEKEND. STARTING SATURDAY BEGAN
TO TRANSITION TO INHERITED WPC ENSEMBLE GRIDS TO THE LARGE
OPERATIONAL MODEL SPREAD. FORECAST CONFIDENCE IS ABOVE AVERAGE
INITIALLY BUT DROP BELOW AVERAGE BY THE WEEKEND.


&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING FOR PKZ041>043-051-052.
         SMALL CRAFT ADVISORY FOR PKZ021-022-033>036-053.

&&

$$

JWA/PRB

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