Area Forecast Discussion
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FXAK67 PAJK 281423
AFDAJK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
523 AM AKST FRI NOV 28 2014

.SHORT TERM...SATELLITE IMAGERY THIS MORNING SHOWING HIGH PRESSURE
OVER THE YUKON AND NORTHERN BRITISH COLUMBIA DRIFTING SOUTHWARD,
AN INVERTED TROUGH EXTENDING NORTHWARD OVER THE PANHANDLE, HIGH
PRESSURE RIDGE OVER THE CENTRAL AK GULF AND A FRONTAL BOUNDARY
EXTENDING FROM THE AK INTERIOR TO THE WESTERN GULF TRACKING
EASTWARD.

THE TIGHT PRESSURE GRADIENT BETWEEN THE YUKON AND BC HIGH AND THE
PANHANDLE INVERTED TROUGH IS WEAKENING AS BOTH FEATURES BEGIN TO
DRIFT SOUTHWARD AND DISSIPATE. THE STRONG OUTFLOW WINDS ON THURSDAY
DROPPED OFF THROUGH THE EVENING AND CURRENTLY THERE ARE ONLY A
FEW OVERLAND LOCATIONS WITH OUTFLOW GUSTS THIS MORNING. INNER
CHANNEL MARINER AREAS, ESPECIALLY N LYNN CANAL AND STEPHENS
PASSAGE STILL GETTING GALE FORCE WINDS BUT THESE WILL DIMINISH
THROUGH THE MORNING AND INTO THE AFTERNOON. WINDS OVER THE
SOUTHERN COASTAL WATERS WILL SHIFT FROM NORTHEAST TO NORTHWEST AS
THE INVERTED TROUGH DROPS TO THE SOUTH.

THE HIGH PRESSURE RIDGE OVER THE CENTRAL GULF WILL TRACKING INTO
THE EASTERN GULF TONIGHT WITH 500 MB VORT MAX RIDING OVER THE
RIDGE. BELOW THIS VORT MAX A WEAK LOW LEVEL TROUGH DEVELOPS OVER
THE NORTHEAST GULF AND EXTENDS ALONG THE COAST THROUGH FRIDAY
NIGHT. THIS WILL BE THE BEGINNING OF THE TRANSITION FROM THE CLEAR
SKIES AND DRY WEATHER. SLIGHT CHANCE TO CHANGE POPS DEVELOP ALONG
THIS FEATURE WITH INCREASING CLOUD COVER, BUT FOR NOW ANY PRECIP
SHOULD BE LIMITED TO COASTAL AREAS WITH A FEW FLURRIES BEING MOST
LIKELY. COLD CONTINENTAL POLAR AIR MASS WILL REMAIN IN PLACE
THROUGH FRIDAY NIGHT. AS WINDS DIMINISHED OVERNIGHT TEMPERATURES
BEGAN TO DROP WITH THE ABSENCE OF DOWNSLOPING. TONIGHT MAY BE BIT
COLDER AS WINDS WILL BE MUCH LIGHTER HOWEVER THERE WILL BE SOME
INCREASED CLOUD COVER WITH THE DEVELOPING TROUGH.

STILL SEEING GOOD AGREEMENT WITH THE NAM/ECMWF FOR THE NEXT 24
HOURS. GFS HAD THE NEXT TROUGH TOO FAR OFF SHORE. THE 06Z NAM AND
00Z ECMWF WERE CLOSE TO INHERITED GRIDS FOR PRESS AND WINDS SO
CHANGES WERE MINOR WITH THE NEW MODELS BLENDED IN. ECMWF AND SREF
POPS LOOKED A BIT HIGH SO KEPT WITH A BLEND OF 06Z NAM AND
PREVIOUS GRIDS. FORECAST CONFIDENCE IS AVERAGE.

.LONG TERM...MORE ACTIVE PATTERN IS IN STORE FOR THE AREA THIS
WEEKEND. LOW PRESSURE WILL DEVELOP OVER THE FAR NW GULF AND MOVE
TO THE NE GULF BY LATE SAT NIGHT...THEN TRACK SEWD ALONG THE OUTER
COAST OR JUST INLAND FROM THE COAST OF SE AK SUN AND SUN NIGHT.
THERE WERE MAINLY MINOR MODEL DIFFERENCES ON THE TRACK AND
STRENGTH OF THE LOW...SO USED MODEL BLEND OF THE 06Z NAM...00Z
ECMWF...AND 00Z GEM FOR SAT-SUN. THE GEM BECAME MORE OF AN OUTLIER
SUN NIGHT SO PHASED IT OUT OF THE BLEND FOR THEN. BLENDED THE
REMAINING MODELS WITH THE PREVIOUS FORECAST FOR MON TO SMOOTH
TRANSITION TO UNCHANGED PART OF THE FORECAST BEYOND THEN.

THE FIRST SIGNIFICANT SNOWFALL OF THE SEASON IS LIKELY FOR MUCH OF
THE AREA SAT NIGHT-SUN NIGHT. AS THE LOW MOVES ACROSS THE
GULF...WARM ADVECTION WILL DEVELOP PRECIP ALONG THE NRN GULF
COAST BY EARLY SAT EVENING...WITH SNOW RAPIDLY DEVELOPING EWD INTO
THE NRN AND CENTRAL INNER CHANNELS AND OUTER COASTAL AREAS LATER
SAT EVENING INTO SAT NIGHT AS LOWER LEVEL FLOW BACKS TO ONSHORE
AND OVERRIDES THE ARCTIC AIRMASS IN PLACE. THERE ARE SOME
DIFFERENCES IN HOW THE SHORTWAVE ALOFT DEVELOPS...WHICH WILL
IMPACT SNOW INTENSITY. THE MODELS IN THE BLEND GENERALLY SUGGEST
THAT A STRENGTHENING JET WILL DEVELOP ON THE BACK SIDE OF THE
SHORT WAVE...WHICH WILL AMPLIFY THE SHORTWAVE AND INCREASE LIFT
DOWNSTREAM OVER THE PANHANDLE. WHERE THE STRONGEST VORT FEATURE
GOES EXACTLY IS AT ISSUE THOUGH. ANOTHER COMPLICATING FACTOR IS
WEAK CONDITIONAL INSTABILITY ABOVE ABOUT 600 MB IN THE MODEL
SOUNDINGS...WHICH COULD CAUSE SOME EMBEDDED CONVECTION TO DEVELOP.
THERE IS ALSO GOOD LIFT FORECAST IN THE DENDRITE GROWTH ZONE
WHICH SHOULD INCREASE SNOWFALL. WHERE THE RAIN/SNOW LINE SETS UP
IS STILL A BIT UNCERTAIN ALTHOUGH IT IS VERY LIKELY THAT THE OUTER
COAST S OF CROSS SOUND WILL CHANGE TO RAIN LATER SAT NIGHT AND
SUN. THE PAJN-PAEL CORRIDOR AS WELL AS THE MORE ERN PARTS OF THE
INNER CHANNELS FROM PAJN TO HYDER WILL SEE THE MOST SNOW OUT OF
THIS SYSTEM. THINK MUCH OF THE INNER CHANNELS FROM PAHN SWD WILL
GET 4 INCHES OR MORE OF SNOW...BUT EXACT LOCATION OF THE HEAVIEST
SNOW AREA IS STILL UNCERTAIN DUE TO THE MODEL DIFFERENCES. ISSUED
A SPECIAL WEATHER STATEMENT TO HANDLE THIS EVENT.

BEFORE THE MAIN EVENT STARTS...WEAK SHORTWAVE WILL MOVE SE ALONG
THE CENTRAL AND SRN OUTER COAST SAT...WHICH MAY GENERATE SOME
LIGHT SNOW ALONG THE COAST AS SHALLOW OVERRUNNING OCCURS. KEPT
POPS AT CHANCE LEVELS THOUGH AS MODELS DIFFER ON WHETHER PRECIP
WILL OCCUR WITH THIS FEATURE.

BEYOND MON...DID NOT CHANGE THE FORECAST AS MAIN FOCUS WAS
UPCOMING WEEKEND SYSTEM. IF ANYTHING...CURRENT FORECAST MAY BE TOO
COLD AS DEEP LAYER TROF SETS UP TO THE WEST OF THE AREA AND RIDGE
AXIS BUILDS JUST DOWNSTREAM. THIS IS GENERALLY A WARM AND WET
PATTERN FOR THE AREA. LATER SHIFTS CAN ADJUST THE FORECAST AS
NEEDED.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING FOR PKZ012-031.
         SMALL CRAFT ADVISORY FOR PKZ013-022-032-034-035-041>043.
         SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ051.

&&

$$

PRB/RWT

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