Small Craft Advisory 769, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
346 PM AKST THU DEC 13 2012

.SHORT TERM...POST FRONTAL ENVIRONMENT OVER THE EASTERN GULF IS
QUITE UNSTABLE THIS AFTERNOON. SITKA WEATHER OBSERVER REPORTED
THUNDERSTORM ACTIVITY SOUTHEAST OF THE AIRPORT. HAVE BEEN
WATCHING VERY PRONOUNCED OPEN CELL CUMULUS CLOUDS IN SATELLITE
IMAGERY ALL DAY. CUMULONIMBUS CLOUDS ALSO VISIBLE IN WEB CAMS
TODAY. CARRIED SCATTERED TS OVER THE COASTAL MARINE ZONES AND
ISOLATED OVER PUBLIC ZONES ALONG THE OUTER COAST THROUGH THE
AFTERNOON. LOWER ATMOSPHERE SEEMS TO BE STABILIZING AND REDUCED
OPEN CELL CUMULUS IS FORMING FARTHER OUT TO SEA. ADDITIONALLY,
MODEL STABILITY PARAMETERS INDICATE GREATEST THREAT FROM
THUNDERSTORMS IS OVER AS OF LATE THIS AFTERNOON. HAVE THEREFORE
NOT INCLUDED ANY REFERENCE TO THUNDERSTORMS IN THE AFTERNOON
FORECAST.

A MAJOR FRONT IS HEADED TOWARDS THE PANHANDLE WITH A MODERATE BUT
CONTINUOUS TROPICAL CONNECTION. IT SEEMS CERTAIN THAT A
SIGNIFICANT PRECIPITATION EVENT IS HEADED OUR WAY. MODEL QPF
VALUES HAVE BEEN RUNNING A LITTLE HIGH SO FAR THIS SEASON AND WE
SAW THIS IN YESTERDAY'S EVENT WHERE ADVISORY LEVEL SNOW FALL ALONG
THE HAINES HIGHWAY WAS A BUST. WITH THIS IN MIND, LOOKED AT BOTH
GFS AND NAM QPF FIELDS TODAY AND THEY WERE BOTH VERY CLOSE. OPTED
TO USE THE NAM GRIDS AND TRIMMED OFF 0.1 INCH OF TOTAL WATER FROM
EACH 6-HOUR GRID. THIS MADE THE DIFFERENCE BETWEEN SNOW TOTALS
TOMORROW IN HAINES AND JUNEAU REQUIRING A WINTER WEATHER ADVISORY
AND NOT. AFTERNOON FORECAST REFLECTS SNOW FOR BOTH THESE
LOCATIONS, BUT LESS THAN 4 INCHES IN 12 HOURS. SNOW LINE HAS BEEN
A VERY FLUID MOVING TARGET THESE PAST FEW DAYS, BUT THE GENERAL
TREND WILL BE TO DRIVE THE SNOW LINE NORTH AND EAST AS THE WARM
SECTOR OF THE FRONT ADVECTS LOW LEVEL WARM AIR INTO THE FORECAST
AREA.

PRE-FRONTAL WINDS ARE VERY STRONG AND WILL PRODUCE WIND GUSTS OF
40 MPH OR MORE ALONG AT LEAST THE OUTER COAST. USED GFS FOR
PRESSURE AND WINDS AND BASED ON THIS MODEL, STRONG WINDS WILL
ARRIVE ALONG THE OUTER COAST LATE TOMORROW AFTERNOON JUST AFTER
THE END OF THE SHORT RANGE FORECAST PERIOD. NAM AND ECMWF WERE
BOTH A LITTLE FASTER WITH THE FRONT, SO IF THESE MODELS VERIFY,
THEN THE STRONG WINDS WILL ARRIVE ON THE OUTER COAST BETWEEN NOON
AND 6 PM INSTEAD.

BESIDES GFS FOR PRESSURE AND WIND, USED NAM FOR POP. MANUALLY
ADJUSTED THE QPF FIELDS DOWN. TEMPERATURES ADJUSTED USING SREF FOR
DAYTIME HIGHS AND GFS40 FOR OVERNIGHT LOWS. SNOW LEVEL MODIFIED
USING GFS. FORECAST CONFIDENCE IS AVERAGE.

.LONG TERM...THE SURFACE FRONT MENTIONED ABOVE WILL BE MAKING
LANDFALL ALONG THE OUTER COAST AT THE BEGINNING OF THE LONG RANGE
PERIOD EARLY SATURDAY EVENING. THIS IS A RELATIVELY FAST MOVING
FRONT AND WILL PRODUCE A 3 TO 6 HOUR PERIOD OF HIGHER WINDS ALONG
THE OUTER COAST AND THE INSIDE CHANNELS AS IT MAKES LANDFALL. WILL
NOT HEADLINE ANY LAND ZONES FOR STRONG WINDS AT THIS
TIME...HOWEVER THERE IS A POTENTIAL GUSTS COULD REACH NEAR 40 MPH
FOR A SHORT TIME SATURDAY EVENING. STEADY PRECIPITATION WILL
CONTINUE TO FALL ACROSS THE ENTIRE PANHANDLE THROUGH EARLY
SATURDAY. STRONG SOUTHERLY WINDS WILL TRANSPORT WARMER AIR INTO
THE REGION WITH MOST AREAS SEEING RAIN. THERE WILL STILL BE ENOUGH
COLD AIR IN THE NORTH-CENTRAL PANHANDLE FOR A RAIN AND SNOW MIX
DURING THE EVENT. FARTHER NORTH...COLD AIR WILL REMAIN ENTRENCHED
WITH SKAGWAY AND HAINES SEEING MOSTLY SNOW. SNOW ACCUMULATIONS ARE
EXPECTED TO REMAIN JUST BELOW ADVISORY CRITERIA WITH 2 TO 4 INCHES
ANTICIPATED FROM FRIDAY NIGHT INTO THE FIRST PART OF SATURDAY.

THE STACKED PARENT LOW OVER THE NORTHERN GULF ON FRIDAY NIGHT
WILL GRADUALLY WEAKEN AND SHIFT INTO THE SOUTHEAST GULF BY EARLY
MONDAY. AS COLDER AIR ALOFT WRAPS IN BEHIND THE FRONT...STEADY
PRECIPITATION WILL TRANSITION TO SHOWERS SATURDAY. DEEP LAYER
MOIST ONSHORE FLOW WILL PERSIST OVER THE WEEKEND WITH NUMEROUS
SHOWERS DEVELOPING OVER THE GULF AND MOVING INTO SOUTHEAST ALASKA.
AS HIGH PRESSURE BUILDS IN FROM THE INTERIOR A NORTHERLY PRESSURE
GRADIENT WILL ESTABLISH ITSELF LATE SATURDAY INTO SUNDAY WITH
NORTHERLY WINDS INCREASING OVER THE NORTHERN AND CENTRAL
PANHANDLE. THIS WILL SHIFT THE RAIN-SNOW LINE FARTHER TO THE SOUTH
OVER THE WEEKEND.

PRECIPITATION CHANCES WILL DIMINISH FOR MUCH OF THE AREA EARLY
NEXT WEEK AS LOW AND MID LEVEL FLOW TURN OFFSHORE. MODELS ARE
INDICATING THIS WILL BE SHORTLIVED AS A STRENGTHENING STORM SYSTEM
CROSSES THE ALEUTIANS AND MOVES INTO THE CENTRAL GULF LATER
TUESDAY INTO WEDNESDAY. THERE STILL CONTINUES TO BE NOTABLE MODEL
DIFFERENCES IN TIMING...STRENGTH...AND EVENTUAL TRACK OF THIS
SYSTEM.

USED THE GFS THROUGH 00Z MONDAY DUE TO ITS INITIALIZATION ON THE
DEVELOPING WAVE NEAR THE ALEUTIANS. AFTER THAT POINT USED MAINLY A
BLEND OF EC/HPC. FORECAST CONFIDENCE IS ABOVE AVERAGE THROUGH MUCH
OF THE WEEKEND BUT DECREASES NEXT WEEK AS MODELS CONTINUE TO HAVE
DIFFICULTY WITH THE PATTERN.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING FOR PKZ022-033-035-036-041>043-051-052.
         SMALL CRAFT ADVISORY FOR PKZ011>013-021-031-032-034-053.

&&

FRITSCH/TPS