Small Craft Advisory 742, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
230 PM AKST TUE DEC 4 2012

.SHORT TERM...SATELLITE IMAGERY THIS AFTERNOON SHOWING THE BROAD
UPPER LOW PRESSURE STILL OVER THE AK GULF WITH ARCTIC HIGH
PRESSURE OVER THE YUKON. THE MAIN LOW PRESSURE CIRCULATIONS AT
THE SURFACE ARE LOCATED NEAR 58N 140 W WITH A SECONDARY
CIRCULATION TO THE SOUTH OVER THE COAST OF SITKA. A WAVE TO THE
NORTHWEST OF HAIDA GWAII LOOKS TO SHOW SOME WEAK CYCLOGENESIS BUT
AT THIS TIME MODELS HAVE NOT INDICATED MUCH MORE WILL HAPPEN WITH
THIS WAVE AS IT SWING TO THE NORTH EAST. THE NEXT LOW IS
CURRENTLY OVER SOUTHERN BRITISH COLUMBIA WITH OCCLUSION MOVING
OVER THE COAST MOUNTAINS AND THE EASTERN PORTIONS OF THE
PANHANDLE. THIS ENTIRE MASS WILL CONTINUE TO ROTATE COUNTER
CLOCKWISE AROUND THE GULF WITH THE CURRENT FEATURES BEGINNING TO
WEAKEN THROUGH WEDNESDAY.

 PRECIPITATION MAINLY OVER THE SOUTHERN PANHANDLE ASSOCIATED WITH
THE LOW OVER BRITISH COLUMBIA AND THE NEXT WAVE IN THE
SOUTHEASTERN GULF. WITH COLD AIR ALOFT ALONG THE BASE OF THE
UPPER LEVEL LOW EXPECTING PRECIP TO BE SHOWERY IN NATURE. THE
ARCTIC FRONT IS EXTENDED FROM CAPE SPENCER ALONG THE PANHANDLE
COAST THEN TO SUMNER STRAIT. NORTH OF THIS LINE SHOWER ACTIVITY
HAS BEEN SCATTERED WITH A FEW CLEAR BREAKS THIS AFTERNOON. SEEING
STEADIER LIGHT SNOW FALL NEAR WRANGLE WITH HEAVIER SNOW NEAR
PETERSBURG. RAIN OVER THE SOUTHERN PANHANDLE HAS BEGUN TO MIX
WITH SNOW AS COLD AIR CONTINUES TO MOVE IN. BIT OF UNCERTAINTY
WITH THE TRACK OF ANY PRECIP BANDS AS THEY MOVE TO THE NORTH. THE
IS A SPLIT BETWEEN MODELS WITH A MAJORITY OF THE PRECIP MOVING
OVER THE EASTERN PANHANDLE WITH THE WAVES SHEARING APART AS THEY
MORE NORTH...OR PRECIP MOVING ALONG THE WESTERN COAST AND
INTENSIFYING AS THEY NEAR CROSS SOUND BY WEDNESDAY. AS NO ONE
MODEL HAS BEEN HANDLING THIS WAVES VERY WELL WENT WITH MORE OF A
SHOT GUN APPROACH. THIS IS MORE AGREEMENT ON QPF AMOUNTS SO FOR
NOW THE THINKING IS 12 HOUR SNOW FALL TOTALS WILL BE LESS THEN
3 INCHES IN THE HEAVIER BANDS.

 STRONGEST WINDS CONTINUE OVER THE NORTHERN PANHANDLE DUE TO THE
PRESSURE GRADIENT BETWEEN THE YUKON HIGH AND THE GULF LOW. AS THE
YUKON HIGH DRIFTS TO THE SOUTHEAST IN TO BRITISH COLUMBIA AND THE
GULF LOW BEGINS TO WEAKEN WILL SEE WINDS DIMINISH THROUGH
WEDNESDAY ALONG WITH THE WEAKENING GRADIENT. CONTINUED FREEZING
SPRAY THROUGH WEDNESDAY MORNING WITH HEAVY FREEZING SPRAY OVER
THE NORTHERN MOST CHANNELS. WITH THE DIMINISHING WINDS AND
INCREASING TEMPERATURES STARTED TO END THE FREEZING SPRAY THROUGH
THE AFTERNOON.

 MODELS WERE IN GENERAL AGREEMENT WITH THE OVERALL SYNOPTIC
PATTERN AND GENERAL WIND FLOW IN THE AK GULF. WITH THE SHOWERY
NATURE OF THE INCOMING PRECIPITATION BANDS NO ONE MODEL HAS
HANDLE THE MESO SCALE FEATURES VERY WELL. USED A BLEND OF GFS/NAM
FOR PRESSURE AND WINDS BUT AN ECMWF/SREF FOR POP. MAINLY AS ECMWF
LOOK TO HAVE SOME ISSUES WITH THE PRESSURE GRADIENT OVER THE
YUKON WHILE THE GFS KEPT MOST PRECIP OFF SHORE THROUGH WEDNESDAY.
NOT VERY HIGH CONFIDENCE IN THE LOCATION SHOWERS AND SNOWFALL
TOTALS.

 &&

 .LONG TERM...EXTENDED FORECAST IS SOMEWHAT OF A MESS AS THE LONG
RANGE MODELS CAN NOT QUITE AGREE ON WHAT IS GOING TO HAPPEN. THIS
IS MORE SO FOR THE WEEKEND RATHER THEN THE LATE WEEK PERIOD THIS
WEEK.

 FOR THE LATE WEEK PERIOD THE LOW THAT HAS BEEN WANDERING AROUND
THE GULF FOR THE PAST FEW DAYS WILL BE APPROACHING THE PANHANDLE
AGAIN. MODELS ARE AT ODDS ON HOW FAST IT WILL APPROACH. NAM IS
THE FASTEST BUT IT ALSO HAS BEEN THE LEAST CONSISTENT ON WHERE
THE LOW GOES ONCE IT GETS HERE. GFS AND ECMWF HAVE BEEN MORE
CONSISTENT ON WHERE IT GOES AND ON WHERE THE ASSOCIATED PRECIP
BANDS FORM UP, BUT THE ECMWF IS A BIT SLOWER WITH LOW PROGRESSION
THAN THE GFS. DECIDED ON A BLEND OF THE ECMWF AND GFS HERE TO GET
A DECENT COMPROMISE FROM BETWEEN THE TWO GLOBAL MODELS. AS FOR
THE RESULTS. A MODERATE STRENGTH BAND OF PRECIP WILL FORM SE TO
NW ALONG THE CENTRAL AND SOUTHERN PANHANDLE THURS EVENING AND MAY
BRING MORE SNOW TO THE CENTRAL PANHANDLE WITH LIGHTER
ACCUMULATIONS FARTHER NORTH.

 FOLLOWING THIS A BRIEF DRY SPELL WILL SHOW UP FOR FRI AFTERNOON
INTO FRI NIGHT BEFORE THE NEXT FRONT CHARGES ACROSS THE GULF.
WITH THIS FRONT MODEL DIVERGENCE BECOMES SEVERE. THE GFS IS MUCH
FASTER WITH BRINGING THE WARM FRONT ACROSS THAN THE ECMWF. IT IS
ALSO A LITTLE FASTER BRING THE COLD FRONT ACROSS THE GULF AS
WELL. THE NEWER 18Z GFS IS FASTER STILL IN BOTH REGARDS. THE
ENSEMBLE MEANS SEEM TO HAVE A MORE CONSISTENT HANDLE ON IT. STILL
THE SPREAD BETWEEN THE INDIVIDUAL MEMBERS IS RATHER LARGE.
DECIDED ON A BLEND WITH THE HPC GRIDS AND THEIR INCORPORATION OF
ENSEMBLE MEANS FOR THE EXTENDED RANGE. IN ANY CASE A WARM UP IS
IN STORE FOR THE PANHANDLE AS WE GET INTO WARM AIR ADVECTION
BEHIND THE WARM FRONT. HOWEVER THE WARM UP WILL NOT BE QUICK
ENOUGH FOR THE PRECIP TO NOT START OUT AS SNOW. AREAS IN THE
CENTRAL AND NORTHERN PANHANDLE HAVE THE POTENTIAL TO SEE ANOTHER
SNOW DUMP EITHER SAT NIGHT OR SUN BEFORE THE WARM AIR ADVECTION
CHANGES THAT SNOW OVER TO A MIX OR ALL RAIN. WINDS MIGHT ALSO BE
A PROBLEM ESPECIALLY ALONG THE OUTER COAST AND SOUTHERN PANHANDLE
IN THE SE FLOW AHEAD OF THE FRONT AND THE SW FLOW BEHIND IT.
BEYOND THE WEEKEND SHOWERS ARE THE RULE OF THE DAY FOR THE REST
OF THE EXTENDED PERIOD.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...STRONG WIND UNTIL 3 PM AKST THIS AFTERNOON FOR AKZ019.
         STRONG WIND THROUGH LATE TONIGHT FOR AKZ018.
MARINE...HEAVY FREEZING SPRAY WARNING FOR PKZ011.
         HEAVY FREEZING SPRAY WARNING FOR PKZ012.
         HEAVY FREEZING SPRAY WARNING FOR PKZ013.
         STORM WARNING FOR PKZ012.
         GALE WARNING FOR PKZ011-013-021-022.
         SMALL CRAFT ADVISORY FOR PKZ031>036-042-043-051-052.
         SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ041.

&&

PRB/EAL

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