Small Craft Advisory 758, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
306 PM AKST MON DEC 10 2012

.SHORT TERM...A LOW IN THE EASTERN GULF IS APPROACHING THE OUTER
COAST BETWEEN CAPE EDGECUMBE AND CAPE SPENCER. SATELLITE IMAGERY
AND MODEL DATA INDICATING THAT THE FRONT IS FULLY OCCLUDED OR
NEARLY SO AS OF LATE THIS AFTERNOON. THE CENTER OF CIRCULATION
HAS MIGRATED INTO THE COLD AIR MASS TO THE NORTHWEST OF THE WARM
SECTOR AND MAXIMUM THICKNESS PACKING IS NOW WELL TO THE SOUTH. IN
ADDITION, A SIGNIFICANT AMOUNT OF LOW AND MID LEVEL COLD AIR
ADVECTION CAN BE SEEN ON THE BACK SIDE OF THE LOW OVER THE
CENTRAL GULF. BASED ON THESE OBSERVATIONS, BELIEVE TEMPERATURES
OVER SOUTHEAST ALASKA WILL SLOWLY FALL THROUGH THE NIGHT AND THEN
CONTINUE TO FALL THROUGH THE DAY TOMORROW. SHOWER ACTIVITY WILL
RETREAT TO THE SOUTHEAST THROUGH THE SHORT TERM FORECAST PERIOD
BUT WILL FOR THE MOST PART STAY AHEAD OF THE RAIN/SNOW LINE AS IT
MOVES SOUTH SO AM NOT EXPECTING TOO MUCH SNOW OUT OF THIS FRONT.
HAVE 1 TO 2 INCHES OVER THE FAR NORTHERN INNER CHANNELS WHERE THE
AIR IS COLDEST, BUT MOSTLY RAIN SHOWERS ELSEWHERE WITH A LITTLE
SNOW MIXED IN AT TIMES.

 THE APPROACHING LOW WILL ALSO ACT TO REVERSE THE SOUTHERLY
PRESSURE GRADIENT THAT WAS SET UP OVER THE NORTHERN INNER
CHANNELS THROUGH THE DAY TODAY. BY MID AFTERNOON, THE SENSOR AT
SKAGWAY WAS ALREADY REPORTING A VARIABLE NORTHEAST WIND AND WINDS
IN GUSTAVUS HAD ALREADY SHIFTED TO THE NORTHWEST. BY LATE
EVENING, BELIEVE WINDS IN HAINES WILL BE NORTHWESTERLY, WITH
NORTHERLY WINDS OVER SKAGWAY AND LYNN CANAL. SITUATION SHOULD
REMAIN AS SUCH THROUGH THE DAY TUESDAY. HAVE SMALL CRAFT WINDS
OVER SEVERAL OF THE INNER CHANNELS. ONLY GALES IN THE AFTERNOON
FORECAST ARE ASSOCIATED WITH THE LOW OFFSHORE AND WILL BE IN THE
SOUTHWEST QUADRANT OF THE STORM OVER MARINE ZONES 310, 41, AND
42.

 USED ECMWF FOR PRESSURE BUT BIASED WIND SPEEDS TOWARDS GFS.
ADJUSTED POP AND QPF FIELDS WITH ECMWF AND GEM. TEMPERATURES
MODIFIED USING A COMBINATION OF GFS AND NAM. FORECAST CONFIDENCE
IS AVERAGE.

&&

.LONG TERM...MEDIUM RANGE MODELS ARE IN GENERAL AGREEMENT THROUGH
THURSDAY EVENING WITH AN UPPER LEVEL RIDGE OVER THE GULF OF ALASKA
SLOWLY DRIFTS EASTWARD ACROSS THE PANHANDLE AS A WEAK UPPER LEVEL
SHORTWAVE TROUGH MOVES OVER THE CENTRAL GULF. THE ECMWF HAS SHOWN
A GOOD CONSISTENCY AND WAS USED TO REFRESH THE INHERITED FORECAST
PACKAGE THROUGH THURSDAY EVENING. THIS UPPER LEVEL RIDGE WILL
GRANT SOUTHEAST ALASKA A SHORT BREAK FOR RELATIVELY DRY WEATHER.
AN INVERTED SURFACE TROUGH OF LOW PRESSURE ACROSS THE EASTERN GULF
ON TUESDAY NIGHT WILL GRADUALLY ERODE AS A FRONTAL
PASSAGE...ASSOCIATED WITH THE WEAK UPPER LEVEL SHORTWAVE...MOVES
INTO THE AREA ON WEDNESDAY.

 THE FRONTAL BOUNDARY WILL SWEEP ACROSS SOUTHEAST ALASKA THROUGH
THURSDAY. SLIGHTLY COLDER AIR MASS EMBEDDED WITHIN THE FRONTAL
PASSAGE WILL APPROACH TO THE PANHANDLE COAST ON WEDNESDAY AND AN
OCCLUDED FRONT WILL MAKE A LANDFALL DURING WEDNESDAY NIGHT.
RAIN/SNOW BOUNDARY STILL EXISTS ACROSS THE CENTRAL PANHANDLE WITH
POSSIBLE SIGNIFICANT SNOWFALL ON WEDNESDAY NIGHT OVER INLAND
LOCATIONS.

 FOR THE LONGER RANGE FORECAST...INTERACTIONS BETWEEN MID TO
UPPER LEVEL ARCTIC SYSTEM AND UNDERCUTTING SOUTHERN STREAM
IMPULSES KEEP SOMEWHAT ACTIVE WEATHER PATTERN OVER SOUTHEAST
ALASKA WITH INCREASING UNCERTAINTY. THE DIFFERENCES OF MODEL
DEPICTION FOR INTENSITY AND TIMING GROW BIGGER BY THE END OF THE
WEEK. LEANED OVER THE HPC ENSEMBLE APPROACH OF HEAVIER WEIGHTING
OVER THE ECMWF FOR THE REMAINING PERIODS. THIS APPROACH MAINTAINS
A GOOD CONSISTENCY FOR THE LONGER RANGE PREDICTION. BASED ON THE
ENSEMBLE MEAN...A LOW PRESSURE SYSTEM WILL MOVE ACROSS THE GULF ON
FRIDAY AND ANOTHER LOW OVER THE WEEKEND. THE PERSISTING WESTERLY
JET WILL KEEP SOUTHEAST ALASKA WITH HIGHER POPS THROUGH EARLY NEXT
WEEK.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING FOR PKZ041-042.
         SMALL CRAFT ADVISORY FOR PKZ011>013-021-022-033>036-043-051-052.
         SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ053.

&&

FRITSCH/AHN