Gale Warning 415, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
129 PM AKST TUE DEC 15 2009

.SHORT TERM...COMPLICATED SHORT TERM FORECAST PERIOD. MAIN
FORECAST CHALLENGE CONTINUES TO BE HOW MUCH SNOW WILL FALL AND
WHERE...FOLLOWED BY IF AND WHEN THE TRANSITION TO MIXED PRECIP
WILL OCCUR. MADE A FEW CHANGES TO THE ENDING TIMES OF THE CURRENT
WINTER STORM ADVISORIES AND ADDED ZONES 17 AND 29. KEPT THE
WINTER STORM WARNING AS IS FOR ZONE 19. ALSO EXTENDED THE WIND
ADVISORY FOR ZONE 29 THROUGH WEDNESDAY...BUT THAT SHOULD BE IT
FOR WIND DOWN THERE AS THE PRESSURE GRADIENT SHOULD BEGIN TO
RELAX AFTER TOMORROW.

 A WEAK LOW JUST OFFSHORE PRINCE OF WALES ISLAND IS CURRENTLY
VISIBLE IN SATELLITE IMAGERY BUT IS FADING FAST AND BECOMING LOST
IN THE CLOUD STRUCTURE ASSOCIATED WITH THE ADVANCING FRONT.
ANOTHER LOW WEST OF WASHINGTON AND SOUTH OF THE QUEEN CHARLOTTES
IS FORECAST TO TRACK NNE OVER THE CHARLOTTES...HECATE
STRAIT...AND OVER THE PANHANDLE VICINITY ZONE 29 LATE WEDNESDAY.
LOW PRESSURE OFFSHORE COMBINED WITH HIGH PRESSURE OVER BRITISH
COLUMBIA CONTINUES TO SUPPORT A VERY STRONG NNE PRESSURE GRADIENT
OVER THE SOUTHEAST PANHANDLE. NOT SEEING ANY SIGNIFICANT WIND AT
OUR ONLY DATA POINT IN ZONE 29...BUT GREY ISLET WINDS ARE
SUSTAINED 50 KTS OUT OF THE NORTHEAST...SO AM CONFIDENT THAT
ADVISORY LEVEL WINDS ARE OCCURRING THERE...ESPECIALLY IN
CHANNELED TERRAIN.

 RADAR CURRENTLY PAINTING PRECIPITATION AS FAR NORTH AS PORT
ALEXANDER. AS OF NOON...NO SNOW YET IN PETERSBURG OR WRANGELL.
CURRENTLY...THE PRECIP LINE LOOKS LIKE IT RUNS FROM PORT
ALEXANDER SSE TO HYDER. THIS IS RIGHT ON TRACK WITH PREVIOUS
FORECAST REASONING. DIFFICULTIES ARISE DUE TO A VERY COLD NEAR
SURFACE ARCTIC AIR MASS BEING OVER RUN BY A WARMER...MORE MOIST
AIR MASS. LOW LEVEL ARCTIC AIR IS DEEP ENOUGH TO ALLOW ALL
ADVISORY ZONES TO START OFF WITH STRAIGHT SNOW AND SURFACE
OBSERVATIONS ARE SUPPORTING THIS. ARCTIC AIR MASS IS NOT THAT
DEEP HOWEVER...SO THE BIG QUESTION IS WHETHER THE SNOW WILL
CHANGE TO RAIN AND IF SO...HOW LONG. IN SOUTHERN PORTIONS OF THE
PANHANDLE...WARMER AIR ALOFT IS EXPECTED TO RESULT IN FREEZING
RAIN AT THE SURFACE OVERNIGHT TONIGHT AND LATEST FORECAST SKEW-T
DATA SUGGEST THAT ZONE 26 MAY SEE SOME TOMORROW MORNING. FARTHER
NORTH IN THE VICINITY OF JUNEAU...BELIEVE THE TRANSITION FROM
SNOW TO MIXED PRECIP WILL OCCUR WITHOUT THE FREEZING RAIN.

 USED NAM FOR PRIMARY GUIDANCE THROUGH THE SHORT TERM. FORECAST
CONFIDENCE IS ONLY AVERAGE DUE TO UNCERTAINTIES REGARDING THE
VERTICAL STRUCTURE OF THE ATMOSPHERE AND HOW LONG THE LOW LEVEL
ARCTIC AIR WILL TAKE TO BE DISPLACED TO THE NORTH.

&&

.LONG TERM...ARCTIC VORTEX OVER SW ALASKA WED WILL DUMBBELL A
FAIRLY DEEP PACIFIC LOW FROM N PAC...ACROSS SE AK...AND INTO NW
CANADA BY THE WEEKEND. THIS WILL MAKE FOR A TRICKY FORECAST GOING
FROM SNOW...TO RAIN AND WIND...AND THE BACK TO A MIX OF RAIN AND SNOW
SHOWERS IN WESTERLY FLOW BY LATE FRI. MODEST AGREEMENT OF DETAILS
BETWEEN MODELS THROUGH THE WEEK. ALSO HAVE MODEST AGREEMENT IN
LONGER RANGE THAT A STRONG AND WARM PACIFIC HIGH WILL BUILD OVER THE
AREA BY EARLY NEXT WEEK. WILL ADJUST GRIDS TO NEW NAM THROUGH DAY
3.5 AND STICK WITH HPC BEYOND.

PACIFIC FRONT SHOULD BE N AND EAST OF AREA BY WEDNESDAY EVENING. A TROUGH
EXTENDING SE FROM ARCTIC VORTEX WILL BE ACROSS THE EASTERN GULF
MONDAY AFTERNOON BUT FORECAST TO STALL ACROSS NORTHERN PANHANDLE
THROUGH THURSDAY. MODELS INDICATE RATHER COLD AIR MASS WEST OF
THIS TROUGH AND A SOMEWHAT MILD AIR MASS OVER SE AK EAST OF
TROUGH. DO EXPECT SOLID SMCR TO GALE SLY TO DEVELOP ON THURSDAY-
FRIDAY WHICH WILL LIMIT IF NOT PREVENT ANY SIGNIFICANT SNOW FOR
ALL BUT FAR NW PORTION OF PANHANDLE AND PLAN TO EMPHASIZE RAIN.
EXCEPTION WILL BE FOR PAYA AND PAHN WERE COLD AIR WILL RE ENTRENCH
AND COULD SEE SIGNIFICANT SNOW BEFORE AND AFTER FRONT.

BY FRIDAY NIGHT CAA IN WESTERLY FLOW SHOULD BRING A RETURN TO SNOW
SHWRS OR MIX OF. STILL HAVE RELATIVELY MILD WATER IN GULF BUT
TRAJECTORY IS NOT TOO LONG. DIFFICULT TO DETERMINE IMPACT OF THESE
SHOWERS AT THIS TIME AND PLAN ON LEAVING THEM MIXED FOR NOW AN
RAISE TEMPS A BIT. A SECOND LOW MOVES ACROSS DIXON ENTRANCE ARE
EARLY SAT WITH ANOTHER ROUND OF LIGHT PRECIP FOR PANHANDLE. THIS
TIME SHOULD HAVE A NORTH SOUTH RAIN/SNOW SPLIT.

RIDGE BUILDS OVR PAC NW THIS WEEKEND AND EXPANDS TO EASTERN GULF
MON. WILL KEEP WEAK ONSHORE IN SOMEWHAT UNSTABLE ENVIRONMENT OVER
THE WEEKEND THAT BECOMES VERY STABLE NEXT WEEK WITH STRONG SURFACE
BASED INVERSION.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WINTER WEATHER ADVISORY UNTIL MIDNIGHT AKST TONIGHT FOR AKZ026.
         WINTER WEATHER ADVISORY UNTIL 6 AM AKST WEDNESDAY FOR AKZ027.
         WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO 6 AM AKST
     WEDNESDAY FOR AKZ021-024-025.
         WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO NOON AKST
     WEDNESDAY FOR AKZ020-022.
         WINTER WEATHER ADVISORY UNTIL NOON AKST WEDNESDAY FOR AKZ028.
         WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO 6 PM AKST
     WEDNESDAY FOR AKZ029.
         WIND ADVISORY UNTIL 6 PM AKST WEDNESDAY FOR AKZ029.
         WINTER STORM WARNING FROM MIDNIGHT TONIGHT TO NOON AKST
     WEDNESDAY FOR AKZ019.
         WINTER WEATHER ADVISORY FROM 6 AM TO 6 PM AKST WEDNESDAY FOR
     AKZ017.
MARINE...GALE WARNING FOR PKZ052.
         SMALL CRAFT ADVISORY FOR PKZ011-021-022-031-033>036-041>043-051.

&&

FRITSCH/MM