Gale Warning 426, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
608 AM AKST FRI DEC 7 2012

.SHORT TERM...BRIEF BREAK IN THE ACTION DURING THE SHORT TERM
FORECAST PERIOD. A MID LEVEL CIRCULATION IS VISIBLE ON SATELLITE
AND RADAR IMAGERY JUST OFFSHORE THE CENTRAL OUTER COAST EARLY
THIS MORNING. ASSOCIATED UPPER LEVEL CLOUDS ARE THINNING AS THE
WHOLE CLOUDS SHIELD MOVES GENERALLY TO THE NORTH INTO CANADA.
LOWER LEVEL OPEN CELL CUMULUS CLOUDS AND RELATED SHOWER ACTIVITY
FROM THIS SYSTEM CONTINUES TO FLOW ASHORE BUT WILL DIMINISH
STEADILY THROUGH THE DAY AS THE LOW ITSELF WEAKENS AND DISSIPATES
BY TONIGHT. CLOSE PROXIMITY OF THE LOW TO THE COAST AND WITH
STRONG HIGH PRESSURE OVER WESTERN CANADA STILL IN PLACE HAS
INTENSIFIED THE OFFSHORE PRESSURE GRADIENT. EARLY MORNING WINDS
IN SKAGWAY OF 30 TO 35 MPH WITH FREQUENT GUSTS TO 50 MPH WITH
BLOWING SNOW ARE A RESULT. ALSO HAVE 40 TO 45 KT NORTHERLY WINDS
IN LYNN CANAL, BUT THESE WINDS AS WELL AS THOSE IN SKAGWAY WILL
DIMINISH THROUGH THE DAY AS THE LOW OFF THE COAST WEAKENS AND
FILLS. SHOWER ACTIVITY WILL MOVE OFF TO THE SOUTHEAST AS IT
DIMINISHES THROUGH TONIGHT.

APPROACHING FRONTAL SYSTEM FROM THE WEST WILL NOT IMPACT
SOUTHEAST ALASKA OR THE INNER CHANNELS DURING THE SHORT TERM
FORECAST PERIOD. PRE-FRONTAL WINDS OVER THE GULF OF ALASKA,
HOWEVER, WILL BE ON THE INCREASE. WESTERLY WINDS ASSOCIATED WITH
THE LOW OFF THE COAST WILL DIMINISH STEADILY THROUGH THE DAY AND
BECOME SOUTHEASTERLY LATE THIS EVENING. WINDS SPEEDS WILL INCREASE
FROM WEST TO EAST ACROSS THE GULF FOR THE REMAINDER OF THE NIGHT
AND REACH GALE FORCE OVER WESTERN PORTIONS OF OFFSHORE MARINE ZONE
310 BY TOMORROW MORNING.

GOOD MODEL AGREEMENT AMONGST ALL MODELS THROUGH THE SHORT TERM.
USED NAM FOR PRESSURE AND WIND IN ORDER TO CAPITALIZED ON BETTER
MODEL RESOLUTION. BLENDED NAM AND GFS FOR POP AND QPF. TEMPS
ADJUSTED USING BIAS CORRECTED MOS. NO SIGNIFICANT CHANGES TO SNOW
LEVELS. FORECAST CONFIDENCE IS ABOVE AVERAGE.

.LONG TERM...AN AREA-WIDE SNOW EVENT WILL BEGIN SAT, FOLLOWED BY
A GRADUAL S TO N SEQUENCE OF CHANGING TO RAIN. THIS SEQUENCE WILL
OCCUR FOR ALL OF OUR AREA OF RESPONSIBILITY EXCEPT THE NORTHERN
PANHANDLE. CHANGES TO THE LONG RANGE FORECAST ARE SMALL. THE
CONFIDENCE IS AVERAGE.

ABOUT 12Z SUN: AN OCCLUDED FRONT IS TO MOVE ONSHORE THE EAST GULF
COAST, AND THE MODELS ARE CONSISTENT ON PROVIDING THIS TIMING.
THIS SYSTEM IS LARGE AND AT 12Z SUN WILL HAVE A PARENT LOW OF
ABOUT 969 MB TRACKING INTO BRISTOL BAY AT THAT TIME. THE ONSHORE
FLOW OF MARITIME AIR, AS DISCUSSED IN THE FOLLOWING PARAGRAPH,
IS ASSOCIATED WITH THE FRONTAL SYSTEM STRUCTURE AND CYCLONE
EVOLUTION.

THE TOUGHEST FORECAST ISSUES ARE ASSOCIATED WITH HOW THE
MARITIME AIRMASS BEING MOVED ONSHORE IS TO OVERRUN THE PRE-
EXISTING MODIFIED ARCTIC AIRMASS. THE ARCTIC FRONTAL BOUNDARY
DOES NOT MOVE N, BUT IS MODIFIED AWAY PROGRESSIVELY INLAND.
PARTIAL THICKNESS VALUES FOR 1000-850 MB HAVE BEEN GENERALLY
FOLLOWED FROM MODEL GUIDANCE, BUT BOUNDARY LAYER CONDITIONS
ARE A TOUGH TOPIC FOR THE MODELS. A SUBJECTIVE ADJUSTMENT
FOR THE COLDER AND LOWER LEVEL AIRMASS WAS USED AND DOES
DELAY THE CHANGEOVER TO MIXED PRECIPITATION BY ABOUT 6 HOURS.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...STRONG WIND UNTIL 9 AM AKST THIS MORNING FOR AKZ018.
MARINE...HEAVY FREEZING SPRAY WARNING FOR PKZ012.
         GALE WARNING FOR PKZ012-013.
         SMALL CRAFT ADVISORY FOR PKZ011-021-022-034-035-041-042-052.
         SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ043.

&&

FRITSCH/JBT