Gale Warning 427, NWS Juneau, AK

763 
FZAK67 PAJK 091242
OFFAJK

OFFSHORE WATERS FORECAST
NATIONAL WEATHER SERVICE JUNEAU AK
400 AM AKST SUN DEC 9 2012 

OFFSHORE WATERS FORECAST FOR GULF OF ALASKA EAST OF 144W

WIND FORECASTS REFLECT THE PREDOMINANT SPEED AND DIRECTION
EXPECTED. SEA FORECASTS REPRESENT THE AVERAGE OF THE HIGHEST
ONE-THIRD OF THE COMBINED WINDWAVE AND SWELL HEIGHT.

PKZ398-100100-
400 AM AKST SUN DEC 9 2012 
.SYNOPSIS FOR THE EASTERN GULF OF ALASKA...
WEAK HIGH PRESSURE WILL BUILD OVER THE EASTERN GULF FOLLOWING A
FRONT THAT MOVED ONSHORE SUN MORNING. A MODERATE STRENGTH LOW
WILL DEVELOP TO 993 MB JUST OFF OCEAN CAPE MON AFTERNOON.  

$$

PKZ310-100100-
GULF OF ALASKA NORTH OF 55 DEGREES NORTH AND EAST OF 144 DEGREES
WEST-
400 AM AKST SUN DEC 9 2012 

.TODAY...SW WIND 30 KT. SEAS 16 FT. RAIN IN THE AFTERNOON. 
.TONIGHT...SW WIND 25 KT. SEAS 19 FT. SW SWELL INCREASING TO W
LATE. RAIN LATE. 
.MON...S WIND 30 KT SHIFTING TO W. SEAS 16 FT. RAIN.
.MON NIGHT...W GALE TO 35 KT. SEAS 18 FT. SHOWERS.
.TUE...W WIND 30 KT. SEAS 16 FT. 
.WED...SW WIND 25 KT. SEAS 14 FT. 
.THU...W WIND 25 KT. SEAS 16 FT. 

$$

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
6 AM AKST SAT DEC 8 2012

.SHORT TERM...THE SNOW EVENT BEGINNING THIS AFTERNOON AND EVENING
WILL BE ALMOST AREA WIDE. THE EXCEPTIONS ARE IN THE SOUTHERN
PANHANDLE WHERE THE ARCTIC AIR MASS HAS ALREADY BEEN MODIFIED AWAY
AND THE PRECIPITATION WILL BE PRIMARILY RAIN. WINTER STORM WARNINGS
ARE IN EFFECT THAT INCLUDE MOST OF THE PUBLIC FORECAST ZONES. PLEASE
SEE THE LISTING AT THE BOTTOM. CONFIDENCE IS AVERAGE.

AT 12Z SUN THE PARENT LOW OF ABOUT 966 MB WILL BE TRACKING INTO
BRISTOL BAY. THE ONSHORE FLOW OF MARITIME AIR, AS DISCUSSED IN THE
FOLLOWING PARAGRAPH, IS ASSOCIATED WITH THE FRONTAL SYSTEM AND
LARGE SCALE CYCLONE EVOLUTION.

THE OCCLUDED FRONT IS TO MOVE ONSHORE THE EAST GULF COAST ABOUT
12Z SUNDAY. THE SNOWFALL AMOUNTS WILL VARY MORE DUE TO IF AND WHEN
A CHANGE TO RAIN OCCURS, AND THIS CHANGE IN TURN WILL BE BASED ON
HOW THE MARITIME AIR MASS BEING MOVED ONSHORE IS TO OVERRUN THE
PRE-EXISTING MODIFIED ARCTIC AIR MASS. THE ARCTIC FRONTAL BOUNDARY
DOES NOT MOVE N, BUT WILL BE MODIFIED AWAY PROGRESSIVELY INLAND,
RESULTING A SEQUENCE OF SNOW CHANGING TO RAIN IN A S TO N
SEQUENCE. THERE IS LITTLE LEFT TO ANY OF THIS UNDERLYING COLDER
AIR MASS IN THE SOUTHERN PANHANDLE, EXCEPT TUCKED AWAY IN MISTY
FJORDS. THE DEEPEST ARCTIC AIR MASS IS IN THE NORTHERN INNER
CHANNELS, AND THE PRECIPITATION SHOULD REMAIN ALL UNMIXED SNOW
THROUGH TONIGHT.

THE MODEL CONSENSUS FOR THIS TIMING IS GOOD, WITH THE GFS, ECMWF,
AND UKMET TOGETHER AND THE NAM SLIGHTLY FASTER. THE NAM, HOWEVER
WAS STILL TO BE CONSIDERED AS THE RELATED MODEL SOUNDINGS WERE
USEFULLY CLOSE TO THE THERMAL PROFILE DATA FROM AIRCRAFT ENTERING
JUNEAU AS WELL AS SITKA.

THE PERSISTENCE OF SNOWFALL IS ASSOCIATED WITH HOW THE MARITIME
AIR MASS THAT IS BEING MOVED ONSHORE IS TO OVERRUN THE PRE-
EXISTING MODIFIED ARCTIC AIR MASS. 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 AIR MASS WAS USED AND DOES DELAY THE CHANGEOVER TO
MIXED PRECIPITATION BY ABOUT 6 HOURS.

ONE FINAL NOTE: ORIENTATION OF THE OCCLUDED FRONT AND THE WINDS
FAVOR NORTH TO EAST WINDS IN THE DEEP BOUNDARY LAYER, AND NOT THE S
TO SE WINDS THAT TEND TO MODIFY AWAY THE ARCTIC AIR MASS AND SPEED
THE RAIN MIX.
&&

.LONG TERM...
DIFFICULT DECISION AS TO SWITCHOVER FROM SNOW TO RAIN ON SUNDAY. WARM
ADVECTION WITH THE FRONT IS STRONG BUT WILL ANY LOW LEVEL WIND OR
LACK OF IT COULD KEEP COLD AIR LONG ENOUGH TO START AS SNOW? HEDGED
ON TIMING TO INCLUDE RAIN IN THE SUNDAY MORNING FORECASTS FOR
SOUTHERN AND COASTAL ZONES BUT KEPT MENTION OF MORNING SNOW MOST
INLAND AND NORTH LOCATIONS. ERROR COULD BE MORE RAPID WARMING FOR
THESE LATTER ZONES. USED TEMPERATURE FORECASTS IN LATER PERIODS TO
DETERMINE SNOW VERSUS RAIN. PATTERN IS DEFINITELY WARMER WITH ONLY
BRIEF COLDER OUTFLOW MENTIONED TUE.

EXPECT WEST TO SW LOW LEVEL WIND IN OUTSIDE MARINE AREAS. ACCEPTED
AKWAVE WAVE HEIGHTS TO 20 FT DUE TO A VERY LONG FETCH BEHIND THE
FRONT. COORDINATED WITH ANCHORAGE ON MODEL SPREAD AND USED AK-GFS-40
TO LOCATE LOW CENTER 00Z TUE BUT NOT AS STRONG. WAS STILL ABLE TO
FORECAST SOME GALES IN THE LATER PERIODS DUE TO FAIRLY STRONG WIND
ALOFT THOUGH ANTICYCLONIC. UPPER RIDGE BUILDING TO THE WEST MON
EVENING TO ALLOW FOR THE OLD LOW CENTER TO MOVE SE FOR THE BRIEF LOW
LEVEL OFFSHORE FLOW MENTIONED ABOVE.

USED DAY SHIFTS FORECASTS FOR MOST OF WEATHER ELEMENTS TUE AND
BEYOND. CONFIDENCE MODERATE TO LOW THROUGH MON ON THE DETAILS.
CONFIDENCE LOW TUE THROUGH FRI DUE TO LARGE MODEL SPREAD.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WINTER STORM WARNING 6 PM SAT TO 6 AM SUN
          AKZ019-021-024>026.
         STRONG WIND THIS EVENING FOR AKZ023-027.
         WINTER WEATHER ADVISORY NOON SAT TO 6 AM SUN AKZ022.
         WINTER STORM WARNING NOON SAT TO 6 AM SUN AKZ017-020.
         WINTER WEATHER ADVISORY 6 PM SAT EVE TO 6 AM SUN
             AKZ018-029.
MARINE...STORM WARNING PKZ052.
         GALE WARNING PKZ022-033-036-041>043-051.
         SMALL CRAFT ADVISORY PKZ021-031-032-034-035.
         SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ053.
&&

JBT/JC