Small Craft Advisory 779, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
350 PM AKST SAT DEC 15 2012

.SHORT TERM...COLD LOW LEVEL AIR ON THE BACK SIDE OF A FRONT
THAT PUSHED OVER THE PANHANDLE LAST NIGHT IS PRODUCING SCATTERED
THUNDERSTORM ACTIVITY TODAY. RECEIVED A REPORT FROM PORT
ALEXANDER OF THUNDER ON THE OUTER COAST SHORTLY BEFORE NOON.
SATELLITE IMAGERY SHOWING PLENTY OF OPEN CELL CUMULUS WITH HIGH
CLOUD TOPS OVER THE EASTERN GULF AND MOVING OVER THE PANHANDLE.
WEATHER RADAR ALSO INDICATING PLENTY OF CONVECTION OFFSHORE, BUT
ONLY A THIRD TO HALF OF THE CELLS ARE SHOWING REFLECTIVITY OF
35 DBZ OR GREATER, WHICH IS PRETTY WEAK FOR THUNDERSTORM
ACTIVITY. NO ACTIVITY ON THE LIGHTNING DETECTION NETWORK, BUT
CONVECTIVE AND INSTABILITY PARAMETERS CERTAINLY MAKE A STRONG
CASE FOR ADDITIONAL THUNDERSTORMS FOR THE REMAINDER OF THIS
AFTERNOON AND EVENING. HAVE INCLUDED SCATTERED THUNDERSTORMS OVER
THE CENTRAL COASTAL MARINE ZONES AND INNER CHANNELS AS WELL AS
ISOLATED THUNDERSTORMS OVER SOUTHERN ZONE 23 AND WESTERN ZONE 26.

A LOW IN THE FAR NORTHERN GULF IS RESPONSIBLE FOR BOTH THE COLD
LOW LEVEL AIR AND THE SHOWER ACTIVITY ACROSS SOUTHEAST ALASKA
TODAY. THE LOW IS FORECAST TO REMAIN NEARLY STATIONARY ALONG THE
SOUTH COAST TONIGHT AND THEN DRIFT SLOWLY TO THE SOUTHEAST THROUGH
SUNDAY WHILE CONTINUING TO WEAKEN. SCATTERED SHOWER ACTIVITY WILL
CONTINUE THROUGH THE SHORT RANGE ACROSS SOUTHEAST ALASKA AS A
RESULT. LITTLE TO NO WARM AIR ADVECTION ASSOCIATED WITH THIS
SYSTEM, SO DAYTIME HIGHS NOT A LOT WARMER THAN OVERNIGHT LOWS
TONIGHT. THERE WILL BE A MIX OF RAIN AND SNOW DEPENDING ON TIME
AND LOCATION WITH NO CLEARLY DEFINED RAIN-SNOW LINE.

HIGH PRESSURE OVER THE ALASKA INTERIOR WILL BUILD TONIGHT AND
THEN BEGIN RIDGING TO THE SOUTHEAST INTO THE YUKON AND BRITISH
COLUMBIA. RESULTANT NORTHEASTERLY OFFSHORE PRESSURE GRADIENT THAT
WE HAVE SEEN A LOT OF SO FAR THIS AUTUMN WILL ONCE AGAIN CAUSE
WINDS TO INCREASE OUT OF THE NORTH FOR HAINES, SKAGWAY, AND LYNN
CANAL, WHICH WILL CONTINUE INTO THE LONG RANGE FORECAST PERIOD.
FOR THOSE LOCATIONS WHERE WINDS WILL NOT PICK UP OVERNIGHT, HAVE
ALSO INCLUDED SOME PATCHY FOG.

USED NAM FOR UPDATES TO PRESSURE AND WIND. BLENDED NAM AND GEM TO
OFFICIAL FOR POP AND QPF. TEMPS ADJUSTED USING BOTH NAM AND GFS.
SNOW LEVEL MODIFIED USING GFS. FORECAST CONFIDENCE IS AVERAGE.

.LONG TERM...THAT LOW IN THE FAR NORTHERN GULF, AS DISCUSSED IN
THE SHORT-RANGE SECTION ABOVE, WILL BEGIN TO MOVE SE AS OUR
WHOLE AREA OF RESPONSIBILITY BEGINS TO CHANGE TO A PROGRESSIVE
PATTERN. AN ARCTIC OUTBREAK, ALSO INTRODUCED ABOVE, WILL PROGRESS
SOUTHWARD THROUGH THE ENTIRE PANHANDLE BY 12Z TUE. CONFIDENCE
IS AVERAGE. PLEASE NOTE THE ARCTIC OUTBREAK WILL ALSO PROVIDE
FREEZING SPRAY IN THE NORTHERN INNER CHANNELS AND BY MON NIGHT
WILL COME OUT OF INTERIOR PASSES INTO STEPHENS PASSAGE.

THAT LOW IN THE NORTHERN GULF WILL MOVE SE TO 100 NM S OF YAKUTAT
00Z MON. IT WILL CONTINUING SE TO BE MOVING ALONG HAIDA GWAII
ABOUT 00Z TUE. THIS TRACK WILL WILL BE PARTIALLY DUE TO INTERACTION
WITH A ZONALLY TRACKING POLAR FRONT ZONE WAVE THAT IS TO MOVE
ONSHORE NEAR SEATTLE 12Z MON.

THE ARCTIC OUTBREAK ALREADY IN THE NORTHERN PANHANDLE WILL RAPIDLY
SPREAD S ALONG THE PANHANDLE MON AS THERE WILL BE A NORTHERLY
GRADIENT SUPPORTED BY A COMBINATION OF THE ARCITC HIGH IN YUKON
TERRITORY AND THE LOW TO THE SOUTH.

THE LARGE-SCALE PATTERN IS BECOMING PROGRESSIVE AND WILL REMAIN
THAT WAY THROUGH AT LEAST 00Z FRI. DURING 00Z FRI TO 12Z SAT THE
MODEL SOLUTIONS RAPIDLY SPREAD OUT. FOR THE GFS A BLOCKING PATTERN
IS SHOWN, AND IT APPROXIMATES AN OMEGA BLOCK, BUT THE EC DOES NOT
FORM IT NEARLY AS WELL.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...SMALL CRAFT ADVISORY FOR PKZ012.
         SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ041>043-051-052.

&&

FRITSCH/JBT