Small Craft Advisory 717, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
557 AM AKST SUN NOV 25 2012

.SHORT TERM...COMPLEX WEATHER PATTERN IN STORE FOR THE NEXT
24 HOURS. SATELLITE IMAGERY SHOWING DOUBLE CENTER LOW LOCATED
NEAR 53N 154W WITH BAROCLINIC LEAF EXTENDING INTO THE AK GULF. IN
THE NORTHERN AK GULF NEAR PRINCE WILLIAM SOUND A SHORT WAVE
EXTENDING TO THE SOUTHEAST WITH TROUGH OF LOW PRESSURE ALONG THE
PANHANDLE COAST. THIS SHORT WAVE IS RIDING ABOVE A RIDGE OF HIGH
PRESSURE THAT IS MOVING EASTWARD ACROSS THE CENTRAL AK GULF.
ARCTIC HIGH PRESSURE CONTINUES TO BUILD OVER THE YUKON AND TRACK
TO THE SOUTH WHILE LOW PRESSURE SYSTEM IN SOUTHERN BRITISH
COLUMBIA WEAKENS AS IT MOVES EAST. THE ENTIRE PATTERN WILL
GENERALLY TRACK EASTWARD THROUGH THE NEXT 24 HOURS.

LEADING SHORT WAVE BANDS ASSOCIATED WITH THE TROUGH OVER THE AK
GULF COAST ARE MOVING OVER THE CENTRAL AND NORTHERN PANHANDLE THIS
MORNING. SHOWERS BEGAN OVER AREAS ALONG A CORRIDOR FROM SITKA TO
JUNEAU. SHOWERS WILL INCREASE ACROSS THE ENTIRE PANHANDLE AS 500
MB VORT MAX ALONG LEADING EDGE OF HIGH PRESSURE RIDGE IN THE GULF
MOVES EASTWARD. AS THE RIDGE MOVES OVER THE PANHANDLE AND LOW
MOVES IN TO THE WESTERN GULF THROUGH THE AFTERNOON A DEFORMATION
BAND FORMS ALONG THE ELFIN COVE TO JUNEAU CORRIDOR INDICATING
DEVELOPMENT OF A NARROW BAND OF PRECIPITATION ORIENTATED FROM WEST
TO EAST. THIS BAND HAS THE POTENTIAL FOR AN OVERRUNNING SNOW
EVENT TO DEVELOP WHICH COULD PRODUCE SOME LOCALLY HIGH SNOW FALL
AMOUNTS. FOR NOW HAVE SNOW AMOUNTS FOR THE CENTRAL PANHANDLE OF 1
TO 2 INCHES OVER A 12 HOUR PERIOD DUE TO UNCERTAINTY IN THE
STRENGTH OF THIS DEVELOPING BAND. WHILE THE DEFORMATION BAND
FORMATION GIVES GOOD INDICATION OF A MORE SIGNIFICANT SNOW FALL
EVENT AND THE COLD SURFACE AIR IN PLACE WITH ADVANCING WARMER
MOIST AIR MASS ALOFT SETS UP FOR OVER RUNNING SNOW THE OMEGA
VALUES IN EACH BAND WERE NOT VERY HIGH AND WERE NOT IDEALLY
LOCATED IN THE -10 TO -20C SNOW GROWTH ZONE.

ARCTIC AIR MASS FROM THE YUKON HIGH HAS PUSHED AS FAR SOUTH AS
SUMNER STRAIT WITH ONLY SOUTHERN AND COASTAL PANHANDLE AREAS ABOVE
34F THIS MORNING. EXPECTING THE COLD AIR TO RETREAT FURTHER
NORTH...MOVING TOWARD FREDERICK SOUND AS HIGH PRESSURE RIDGE FROM
THE GULF ADVANCES. SHOULD START TO SEE A MIX OF SNOW AND RAIN AT
TIMES IN THE AFTERNOON. THE ARCTIC AIR WILL REESTABLISH TONIGHT
WITH TEMPS IN THE UPPER TEENS TO LOW 20S OVER THE CENTRAL AND
NORTHERN PANHANDLE AND TEMPS IN THE LOW TO MID 30S FOR THE
SOUTHERN PANHANDLE.

WINDS OVER MOST OF THE PANHANDLE WILL BE LESS THAN 20 MPH WITH
THE EXCEPTION OF THE NORTHERN PANHANDLE WHERE INCREASING PRESSURE
GRADIENT AS THE YUKON HIGH DROPS TO THE SOUTH WILL PICK WINDS BACK
UP TO 25 MPH. SMALL CRAFT WINDS WILL DEVELOP AGAIN FOR THE
NORTHERN INNER CHANNELS THROUGH THE DAY WITH SMALL CRAFT WINDS IN
THE NORTHERN AK GULF ALONG THE DEFORMATION BAND.

MODEL CONSENSUS WAS GOOD WITH RESPECT TO THE PRESSURE PATTERN AND
FOR POP FIELD ALONG THE 500 MB VORT MAX AND DEFORMATION BAND ALONG
THE ELFIN COVE TO JUNEAU CORRIDOR. HAD LESS CONSENSUS WITH SURFACE
TEMPERATURES...THE SOUTHERN EXTENT OF THE ARCTIC AIR MASS AND SNOW
FALL AMOUNTS. USED A BLEND OF 06Z NAM WITH INHERITED GRIDS. MAIN
CHANGES TO THE FORECAST WERE THE LOCATION OF PRECIP BANDS OVER THE
CENTRAL PANHANDLE. WHILE THERE IS GOOD CONFIDENCE IN THE SYNOPTIC
PATTERN...DEALING WITH THE SNOW AMOUNTS FROM SHOWERS IS VERY TRICKY
AND CONFIDENCE IS LOW.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...THE BAND OF SNOW MENTIONED
IN THE SHORT TERM DISCUSSION WILL CONTINUE INTO MONDAY MORNING
ALONG THE CENTRAL PANHANDLE WITH AN ADDITIONAL 1-2 INCHES THROUGH THE
MORNING. BY MONDAY EVENING PRECIP CHANCES DROP OFF WITH INCREASING
OFFSHORE FLOW DUE TO A LOW APPROACHING FROM THE NORTH PACIFIC.

FOR MUCH OF THE WEEK...THE UPPER LEVEL PATTERN HAS A RIDGE OF
HIGH PRESSURE OVER THE WEST COAST...BLOCKING MOVEMENT OF SYSTEMS
TO THE EAST WITH A COL OVER THE GULF OF ALASKA. LATE TUESDAY/EARLY
WEDNESDAY THE RIDGE SHIFTS TO THE EAST SLIGHTLY AND ALLOWS THE UPPER
LEVEL LOW MENTIONED ABOVE TO MOVE INTO THE SE GULF THROUGH WEDNESDAY
NIGHT...HOWEVER WEAKENING AS IT DOES. THIS FEATURE LINGERS THERE
THROUGH THE REST OF THE WEEK WITH A LARGE BROAD TROUGH ACROSS THE
GULF. THE LOCATION OF THE LOW IN SE GULF WILL CONTINUE TO BRING
CHANCES OF RAIN TO THE SOUTHERN PANHANDLE AND OFFSHORE FLOW OVER
THE NORTH.

AT THE SURFACE THE PATTERN WILL BE SIMILAR...BUT WITH A HIGH
PRESSURE ANCHORED OVER THE YUKON AND NORTHERN B.C. WHEN THE SURFACE
LOW DRAWS NEAR THE SE GULF THE COMBINATION WITH THE YUKON HIGH
WILL CAUSE THE NORTHERLY WINDS TO INCREASE THROUGH THE INSIDE
WATERS. AS A RESULT HAVE CONTINUED THE STORM FORCE WINDS
FORECASTED FOR NORTHERN LYNN CANAL DURING WEDNESDAY AND THURSDAY.

MODELS DIVERGE ON EXACT PLACEMENT OF THE LOW IN THE SE GULF FOR THE
SECOND HALF OF THE WEEK...HOWEVER THE GENERAL IDEA IS THE SAME. THIS
INCLUDES LIKELY PRECIP OVER THE SOUTH AND CHANCE OR LESS WITH
NORTHERLY FLOW FOR THE NORTHERN HALF OF THE REGION. OVERALL MODELS
WERE IN GOOD ENOUGH AGREEMENT FOR AN ENSEMBLE APPROACH FOR
PRESSURE AND WINDS...KEEPING FORECAST CONFIDENCE SLIGHTLY ABOVE AVERAGE.
FOR TEMPERATURES AND PRECIP TYPE ON THE OTHER HAND CONFIDENCE IS
LOWER AND NOT AS SURE HOW FAR SOUTH THE COLD NORTH AIR WILL GET
LATER IN THE WEEK ESPECIALLY WITH THE SE GULF BRINGING IN WARMER
AIR ALOFT.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...SMALL CRAFT ADVISORY FOR PKZ011>013-021-022-043-051>053.
         SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ041-042.

&&

PRB/HANNON