Small Craft Advisory 422, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
239 PM AKDT WED JUL 18 2012

.SHORT TERM...SATELLITE IMAGERY THIS AFTERNOON SHOWING WEAKENING
RIDGE OF HIGH PRESSURE IN THE EASTERN GULF OVER LINGERING MARINE
STRATUS LAYER WITH A DEVELOPING LOW PRESSURE SYSTEM MOVING INTO
THE CENTRAL GULF. THE ADVANCING LOW WILL DEEPEN AS IT TRACKS INTO
THE SOUTHEASTERN GULF OVERNIGHT. THE ASSOCIATED WEATHER FRONT
WILL CROSS THE GULF AND MOVE OVER THE SOUTHERN PANHANDLE LATE
WEDNESDAY NIGHT INTO THURSDAY. RAIN SHOULD BE LIMITED TO AREAS
FREDERICK SOUND SOUTH WITH PRECIP DEVELOPING THOUGH THE LATE
EVENING AND OVERNIGHT HOURS.

FOR THE REST OF THE PANHANDLE THE LINGERING MARINE STRATUS LAYER
HAS BEGUN TO DISSIPATE AS WINDS SHIFT FROM ONSHORE TO OFFSHORE
FLOW. THE BREAKING APART OF THIS CLOUD LAYER WILL MOVE FROM NORTH
TO SOUTH NEARING FREDERICK SOUND WHERE CLOUD COVER WILL THEN BE
ASSOCIATED WITH THE INCOMING FRONT.

WINDS HAVE BEEN GENERALLY LIGHT TODAY AND WILL SEE A DIRECTION
SHIFT TO THE SOUTH AND EAST TONIGHT. WINDS OVER THE SOUTHERN
PANHANDLE WILL PICKUP TO 20 KT WITH THE ADVANCING FRONT. FOR THE
NORTHERN PANHANDLE DEVELOPING THERMAL GRADIENT WILL INCREASE WINDS
TO SMALL CRAFT LEVEL OVER NORTHERN LYNN CANAL. THESE WINDS WILL
DROP OFF OVERNIGHT AS THE GRADIENT WEAKENS.

NEW MODEL RUNS WERE IN FAIR AGREEMENT. 20Z SATELLITE IMAGE SHOWED
A MOISTURE PLUME NEAR 144 W THAT IS A BIT FURTHER NORTH THAN
MODELS WERE PREDICTING. THIS MAY ALLOW RAIN FALL TO PUSH FURTHER
NORTH THAN FORECAST. HOWEVER AN EARLIER MOISTURE PLUME THAT
EXTENDED TO THE NORTH DISSIPATED AS THE LOW PROGRESSED AND
EASTERLY OFFSHORE FLOW INCREASED. USED A BLEND OF 12Z NAM/GFS WITH
INHERITED GRIDS...OVERALL CHANGES WERE MINOR WITH MAIN CHANGE
BEING REDUCED CLOUD COVER OVER THE NORTHERN PANHANDLE THURSDAY
MORNING. FORECAST CONFIDENCE IS AVERAGE FOR SYNOPTIC PATTERN WITH
LOWER CONFIDENCE WITH RESPECT TO CLOUD COVER.

&&

.LONG TERM...VERTICALLY STAKED LOW JUST SW OF HAIDA GWAII THURSDAY
NIGHT WILL REAMAIN NEARLY STATIONARY THROUGH SATURDAY AND WEAKEN
AS IT MOVES OFF TO THE EAST SUNDAY. THIS LOW WILL WRAP A SERIES OF
FRONTS NORTHWARD INTO THE SOUTHERN PANHANDLE THAT DISSIPATES IN EAST
TO SOUTHEAST FLOW OVER THE NORTHERN PANHANDLE. A WEAK SURFACE
RIDGE BUILDS OVER THE CENTRAL GULF THROUGH FRIDAY THEN SHARPENS UP
OVER THE EASTERN GULF SATURDAY AND MOVES ACROSS SOUTHEAST ALASKA
SUNDAY. A WEAKENING FRONT MOVES INTO THE EASTERN GULF SUNDAY AND
OVER SOUTHEAST ALASKA MONDAY.

EXPECT PERIODS OF LIGHT RAIN ACROSS THE SOUTHERN ZONES WITH LIGHT
RAIN LIKELY FOR SOUTH CENTRAL AREAS FRIDAY NIGHT INTO SATURDAY
MORNING. EXPECTING RAINFALL AMOUNTS FROM A TENTH TO THREE QUARTERS
OF AN INCH IN THE WETTER TERRAIN FAVORED LOCATIONS. APPEARS FRONT
HANGS UP JUST SOUTH OF JUNEAU AND WILL KEEP THIS AREA MOSTLY
CLOUDY WITH JUST A SLIGHT CHANCE OF RAIN. PAYA IN BEST SPOT TO
PICK UP OFFSHORE FLOW AND PARTLY CLOUDY SKIES. COULD STILL HAVE
LEFTOVER EASTERLY SMCR IN SOUTHEAST GULF BUT EXPECT WINDS TO EASE
FRIDAY NIGHT. NORTHERN INNER CHANNELS SHOULD SHIFT TO A LIGHT
NORTHERLY WIND WHILE A SOLID SE 20 KT FOR SOUTHERN INNER
CHANNELS.

A SECOND WAVE WRAPS INTO THE SOUTHERN HALF SATURDAY NIGHT WITH
ANOTHER SHOT OF A HALF INCH OF RAIN. HAVE A WEAK PRESSURE GRADIENT
WITH THIS PART OF THE SYSTEM UNDER AND LOW LEVEL INVERSION SO MAINLY
LIGHT WINDS FOR INNER CHANNELS BUT GO TO NW 20 KT FOR SE GULF
SATURDAY NIGHT AS LOW MOVES OFF TO E AND RIDGE BUILDS OVER GULF.

MARINE LAYER EXPECTED TO BUILD OVER THE GULF THIS WEEKEND AND
COULD BE ADVECTED ONSHORE IN WESTERLY FLOW THROUGH EARLY NEXT WEEK
BUT ALSO HAVE SOME SINGS FOR PARTLY CLOUDY CONDITIONS AND WARMER
TEMPS OVER THE NORTH WITH MID LEVEL OFFSHORE FLOW. FOR NOW KEPT
THIS AREA DRY WITH MOSTLY CLOUDY SKIES.

MODELS WERE IN FAIRLY GOOD AGREEMENT WITH THE SYNOPTIC PICTURE
THROUGH THE LONG TERM. OPTED TO USE A GFS/EC BLEND FOR GRIDS TODAY
WITH LITTLE OVERALL CHANGE. DID USE COOLER EC MOS FOR TEMPS WHICH
HAS BEEN VERIFYING A LITTLE BETTER LATELY.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...SMALL CRAFT ADVISORY FOR PKZ012.

&&

PRB/MM