Small Craft Advisory 647, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PST WED FEB 16 2011

.SYNOPSIS...A COLD UPPER TROUGH OVER THE PACIFIC NORTHWEST WILL KEEP
SHOWERS IN THE FORECAST THROUGH FRIDAY NIGHT. THE AIR MASS WILL
GRADUALLY COOL WITH THE SNOW LEVEL FALLING TO 500 FEET OR LOWER
TONIGHT. THE TROUGH WILL MOVE SOUTH FRIDAY NIGHT AND DRIER NORTHERLY
FLOW WILL PREVAIL SATURDAY. ANOTHER COLD UPPER TROUGH WILL MOVE
ACROSS THE REGION FROM THE NORTHWEST SUNDAY OR EARLY NEXT WEEK.

&&

.SHORT TERM...LATEST IR SATELLITE IMAGERY SHOWS NUMEROUS COLD TOPPED
CU ACROSS THE PAC NW AND OFFSHORE UNDER A BROAD TROUGH WHICH HAS
SETTLED OVER THE REGION. THE BEST UPPER LEVEL DYNAMICS HAVE SHIFTED
WELL S OF THE THE AREA BUT WITH 500 MB TEMPS AT -36C THERE ARE
PLENTY OF INSTABILITY SHOWERS ACROSS THE REGION. THE GREATER PUGET
SOUND REGION APPEARS TO BE IN SOMEWHAT OF A SUCKER HOLE THIS MORNING
DUE TO SOME SHADOWING EFFECTS OFF THE OLYMPICS. HOWEVER...MODELS
INDICATE THAT DIURNAL INSTABILITY WILL INCREASE THROUGH THE DAY WITH
SHOWERS BECOMING MORE NUMEROUS DURING THE AFTERNOON AND FILLING IN
ACROSS MUCH OF THE AREA. MODEL FORECAST LI'S ARE DOWN NEAR 0 TO -1C
BY 21Z WHICH SUGGESTS AT LEAST SOME THREAT OF ISOLATED
THUNDERSTORMS...THOUGH NOTHING TOO ORGANIZED WITH THE MAIN DYNAMICS
S OF WA. THE CURRENT FORECAST APPEARS ON TRACK TODAY WITH NO UPDATES
PLANNED.

THE AIR MASS COOLS A BIT MORE TONIGHT AND THURSDAY WITH 850 MB TEMPS
DROPPING DOWN TO AROUND -7C AND 1000-850 MB THICKNESS VALUES JUST
BELOW 1300M. DURING THIS PERIOD...IT APPEARS PRECIP WILL BE MOSTLY
IN THE FORM OF SHOWERS WITH ONSHORE FLOW ACROSS THE AREA MODERATING
THE LOWEST LEVELS. WHILE SOME SNOW OR MIXED PRECIP MAY BE OBSERVED
IN PASSING SHOWERS ABOVE 500 FEET...THE LOWLANDS FOR THE MOST PART
WILL LIKELY JUST SEE COLD RAIN SHOWERS MIXING WITH SOME WET SNOW
DURING THE NIGHT/MORNING HOURS WITH ACCUMULATION UNLIKELY. HILLTOPS
ABOVE 500 FEET COULD SEE A DUSTING BUT THIS WILL BE HIT AND MISS.

ONSHORE FLOW WILL INCREASE ON THURSDAY NIGHT AS ANOTHER UPPER TROUGH
CLOSES OFF OVER SW B.C. AND A SURFACE LOW DEVELOPS OFF VANCOUVER
ISLAND. ALL MODELS HAVE SHIFTED WWD WITH THE TRACK OF THE UPPER/SFC
LOW DIVING SWD OFF THE WA COAST FRIDAY. PREVIOUS GFS RUNS WERE
DROPPING THE SYSTEM SWD OVER WA PULLING DOWN A COLD ARCTIC BOUNDARY
WHICH COULD HAVE BEEN A SNOW PRODUCER FOR THE LOWLANDS. THE NEW
SUITE OF MODELS INCLUDING THE 12Z GFS NOW KEEP MOST THE RAIN/SNOW
SHOWERS LIMITED TO THE COAST WWD AND NOT DRAGGING DOWN NEARLY AS
MUCH COLD AIR...THOUGH STRONG FRASER OUTFLOW STILL SEEMS LIKELY
ACROSS THE FAR N INTERIOR WHERE MUCH COLDER TEMPS ARE EXPECTED.
THERE IS STILL ENOUGH UNCERTAINTY TO LEAVE THE FORECAST UNCHANGED
FOR THE TIME BEING.  MERCER

.LONG TERM...PREVIOUS DISCUSSION...ALL OF THE EXTENDED MODELS AGREE
ON A DRY DAY FOR SATURDAY WITH NORTHERLY FLOW ALOFT OVER WESTERN
WASHINGTON. AS WAS THE CASE WITH THE 00Z RUNS LAST NIGHT THE GFS
BRINGS A SHORTWAVE DOWN IN THE NORTHERLY FLOW ALOFT SATURDAY NIGHT
INTO SUNDAY MORNING WHILE THE ECMWF IS MUCH SLOWER AND FURTHER TO
THE WEST WITH THIS FEATURE. LITTLE AGREEMENT IN THE MODELS FOR THE
REMAINDER OF THE EXTENDED PERIOD WITH THE ECMWF DIGGING A TROUGH OFF
THE COAST FOR THE FIRST PART OF NEXT WEEK KEEPING THINGS DRY UNTIL
TUESDAY WHILE THE GFS HAS THIS FEATURE ABOUT 5 DEGREES FURTHER EAST
WHICH KEEPS THE POSSIBILITY OF SNOW IN THE FORECAST FOR SUNDAY
THROUGH TUESDAY. THE LACK OF CONSISTENCY BETWEEN THE MODELS DOES NOT
LEND MUCH CONFIDENCE TO EITHER SOLUTION AND BOTH SOLUTIONS ARE
PLAUSIBLE SO FOR NOW WILL HAVE TO STAY WITH A VERY BROADBRUSH CHANCE
POPS FORECAST WITH RAIN OR SNOW MENTIONED THROUGH THE FIRST PART OF
NEXT WEEK. FELTON

&&

.HYDROLOGY...THERE IS NO INDICATION THAT A FLOOD PRODUCING PATTERN
WILL DEVELOP DURING THE NEXT TEN DAYS. ON THE CONTRARY...COOL
WEATHER WILL PRECLUDE ANY THREAT OF FLOODING ON THE GREEN OR ANY
OTHER WESTERN WASHINGTON RIVERS.

&&

.AVIATION...AN UPPER TROUGH REMAINS OFFSHORE WITH LIGHT SW FLOW
ALOFT.  THE AIR MASS REMAINS MOIST AND SLIGHTLY UNSTABLE TODAY.
WINDS WILL REMAIN BLUSTERY IN PLACE TODAY AS A 990 MB LOW MOVES INTO
SOUTHERN BRITISH COLUMBIA TODAY. SHOWERS AND ISOLATED THUNDERSTORMS
WILL LOWER CEILINGS AND VSBYS DOWN INTO THE MVFR CATEGORY AT
TIMES...OTHERWISE VFR CONDITIONS EXPECTED.

KSEA...SOME SHOWERS ARE EXPECTED TO DEVELOP THIS AFTERNOON WITH A
SHOT AT A LIGHTNING STRIKE NEARBY THIS AFTERNOON OR EARLY THIS
EVENING.  HOWEVER NOT ENOUGH OF A CHANCE TO WARRANT MORE THAN A CB
IN THE CLOUD GROUP AT THIS TIME.  SLY WINDS ARE EXPECTED TO PICK UP
A BIT THIS AFTN BEFORE EASING THIS EVENING.  CERNIGLIA

&&

.MARINE...THE LOW OFF THE COAST IS RIGHT ABOUT 990 MB AT THIS TIME
AND IS RIGHT ON THE CENTRAL VANCOUVER ISLAND COAST THIS MORNING.
THIS LOW WILL SLOWLY MOVE NE TODAY AND GRADUALLY FILL WHICH WILL
ALLOW THE SURFACE PRESSURE GRADIENTS OVER THE REGION TO RELAX.

WINDS HAVE RELAXED ENOUGH OVER THE COASTAL WATER TO DROP THE GALE TO
A SCA FOR NOW.  THERE ARE A FEW AREAS IN THE NORTHERN WATERS THAT
HAVE PUSHED INTO THE LOW 30S...BUT WILL LEAVE THE 20-30 KT FORECAST
UP FOR NOW AS THAT IS REPRESENTATIVE OF THE OVERALL CONDITIONS.

LIGHTER WINDS AND SEAS THU.  NEXT LOW WILL SLIDE DOWN THE COAST THU
NIGHT AND FRI AND REMAIN OFFSHORE.  THIS IS NOT A STRONG SYSTEM BUT
IT WILL LIKELY BE GOOD FOR A FEW SCA TYPE WINDS.  CERNIGLIA
&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...
PZ...SMALL CRAFT ADVISORY ALL WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR CONDITIONS.

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