Small Craft Advisory 1008, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
920 PM PDT TUE JUN 12 2012

.SYNOPSIS...AN UPPER TROUGH COMBINED WITH ONSHORE FLOW WILL GIVE
SOME SHOWERS TO THE AREA TONIGHT AND WEDNESDAY...ESPECIALLY IN THE
PUGET SOUND CONVERGENCE ZONE AND OVER THE MOUNTAINS. SHOWERS IN THE
LOWLANDS WILL END LATE WEDNESDAY AFTERNOON. ANOTHER UPPER TROUGH
WILL MOVE ACROSS THE AREA THURSDAY GIVING A CHANCE OF SHOWERS TO
NORTHERN PORTIONS OF THE AREA. DRY AND WARMER WEATHER WILL DEVELOP
FRIDAY AND SATURDAY AS HIGH PRESSURE BUILDS OVER THE AREA.

&&

.SHORT TERM...A WEAK SHORT WAVE TROUGH SITTING FROM OFFSHORE OF
VANCOUVER ISLAND INTO THE OREGON COASTAL WATERS WILL SWING ACROSS
THE AREA THIS EVENING THROUGH EARLY WEDNESDAY. THIS
TROUGH...COMBINED WITH LOW LEVEL ONSHORE FLOW IS BRINGING SOME
SHOWERS TO THE AREA...ESPECIALLY AROUND A PUGET SOUND CONVERGENCE
ZONE...AN AREA OF CONVERGENCE TO THE LEE OF VANCOUVER ISLAND INTO
THE SAN JUANS AND WESTERN WHATCOM COUNTY...AND OVER THE MOUNTAINS.

WEAK AND FLAT SHORT WAVE RIDGING WILL MOVE ACROSS THE AREA LATE
WEDNESDAY AFTERNOON AND EVENING. THIS WEAK RIDGE WILL ALLOW ISOLATED
SHOWERS TO BECOME CONFINED TO THE CASCADES WHILE THE LOWLANDS STAY
CLOUDY IN ONSHORE FLOW. THERE WILL BE SOME BREAKS IN THE CLOUDS
DURING THE AFTERNOON HOURS AND TEMPERATURES WILL RISE INTO THE LOWER
TO MID 60S.

GRIDDED AND TEXT FORECASTS FOR TONIGHT THROUGH WEDNESDAY NIGHT WERE
UPDATED TO REDUCE POPS IN THE LOWLANDS OUTSIDE OF THE CONVERGENCE
ZONE AREAS. OTHERWISE THE SHORT TERM FORECASTS ARE ON TRACK.

THE INCOMING NAM12 SOLUTION CONTINUES TO SHOW THE PASSAGE OF ANOTHER
SHORT WAVE TROUGH ON THURSDAY. THIS WILL LIKELY INCREASE THE
PROBABILITY OF SHOWERS OVER THE NORTHERN PORTIONS OF THE AREA AND
WILL KEEP TEMPERATURES RUNNING 4-8 DEGREES BELOW NORMAL ON THURSDAY.
ALBRECHT

.LONG TERM...PREVIOUS LONG TERM DISCUSSION FOLLOWS...HIGH PRESSURE
ALOFT BUILDS OVER THE PAC NW FROM THE WEST ON FRIDAY. THIS WILL BE
THE START OF A PERIOD OF DRY AND WARMER WEATHER. HIGH TEMPERATURES
ON FRIDAY SHOULD REACH INTO THE LOW 70S AND POSSIBLY THE MID OR
UPPER 70S FOR THE GREATER PUGET SOUND REGION BY SATURDAY. LONG RANGE
MODELS HAVE VARIED CONSIDERABLY THE PAST FEW DAYS. HOWEVER...THE
GFS/ECMWF BOTH AGREE TODAY ON A STRONGER RIDGE PATTERN. THE FORECAST
WAS ADJUSTED TO WARM TEMPS AND REMOVE THE MENTION OF RAIN. SUNDAY IS
TRICKY AS WELL. THE FLAT RIDGE COULD BE STRONG ENOUGH TO KEEP
PRECIPITATION TO THE NORTH. BUT THE ECMWF DOES SHOW LIGHT QPF FROM
ROUGHLY NORTH OF PUGET SOUND AND ALSO OVER THE COASTAL AREAS AND
MOUNTAINS. LEFT CHANCE POPS THERE. HIGH TEMPS MAY BE A BIT LOWER
THAN SATURDAY'S HIGHS BUT SHOULD STILL REACH 70 ACROSS THE SOUTHERN
AREAS WITH WEAK NW PRESSURE GRADIENTS. BY EARLY NEXT WEEK ANOTHER
TROUGH COULD DIG OVER THE REGION. MODELS HAVE BACK WAY OFF ON THE
COLD UNSETTLED PATTERN BUT STILL INDICATE SOME THREAT OF SHOWERS.
DECIDED TO KEEP A CHANCE OF SHOWERS IN THE FORECAST MON/TUE ALTHOUGH
THIS COULD CHANGE GIVEN THE UNCERTAINTY.  MERCER

&&

.AVIATION...A WEAK UPPER TROF WILL REMAIN OVER THE REGION. CONTINUED
WLY FLOW ALOFT. THE LOW LEVEL ONSHORE PRES GRADIENT WILL TIGHTEN
FURTHER OVERNIGHT. CIGS /FOR THE MOST PART/ WILL PRIMARILY BE IN THE
1-3K FT RANGE OVERNIGHT. THE EXCEPTION WILL BE THE COAST AND WITHIN
THE PUGET SOUND CONVERGENCE ZONE WHERE THERE WILL BE AREAS OF VLIFR
CIGS AND VSBYS /AT OR BELOW 300 FT AND BELOW 1SM...RESPECTIVELY/.

THE PUGET SOUND CONVERGENCE ZONE WAS BETWEEN KPAE AND KAWO BUT WILL
SHIFT SWD OVERNIGHT TO ITS TYPICAL LOCATION...ALONG THE KING/
SNOHOMISH COUNTY LINE.

KSEA...CIGS SHOULD REMAIN IN THE 1-2K FT RANGE OVERNIGHT BUT EXPECT
THE LOWER CLOUD DECK TO OCCASIONALLY SCATTER TIL ABOUT 0900 UTC. SW
WINDS OF 10-15 KT WILL CONTINUE OVERNIGHT.

&&

.MARINE...A 1030 MB HIGH WELL OFFSHORE AND LOWER PRES OVER THE GREAT
BASIN WILL RESULT IN MODERATE ONSHORE FLOW THROUGH WED. THERE IS A
POSSIBILITY THAT WINDS MAY REACH GALE FORCE OVER THE CENTRAL STRAIT
OF JUAN DE FUCA ON WED. A 1020 MB HIGH WILL DEVELOP OVER THE COASTAL
WATERS ON THU FOR LIGHT ONSHORE FLOW THAT WILL PERSIST THROUGH
FRI.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY UNTIL 3 AM PDT THURSDAY FOR THE
      CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA.

&&

YOU CAN SEE AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.