000
FXUS66 KSEW 060348
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT SUN SEP 5 2010
.SYNOPSIS...A FRONTAL SYSTEM WILL BRING RAIN TO WESTERN WASHINGTON
MONDAY. A COOL SHOWERY UPPER LEVEL LOW WILL BE OVER THE AREA
TUESDAY...THEN MOVE SOUTH WEDNESDAY. AFTER IMPROVING WEATHER THROUGH
THURSDAY...THE WEATHER PATTERN WILL BECOME UNSETTLED AGAIN BY NEXT
WEEKEND.
&&
.SHORT TERM...LATEST RADAR LOOP INDICATES SHOWERS ARE GENERALLY
DISSIPATING OVER THE CASCADES THIS EVENING AS WE LOSE DIURNAL
INSTABILITY. THIN HIGH CLOUDS ARE BEGINNING TO SPREAD ACROSS THE
AREA IN ADVANCE OF THE NEXT LOW PRESSURE SYSTEM TO AFFECT WRN WA THE
NEXT COUPLE DAYS. THE LEADING FRONTAL BAND ASSOCIATED WITH THE
DEEPER MOISTURE IS NOW OVER VANCOUVER ISLAND WITH PRECIP BEING
REPORTED ON THE N END OF THE ISLAND. IR IMAGERY SHOWS THIS BAND
HEADING SE TOWARD WA WITH THE LAST VISIBLE PICTURE EARLIER SHOWING A
SOLID MASS OF STRATUS TO FOLLOW BEHIND THE FRONT. ONSHORE FLOW WILL
INCREASE TONIGHT AND BECOME STRONG MONDAY. LOW CLOUDS WILL SPREAD
INLAND LATER TONIGHT AS THE FRONTAL BAND REACHES THE COAST. THE
FRONTAL BAND WILL BRING RAIN TO THE COAST LATER TONIGHT...SPREADING
ACROSS THE INTERIOR DURING THE MORNING. LOOKS LIKE A RAINY DAY FOR
MOST THE AREA. THE NAM IS ONE OF THE DRIER MODELS AT LEAST EARLY ON
FOR THE N/CENTRAL SOUND BUT THIS IS AN OUTLIER WITH OTHER MESO
MODELS LIKE THE MM5GFS SHOWING LIGHT PRECIP THROUGH DAY ACROSS ALL
OF THE AREA.
THE UPPER TROUGH DROPS SE ACROSS THE AREA MONDAY NIGHT AND TUESDAY
WITH MODELS IMPLYING RATHER WIDESPREAD SHOWER ACTIVITY. MESO MODELS
ALSO HINT AT CONVERGENCE ZONE WHICH MAY MEANDER BETWEEN WRN
SNOHOMISH AND KING COUNTY. BY TUESDAY AFTERNOON A WEAK SURFACE LOW
OR OPEN SURFACE TROUGH SWINGS THROUGH SRN WA/NRN OREGON SPREADING
ADDITIONAL SHOWER ACTIVITY NWD ACROSS MOST OF WRN WA.
THE TROUGH BECOMES ELONGATED AND STALLS OVER THE AREA ON WEDNESDAY
BEFORE SHIFTING EWD WEDNESDAY NIGHT. THERE WILL STILL BE A CHANCE OF
SHOWERS...ESPECIALLY WITH DIURNAL INSTABILITY MORE OF A PLAYER
DURING THE AFTERNOON/EARLY EVENING. SHOWERS SHOULD DECREASE OR END
FOR A PERIOD WEDNESDAY NIGHT BETWEEN SYSTEMS. MERCER
.LONG TERM...PREVIOUS DISCUSSION FOLLOWS...THERE IS A GOOD CHANCE
THAT THU REMAINS DRY WITH A BREAK BETWEEN WEATHER SYSTEMS AND THE
SORT OF COOL...SOMEWHAT MOIST AND UNSTABLE AIR MASS...SUCH AS WHAT
WAS OVER THE REGION TODAY. THE OVERALL WEATHER PATTERN DOES NOT
CHANGE MUCH INTO NEXT WEEKEND...SO THE EXTENDED PERIOD WILL REMAIN
COOL WITH A CHANCE OF SHOWERS FROM TIME TO TIME...THE BEST CHANCE OF
SHOWERS SEEMS LIKELY TO BE THE CASCADES AND IN THE PUGET SOUND
CONVERGENCE ZONE AS THE GLOBAL MODELS ARE ADVERTISING WNW FLOW ALOFT
AND AT LEAST A COUPLE OF EMBEDDED WEAK SHORTWAVES FRI AND BEYOND.
ANY OF THESE SYSTEMS MAY END UP PULLING IN ENUF MOISTURE BEFORE THEY
GO UP AND OVER THE MID PACIFIC RIDGE TO END UP SIMILAR TO THE SYSTEM
WHICH IS ON TAP FOR MONDAY...BUT IT IS TOO FAR AWAY TO TIME ANYTHING
OR COMMIT TO HIGHER POPS THAN CHC/SCATTERED SHOWERS. 19
&&
.AVIATION...A COLD FRONT FROM THE NORTHWEST TIP OF VANCOUVER ISLAND
SOUTHWESTWARD INTO THE PACIFIC WILL MOVE TO NEAR THE WASHINGTON
COASTLINE EARLY MON MORNING THEN WILL SHIFT INLAND DURING THE DAY AS
IT SLOWLY DISSIPATES. A LARGE AREA OF STRATUS CAN BE SEEN ON THE
SATELLITE IMAGERY AHEAD OF THE FRONTAL SYSTEM...THIS WILL MOVE
RAPIDLY INLAND LATER TONIGHT. AT FIRST GUESS BASED ON STRATUS
REACHING THE NORTH COAST THIS EVENING...CIGS ARE EXPECTED TO BE
AROUND 010-015 AT MOST WESTERN WASHINGTON TERMINALS AFTER ABOUT 10Z.
THE FRONTAL CLOUDS AND MOISTURE WILL LIKELY REACH THE COAST AND
NORTH INTERIOR AROUND 12Z THEN SPREAD INTO THE REMAINDER OF THE AREA
AROUND 15Z. FLOW ALOFT IS W/NW WITH THIS FRONT AND LOW LEVEL FLOW
INCREASES AROUND THE FRONT...SO EXPECT THAT CIGS MAY ACTUALLY
IMPROVE SOMEWHAT IN THE LOWLANDS EAST OF THE COASTAL RANGES AND
OLYMPICS DURING THE MORNING HOURS AS THE FRONTAL PRECIPITATION MOVES
IN. WITH EXCEPTION TO THE COAST AND WEST SLOPES OF THE
MOUNTAINS...EXPECT SHADOWING TO RESULT IN RATHER LIGHT PRECIPITATION
AMOUNTS WITH THE FRONT IN THE PUGET SOUND LOWLANDS. BEHIND THE FRONT
LATER MON AFTERNOON...ABOUT 00Z TUE...EXPECT A PUGET SOUND
CONVERGENCE ZONE TO SET UP BETWEEN KPAE AND KSEA AND CONTINUE THRU
MUCH OF MON NIGHT. TERRAIN WILL BE MAINLY OBSCURED AFTER 10-12Z
EARLY MON AND REMAIN THAT WAY THROUGH MON NIGHT.
KSEA...STRATUS AT OVC015 WILL SURGE INLAND AND TO THE TERMINAL BY
10Z. FRONTAL CLOUDS AND PRECIPITATION WILL ARRIVE AROUND 15Z...BUT
CIGS MAY IMPROVE A BIT AFTER ABOUT 15Z TO SCT015 BKN025 OVC045 AS NW
FLOW ALOFT INTERACTS WITH THE OLYMPICS GIVING SOME RAIN SHADOWING. A
PUGET SOUND CONVERGENCE ZONE IS EXPECTED TO FORM AROUND 00Z TUE TO
THE NORTH OF THE TERMINAL. AT THIS TIME IT IS EXPECTED TO REMAIN TO
THE NORTH. EXPECT LIGHT S WIND TO INCREASE TO 10-14 KT 20Z MON AS
THE AIR MASS DESTABILIZES SOMEWHAT AND FLOW ALOFT INTERACTS WITH THE
OLYMPICS. ALBRECHT
&&
.MARINE...PER BUOY 46088 IT APPEARS THAT MARGINAL SMALL CRAFT
ADVISORY CONDITIONS ARE OCCURRING IN THE CENTRAL STRAIT OF JUAN DE
FUCA AND AT THE EAST ENTRANCE. EXPECT THESE WINDS TO RELAX AFTER
ABOUT MIDNIGHT...THEN TO BACK TO S/SE 10 TO 20 KT MOST WATERS BY
MORNING AS A FRONTAL SYSTEM NOW EXTENDING FROM NORTHERN VANCOUVER
ISLAND SW INTO THE PACIFIC DROPS TO NEAR THE WASHINGTON COAST 15Z
MON. THE FRONT WILL MOVE INLAND AND DISSIPATE DURING THE DAY MON
WITH S WINDS BECOMING W 10-15 KT BEHIND THE FRONTAL SYSTEM. AT THIS
POINT IT APPEARS THAT WINDS WILL REMAIN JUST BELOW SMALL CRAFT
ADVISORY CRITERIA WITH THIS FRONTAL SYSTEM.
AN UPPER LEVEL LOW AND ITS SURFACE REFLECTION DROPS INTO THE SW WA
AND OREGON COASTAL WATERS TUE GIVING LIGHT PRES GRADIENTS. FLOW THEN
BECOMES WEAKLY ONSHORE WED THRU MUCH OF THE UPCOMING WEEK. ANOTHER
WEAK FRONT MAY DROP INTO THE COASTAL WATERS LATE WED NIGHT OR THU.
ALBRECHT
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL/EAST STRAIT UNTIL 1 AM MON.
$$
WWW.WEATHER.GOV/SEATTLE
NWS SEW Office Area Forecast Discussion
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