Severe Thunderstorm Warning 9, NWS Seattle, WA

AREA FORECAST DISCUSSION...CORRECTED ONE TYPO IN THE SYNOPSIS
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM FRI AUG 8 2008

.SYNOPSIS...A WEAK FRONT WILL INCREASE CLOUDS OVER WESTERN
WASHINGTON TONIGHT AND BRING A CHANCE OF LIGHT RAIN AN DRIZZLE LATE
TONIGHT AND SATURDAY MORNING. THE TRAILING UPPER LEVEL LOW WILL
BRING SHOWERS TO THE AREA MAINLY SATURDAY AFTERNOON AND NIGHT. AN
UPPER LEVEL RIDGE WILL BRING WARMER AND DRY WEATHER EARLY NEXT WEEK.
A WEAK UPPER LEVEL TROUGH MAY BRING SOME SHOWERS TUESDAY NIGHT AND
WEDNESDAY...BEFORE ANOTHER STRONGER RIDGE FOLLOWS WITH A WARMING AND
DRYING TREND LATE NEXT WEEK.

&&

.SHORT TERM...THE LARGE UPPER LEVEL LOW NEAR 49N/133W WILL DOMINATE
THE WEATHER PATTERN THROUGH THIS WEEKEND. A VERY WEAK FRONT IS ABOUT
90 MILES OFFSHORE THIS AFTERNOON AND WILL PROBABLY DISSIPATE OVER
THE AREA TONIGHT AS IT TRIES TO MOVE INLAND. MODELS HAVE BACKED WAY
OFF ON THE POPS TONIGHT...BUT WITH ONSHORE FLOW AND LOW LEVEL
MOISTURE AVAILABLE ALONG THE COAST WILL KEEP KEEP A SMALL CHANCE OF
LIGHT RAIN AND DRIZZLE LATE TONIGHT.

THE APPROACHING SYSTEM SHOULD PUSH STABLE AIR ACROSS THE CASCADE
CREST TONIGHT...EFFECTIVELY ENDING THE THUNDERSTORM THREAT ALONG THE
CREST. HOWEVER THIS MIGHT BE A CLOSE ONE. NOTHING HAS DEVELOPED YET
OVER THE CREST THIS AFTERNOON...BUT SOME CELLS NEAR REDMOND OREGON
TRIGGERED BY A VORT MAX MOVING NNE ACROSS EASTERN OREGON INDICATE
THAT CELLS COULD COME CLOSE TO THE CASCADE CREST THIS EVENING.

LATEST NAM/GFS/ECMWF ARE FINALLY IN GOOD AGREEMENT BRINGING THE
UPPER LEVEL LOW TO THE NORTH OR CENTRAL VANCOUVER ISLAND COAST
SATURDAY AFTERNOON. ALL SHOW A SIGNIFICANT SHORTWAVE TROUGH ROUNDING
THE LOW AND MOVING INLAND OVER W WA SATURDAY AFTERNOON. HAVE BOUGHT
INTO THIS IDEA AND TRANSITIONED FROM A CHANCE OF LIGHT RAIN/DRIZZLE
IN THE MORNING TO SHOWERS LIKELY IN THE AFTERNOON. MOS SUPPORTS
LIKELY OR BETTER POPS. THE SHORTWAVE SWINGS NE DURING THE EVENING SO
HAVE DROPPED BACK TO CHANCE POPS OVERNIGHT.

CONFIDENCE FOR THE SUNDAY FORECAST ISNT THE BEST. THE UPPER LOW IS
EXITING TO THE NE BUT THERE COULD STILL BE SOME HANG BACK SHOWERS
OVER THE NE PART OF THE AREA. THE MAIN COMPLICATION IS THE BATCH OF
MOISTURE OUR NEAR 50N/150W THAT THE 12Z GFS BRINGS OVER THE SW PART
OF W WA SUNDAY MORNING. THE GFS WRINGS SOME PRECIP OUT OF THIS BUT
CONFIDENCE REALLY ISNT GOOD. DYNAMICS COULD POSSIBLY COME FROM
ANOTHER WEAKER SHORTWAVE ROUNDING THE OLD LOW. NET EFFECT WAS TO
KEEP A CHANCE OF SHOWERS GOING OVER THE WHOLE AREA IN THE MORNING
THEN DRY EVERYWHERE BUT THE FAR SW OUT IN THE AFTERNOON.

AN UPPER LEVEL RIDGE STILL MOVES OVER THE AREA MONDAY FOR A SOLID
WARMING DRYING CLEARING TREND. KAM

.LONG TERM...MODELS STILL MAINTAIN THE EFFECTS OF THE RIDGE OVER THE
AREA ON TUESDAY. NEW WRINKLE IS THAT BOTH GFS/ECMWF BRING A
SHORTWAVE TROUGH ACROSS THE AREA TUESDAY NIGHT WEDNESDAY. GIVEN THE
AGREEMENT IN THE MODELS A SMALL CHANCE OF SHOWERS WAS ADDED TO THE
FORECAST. OF COURSE SINCE THIS IS STILL 4-5 DAYS OUT THE MODELS
COULD STILL CHANGE COURSE...EVEN WITH THE GOOD AGREEMENT THIS
AFTERNOON. BOTH MODELS BUILD AND EVEN STRONGER RIDGE OVER THE AREA
THURSDAY THROUGH SATURDAY. IF THIS WORKS OUT AS PROGGED COULD SEE
SOME 90S AGAIN...BUT STILL TOO EARLY TO BET ON IT. IF THE MODELS
STICK WITH THIS SOLUTION...MAX TEMPS THU-FRI WILL NEED BOOSTING. KAM

&&

.AVIATION...BROAD UPPER LEVEL LOW WILL CONTINUE TO SPIN OVER THE
OFFSHORE WATERS THIS EVENING WITH S/SW FLOW ALOFT OVER W WA.
SHOWER/T-STORM ACTIVITY SO FAR HAS STAYED IN EASTERN WA...AND THREAT
FOR ADDITIONAL STORMS THIS EVENING WILL CONTINUE TO BE EAST OF THE
CASCADES. WEAK FRONT OVER THE COASTAL WATERS WILL DISSIPATE AS IT
MOVES INLAND THIS EVENING BUT MAY SEE LIGHT RAIN OR DRIZZLE IN THE
INTERIOR DURING THE EARLY MORNING. SHOWERS ACROSS W WA WILL INCREASE
SAT AFTERNOON AS THE UPPER LEVEL LOW MOVES OVER THE AREA.

KSEA...SKIES CLEARED UP ACROSS PUGET SOUND THIS AFTERNOON...BUT
EXPECT AN INCREASE IN CLOUDS THIS EVENING AS WEAK FRONT MOVES INLAND
AND DISSIPATES. MAY SEE LIGHT RAIN OR DRIZZLE BY MORNING...WITH AN
INCREASE IN SHOWERS SAT AFTERNOON AS THE UPPER LEVEL LOW MOVES
INLAND. WINDS WILL REMAIN S/SW 10-15KT THIS EVENING...THEN 05-10KT
OVERNIGHT. 33

&&

.MARINE...STRONGEST WINDS ACROSS W WA THIS EVENING WILL BE ACROSS
THE CENTRAL/EASTERN PORTIONS OF THE STRAIT OF JUAN DE FUCA...AND
WILL LEAVE A 06Z EXPIRATION TO THE SMALL CRAFT ADVISORY INTACT AS
WINDS ARE EXPECTED TO DIMINISH. WINDS ACROSS ADMIRALTY INLET ALSO A
CONCERN WITH NORTHERLIES NORTH PART...AND SOUTHERLIES SOUTH PART.
WILL STICK WITH WINDS EVENTUALLY BECOMING N THIS EVENING THEN
BECOMING VARIABLE ONCE AGAIN AFTER MIDNIGHT. OTHERWISE...EXPECT
LIGHT ONSHORE FLOW ACROSS W WA OVER THE WEEKEND AS SFC HIGH PRES
RIDGE REMAINS ALONG THE WA COAST. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...SMALL CRAFT ADVISORY CENTRAL AND
EAST STRAIT OF JUAN DE FUCA.

WEATHER.GOV/SEATTLE