Severe Thunderstorm Warning 1, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
340 PM PDT WED JUL 2 2008

.SYNOPSIS...SOUTHERLY FLOW ALOFT WILL BRING A MOISTER AND MORE
UNSTABLE ATMOSPHERE TO WESTERN WASHINGTON THROUGH THURSDAY. A BROAD
UPPER LEVEL LOW PRESSURE TROUGH WILL CROSS THE REGION ON
INDEPENDENCE DAY. ANOTHER TROUGH WILL PASS BY ON SUNDAY. FLAT UPPER
LEVEL HIGH PRESSURE RIDGING SHOULD THEN DOMINATE FOR THE FIRST HALF
OF NEXT WEEK.

&&

.SHORT TERM...AN UPPER LEVEL RIDGE AXIS OVER ERN WA WILL CONTINUE TO
SHIFT EAST...OPENING THE DOOR TO INCREASING INFLUENCE FROM AN UPPER
LOW NOW NEAR 43N 134W. SOUTHERLY FLOW ALOFT HAS BEEN INCREASING
TODAY AND THE MID LEVELS ARE UNSTABLE WITH THE EXIT OF THE UPPER
RIDGE. AMSU PW IMAGERY SHOWS INCREASING AMOUNTS OF MOISTURE BEING
ADVECTED FROM THE SOUTH INTO THE MIX...ALL POINTING TO UNSETTLED
CONDITIONS OVER THE NEXT 24 HOURS. FOR THIS EVENING...THE MOIST AND
UNSTABLE CONDITIONS HAVE LED TO SOME CONVECTIVE DEVELOPMENT OVER THE
HIGHER TERRAIN BETWEEN MOUNT RAINIER AND SNOQUALMIE PASS...AND
FURTHER DEVELOPMENT COULD TAKE PLACE OVER THE CASCADES THROUGH
SUNSET.

WATER VAPOR LOOPS SHOW AN UPPER LEVEL WAVE NOW MOVING NORTHWARD OFF
THE SRN ORE COAST. OVERALL CLOUD COVERAGE HAS BEEN INCREASING IN
ADVANCE OF THIS FEATURE AS LARGE-SCALE LIFT IS ENHANCED. AS THIS
FEATURE MOVES NORTH...FURTHER SHOWER DEVELOPMENT IS PROBABLE
OVERNIGHT AND ON THU MORNING. INSTABILITY IS STRONG ENOUGH THAT
ISOLATED THUNDER IS POSSIBLE FOR ALL BUT THE COAST IN THE MORNING
BEFORE THE WHOLE SETUP SHIFTS INTO THE CASCADES IN THE AFTERNOON IN
TIME FOR PEAK DAYTIME HEATING.

BY THURSDAY NIGHT...THE UPPER LOW OVER THE ERN PAC OPENS UP AND
SPREADS ITS BROAD REMNANT TROUGH AXIS ACROSS WRN WA ON FRIDAY. THIS
WILL KEEP THE HOLIDAY ON THE CLOUDY SIDE WITH SOME MORNING DRIZZLE
THROWN IN ALONG THE COAST. MODERATE ONSHORE FLOW WILL FAVOR
CLOUDY/FOGGY CONDITIONS ALONG WINDWARD MTN SLOPES WITH SOME LIGHT
SHOWERS MOST ANYWHERE IN THE MORNING. THE TROUGH AXIS SHOULD HAVE
MOVED INTO ERN WA BY FRI EVENING...SO THE HOPE IS THAT THERE WILL BE
MORE BREAKS IN THE CLOUD COVER IN TIME FOR EVENING ACTIVITIES.

ONSHORE FLOW WILL CONTINUE OVER THE WEEKEND WITH MORNING CLOUDS AND
AFTERNOON SUN BREAKS. SOMEWHAT ACTIVE WESTERLY FLOW ALOFT WILL
OCCASIONALLY SUPPORT LIGHT OR WEAK SHOWERS...MOST NOTABLY ON SUNDAY
AS ANOTHER WEAK UPPER TROUGH AXIS PASSES BY.    HANER

.LONG TERM...BOTH THE GFS AND ECMWF SEEM TO AGREE THAT FLAT UPPER
RIDGING WILL BUILD IN FOR THE FIRST HALF OF NEXT WEEK WITH 500 MB
HEIGHTS RISING INTO THE 580+ DAM RANGE. THIS WILL CAUSE A REBOUND OF
INTERIOR LOWLAND TEMPERATURES BACK TO NEAR 80. SHALLOW MARINE CLOUDS
WILL BE CONFINED TO THE IMMEDIATE COASTLINE.        HANER

&&

.AVIATION...COASTAL FOG AND LOW STRATUS WILL SPREAD ONSHORE THIS
EVENING...AND THRU THE STRAIT AND CHEHALIS GAP LATER TONIGHT. SOME
AREAS OF FOG WILL LIKELY FORM AGAIN OVER THE INLAND WATERS BY
DAYBREAK. ALSO INCREASING MID AND HIGH CLOUDS AND A FEW SHOWERS AND
ISOLATED THUNDERSTORMS WILL WORK NORTH OVER WESTERN WA TONIGHT.
SHOULD BE A BIT OF A HODGE PODGE OF CONDITIONS BY DAYBREAK. 19

KSEA...EVENING NORTHERLY BREEZE BECOMES SOUTHERLY OVERNIGHT. SOME
SHOWERS BREAK OUT AROUND MIDNIGHT AND THE MODELS...WILL JUST KEEP
THE VCSH IN THE FCST AND WILL SEE WHAT SHOWS UP ON RADAR LATER THIS
EVENING. WITH GRADIENTS GOING ONSHORE THERE IS A CHANCE THAT SOME
LOW STRATUS OR FOG WILL BE OVER PUGET SOUND AGAIN...BUT LIKE TODAY
IT MAY NOT MAKE IT TO THE AIRPORT. HOWEVER WITH THE MODELS IN
GENERAL MOISTENING UP THE AIR MASS THRU THE ENTIRE DEPTH JUST ABOUT
ANYTHING COULD BE IN PLACE THU MORNING. 19

&&

.MARINE...THE LIGHT OFFSHORE GRADIENTS ARE COMING AROUND TO ONSHORE
SO WILL PUT A SMALL CRAFT UP FOR THE CENTRAL STRAIT TNGT. THE AREAS
OF FOG MAY SWITCH OVER TO JUST STRATUS WITH A LITTLE LIFT AS THE AIR
MASS DESTABILIZES. HOWEVER FOR NOW THE MARINE INVERSION IS STILL
PRETTY STRONG AND THERE IS NOT EVIDENCE OF GOOD MIXING SO KEEPING
AREAS OF FOG IN THE FCST TONIGHT AND AT LEAST PATCHY FOG THU
MORNING.19

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL STRAIT OF JUAN DE FUCA.

WEATHER.GOV/SEATTLE