590 
fxus66 ksew 081552 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
850 am PDT Friday Aug 8 2008 


Synopsis...onshore flow will continue today for more clouds and 
cooler temperatures. With south flow aloft a few thunderstorms may 
develop along the Cascade crest. A weak front will bring clouds and 
a chance of showers tonight as it moves slowly inland and 
dissipates. The trailing upper level low will maintain a chance of 
showers through Saturday night. An upper level ridge will bring 
warmer and dry weather early next week. 


&& 


Short term...south-southwest flow aloft continues over West Washington today with the 
unstable air mass over eastern Washington running up to the Cascade crest. 
Water vapor imagery confirms the weak vorticity maximum the GFS shows over 
central Oregon that is moving north near the crest. Some active 
cells near The Dalles will move north along the crest this morning but 
there isn't anything going on to the S yet. A weak vorticity maximum over S 
central Oregon could do something this afternoon...or not. Will keep 
the chance of thunder along the crest today...but may end up 
dropping it for this evening since the GFS shows stable air with 
higher lifted indices spreading east across the crest this evening. 


Onshore flow overnight has actually covered more of West Washington than 
yesterday...it just left a Big Hole from about sea-tac up through 
Everett this morning. Still some chance that with S winds some 
patches of stratus could drift up over Seattle for a while this 
morning. The marine layer is a little deeper this morning and with 
weak onshore pressure gradients continuing today and the 
cross-Cascade gradient increasing it will be a little slower 
burn-off this afternoon...and probably won't burn off all the way to 
the coast. 


The front over the offshore waters is only about 100 nm 
wide...fairly skinny and looking pretty weak. Over the past few days 
the GFS/NAM have been waffling a bit on how much precipitation this feature 
will generate. At this point the models show hardly any...and MOS 
probability of precipitation are quite low. Will see what the 12z runs have...but the 
tentative plan will be to cut tonights probability of precipitation back a bit. With a 
continuous sheet of stratus between the coast and the front that the 
system can sweep inland...the very moist air mass the models show 
moving inland tonight looks realistic. This should be very stable 
and will change the precipitation type from showers to light rain and 
drizzle. 


Best shot of precipitation for West Washington will be when the upper level low near 
49n/134w moves inland over S b.C. On Saturday then moves east 
through early Sunday. 60 percent MOS probability of precipitation for Saturday at ksea are 
the highest yet...but will wait for the 12z data before boosting the 
forecast probability of precipitation. The models have also waffled on precipitation with this 
system for Saturday night and Sunday. GFS/NAM have different timing 
on when the best period is for precipitation...so may have to boost 
Saturday night probability of precipitation as well. Still leaning toward a dry Sunday over 
The Lowlands but this may change. Kam 


Long term...previous discussion from the 330 am afd...an upper 
level ridge moves over the region in the wake of the departing low 
for a return to warmer and dry weather. Monday will clear out with a 
good amount of warming over Sunday. The ridge shifts to the east 
through the week but the models hold reasonably high heights over 
the northwest with light flow in the low levels...possibly even offshore 
Thursday. Overall...mostly clear...dry with normal to above normal 
temperatures which is what is in the current forecast. Only made a 
few minor tweaks to the temperatures through this period. Cerniglia 


&& 


Aviation...broad upper level low will continue to spin of the Washington 
coast today with S/SW flow aloft over West Washington. Shower activity this 
morning remains in eastern Washington although there is potential for 
showers and isolated thunderstorms this afternoon again along the 
Cascade crest. Instability will decrease this evening with 
showers/T-storms moving into East Washington. 


Other concern this morning is low level clouds across The Lowlands 
with onshore flow. While the fog has lifted in some spots...north 
interior...still seeing lower ceilings across most areas as stratus 
continues to fill in. Visible imagery shows a solid marine layer 
across the entire coast/Strait and SW interior where MVFR to IFR 
conditions persist. Will indicate some lifting this afternoon but 
not expecting any clearing. 


Ksea...low clouds will linger across Puget Sound sound this morning with 
MVFR conditions through 18z. Low clouds will return overnight around 
09z as marine layer filters back in. Winds today will remain S/SW 
05-10 knots. 33 


&& 


Marine...while pressure gradient through the Strait of Juan Delaware fuca 
fairly weak...still seeing Small Craft Advisory winds in the 
central and east portion. Will continue Small Craft Advisory headlines through 06z 
with winds easing early Sat. Onshore flow will continue through the 
weekend . 33 


&& 


Sew watches/warnings/advisories...Small Craft Advisory central and 
east Strait of Juan Delaware fuca. 


$$ 


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