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fxus66 ksew 300418 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
920 PM PDT Friday Aug 29 2008 


Synopsis...strong onshore flow behind a dissipating cold front will 
produce convergence zone showers over Puget Sound sound tonight and 
scattered showers over the Cascades. An upper level trough will move 
south over the area Saturday and Sunday bringing an unseasonably 
cool air mass and a few showers. An upper level ridge will move over 
the area Monday as the trough moves east...bringing drier and warmer 
conditions. A weak upper level trough may bring some showers to the 
area Tuesday. A strong upper level ridge may build over the area 
late this week. 


&& 


Short term...the cold front moving across West Washington late this afternoon 
has pretty much fizzled this evening. Main precipitation was over the north 
Cascades with very little over The Lowlands except for parts of the 
north interior. Main feature tonight is the Post-front strong onshore 
flow. The resulting Puget Sound sound convergence zone is up north between 
Arlington and Mount Vernon but should redevelop further south later 
this evening as strong flow through the Strait of Juan Delaware fuca 
begins to weaken. The only worry here is that the new 00z NAM hardly 
shows any convergence zone precipitation over central Puget Sound sound 
tonight...after consistently showing precipitation the past couple of days. 
West flow aloft is also producing showers over the Cascades...which 
should taper off as the 850 mb flow weakens and shifts northwest. 


Models have a large upper level trough easing down over West Washington 
Saturday then moving directly overhead on Sunday. Both NAM/GFS have 
minimal shower activity over the area Saturday. Forecast looks good 
with partly sunny and low chance probability of precipitation. NAM shows showers developing 
over the Olympics in the northwest flow between 850 mb-700 mb then drifting 
southeast across S Puget Sound sound. This is a subtle feature that the models 
sometimes have Success with. Will leave this for the midshift to buy 
off on...or not. Forecast soundings show an unstable air mass over 
the area Sunday while the upper level trough is directly overhead. 
GFS lifted indices are zero or 1. Neither model shows much in the 
way of precipitation and neither does MOS. With the instability it's hard 
to not have at least a minimal chance of showers. 


GFS still shows an upper level ridge moving over the area on Monday 
after the big Sunday trough moves east. GFS has been a little 
inconsistent in pinning down the strength and orientation of the 
ridge over the past couple of days. Main question is how much 
sunshine will develop...especially with clouds from another 
approaching trough right behind the ridge. Should be dry and a bit 
warmer...but only with maximum temperatures in the 60s. Kam 


Long term...previous discussion from the 3 PM afd...the ridging 
aloft is transitory as forecasts agree on bringing the next shortwave 
onshore Tuesday. The real question will be how much energy this system 
will have. So have increased the clouds and renewed the threat of 
precipitation Tuesday lingering into Wednesday. Extended forecasts agree on ridging later 
in the week. Again the question is how strong will the ridge aloft 
be along with its corresponding low level offshore flow. For now..am 
including a warming and clearing trend Thursday and Friday. We will have 
time to fine-tune the details as we get closer to that time frame. 
Buehner 


&& 


Aviation...cold front over the Cascades this evening. Convergence 
of the central sound lowering ceiling to 1500 to 2500 feet until the 
early morning hours. Outside of the convergence just scattered 
clouds. Low level onshore flow weakening overnight with cool upper 
level trough moving over western Washington on Saturday. Still 
enough low level moisture over the area to have ceilings reform 
outside of the convergence during the early morning hours in the 
3000 to 4000 foot range. With the unstable air mass a few showers 
over the area Saturday but scattered enough to leave out of the 
terminals. 


Ksea...sct-bkn layer around 2500 feet through the early morning 
hours. Ceilings lifting up to 4000 feet Saturday morning. Winds 
northerly 10-15 knots for the next couple of hours. Winds easing to 
northerly 4 to 8 knots and then light and variable by 15z Sat. Felton 


&& 


Marine...gale force winds in the central and eastern Strait until 
early Saturday morning. Kuil-kbli gradient peaked near 4 mb around 
00z is down to near plus 3 mb at 04z. This trend will continue into 
Saturday morning. Small Craft Advisory winds in Admiralty Inlet and 
at the West Entrance will also diminish overnight. 


With a deep upper trough moving across the area...the onshore flow 
will continue through the weekend with Small Craft Advisory winds in 
the central and eastern Strait during the late afternoon and evening 
hours Saturday and Sunday. 


A weak front is expected to approach the coastal waters late Monday 
night and dissipate over the area into Tuesday. Felton 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Gale Warning central and eastern Strait. 
Small Craft Advisory Admiralty Inlet and West Entrance to the 
Strait. 
Small Craft Advisory in effect for rough Grays Harbor 
bar conditions. 


$$ 


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