911 
fxus66 ksew 052102 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
200 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...some moderate cumulus over The Lowlands early this 
afternoon...towering cumulus with showers so far are confined to the Cascades 
from southern Skagit through Snohomish and King counties. It does not seem 
likely that much shower activity will develop across The Lowlands 
this evening as the air mass begins to stabilize ahead of the next 
frontal system...so I have low probability of precipitation except on the coast and in the 
Olympics where rain will develop overnight. For the inland portion 
of western Washington the rain will develop around daybreak...and the significant 
rainfall should be on the coast...in the mountains...and in the 
north. The areas of rain will taper off to showers through Monday night 
with scattered showers a good bet through Tuesday night. The east-west 
oriented band of heavier precipitation advertised by the NAM that looks like 
a pscz...with something else going on over the north part f the 
Olympics...is a little odd looking. The uw wrfgfs forecast has a more 
typical pscz like band of showers from the NE Olympics extending east-northeast 
through southern whidbey then up through northern Snohomish County Monday 
evening...that pscz shifts south and breaks up by midnight Monday night. 
Scattered showers continue Tuesday with the upper trough 
overhead...and then by Wednesday there will probably just be afternoon 
scattered mountain showers. 19 


Long term...there is a good chance that Thursday 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 sound convergence zone as the global 
models are advertising west-northwest flow aloft and at least a couple of 
embedded weak shortwaves Friday and beyond. Any of these systems may 
end up pulling in enough moisture before they go up and over the middle 
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 
probability of precipitation than chance/scattered showers. 19 


&& 


Aviation...afternoon cumulus over the region...except some towering cumulus in the 
Cascades with scattered showers locally obscuring the Cascades and 
areas of broken-overcast ceilings obscuring the higher elevations. The Olympics 
will become obscured by midnight tonight...and the Cascades should 
be obscured in merging layers of clouds and rain by daybreak Monday. 
For now the the air is cool and unstable...but will stabilize 
overnight with increasing moisture ahead of the approaching frontal 
system. The front should be through the coast before noon and through the 
interior marine areas around middle afternoon. Expect areas of low ceilings 
and visibility on Monday most of western Washington with this front. Northwest flow aloft tonight 
will back to west flow aloft Monday night. 19 


Ksea...afternoon scattered-broken cumulus will probably scatter out this evening 
for awhile as the cumulus collapse and dissipate. But middle level moisture 
will arrive before midnight...and by daybreak the air is moist. 
Monday will should be a typical rainy MVFR day with a chance that 
ceilings descend to IFR any time after the rain starts 12-15z. 19 


&& 


Marine...the 4km wrfgfs has w20kt in the central and east Strait 
this evening and a small area of w25kt just southeast of Victoria...so we 
have a Small Craft Advisory up for 15-25kt winds this evening. The 
westerly will give out quickly and winds will back to south-southeast most waters and 
rise to 10-20kt by daybreak. The 4km model has a few spots reaching 
25kt...just off Long Beach at daybreak...and also between Lummi 
Island and pt Roberts...so this front may warrant some small craft 
advisories. Frontal passage should be through the coast before noon and through the 
interior marine areas middle afternoon. The westerly flow behind the front 
is not particularly strong and should not need advisories. 19 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Small Craft Advisory central/east Strait this evening. 


$$ 


Www.Weather.Gov/Seattle