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fxus66 ksew 191540 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
840 am PDT Friday Mar 19 2010 


Synopsis...an upper level ridge will give sunny and dry weather 
today. The ridge will shift inland Saturday as cold front and upper 
trough approach the region. Cool moist and unstable air following 
the cold front will maintain a showery pattern Sunday and Monday. 
High pressure will build on Tuesday bringing a period of dry weather 
before the next front brings unsettled weather Wednesday. 


&& 


Short term...a strong ridge of high pressure offshore this morning 
will slowly move slowly move east tonight and Saturday. Models 
indicated the ridge axis will be over Puget Sound sound 12z Saturday and 
over eastern Washington 00z Sunday. The upshot is dry weather both 
days...with sunny conditions today and maybe some thin cirrus 
Saturday. Patchy fog in the Snohomish valley...apparent on visible 
satellite imagery...will be gone shortly. 


With offshore surface flow today high temperatures will touch 60 
degrees or so in most locations...middle and upper 50s north part. On 
Saturday high temperatures will be in the 60s most areas...but on 
the coast as onshore flow develops they will probably be a few 
degrees lower than today. 


Models generally bring a little rain inland Saturday night as the 
next front approaches. The front moves inland Sunday for more 
widespread rain. Currently probability of precipitation are likelies inland and categorical 
west part...with high temperatures 10 degrees or so lower. Burke 


Long term...previous discussion...the upper trough remains parked 
over the Pacific northwest Sunday night and Monday with a cool...unstable... 
showery pattern in place. Snow levels will fall below 3k feet but 
accumulations appear light with not much flow over the area. Could 
see a strike or two but will hold off on putting any thunder in the 
forecast. Drier northwest flow develops Monday night as the trough begins 
to pull east allowing showers to decrease. A temporary upper ridge 
transverses the area on Tuesday with a period of dry weather 
expected. The progressive pattern continues into middle week with the 
next trough and associated front quickly moving into western Washington on 
Wednesday. While not strong...this system will usher in moisture... 
cool temperatures...and a chance of showers. Mercer 


&& 


Hydrology...previous discussion...the flood risk across western 
Washington and for the Green River is very low for the next 10 days. 


High pressure will maintain dry weather across the area through 
Saturday. A frontal system will move across western Washington Saturday night 
followed by a cool trough Sunday and Monday. This system will lack 
substantial moisture with weak flow not aiding orographics. The snow 
level will also fall below 3 knots feet by Sunday night. Bottom line... 
light rainfall/snowfall amounts are expected with no flooding 
concerns. Another system on Wednesday will also be fast moving and 
rather weak. Rainfall amounts will also be light with this system. 
Mercer 


&& 


Aviation...northwest flow aloft over western Washington will 
continue through this evening ahead of an upper level ridge over the 
offshore waters. The flow will back to westerly later tonight as the 
ridge shifts inland. Surface high pressure over southern British 
Columbia will produce low level offshore flow through the period... 
which will strengthen tonight as a frontal system just upstream of 
the ridge approaches the region from the west. Clear skies will 
prevail through tonight except for patchy...highly localized stratus 
during the late night and morning hours. 


Ksea...VFR. North to northeast wind 8-12 knots easing to 5-9 knots midday 
through tonight. Mcdonnal 


&& 


Marine...offshore flow will increase today in response to a surface 
ridge building over southern British Columbia. The offshore flow 
will strengthen further tonight into Saturday as pressures fall over 
the northeast Pacific. Winds might approach gale force near the West 
Entrance to the Strait early Saturday morning. A cold front will 
sweep across the area Saturday night with probable Small Craft 
Advisory wind for much of the waters. Onshore flow will persist 
behind the front Sunday into Monday as a surface ridge builds over 
the waters. Mcdonnal 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Small Craft Advisory West Entrance to the Strait. 


$$ 


Www.Weather.Gov/Seattle 


For an illustrated version of the forecast discussion please see 
www.Weather.Gov/Seattle/gafd/latest_webafd.Html (all lowercase).