Weather

904 
fxus66 ksew 130956 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
300 am PDT Monday Oct 13 2008 


Synopsis...a weak warm front move across the area today. A cold 
front will sweep across the area early this evening. Expect Post 
frontal showers and onshore flow tonight with a Puget Sound sound 
convergence zone. Tuesday through early Wednesday will be dry with 
weak high pressure over the area. A weak disturbance will bring 
clouds and a chance of a little rain to the area Wednesday afternoon 
and Wednesday night. Westerly flow aloft will bring more systems 
into the region later in the week. 


&& 


Short term...the zonal index across the Pacific is increasing with 
time. With that...the model solutions are increasingly 
diverging...particularly in the extended range. 


A weak warm front is spreading light rain onto the coast and into 
the north interior early this morning. This will gradually spread 
inland during the early morning hours. There may be a bit of a break 
in precipitation to the Lee of the Olympics as warm advection 
decreases and increasing westerly flow over the terrain induces some 
Lee side subsidence. Rainfall amounts with the warm front will be 
quite light...on the order of 0.1-0.15 inches coast and 
mountains...and 0.05 inches or so in the interior lowlands. 


A cold front now nearing 130w will sweep across the area late this 
afternoon and evening. This front is tapped into a moderately moist 
subtropical plume extending NE from north of Hawaii and containing 
precipitable water amounts 1-1.4 inches. Quantitative precipitation forecast from a blend of global 
and mesoscale models gives 0.6-0.9 inches of rain in the central 
Cascades and east central Puget Sound sound lowlands in the wake of the 
cold front this evening. Much of this precipitation will be 
associated with an active Post frontal Puget Sound sound convergence zone. 


Conditions rapidly dry to that only clouds are left Tuesday morning. 
Moist ground and cooling will give patchy morning fog to favored 
locations Tuesday morning. Expect partly sunny skies Tuesday afternoon as 
weak high pressure builds into the area. 


Radiational cooling and light pressure gradients will result in an 
increase in coverage and duration of fog late Tuesday night into Wednesday 
morning. Beginning Wednesday afternoon models show increasing divergence 
in solutions as the GFS brings a warm front or weak low NE from the 
Oregon waters and into southern portions of the area Wednesday evening. 
The European model (ecmwf) and NAM are dryer and weaker with this feature. At this 
point will increase clouds Wednesday afternoon and evening and give a 
chance of rain to the area. Later shifts can refine this as we get 
closer to the event and models converge. Albrecht 


Long term...we move into a more active period Thursday and beyond as 
increasing westerly flow brings a series of fronts across the area. 
The problem is timing. The GFS is faster in bringing a front into 
the area sometime Thursday while the European model (ecmwf) is slower. At this time the 
forecast is a broadbrush with clouds and a chance of rain...likely 
probability of precipitation on the coast. Will not make any changes to this thinking at 
this time. Albrecht. 


&& 


Aviation...upper level ridge will break down today as cold front 
currently located across southern British Columbia tracks southeast and into West Washington. This 
front will bring an increase in rain this morning /MVFR conditions/ 
along with breezy southerly flow...especially across the interior. 
The cold front will pass through the coast late morning...then move 
through the interior this afternoon. Showers are expected behind the 
front with associated short wave trough aloft. Models indicating 
Puget Sound sound convergence zone forming near kpae/kawo this 
evening...that will sag south into King Colorado as the night GOES on. 
Showers will be mainly confined to the Cascades during the overnight 
hours. 33 


Ksea...ceilings will gradually lower this morning with MVFR conditions 
expected. Light rain also possible early...with an increase in 
activity by middle morning. In addition...southerly winds will rise to 
10 to 20 knots. Expect showers in the vicinity this evening with the 
developing pscz. 33 


&& 


Marine...blustery day ahead as cold front over southern British Columbia slides 
southeast through West Washington. Expecting Small Craft Advisory winds across all 
waters today. Northwest/west onshore flow will increase this afternoon behind 
the front leading to a strong westerly surge through the Strait of Juan 
Delaware fuca. Meanwhile...seas will also build to near 10 feet this 
evening. Small Craft Advisory winds will continue through the Strait and north inland 
waters this evening with winds gradually weakening early Tuesday 
morning. Broad area of high pressure will shift into the coastal waters 
on Tuesday with light onshore flow expected. Another frontal system will 
approach West Washington Wednesday afternoon/evening. 33 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Small Craft Advisory for all waters. 


$$ 


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