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Area forecast discussion 
National Weather Service Seattle Washington 
900 PM PDT Monday Oct 6 2008 


Synopsis...a vigorous frontal system will sweep through western 
Washington tonight. A strong low pressure trough will bring windy 
weather by daybreak. The strongest winds are expected to be near the 
Strait of Juan Delaware fuca and north of Everett. The snow level will 
fall to 4000 feet in the mountains on Tuesday behind the front. 
Unsettled weather will persist through the middle of the week. High 
pressure will build for next weekend. 


&& 


Short term...models seem to have a good handle on the strength and 
track of the deep 985 mb surface heading toward North Vancouver Island. 
WV imagery shows a very nice dry slot behind the trailing cold front 
associated with a very strong 155 knots westerly jet aiming toward northern Washington. 
The bent back occlusion looks fairly expansive on the infrared imagery. 
Although the surface low tracks further north than the previous 
system...the southern extent of The Bend back should still cross the area 
later tonight and Tuesday morning. Southeasterly winds ahead of the front are 
already picking up with strong gales likely along the coast and from 
the East Entrance northward. Wind Advisory southeasterly winds look on track for land 
areas from around Everett northward tonight just head of and along the 
front. Of greater concern is the westerly surge through the Strait that 
picks up after 09z tonight through roughly 15z Tuesday morning per the 
earlier 4km mm5wrf. A very strong westerly jet with strong 850 mb winds 
near 50 knots line up nicely down the Strait which could mix down to 
the surface in gusts. The air mass is quite unstable...evident by a 
few lightning strikes along the coast the past hour. This often 
signals a higher likelihood of stronger wind gusts. The 00z NAM-12 
is not nearly as strong with the westerlies down the Strait so i'm 
hesitant to issue a warning for the Whidbey Island area at this 
point. However...I will refresh the Wind Advisory statement to 
emphasize the change from southerly to westerly wind by Tuesday morning and 
emphasize strong gusts. Will need to watch this area closely later 
tonight. Winds should begin to subside by middle-late Tuesday morning as 
pressure rises subside as the low moves eastward. 


Strong westerly flow aloft will persist through the day. The Post frontal 
air mass will not be terribly moist but cold air advection and 850 
mb westerly flow of 30-40 knots into the Cascades should produce very 
efficient orographic enhancement. Snow levels fall to around 4500 
feet early Tuesday morning...then down to 3500-4000 feet by midday. 
Model quantitative precipitation forecast suggests 2-4 inches of wet snow at the higher 
passes...with highest amounts from Stevens Pass northward. Higher 
elevations of the north Cascades such as Montana Baker should be most favored 
in this pattern with 6+ inches possible by Tuesday evening. A Winter 
Weather Advisory remains in effect. 


Westerly flow persists into Tuesday night then a broader upper trough 
develops over the area Wednesday in response to strong ridging offshore. 
Expect showers and cool temperatures over the area on Wednesday into Thursday before a 
drying trend the end of the week. Mercer 


Long term...previous discussion...did not spend time on the 
extended grids and for Friday-Monday a much more benign weather pattern is 
likely. After upper trough moves through the region Wednesday/Thursday a ridge 
builds in. Isolated showers still possible Friday. Partly sunny at 
least Sat sun. The European model (ecmwf) has consistently kept a much deeper upper 
trough over the western U.S. Through the end of the week...but does 
finally build heights over western Washington to the 570s late sun. Temperatures will 
need some work in the extended...a good shot of getting well up int 
he 60s and even lower 70s Sunday afternoon in the southwest 
interior. But I did not have time to address that today. 19 


&& 


Aviation...985 mb low near 49n/129w will move onto northern 
Vancouver Island shortly. An associated frontal system 50 nm west of 
the Washington coast at 330z will move onshore by 05z then move 
through the Puget Sound sound lowlands around midnight. 


Ceiling and visibilities are all over the place this evening. There 
is quite a bit of flow interacting with the Olympics and coastal 
terrain...and that is causing local low level clearing to the Lee of 
the terrain. Behind the front expect much of the same as strong west 
winds aloft interact with the terrain. The west slopes of the 
Olympics and Cascades will see cloud bases 030 mean sea level but there will be 
holes to the Lee of the Olympics and coastal hills through Tuesday. 


The storm system wind field is impressive and will be the main 
impact on terminals overnight through Tuesday morning. A 155 knots 300 mb 
jet is aimed into the Olympics...and an impressive bent back 
occlusion to the south of the low extends south to 47n at 130w. Buoy 
46005 near 47n/131w has reported west wind 39g50 knots with this bent back 
occlusion. Satellite trajectories take this feature east-northeast through the 
northern half of the forecast area late tonight through Tuesday morning. 
The strong flow...interacting with terrain...will produce strong 
mechanical turbulence ahead of the front in places like kbli. Behind 
the front expect the same at places like kbfi. Winds will decrease 
rapidly after during the morning hours. 


Ksea...strong west flow aloft...interacting with the Olympics...will 
occasionally scatter low clouds this evening ahead of the front. 
Behind the front expect wind 20020g33kt 10z-15z at the terminal with 
occasional clearing of the low level moisture. Wind will settle down 
to SW 10g20kt by 18z with occasional showers but clearing at times. 
Ceilings generally 035-050. Any convergence zone activity Tuesday afternoon 
will likely remain north of downtown Seattle. Albrecht 


&& 


Marine...985 mb low near 49n/129w will move onto northern Vancouver 
Island shortly. An associated frontal system 50 nm west of the 
Washington coast at 330z will move onshore by 05z then move through 
the inland waters by midnight. 


The storm system wind field is impressive and will give gales to all 
waters through Tuesday morning. The offshore buoy near 47n/131w has been 
reporting west wind 35-40 knots with occasional gusts to 50 knots with the 
impressive bent back occlusion to the south of the low extends. This 
feature will lift east-northeast across the Strait and northern inland waters 
late tonight and Tuesday morning. 


A west/northwest swell to near 20 feet offshore will decay to 16 feet in the 
coastal waters late tonight through Tuesday. 


Deep low fills and moves into inland British Columbia fast...so wind 
should diminish considerably by Tuesday afternoon. A short wave 
surface ridge on Wednesday will keep winds relatively light. A 
trough on Thursday will boost onshore flow. A weak system will move 
approach the waters Saturday. Albrecht 


&& 


Sew watches/warnings/advisories... 
Washington...Wind Advisory in effect from 2 am to 9 am PDT Tuesday for the 
north part of western Washington including the Strait of Juan Delaware 
fuca and areas north of Seattle. 
Winter Weather Advisory in effect for Tuesday for the Cascade and 
Olympics mountains above 4000 feet 
pz...Gale Warning all waters. 
Small Craft Advisory Grays Harbor for rough bar conditions. 


$$ 


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