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Area forecast discussion 
National Weather Service Portland or 
241 PM PST Tuesday Dec 02 2008 


Synopsis...northwesterly flow behind last nights front will continue 
to bring marine clouds inland this afternoon and evening with 
isolated to scattered light showers possible in the mountains. Fog 
and low clouds will form in The Lowlands overnight and persist 
through midday Wednesday as a ridge aloft builds over the area. The 
lingering low clouds will clear out Wednesday afternoon as northeast 
winds begin to blow. The northeast winds will increase Wednesday 
night and Thursday for a sunny and windy Thursday. Dry weather will 
continue Friday and Saturday but valley inversions will become 
increasingly strong as surface winds lighten. The next frontal system 
will move into the area on Sunday. 


&& 


Short term...the upper level low swept inland across Washington and 
Oregon last night leaving the area in northwesterly flow aloft behind 
it. The dry slot behind the low is rather impressive on the water 
vapor imagery and now centered over California and Nevada. The northwest 
flow aloft and subsidence as the ridge builds will minimize showers 
keeping them light and limited to orographically enhanced area this 
evening. 


As the ridge aloft continues to strengthen over the area tonight 
skies will clear just enough for fog and low clouds to form in the 
moist layer trapped near the surface and become increasingly 
widespread over The Lowlands. Surface winds will weaken as the 
pressure gradient flattens in the evening and becoming light and 
variable overnight. With very little wind Wednesday morning low 
clouds and fog will be slow to mix out lasting through early 
afternoon. 


A northerly pressure gradient will develop over the area Wednesday 
afternoon as a thermal trough builds north from northern California 
into western Oregon and an Arctic surface high in Canada edges into 
eastern Washington. The resulting northerly winds should stir things 
enough to mix out the Willamette Valley inversion by late Wednesday 
afternoon. The northeasterly pressure gradient will increase 
substantially Wednesday night through Thursday morning so once the 
fog is scoured out late Wednesday it is unlikely to come back. 


Breezy northeast winds will increase on Thursday over much of the 
area. The strongest winds will be out of the east in the western 
gorge in the 40 to 45 knot range. However given the strength of the 
thermal trough along the coast and the northerly component to the 
winds in addition to the more typical easterly gradient...north to 
northeast winds will quite a bit more widespread than normal for an 
offshore flow pattern. Ruthford 


Long term...by Thursday night the surface pressure gradients turn 
straight east as the ridge axis aloft tilts inland. This will confine 
winds back into the east to west running gaps in the Cascades and 
Coast Range. Winds will decouple over the Willamette Valley allowing 
a strong inversion to form late Thursday night and Friday morning. By 
this point however the NE winds from Wednesday night and Thursday 
will have dried the lower atmosphere substantially so fog will be 
limited in extent despite the strong inversion. With light winds on 
Friday and weakening offshore gradient it is unlikely the inversion 
will do much more than lift slightly in the afternoon. Light 
winds...stable conditions and limited mixing will continue into 
Saturday but should improve significantly on Sunday as an upper level 
shortwave moves overhead. This feature destabilizes the atmosphere 
bringing cool showery weather as well as triggering a strong push of 
marine air. Weather pattern begins to turn more active next week as 
the jet stream starts migrating southward...therefore increased probability of precipitation 
accordingly. Ruthford/Wolfe 


&& 


Aviation...the coast continues to be plagued with areas of MVFR and 
IFR conditions this afternoon while most inland areas have improved 
to VFR. Weak onshore flow is expected to continue through the day 
today. Onshore flow will weaken overnight as high pressure builds 
aloft. This should provide a nice setup for fog development late 
tonight through much of Wednesday. 


Kpdx and approaches...VFR conditions will continue through early 
this evening. Fog is likely to impact kpdx area terminals tonight... 
possibly as early as 06z as winds die down. Conditions will improve 
Wednesday afternoon as northerly low level flow develops. 


&& 


Marine...seas will remain near 10 feet through early this evening 
before subsiding. Weak high pressure will build across the waters 
tonight. Another long-period swell train will likely begin to impact 
coastal waters by Wednesday night...though this one should not be quite as 
large as the swell that impacted the waters this past weekend. 


&& 


Pqr watches/warnings/advisories... 
or/WA...Small Craft Advisory for rough Columbia bar through this 
evening. 
Small Craft Advisory for hazardous seas Cape Shoalwater to 
Florence through early this evening. 


&& 


$$ 


More weather information online at... 
http://weather.Gov/Portland 


This discussion is for northwest Oregon and southwest Washington from 
the Cascade crest to 60 nautical miles offshore. The area is commonly 
referred to as the forecast area.