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Area forecast discussion 
National Weather Service Portland or 
245 PM PST Friday Nov 21 2008 


Synopsis...a vigorous Pacific cold front is approaching the 
Washington and Oregon coast this afternoon. The front will move 
onshore by early evening...spreading rain and mountain snow across 
the district tonight. Snow levels will fall below Cascade Pass levels 
tonight...with a couple inches of snow accumulation expected on the 
passes by Saturday morning. High pressure will build across the 
district Saturday...causing showers to decrease. Gusty east gorge 
winds will be possible again Sunday and Monday as high pressure moves 
east of the Cascades. Region stays mainly dry Monday...then the next 
cold front brings another chance of rain for Tuesday. Long range 
models are hinting at a chance of rain again for Thanksgiving day. 


&& 


Short term...persistent fog and low clouds held temperatures down 
today for the western Willamette and Tualatin valleys...where many 
locations have been stuck in the middle to upper 40s for highs so far. 
Eastern portions of the Willamette Valley saw more sunshine early on 
today and were thus a bit warmer...as were the coast and lower hills. 
East winds have picked up a bit through The Gorge today as high 
pressure moves east of the Cascades and vigorous cold front 
approaches the coast. 


This vigorous front appears to be getting a dynamic boost from a 90- 
100 knots punch of jet energy injecting into the base of the upper level 
trough associated with the front. As a result...numerous lightning 
strikes have shown up on lightning detection along the front...about 
100 miles offshore. Have added a slight chance of thunderstorms to 
the northern coastal waters...but by the time the front reaches land 
it appears the dynamics will weaken somewhat and therefore decided 
not to include thunder in north coast forecast. As the front moves 
onshore...rain will spread across the area...probably bringing 
respectable rainrates but only for a short period of time. 


Timing of rain appears to be around 00z-02z this evening along the 
coast...spreading onshore and reaching the kpdx area by 04-05z. Winds 
ahead of the front are pretty gusty...as shown by buoys 46005 earlier 
and 46089 more recently...which have gusted around 40 knots. For now 
will stick with winds gusting to 50 miles per hour along the coastal headlands 
ahead of the front. Snow levels with front likely start out slightly 
above pass level...but come down overnight as cooler air moves in 
with the front. A couple inches of accumulation are likely on the 
passes...but for now it appears accumulations will remain below 
advisory criteria. 


High pressure builds in quickly Sat...shutting off the orographic 
flow for the Cascades. This should allow showers to taper off quickly 
and a rather dry afternoon throughout. Sat night looks to be another 
mainly clear calm night...so fog and near freezing temperatures will 
again be a possibility for the outlying valleys. Gradients turn 
offshore Sunday...causing another easterly wind event especially 
through The Gorge. Dry weather and gusty east gorge winds may linger 
into Monday as strong ridge remains over the interior northwest. 
Weagle 


Long term...models show a split upper level flow pattern over the 
region at times during the extended. Did not change the forecast too 
much as timing is often an issue with these types of patterns. The 
12z GFS and Euro both pinch off a portion of an upper level trough 
Monday evening and form a cut off upper level low off the coast of 
California. The result is a weakening front that moves through the 
region on Tuesday. Another strong upper ridge will build in Wednesday and 
give dry conditions to the forecast area. Another trough approaches 
the Pacific northwest on Thursday night. Dalton 


&& 


Aviation...generally VFR conditions over the forecast area should 
continue through the rest of the afternoon. As the front offshore 
approaches...widespread MVFR ceilings will reach the coast early in the 
evening then spread inland overnight. Rain accompanying the front 
will pass through the area rather quickly overnight...leaving 
showers over the coast by early Saturday morning and inland by Sat 
afternoon. Ceilings are likely to stay MVFR with areas of IFR in heavier 
rain. 


Kpdx and approaches...VFR conditions should continue through the 
evening. Good easterly low level winds will begin to shift to the 
south around 00z as front offshore approaches. Expect MVFR ceilings by 
06z Sat along with rain development...and should continue through 
early morning Saturday. Brown 


&& 


Marine...winds increasing this afternoon approaching gale force at 
least in the northern waters. Should see strongest winds with the 
frontal passage early tonight. This will be a quick-hitter...with 
about 6 hours of gale force wind. This will be a pretty decent 
front...with gusts expected around 40 knots over the waters and to 45 
knots on The Headlands. Wind switches to northwesterly behind the front and 
will drop to small craft levels. Wind speeds to remain 20 knots or less 
Sat through at least the middle of next week. 


Seas running at 10 to 12 feet or so this afternoon and will begin to 
increase this evening. Combined seas likely to hit 15 feet or so just 
ahead and with the front...and continue through Sat morning. Rough 
bar conditions through at least Sat afternoon. Weishaar/Brown 


&& 


Pqr watches/warnings/advisories... 
or/WA...Small Craft Advisory for rough Columbia River bar through 
Sat afternoon. 
Gale Warning Cape Shoalwater to Florence through late 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.