Weather

170 
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afdpqr 


Area forecast discussion 
National Weather Service Portland or 
225 PM PDT Friday Aug 29 2008 


Synopsis...a weakening Pacific front will approach the northern 
coast late this afternoon and move across the area tonight. A cool 
upper level trough will sink south into the area Saturday and Sunday 
the air mass will be slightly unstable on Sunday as the coldest part 
of the upper trough moves through. The upper low will sink south into 
Great Basin Monday with continued onshore flow. The next front will 
brush across the area Tuesday. Northwest flow will continue over the 
area Wednesday as an upper level ridge begins to build offshore. Long 
range models vary on whether this ridge builds over the Pacific 
northwest or just offshore. Either way...Thursday and Friday will be 
dry with significant warming a possibility. 


&& 
Short term...satellite imagery continues to show a weakening front 
crawling toward the forecast area. While not impressive on satellite 
the front should have enough punch for some light rain threat late 
this afternoon on the north coast and across much of the area 
overnight. However...the front will be strong enough to lift the 
shallow marine layer up and push it inland tonight. The frontal band 
will be clear of the area by Sat morning. Initially not much in the 
way of clouds behind the front even with onshore flow. In fact... 
coastal areas should see some clearing later tonight. A cool upper 
trough will then slowly sink over the area Saturday. The initial 
shower threat will be near the coast Saturday morning and then will 
spread inland into SW Washington Saturday afternoon as the initial 
part of the trough moves over the area. The brunt of the cold pool 
will arrive Saturday night with shower chances spreading across much 
of the forecast area. The next trough axis will swing across the area 
Sun morning continuing the shower threat. Despite 500 mb temperatures 
near -24 on Sunday models show very little instability. Will keep 
thunderstorms out of the forecast for now. Ridging should begin to 
build offshore on Monday for ending the shower threat. 
However...temperatures should remain slightly below normal with 
onshore flow and a residual cool air mass. 


Long term...a weak disturbance will move across British Columbia on 
Tuesday with the tail end brushing the northern parts of the forecast 
area. Moisture looks shallow and limited and will leave slight chance 
probability of precipitation across the north. Guidance varies substantially on the remainder 
of the extended. However...all guidance keep the area dry. The 12z 
GFS suggests a Big Warm up by Thursday and Friday. The European model (ecmwf) is much 
cooler. At this point will warm high temperatures back to 
climatology. 
&& 


Aviation...IFR ceilings persisting along the coast this 
afternoon...will gradually lift to MVFR and the Pacific front moves 
onshore and lifts the marine airmass. Ceilings inland will remain 
VFR with occasional MVFR as the front moves through around 04z-06z. 


Kpdx and approaches...predominately VFR conditions. Some MVFR 
ceilings lowering to around 2500 feet are possible with some light 
isolated showers as the front moves through this evening. 
&& 


Marine...expect marginal small craft winds to develop later this 
afternoon and evening...then continuing into tonight as a cold front 
drops southeast through the waters. Another chance at Small Craft Advisory 
winds late Sat night and sun. Seas will stay below advisory 
criteria for hazardous seas. Tw 


&& 


Pqr watches/warnings/advisories... 
or/WA...Small Craft Advisory for wind Cape Shoalwater to Florence 
tonight. 
Small Craft Advisory for rough Columbia bar early Sat 
morning. 
&& 


Preliminary point probability of precipitation (probability of measurable precipitation) 
AST 53321- pdx 31221- sle 31221- eug 21221- 


$$ 


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.