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Area forecast discussion 
National Weather Service Portland or 
917 am PST Sat Nov 21 2009 


..updated for high wind watch and possible Wind Advisory in the 
Willamette Valley tonight... 


Synopsis...a warm front will approach the Oregon coast this 
afternoon then push inland this evening. A cold front will quickly 
follow tonight as a surface low pressure system moves inland near the 
Washington Oregon border. A weak ridge of high pressure will follow Sun 
afternoon and night...before the next frontal system arrives Monday. 
&& 


Short term...latest model runs continue to exhibit differences in 
track of the surface low coming in tonight. Upper features appear 
pretty well agreed on...with a strong shortwave moving across the 
forecast area tonight...digging to the east-southeast. Given the tract of the 
upper vorticity maximum and the negative tilt of the upper trough...continue 
to prefer a more southerly track of the surface low...as such prefer 
the 00z run of the European model (ecmwf) which takes a surface low east roughly across 
kkls at 12z sun. Timing of the cold front in all models indicates 
frontal passage on the coast not until sometime tonight. Given the 
continued uncertainties in the models...will extend the high wind 
watch for the north and central Oregon coast to include the southern 
Washington coast...and extend it till 6am. Winds inland look gusty 
with and behind the front as there are strong pressure rises behind 
the front...expect an advisory for some portion of the Willamette 
Valley for late tonight will be needed...will probably issue with 
afternoon package. 


Doppler radar continues to show showers spreading over most of the 
forecast area this morning as westerly flow aloft follows the upper 
trough. With upper ridging later today...and low level flow backing 
more to the S...should see showers drop off today. Overnight snowfall 
amounts in the Cascades since last evening have ranged from roughly 4 
to 9 inches. For now will hang onto the Snow Advisory for the north 
Oregon and S Washington Cascades today...although will likely see a 
significant dropoff in snowfall later this morning. Will continue 
the Winter Storm Warning for the Cascades as moisture...strength of 
the front and the Post frontal flow all favor significant snowfall 
later tonight and sun. 


Current model runs indicate best chances for precipitation Monday 
will shift north into Washington with warm front overrunning. The upper ridge 
holds cold front at Bay until Monday night. Likens 


Long term...no changes. Previous discussion follows...weak high pressure 
builds over the forecast area Monday night. Models show a weak system 
approach the coast early Tuesday...but it tracks further north and 
lightly brushes the far northwest corner of the County Warning Area. This ridge is 
more persistent than previous model runs suggested. It manages to 
delay the arrival of a second system through Thursday. By middle next 
week...models disagree on the solution for the weekend. GFS model 
runs are inconsistent...the 06z run held back on precipitation through Thursday 
night but the current run shows rain extending to the Cascades Thursday 
night. So far...models suggest a fairly nice weekend as another upper 
high pressure temporarily sets up over the Pacific northwest. Lramirez 


&& 


Aviation...little change in overall conditions this morning. 
Predominately VFR with scattered showers...which may bring very 
brief MVFR ceilings. Conditions remain VFR through today...though will 
see increasing MVFR along the coast after 20z as warm front swings 
across the region. Cold front tonight will bring increasing MVFR 
after 04z...and increasing southerly winds. 


Kpdx and approaches...VFR conditions through 05z...then increasing 
MVFR ceilings as cold front approaches. Watch for potential of low level 
wind shear between 00z and 06z tonight as will see increasing south 
winds aloft...blowing over the low level easterly flow coming from 
gorge.Rockey/Wolfe 
&& 


Marine...not much change. Conditions will deteriorate later today 
as warm front lifts across the region. This will bring gale force 
south winds. However...strong winds arrive tonight as rather strong 
low pressure passes across the north Oregon coastal waters. Storm Warning 
for the coastal waters for tonight. Winds turn more west to northwest on 
Sunday and begin easing. Seas tonight likely to peak in the 20 to 25 
feet range...but will drop quickly on sun. 
&& 


Pqr watches/warnings/advisories... 
WA/OR...Gale Warning for coastal waters from Cape Shoalwater to 
Florence for this afternoon. 
Storm Warning for coastal waters from Cape Shoalwater to 
Florence for tonight. 
Small Craft Advisory for rough Columbia River bar conditions 
through sun. 
Snow Advisory for the South Washington and north Oregon 
Cascades through 4 PM. 
Winter Storm Warning for the S Washington and north and central Oregon 
Cascades tonight and Sunday. 
High wind watch for the southern Washington/northern and 
central Oregon coast tonight. 
&& 


$$ 


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.