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Next 12 Hours

Forecast data from the National Digital Forecast Database.

5 Day Forecast

• Wednesday
• 
• High: 59 °
• Low: 52 °
• Rain Showers

• Thursday
• 
• High: 59 °
• Low: 50 °
• Chance of T-Storms

• Friday
• 
• High: 73 °
• Low: 52 °
• Partly Cloudy

• Saturday
• 
• High: 77 °
• Low: 50 °
• Mostly Cloudy

• Sunday
• 
• High: 63 °
• Low: 48 °
• Partly Cloudy

Area forecast discussion 
National Weather Service Portland or 
908 am PDT Wednesday may 23 2012 


Synopsis...an upper low will remain anchored in the southeast Gulf 
of Alaska today and early tonight...directing moist onshore flow and 
showers into southwest Washington and northwest Oregon. The upper low 
will drop south into the Pacific northwest and across western Oregon 
Thursday for more showers...continued cool weather...and possibly a 
few thunderstorms. The upper low will drop south on Friday with a few 
lingering showers especially near the mountains...but with temperatures 
rebounding closer to normal with more sunshine in the afternoon. A 
weak disturbance will brush the area Sunday...followed by a couple of 
more systems early next week. 
&& 


Short term...an upper low remains anchored in the southeast Gulf of 
Alaska in the wake of the rather strong surface low that moved 
through the Pacific northwest on Tuesday. This upper low is directing 
a west to northwest oriented onshore flow into southwest Washington 
and northwest Oregon. One short wave was moving through this 
morning...with perhaps a weaker disturbance later today and early 
tonight. There is some rain shadowing in the Lee of the Coast Range. 
The best quantitative precipitation forecast today and early tonight will be in the mountains...where 
snow levels in the Cascades will hover around 4500 feet or so. 


Another stronger disturbance out near 53n/145w will swing below the 
Gulf of Alaska upper low and drag the whole upper south into 
southwest Washington and western Oregon Thursday. This will increase 
showers and keep temperatures nearly 10 degrees below normal. Snow 
levels will remain close to pass elevations. The upper cold pool of 
minus 26 degrees c will help increase the instability over the area 
Thursday. Some of the showers could have heavy rain and small hail. 
There is a slight chance of thunder...but this may be dependent on 
how solid the cloud cover is. The main jet sags south Thursday... but 
the models indicate enough quantitative precipitation forecast to support the categorical MOS probability of precipitation 
versus the likely probability of precipitation of our current forecast. 


The upper low sags south of our area by Friday with decreasing 
showers. Will likely still see a diurnal increase in the showers 
especially near higher terrain Friday afternoon. With a slightly 
warmer air mass and more sun breaks on Friday...afternoon temperatures will 
have a chance to get close to 70 over the inland areas Friday. 
Tolleson 


Long term...no changes. Previous discussion follows... 
the next strong shortwave will continue to push south into California 
late Friday. Wrap around moisture is possible through the Holiday 
weekend but looks limited to the higher terrain. Models are in good 
agreement building a weak ridge over the area Saturday night into 
Sunday...allowing showers to diminish through Sunday evening. Have 
decreased pop amounts even more due to increasing confidence in model 
solutions. An upper level trough moves to the north of the region by 
Monday...bringing yet another chance for measurable precipitation over the 
area. This is short lived as a stronger ridge of high pressure 
amplifies over the Pacific northwest by middle week. 27 
&& 


Aviation...seeing a mix of MVFR and VFR conditions across the 
region. Around konp...do expect prevailing IFR ceilings to continue but 
will see occasional breaks in the low stratocumulus. MVFR should 
become more widespread again around late morning as a weak trough 
enhances showers across the district. Better mixing under some sun 
breaks during the late afternoon should allow ceilings to lift into the 
low VFR range and most likely for inland terminals. Do expect periods 
of MVFR ceilings under showers through this evening. An upper low 
descending along the coast brings additional onshore moist flow 
overnight and likely bring a return to coastal IFR and inland MVFR 
ceilings. 


Kpdx and approaches...feel MVFR ceilings around 1800 to 2000 feet will 
continue through early afternoon as moisture packs up along the 
Cascades. Ceilings should lift and break into a scattered/broken deck with MVFR 
ceilings possible under showers for the rest of the afternoon. Will see 
ceilings around 2000 feet return overnight as another onshore surge of 
moisture returns. /Jbonk 
&& 


Marine...westerly winds will continue for the next 12 to 18 hours 
ahead of an upper low descending down the West Coast. Speeds will 
generally range 10 to 15 knots with periods getting a bit higher but 
they don't appear particularly gusty at this time. Combined seas will 
hover around 8 feet with swell periods of 10 to 11 seconds. Seas will 
gradually diminish becoming more wind wave driven over the next day 
or so. Periods will also diminish but appear to retain average 
steepness. Westerly flow will shift more northerly Thursday as the 
upper low dives south across the Pacific northwest. Combined seas 
under a northwest swell may reach 10 feet in the outer waters later 
Thursday/Friday behind this system. /Jbonk 
&& 


Pqr watches/warnings/advisories... 
or...none. 
Washington...none. 
Pz...none. 


&& 


$$ 




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.