Over the small area covered by the city of Portland and its surrounding areas, there are perhaps 8 different climate areas, and elevations that range from sea level to slightly over 1,000 feet (about 300 meters). Some areas can be encrusted in ice while other areas are reasonably warm and possibly have sun.
Weather reports seem to usually use the Portland airport as the monitoring station, but the Portland airport sits in the wind blast zone from the Columbia River Gorge and is also located almost at sea level. Thus, frequently the weather reported for "Portland" doesn't match what is actually happening in the majority of the city. Most of the time there isn't that much difference. Other times, there is a huge difference.
Photo 1 shows a recent photo of the West Hills. In the level section of northwest Portland, the nice green grass shows no sign of accumulated snow. 400 feet above, in the hills, there is significant snow accumulation and signs have been folded down on the roads leading there saying that tire chains are being required. This is one of the best illustrations of how suddenly the weather can change in a short distance in Portland. Photo 2 shows what this looks like after walking up into the hills themselves.
Now, here is the painful reality: when the temperature drops just slightly in the evening, all this mess that is here just wet streets will turn to ice, and make a huge mess for those trying to drive anywhere. When both of these photos were taken in early 2008, the temperature was just slightly above freezing. Walking was easy during the day, as the water was liquid. Getting down these hills after it all freezes, either on foot or by driving, will not be an easy undertaking.
Here are some basic climate zones in the Portland area you may hear on weather reports:
"East County": this is a highly subjective term, and can be used to describe anything east of Interstate 205 to anything east of Gresham. Areas along the Columbia River get wind and cold winter weather the wind brings. Further south into Gresham and further east areas are in a cold weather zone from the Cascade mountains, and a wind from the east can drop the temperature a lot.
"West Hills": the ridge west of downtown Portland, and runs from West Linn north all the way to Longview, but generally the term only referrs to that area immediately west of downtown Portland.
"Downtown": generally west of the Willamette River and east of I-405, but the weather zone extends east all the way to Mt Tabor (SE 60th Avenue or so). This hill protects areas west from direct hits from weather from the east.
"Tualatin Valley": this area is west of the West Hills, and includes Beaverton and Hillsboro. It is protected from cold weather from the east wind, but it is higher in elevation than downtown Portland. One can be freezing while the other well above freezing.
The following areas are generally never referred to in local weather reports, but have separate weather systems:
Milwaukie to Oregon City and West Linn: generally these areas are somewhat further protected from cold weather by a ridge of hills on the east side of the metro area.
Clackamas: generally falls into the same category as the above, but if the weather is coming off the mountains it will come down a series of valleys and make Clackamas much different. Around April of 1993, there was a freak snow storm that happened in Clackamas, while the rest of the Portland area was enjoying a nice, warm spring day.
Troutdale (includes Wood Village and Fairview): this area receives the strongest winds from the Columbia River, and may be significantly colder (or at least feel that way!) than the general "East County" term.
Upper level of Oregon City and West Linn: these areas will get snow and ice when the freezing level is extremely low, but they are also be subject to fairly severe fog during the winter due to humidity from Willamette Falls.
The link below is to another tip I wrote about Portland weather, which features some links to weather information.
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