(originally published to Helium writing site, now gone)
The city of Portland, Oregon has a marine west coast climate with a relatively narrow annual temperature range and rain through the year. Similar climates are found in the United Kingdom, much of Europe, along much of the western coast and some of the eastern coast of South America, southeast Australia, and New Zealand, as well as the northwest US coast.
Portland is 20-50 feet above sea level in the Willamette River valley, 65 miles from the Pacific Ocean, between the Coastal Range to the city’s east and the higher Cascades Range to its west. Rainfall averages about 40 inches a year, similar to most US east coast cities, and less than the 60-80 inch average for west coast cities and the 180-200 inches for the Coastal Range. Rain is usually light or moderate rather than heavy and residents often call it Portland mist. The rain pattern keeps things green all year and is ideal for gardeners and vegetable and fruit growing.
The Cascades push moisture-laden westerlies upwards resulting in much of the area’s rain. Most of the rain falls in winter, with close to 90 per cent of it falling in the seven months from mid October to mid May. The months of July and August are the driest, each averaging less than an inch. Nevertheless, heavy rain can occur in summer. In 1948, Vanport just north of Portland was flooded and the Columbia River swollen for 40 days in the worst floods since 1894. Rainfall varies across the metropolitan area. The elevated West Hills region averages 60 inches, whereas 10 miles away, the airport to the north-east of the city next to the Columbia River gets 36 inches.
Most of the precipitation is rain. Measurable snow falls on about four days a year. Snow accumulation of over two inches is rare and usually melts in less than a day. Snow is more likely in the West Hills area and also at Troutdale east of the airport on the Columbia River. Record snowfall for Portland was 61 inches in the winter of 1892-93. The 2008-09 winter had the third highest snow reading on record with 24 inches.
Winters are cool to cold with cloudy skies. Winds are usually southerly during milder, rainy periods and easterly in colder dry times. Extreme cold is rare. The Cascade Ranges usually prevent the cold inland air masses originating in the Arctic from reaching Portland. However, occasional Arctic winds find their way over the Cascades resulting in cold blustery conditions. Slightly warmer winds find their way to Portland via the Columbia River Gorge canyon east of the city. At other times, cold air flows out along the canyon.
When easterly winds coincide with rain, a layer of below freezing air forms along the Columbia River, resulting in snow in the eastern and northern parts of the city. Easterlies often bring subfreezing temperatures to Portland in winter. On average, daytime temperatures reach about mid 40s to low 50s Fahrenheit in the winter, falling to the 30s overnight. Temperatures can fall below freezing between early November and late March. Frosts can occur from late October to late April. The record low temperature was minus 3 degrees Fahrenheit (or minus 19 degrees Celsius) on 2 February 1950.
Spring is cool and can be fairly wet, especially early spring. Temperatures in March average about 40 degrees Fahrenheit overnight to mid 50s by day, rising to a range of about 50 to 70 in May. By June, daytime temperatures can sometimes be in the 80s and 90s, although the average is mid 70s. May and June have less rain but cloudy days are common.
The height of summer in Portland arrives in July. High pressure systems in the Pacific bring north-westerly winds during afternoons and evenings. These highs prevent much moisture from forming, resulting in warm dry summers. Temperatures usually range from the 50s overnight to high 70s in the daytime. It can be hotter though, with temperatures in the 90s reasonably common but infrequently exceeding 100 degrees. Occasionally, temperatures can go over 100 in any of the months from May to September. Memorial Day (late May) 1983 was 100 degrees. Portland’s record high temperature was 42 degrees on 30 July 1965 and on 8 and 10 August 1981. The heat doesn’t last long as cooler ocean air blows in.
Early autumn is warmer than late spring but temperatures soon cool to the point where late autumn is cooler than early spring. September is warm with daytime temperatures in the mid 70s, falling to mid 60s in October. Cooler clear nights result in fog across the valley which can be thick at night and in the early morning. Fog can last several days.
The city gets few destructive storms. The Coastal Range partly shelters it from ocean storms. Surface winds occasionally exceed 50 mph (gale force) but rarely above 75 mph. Thunderstorms are not common. Winter and spring storms are weak and might produce gusty winds and small hail. Summer storms can be fiercer with frequent lightning, stronger winds, and larger hail. Funnel clouds occasionally form but the city has few tornadoes. Its worst tornado was on 5 April 1972, killing six people, injuring over 300, and causing $4 million in damage.
In summary, Portland has a mild climate with a relatively small temperature range. Most of its rain is in winter. Its topography usually prevents it from suffering weather extremes.