Portland Weather

Portland Rain

Portlanders call the rains Portland mist. The mist brings us a lush green year around and an ideal climate for gardeners.

The City of Portland is not the rainiest place in the metro area. Parts of the metro area receive 64 plus inches per year — fifteen inches more than Portland’s official rainfall measurements, which are taken at the airport, one of the driest spots in town.  Downtown Portland receives just over 44 inches of precipitation annually. Note the image on the left and the dark green shows where the rainfall is the highest — Damascus and Happy Valley.

Despite its reputation for rain, Portland doesn’t even make the top ten U.S. cities with the highest annual rainfall. However, Portland ranks third on the list of cities with the rainiest days in the country with 164 rainy days a year. So it’s not that Portland gets a lot of rain, it just rains often. Whereas it can rain 2-3 inches in an hour or two on the east coast or Midwest, it will take days and often weeks to accumulate 2-3 inches in Portland. It will be raining for hours, and suddenly the sun will come out for a short time and then it’s back to rain. Then the rain stops for either a few minutes or hours as the rain clouds move on — view the radar map as it’s a good way to understand the weather pattern in the metro area.

A water year is defined as the 12-month period beginning October 1 of any year and continuing through September 30 of the following year. The water year is designated by the calendar year in which it ends and which includes 9 of the 12 months. Thus, the water year ending September 30, 2016, is called the 2016 water year.

The HYDRA rainfall network is operated and maintained by the City of Portland Bureau of Environmental Services, and there are 38 gauges throughout Portland where rainfall is measured — it measures rainfall for 1-day, 3-days, 5-days, month and water year.  The water year average for these 38 gauges is 42.77 inches.

We have four months of very rainy weather, four months of 50-50 rainy days, and four months of very dry weather.  Here are the rain numbers:

• Fifty-five percent of the rain comes in four months:  November, December, January, and February.
• Another 32% comes in March, April, May, and October.
• In the other four months (June, July, August, and September), Portland receives about five inches of rainfall.
• The end of May marks the end of Portland and Oregon’s rainy season. On average, Portland receives 88 percent of its precipitation from October 1 through May 31. That means that just 12 percent of the city’s precipitation falls during the summer months.

Here are some links for further information about Portland rainfall:

• Historical average temperature and precipitation by month and day.
• Ten-day weather forecast.
• Interactive radar map shows rain, snow, and mix.

Coping with the Rain

Don’t let anyone kid you.  It can get disheartening.  Coping with the rain during the winter months is the challenge.  Portlanders read, go to the movies, and find dry places like Eastern Oregon.  It’s a good time to travel.  The good news is that it is never really cold as evidenced by the Average Temperatures chart below. Think positive.  Rain is good for the complexion as your mother may have told you as a kid when you couldn’t go outside to play because of the rain.  Dolly Parton put a positive twist on it, “The way I see it, if you want a rainbow, you gotta put up with the rain.” The comic strip, BC, added their dry sense of humor with the strip above. Carl Abbott, professor of Urban Studies at Portland State University has this to say about Portland weather in his book called Greater Portland: Urban Life and Landscape in the Pacific Northwest:

The most common understanding of Portland’s climate is grayness.  Because the sunny weeks of August lack even the drama of lightning storms, it is the low winter sky that attracts notice – the gray blanket of drizzle, the short winter light. Clouds are great gray sponges wrung out against the wet slope of the Cascades.  The winter weather can nourish deep depression.  But the gray months can also be soothing, muffling, twilight weather, thinking weather.  Portlanders outpace most of the nation in magazine subscriptions.  They are avid bookworms and science fiction fans who spend 37 percent more than average Americans on reading matter.

Lauren Kessler, writer, and director of the Literary Nonfiction Program at the University of Oregon puts a romantic twist on the rain.  Lauren admits it took her a few years to appreciate a Western Oregon winter. She moved to Oregon in the late 70s.  Here are Laurel’s words:

I love the rain. I don’t mean I grudgingly appreciate its ecological necessity. I don’t mean I’ve learned to tolerate it. I don’t mean I wait it out, flipping through the calendar to see how many more pages until the sun might break through. I mean I love it.  I love everything about it. I love falling asleep under a down comforter in the dead of winter with the windows thrown open to the hiss of rain. I love waking up to the soft aqueous light that is a painter’s dream and listening to the rush of water in the culvert. I love the thrum of rain against the house on a dark afternoon with potato leek soup simmering on the stove.

Sallie Tisdale’s article in The Oregonian on December 16, 2007, is entitled, “Our Blessed, Bountiful, Horrible Rains.”  Below is a paragraph from her story:

I spend time regularly in a cabin near Mount Hood, and more than anything this has changed the way I see the rains. Winter and rain are best shared with trees and stones. I listen to the peculiarly comforting sound of rain falling on the cones and needles and brushy bark, running in rivulets down to the river below. Thomas Merton also spent rainy winter nights alone in a cabin. There he wrote of the “enormous virginal myth” that is rain — “a whole world of meaning, of secrecy, of silence, of rumor.”

We’ll move on with after this quote by Langston Hughes, “Let the rain kiss you. Let the rain beat upon your head with silver liquid drops. Let the rain sing you a lullaby.”

Snow in Portland

Yes, we get snow in Portland.  If you read the official record from the National Weather Service, you will not get an accurate picture of what happens about snow in Portland.  The official record doesn’t tell the entire story because the National Weather Service office is in Northeast Portland and at a low elevation. It is an entirely different story at higher elevations.  Snow forecasts always bring with them an “elevation” factor such as, “1-3 inches of snow is forecast for late afternoon at elevations of 500 feet and higher.” 

Here is a good example that occurred in late December 2003: On Monday, some parts of the West Hills received seven inches, Salem got six inches and Oregon City five and a half inches. Moving toward the Coast Range, Buxton recorded 13″, while in the shadows of Mount Hood, Government Camp got 28″ and Timberline 42″.  By late afternoon Wednesday, 3.7 inches of snow had fallen at the National Weather Service office in Northeast Portland. With the half-inch that fell Monday (December 29), 4.2 inches was the most snow recorded for a single month in Portland since February 1993, when 6.6 inches fell. By late Wednesday morning, the snow had turned to rain in downtown Portland but continued to pile up at higher elevations and closer to the gorge. So the official record will show 4.2 inches. 

But if you live in the West Hills (elevations above 500 feet), you were shoveling seven inches or more. January 1950 was a very cold month statewide, with frequent snowstorms. For the state as a whole, snow was the heaviest during this January than ever before since the beginning of weather record keeping, which began in 1890.  Portland received close to two feet of snow in ten days. 

Portland’s record low was also set in 1950 when it reached three below zero on February 2. Portland’s snowfall for December 2008 totaled 18.9 inches and December 2008 also marked the second-snowiest month in Portland’s recorded history, bested only by the record set in the Rose City in January 1950 with 41.4 inches. During 1871 to the present, Portland’s all-time records for snowfall include, at number one, the winter of 1892-93, with 60.9 inches of snow. The winter of 1949-50 came in third with a total of 44.5 inches, while the winter of 2008-09 totaled 23.6 inches.

Climate scientists predict significantly less snow for us in coming years as the planet’s warming climate tilts the Northwest’s precipitation more toward rain. Snow Christmas Day  The chances of snow on Christmas Day in the Portland area is about 1 in 100, according to an Associated Press report based on National Climatic Data Center records from 1988-2005.

Visit the National Weather Bureau’s Web page entitled Some of the Area’s Snowstorms.  It provides an overview of major storms in Oregon during the 1990s.  The record:  224 inches at Timberline Lodge on Mt. Hood during January 1950.

Winter’s Pineapple Express and Then Summer

Mosaic of Microclimates  Usually the weather stats are reported from the airport (northeast part of the city) and rainfall and temperature can vary from the airport to other parts of the city.   On the same winter day, you can get rain, hail, and sun.  Even within the same hour. The West Hills may experience a significant snowfall that brings trace amounts of snow to other areas.  In the summer, traveling from the West Hills (lots of trees and wind currents in the hills) heading into Beaverton (West suburb) or across the river into East Portland, you will notice an increase in the temperature.

Winter  Portlanders watch the northern Pacific for their weather.  The winter rain happens when low-pressure builds in the Gulf of Alaska and the jet stream drops southward to sweep across the northern states.  The counterclockwise swirl around the deep atmospheric low pumps moist Pacific air across Oregon from the west and southwest, driving ashore band after band of clouds. Pineapple Express is the shorthand for especially juicy storms of warmer air that pick up moisture from as far to the southwest of Hawaii and drench the valleys and mountains.  When a high-pressure area builds off the coast, and the jet stream moves north, we are the guarantee dry weather.

Summers are Grand  Summers are dry and weeks go by without rain.  Mild temperatures and low humidity.  It is one of the best places in the USA during much of June, all of July and August, and a good share of September. June average seven days of temperatures to reach or exceed 80 degrees, July usually sees 15 days, and August averages 15. Portland also sees an average of 12 days of 90 degrees or above. The first frost is in early November, and the last frost in early April.

280 Growing Days  Portland has close to 280 growing days according to Western Gardens by Sunset Publications.  Visit Timber Press gardening links to learn more about Pacific Northwest gardening.

Average Days per Month: Clear, Cloudy, and Rainy Skies

Type of Day	Jan.	Feb.	Mar.	Apr.	May	June	July	Aug.	Sept.	Oct.	Nov.	Dec.
Clear Days	2.9	2.9	3.3	3.5	5.0	6.2	12.6	11.4	10.3	5.4	2.8	2.1
Partly Cloudy	3.6	3.8	4.9	5.8	7.2	7.7	8.5	9.6	8.1	7.6	4.3	3.3
Cloudy Days	24.4	21.5	22.8	20.7	18.9	16.1	9.9	10.1	11.6	18.0	22.9	25.5
Rainy Days*	18	15.6	16.9	14.4	11.8	9.2	3.9	4.9	7.5	12.3	18.0	18.7

*.01 inches or more of precipitation.
Source:  Western Regional Climate Center in Reno, Nevada.  The data is based on daylight hours only. A clear day denotes zero to 3/10 average sky cover. Partly cloudy is 4/10 to 7/10 tenths. Cloudy is 8/10 to 10/10 tenths.

Portland Weather Historical Data

NOAA (National Oceanic and Atmospheric Administration) keeps weather data for over 50 years in Portland.  Here are some of the numbers:

• Annual Sunshine – The suns shines about 48% of the time.  Out of a possible 267,955 minutes, Portland averaged 128,618 minutes of the sunshine between 1951 and 1995.
• Winds – The prevailing winds in the summer (April – September) is NNW at 6.5 -7.6 mph.  The winter months bring winds from the ESE.
• Relative Humidity – Average is 73% at 10 AM and 59% at 4 PM.  This data from 1961-1990.
• Rain – Nearly 90 percent of the annual rainfall occurs from October through May.
• Hottest Day of the Year – The average high on August 9 is 84 degrees, from 60 years of data (1949-2009) recorded at the Portland Airport Weather Station. That means there’s roughly a 50-50 chance it will get hotter than 84 on August 9.

Record Rainfall  On December 12-13, 1882, Portland received 7.66 inches of rain in one 24-hour period and saw 10.75 inches in two days. Portland’s single-month rainfall record occurred in November 2006 when 11.92 inches fell.  These facts were obtained from The Oregon Weather Book by George Taylor and Raymond R. Hatton, Oregon State University Press, ISBN 0-87071-467-8 except for the single-month record which happened after the book was published.

The National Weather Bureau’s Web site has a section entitled Oregon’s Top 10 Weather Events of the 1900s which track floods, snowstorms, tornadoes, and wind storms.

In 1998, the Department of Geology at Portland State University published a report documenting the extent of landslides following the 1996 flood and evaluating the causes.  The greatest concentration of landslides in Portland was in the West Hills in the wind-blown loess of the Portland Hills Silt Formation. A total of 705 slides were studied in the project.

The Weather Café™ by Rufus provides uniquely informative long-range forecasts for specific patrons in the Pacific Northwest.  Rufus’ forecasts have information of value, but even more, they show a sense of humor and are fun to read.  It is a free service for patrons from British Columbia to northern California.

Record Heat  The summer of 2015 will be remembered not for the occasionally hot day, but for the sheer number of hot days and overnight lows that sometimes never dipped below 70 degrees. Perhaps the biggest record to fall was the number of 90 degree or greater days, a whopping 29 days, 17 more than average. Those prolonged hot spells helped set Oregon’s forests ablaze, and when the wind was right, brought smoke from those fires right to our doorstep. The lure of rivers, lakes and swimming holes sent drowning deaths to 36 in the region between Memorial Day and Labor Day, the highest number in nearly a decade.

Portland Average Rainfall and Temperatures

Below you will find information about Portland rainfall and temperatures from World Climate.

• Average Rainfall –  Located at about 45.51°N 122.68°W.  Height about 48m/157 feet above sea level.
• Average Temperature – Located at about 45.60°N 122.60°W. Height about 12m/39 feet above sea level.
• Average Maximum Temperature – Portland International Airport is located at about 45.60°N 122.60°W. Height about 8m/26 feet above sea level.
• Average Minimum Temperature – Portland International Airport is located at about 45.60°N 122.60°W. Height about 8m/26 feet above sea level.
• Heating Degree Days – The cumulative number of degrees in a month or year by which the mean temperature falls below 18.3°C/65°F.  Data from Portland International Airport.
• Cooling Degree Days – The cumulative number of degrees in a month or year by which the mean temperature is above 18.3°C/65°F.  Data from Portland International Airport.

According to the National Weather Service, winter precipitation in Portland falls mostly as rain, with an average of four days with measurable snow. Accumulations rarely exceed two inches and melts rapidly especially if it rains after the snow shower. High temperatures for December through February average in the upper 40s, with lows in mid-30s. The 3-month period averages 14.03 inches of precipitation.

Factors Responsible for Portland’s Climate

The mountains (Coastal to the west and Cascades to the east)  along with Portland’s latitude and proximity to the Pacific Ocean, determine the climate.  Here are some of the factors mixed with general information that create the weather in Portland.

Air Flow

• Air that crosses the Coast Range cools as it moves east, dropping a large amount of precipitation on the coastal mountains.
• The air that flows over Portland is drier than the air that originally moved in from the ocean.
• Winds are predominately southerly during with mild, rainy spells in the winter.
• In the winter, the cold easterly winds also bring the coldest air to the Portland area.
• During the summers, northwesterly winds bring cool air from the Pacific Ocean down along the Columbia River.

Rain

• The lush conifer forests that flourish in Western Oregon receive most of the moisture from the storms that roll in off the Pacific Ocean.  Portland receives the remaining moisture and Eastern Oregon very little.
• Many areas in the Coast Range receive between 180-200 inches of rain per year.  Average rainfalls for coastal cities is 60-80 inches per year.
• In parts of southeast Oregon (high desert), average rainfall can be as low as 6-8 inches per year.

Temperatures

• Temperatures below zero degrees are rare, occurring only six times over the last 125 years!
• Most temperatures during the winter reach the 40s during the day and fall back into the low to middle 30s at night.
• Summer can be quite warm, with the temperatures reaching the middle 90s, although these warm days do not last long before the cool marine air arrives with temperatures in the 70s. Portland averages 12 days a year when temperatures reach 90 degrees or higher.
• Temperatures above 100 degrees are rare.

Mountains

• The Coast Range provides the Portland area limited shielding from the Pacific Ocean storms.
• The Cascades offer a steep slope for an orographic lift of moisture-laden westerly winds, resulting in moderate rainfall for the region.
• The Cascades also act as a barrier, preventing the colder continental air masses originating in the arctic areas of Canada from invading western Oregon. Occasionally, however, cold air does work its way into western Oregon through the Columbia River Gorge.

 Storms

• Destructive storms are infrequent in the Portland area. Surface winds seldom exceed gale force (wind speeds sustained at 50 mph or greater) and have rarely exceeded 75 mph. Thunderstorms can occur during any month but are not common.
• Occasionally, thunderstorms produce funnel clouds, but tornadoes are exceedingly rare.

El Nino, La Nina, and La Nada

Sea surface temperatures in the equatorial Pacific have a huge impact on not only weather in the Pacific Northwest, but across the U.S. and even globally. Above-average sea temperatures usually lead to El Nino, which often translates into a milder and drier winter for the Pacific Northwest. Below-average temperatures, a La Nina, mean our winters will be colder and wetter than average. When neither condition predominates, meteorologists sometimes call it La Nada. Whether El Nino, La Nina or La Nada, Pacific Northwest winters usually can be counted on for periods of heavy rain, high winds, and heavy mountain snows.

Portland Seasons

The winter season is characterized by mild temperatures, cloudy skies, and rain.  Winds are predominately either southerly with mild, rainy spells, or easterly during colder dry spells. Outbreaks of cold arctic air from east of the Cascades will occasionally spill into the Portland area, bringing cold, blustery east winds.  If the east winds occur when the rain is falling over the metropolitan area, a shallow layer of cold air forms along the Columbia River.  In and near this cold sub-freezing air, freezing rain and even snow will occur over eastern and northern Portland.

Spring is a transitional time as the weather patterns shift from winter to summer. As a result, March and April are wet and cool, while May and June turn drier.  Temperatures during May and June often take a roller coaster ride, ranging predominantly in the 60s and 70s, occasionally reaching the 90s for a day or two.  Even though the number of rain days decreases in May and June, there are still many cloudy days.

Summer finally arrives in the middle to late June, when the temperature is finally able the reach the 80s on a daily basis. Northwesterly winds bring cool air from the Pacific Ocean down along the Columbia River.  Summer can be quite warm, with the temperatures frequently reaching the middle 90s, although these warm days do not last long before the cool marine air arrives with temperatures in the 70s.  Temperatures above 100 degrees are rare.

Autumn is the reverse of spring, with many warm days in September. By the middle of October, the rains are beginning to arrive. Also, cooler temperatures arrive, with afternoon highs in the 50s and 60s.  Fog begins to occur on a nightly basis during late October and November, with visibilities often under one mile.  However, fog varies by location, with the difference frequently depending on the altitude.