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November-December 2011

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Oregon's Weather and Climate: Wet, Dry, Hot, and Cold

Caption: Portland, Oregon, and Mount Hood on an October afternoon.

Caption: Portland, Oregon, and Mount Hood on an October afternoon.

… even the Easterly winds which have heretofore given us the only fair weather which we have enjoyed seem now to have lost their influence in this respect.

Meriwether Lewis

(part of the Corps of Discovery Expedition*, March 6, 1806, at Fort Clatsop

Meriwether Lewis's comment above, written during his expedition to find a “Northwest Passage,” showed an astute awareness for someone who had only been on the Oregon coast for one winter. What Lewis observed was a typical annual change in the spring months of the year along the Oregon coast. It offered testimony to the weakening of the predominant wintertime low pressure in the Gulf of Alaska, called the Aleutian Low, and marked its replacement by a Pacific area of high-pressure center moving north. This gradual reversal of pressure gradients is evident in all of Oregon's local wind charts. Simply stated, the Pacific Ocean exerts an incredibly strong influence on Oregon's weather.

The Coastal Strip

The winds from the Land brings us could [sic] and clear weather while those obliquely along either coast or off the Oceans bring us warm damp cloudy and rainy weather. The hardest winds are always from the S.W.

Meriwether Lewis, January 31, 1806, at Fort Clatsop

Such an observation is completely true in the winter. However, as the aforementioned high-pressure area moves north and summer sets in, the winds along the coast shift from somewhat offshore (southeasterly) to onshore (northwesterly). The northwest winds are often referred to as “nature's air conditioning.” Had the Corps of Discovery (the name attributed to the Lewis and Clark Expedition to the Pacific Northwest from 1804–1806) experienced a summer along the Oregon coast, they would certainly have noted the changes. The prevailing winds shift to northwesterly along the coast throughout the spring, as Lewis noted in his diary. Even the copious precipitation that plagued the party during the winter of 1805–1806 falls off remarkably along the entire coast during the summer months. Total yearly amounts that approach 150 inches in the Coast Range Mountains fall to a total of fewer than 10 inches during the three meteorological summer months (June-August). Temperatures along the coastal strip do not show much variation from winter to summer, due mainly to the moderating effect of ocean waters and upwelling along the coast.

But there are exceptions! When Lewis's winds shift to offshore in the summer months, hot air, which comes from interior sections of Oregon, floods the coastal strip, pushing the cool marine layer of air a short way offshore. This results in temperatures climbing into the 90s along the coast, as the thermal trough from California noses north along the coast of Oregon. Such conditions last for one, maybe two, days, and the onshore winds return, often abruptly, as the thermal trough shifts inland. This warming is greatly aided by downslope conditions, especially along the extreme southern Oregon coast. Here the warming is called the “Brookings effect,” as air from near 4,000 feet is brought down to sea level by easterly winds, warming at the adiabatic rate of 5.4 degrees for every 1,000 feet it descends. Brookings, within Oregon's coastal “banana belt,” has recorded temperatures of 80°F, even in January.

As Meriwether Lewis observed during his encampment along the coast, it can get windy. Storms approach the coast from primarily a westerly direction. In this case, strong southerly winds occur ahead of the storm's advancing frontal zone, as higher pressure to the south tries to compensate for falling pressures to the north. Once the front passes inland, winds shift to westerly. Occasionally, however, storms approaching the coast from the west at around 40° N latitude, will curve north just off the coast. Such was the case on October 12, 1962, when winds of over 140 mph were recorded along the coast and over 100 mph inland.

Caption: Wild and scenic Rogue River in southern Oregon in fall.

Caption: Wild and scenic Rogue River in southern Oregon in fall.

Caption: Figure 1: An offshore component to the wind in extreme southwest Oregon warms air adiabatically as it descends from 4,000 feet over the interior to sea level.

Caption: Figure 1: An offshore component to the wind in extreme southwest Oregon warms air adiabatically as it descends from 4,000 feet over the interior to sea level.

Inland Valleys

Oregon is divided into three distinct valley segments west of the Cascade Mountains. The Willamette River drains the Willamette Valley and flows north, meeting the Columbia River in the Portland area. Farther south, the Umpqua River drains the valleys around Roseburg and flows west into the Pacific Ocean. Still farther south, near the California/Oregon border, the Rogue River drains the valley around the cities of Medford and Grants Pass. It also reaches the Pacific Ocean along the southern Oregon coast. The same pressure pattern that affects the coastal strip exerts its influences on the inland valleys. Precipitation reaches a maximum in the winter and falls to less than five inches during the summer months. Annual totals range from around 20 inches in the valley around Medford to near 50 inches in places around the Portland area. Elevation can also greatly affect precipitation values.

Oregon is somewhat protected from cold air from northern latitudes on the North American continent by the Rocky Mountains. Occasionally, when the upper level flow is out of the north, these Arctic air masses move down west of the Rocky Mountains and spill into Oregon. Arctic air will flow down from Canada into eastern Oregon and into western Washington from the Frazer River Valley. As a result, it can work its way into western Oregon from the north or down through the Columbia River Gorge. Snowfall will often accompany these cold, Arctic air masses, especially as westerly winds begin to affect the area. Generally lasting less than one week, the influence of Arctic air can be prevalent off and on through the entire winter, as the following quote from The Statesman (Salem, Oregon) on February 27, 1862, demonstrates: “The past week closed with one foot of snow generally on the level country and from one and one-half to two feet, on the higher grounds, with an ‘upward tendency.’”

As cold air slowly relinquishes its hold on Oregon's weather, the ensuing condition usually results in freezing rain. Often called a “silver thaw” in western Oregon, rain falling through a warm layer and striking a surface that is still below freezing causes the rain to freeze on contact. Severe ice storms can result, and they usually bring transportation in major cities to a standstill. Some localities near the mouth of the Columbia River Gorge, such as the Portland metropolitan area, are more susceptible to these storms, as the cold air is replenished throughout an otherwise warming event from its source east of the Cascade Mountains.

This same coastal thermal trough brings hot temperatures to the western valleys in the summer. Aided by a very warm ridge of high pressure aloft, this thermal trough may stay for two to three days in western valleys. On rare occasions, it can result in temperatures hovering in the 90s to around the 100-degree mark, and even longer as it did from August 6–11, 1981, and September 6–11, 2011 in the Portland area. It was a thermal trough that was in part the cause of the devastating Tillamook Burn in August 1933, which scorched over 350,000 acres of Oregon forests before marine air dampened the event.

Caption: The Columbia River Gorge, because of the strong prevailing westerly winds from April through October, attracts wind surfers from all over the globe. Waves from 3 to 5 feet are common.

Caption: The Columbia River Gorge, because of the strong prevailing westerly winds from April through October, attracts wind surfers from all over the globe. Waves from 3 to 5 feet are common.

Columbia River Gorge

Oregon is separated into a western and an eastern section by the Cascade Mountains. Flowing through this range of mountains, as it has done for millions of years, is the Columbia River. Along its banks is what is called the Columbia River Gorge. It is a lesson in climatology. At the western end, beginning near Portland, annual precipitation is near 40 inches. In less than 25 miles, this figure doubles to around 80 inches, due to the mountain effect. From that point eastward, precipitation falls to around 30 inches in less than 25 miles at Hood River and drops off to around 10 inches another 25 miles just beyond the city of The Dalles.

Air flowing through a restriction, like the gorge, has a tendency to accelerate. Such is the case here. When high pressure is prevalent east of the Cascade Mountains, especially in winter, winds can gust to 100 mph at the west end of the gorge near Crown Point. In summer, as the Pacific high pressure becomes settled off the coast and lower pressures prevail east of the mountains, westerly winds replace the easterly winds, creating excellent wind surfing conditions. This change is manifest in the trees. At the west end of the gorge, trees exposed to the wind have few or stunted branches on the east side. At the east end of the gorge, the reverse is true.

The Columbia River Gorge, much like Portland on its western flank, is also susceptible to ice. However, if the cold air is sufficiently deep, heavy snows can occur.

Caption: As a thermal trough moves east from near the coast, marine air spreads to inland western valleys through various gaps in the Coast Range.

Caption: As a thermal trough moves east from near the coast, marine air spreads to inland western valleys through various gaps in the Coast Range.

Caption: A typical wind storm approaching the Oregon Coast. Note closeness of isobars ahead of the front that cause strong southerly wind.

Caption: A typical wind storm approaching the Oregon Coast. Note closeness of isobars ahead of the front that cause strong southerly wind.

Eastern Oregon

East of the Cascade Mountains, Oregon is broken up by a series of ridges and mountains. The Blue Mountains dominate the northeast quarter and rise to elevations of over 10,000 feet. In the southeast portion of the state, the Steens Mountain chain rises from the desert floor of around 4,000 feet to elevations of close to 10,000 feet. In the north-central part of eastern Oregon, the Ochoco Mountains, Aldrich Mountains, and Strawberry Mountains have peaks that range from 6,000 feet to near 9,000 feet. Vegetation varies over this section of the state, as the amount of precipitation and temperature varies with elevation. From a low of below 10 inches in the Alvord Desert just east of the Steens Mountain, some of the higher elevations can experience as much as 60 inches of precipitation annually. This creates a series of climatic regimes. At Steens Mountain alone, vegetation varies from desert at the base to alpine near the summit.

Most of the precipitation east of the Cascade Mountains falls as snow during the winter months, especially at the higher elevations. There is a secondary, but smaller maximum, in the spring months of May and June, when the area can experience thunderstorms. May and June are flash flood months in eastern Oregon. Several disastrous floods have occurred during this period, the most destructive of which almost destroyed the town of Heppner on June 14, 1903, killing around 265 people. Other floods have afflicted the area, as well. Driving through eastern Oregon, one can frequently see debris at the mouth of a canyon area from heavy rain that fell one to two miles distant.

Eastern Oregon does have some quirks as far as weather is concerned. When southerly winds occur over the Blue Mountains in the winter, adiabatic downslope warming can raise temperatures on the north side of this range well into the 60s, while cold air is often trapped in low valleys to its south. When cold Arctic air dominates the weather picture, Oregon's coldest temperatures occur. Minimums of −10°F to −30°F are common with an arctic air mass in place, Ukiah, Bates, and Seneka shivered at −54°F, the state's record low.

But it can get hot in eastern Oregon also. Such was the case from August 7–11, 1898, when the temperature rose to 119°F at both Prineville and Pendleton.

Caption: The area around Long Creek, Oregon is typical of eastern Oregon terrain with high mountain valleys surrounded by forested hills.

Caption: The area around Long Creek, Oregon is typical of eastern Oregon terrain with high mountain valleys surrounded by forested hills.

Caption: This MODIS image of the Oregon coastal region was obtained from NASA's polar orbiting Terra satellite on September 23, 2009 (12:30pm PDT). At the time of this image, coastal weather stations were reporting overcast stratus clouds with temperatures around 60 degrees; just inland, including Portland, along the the Columbia River, temperatures were in the lower to middle 80s. Later in the afternoon, Portland soared to 93°F, just missing the record for the day (94°F set in 1974). At the same time, less than 100 driving miles to the northwest, Astoria, was reporting a temperature of 63°F. Smoke from California wildfires appears at right.

Caption: This MODIS image of the Oregon coastal region was obtained from NASA's polar orbiting Terra satellite on September 23, 2009 (12:30pm PDT). At the time of this image, coastal weather stations were reporting overcast stratus clouds with temperatures around 60 degrees; just inland, including Portland, along the the Columbia River, temperatures were in the lower to middle 80s. Later in the afternoon, Portland soared to 93°F, just missing the record for the day (94°F set in 1974). At the same time, less than 100 driving miles to the northwest, Astoria, was reporting a temperature of 63°F. Smoke from California wildfires appears at right.

A State of Extremes

Given its diverse geography, it is not surprising that Oregon is a state of weather extremes, from intense heat to thunderstorms and floods.

Although severe thunderstorms are not as frequent in Oregon as in many states east of the Rocky Mountains, they do occur. On June 6, 1948, hailstones as large as hen's eggs and some probably as large as baseballs fell near the western Oregon city of Mapleton. Tornadoes also occur and have been observed in every month of the year. A total of 147 have been observed since 1876, including one EF2 tornado on December 14, 2010, near Aumsville. Two of the recorded tornadoes produced fatalities. The first touched down near the small community of Lexington in eastern Oregon on June 14, 1888, and the second occurred just six years later when over half of the small town of Long Creek, also in eastern Oregon, was devastated on June 3, 1894. At least four deaths were attributed to these storms. Most of the tornadoes that occur in Oregon are of the EF0 variety, but occasionally an EF2 is documented. East of the Cascades, the maximum occurs with the thunderstorms in the spring months, while in western Oregon, a fall maximum, associated with an active jet stream and cold upper level troughs, dominates.

Flash floods in eastern Oregon have been mentioned in historical records, but west of the Cascades, severe river flooding occurs due to the combined effects of heavy rain and snowmelt. Heavy fall and winter precipitation that lasts for several days can send coastal and inland valley streams rising to above flood stage. When those warm rains fall on an early snowpack in the mountains, what meteorologists and hydrologists call a “rain on snow” event happens. Such was the case in December 1861 when several towns in the Willamette Valley were submerged. A similar event happened more recently in February 1965. In spring, a rapidly melting mountain snowpack can send the Columbia River rising to above flood stage. This event has happened several times in the past, but June 1894 was perhaps the worst, when the flooding Columbia River caused the Williamette River to rise, flooding a large portion of downtown Portland for as much as two weeks.

Caption: A fog bank along the Oregon coast about to move inland.

Caption: A fog bank along the Oregon coast about to move inland.

Caption: Comparison of temperatures (by months) at a coastal location (Seaside) and an inland location (Burns).

Caption: Comparison of temperatures (by months) at a coastal location (Seaside) and an inland location (Burns).

Caption: Average annual precipitation in Oregon from 1971–2000.

Caption: Average annual precipitation in Oregon from 1971–2000.

Rain Days

Members of the Corps of Discovery team recorded 128 “rain days” in their journals for November 1805 through March 1806 on the northwest Oregon coast. This value compares very well with 127 rain days at Astoria (including measurable and traces) during the winter of 2003–2004 when precipitation was near normal. During the winter of 2005–2006 there were 114 rain days and the total for the same period was some 12 inches above normal. Despite the claim of Lewis and Clark that the winter of 1805–1806 was “very wet” (perhaps by East Coast standards, as that was the only guide they had at that time), the amount of rain days they experienced seems near average, using present day data.

Caption: This picture was taken at near the mid-point of the Columbia river gorge. Note the absence of limbs on the west side (prevailing westerly winds) and the absence of limbs on the east side (easterly winds). The photo is taken looking due north.

Caption: This picture was taken at near the mid-point of the Columbia river gorge. Note the absence of limbs on the west side (prevailing westerly winds) and the absence of limbs on the east side (easterly winds). The photo is taken looking due north.

Oregon's weather and climate can really be summarized as a “state of extremes,” thanks to its topography, northern latitude, and proximity to the Pacific Ocean. This was recognized by the Corps of Discovery more than 200 years ago, and it holds true today. The fact that we have historical data to back up our modern observations about the weather is a testimony to the record-keeping of the early explorers, their willingness to provide such data to another weather observer, Thomas Jefferson, and the dedication of many weather observers in the years since.

GEORGE R. MILLER served as Area Manager/Meteorologist in Charge of the National Weather Service Forecast office in Portland for 12 years. He has authored two books on Pacific Northwest weather.

H. MICHAEL MOGIL is a Certified Consulting Meteorologist and science writer living in Naples, Florida. He has authored several books on extreme weather.

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