The Weather of the Pacific Northwest
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The Weather of the Pacific Northwest

Clifford Mass

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eBook - ePub

The Weather of the Pacific Northwest

Clifford Mass

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About This Book

The Pacific Northwest experiences the most varied and fascinating weather in the United States, including world-record winter snows, the strongest non-tropical storms in the nation, and shifts from desert to rain forest in a matter of miles. Local weather features dominate the meteorological landscape, from the Puget Sound convergence zone and wind surges along the Washington Coast, to gap winds through the Columbia Gorge and the ?Banana Belt? of southern Oregon. This book is the first comprehensive and authoritative guide to Northwest weather that is directed to the general reader; helpful to boaters, hikers, and skiers; and valuable to expert meteorologists.

In The Weather of the Pacific Northwest, University of Washington atmospheric scientist and popular radio commentator Cliff Mass unravels the intricacies of Northwest weather, from the mundane to the mystifying. By examining our legendary floods, snowstorms, and windstorms, and a wide variety of local weather features, Mass answers such interesting questions as:

o Why does the Northwest have localized rain shadows?

o What is the origin of the hurricane force winds that often buffet the region?

o Why does the Northwest have so few thunderstorms?

o What is the origin of the Pineapple Express?

o Why do ferryboats sometimes seem to float above the water's surface?

o Why is it so hard to predict Northwest weather?

Mass brings together eyewitness accounts, historical records, and meteorological science to explain Pacific Northwest weather. He also considers possible local effects of global warming. The final chapters guide readers in interpreting the Northwest sky and in securing weather information on their own.

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1

THE EXTRAORDINARY WEATHER OF THE PACIFIC NORTHWEST

ON COLUMBUS DAY 1962, THE STRONGEST NONTROPICAL CYCLONE TO STRIKE the continental United States during the past one hundred years pummeled the West Coast from northern California to southern British Columbia. With winds gusting to nearly 200 miles per hour over the coastal headlands and 100 miles per hour around Puget Sound and the northern Willamette Valley, the damage was devastating (figure 1.1). Power outages extended over most of the region, more than fifty thousand buildings were damaged, and forty-six people lost their lives. How could such a storm strike a region known for its benign weather and the velvet softness of its clouds, fog, and incessant light rain? Northwest weather is often surprising, both in its intensity and in the startling contrasts between nearby locations.
1.1. On the state capitol grounds in Salem, Oregon, the bronze statue The Circuit Rider was toppled by hurricane-force winds during the 1962 Columbus Day Storm. Photo by Hugh Stryker and provided courtesy of the Salem Public Library Historic Photograph Collections.
The weather of the Pacific Northwest is exceptional in many ways. While much of the eastern two-thirds of the United States endures warm, humid summers and cold, often snowy winters, the western side of the Northwest enjoys mild, dry summers and temperate, wet winters. In much of the country, weather varies gradually from one location to another; in contrast, rapid changes and localized weather are the norm in the Northwest, where radically different weather conditions are often separated by a few miles. While thunderstorms are a major feature of the weather in most of the country, in the Northwest they are infrequent events, with strong thunderstorms, tornadoes, or hail a rarity. Although hurricanes can strike the Atlantic and Gulf coasts of the United States and greatly influence the weather far inland, the Northwest is never affected by such tropical storms. Not only is the weather different in the Northwest, but so is its prediction. While weather forecasts for the central and eastern portions of the nation are enhanced by the dense observing network over North America, Pacific Northwest predictions are degraded by the sparsity of observations to the west, since nearly all West Coast storms originate over the relatively data-poor North Pacific.
Although Northwest weather is usually gentle and benign, some of the most severe weather of the continent is experienced here. Intense Pacific low-pressure systems, like the Columbus Day Storm, packing hurricane-force winds and extending over considerably larger areas than tropical storms, can bring destruction to wide swaths of the region. Localized windstorms, often associated with gaps in the high Northwest mountains or air flowing across major terrain features, have produced severe small-scale winds reaching 100 miles per hour or more. One such event destroyed the Hood Canal Bridge in 1979 at a cost of over 140 million dollars, and others have peeled off roofs in the Cascade foothills town of Enumclaw. While snow is infrequent and generally light over the Northwest lowlands, the heaviest measured snowfall in the world strikes the Cascade Mountains, resulting in buried roads and avalanches. During the last week of December 1996, such heavy snow closed all Cascade passes in Washington and resulted in widespread building collapses on both sides of the mountains. Although rainfall amounts are usually light to moderate during Northwest winters, Pineapple Express rainstorms, associated with rivers of atmospheric moisture originating north of Hawaii, can bring several feet of rain to Northwest communities over a few days, resulting in catastrophic flooding and mudslides. Such conditions hit the region with full force during November 2006, with Mount Rainier National Park experiencing the most severe damage since its inception (figure 1.2) and losses from flooding in Oregon and Washington totaling hundreds of millions of dollars. Billion-dollar storms have occurred several times in the Northwest since 1980 and all of them have been associated with severe flooding.
Startling weather contrasts over small distances are some of the most singular aspects of Northwest weather. The high terrain of the region often separates radically different climate and weather regimes, with transitions occurring over a matter of miles. The Olympic Mountains are a prime example: rain-forest conditions and annual precipitation approaching 200 inches a year are found on its western slopes, such as within the Hoh River valley, while a few dozen miles away, on the mountains’ northeastern side, Sequim typically receives about 15 inches a year. It is easy to see why the latter is a magnet for retirees in search of California-like conditions in the Northwest. Large contrasts similarly occur over the Columbia Gorge, with the change from the wet, lush forest environment near Cascade Locks to arid, barren conditions just east of Hood River occurring in a little over 20 miles and less than a half hour’s drive on Interstate 84. On December 18, 1990, an unexpected foot of snow crippled the city of Seattle during rush hour, while 20 miles to the north and south the ground remained bare. Northwest winds can also vary greatly over short distances. An extreme case occurred on the night of December 24, 1983, during a severe cold spell over the region. Air rushed westward through a gap in the Cascades and descended toward Enumclaw and vicinity, bringing wind gusts of over 120 miles per hour that tore off roofs and crumpled high-tension power-line towers. In contrast, 25 miles to the northwest in Seattle the winds were dead calm. No wonder local TV stations love to describe Northwest weather as “weird” or “wacky.”
1.2. Record-breaking rains on November 6, 2006, caused catastrophic slope and road failures across Mount Rainier National Park, resulting in the closure of much of the park for months. This picture shows damage to Nisqually Road at Sunshine Point in the southwestern portion of the park. Photo courtesy of the National Park Service.
The Northwest is also home to notable weather anomalies. When air descends the steep terrain that bestrides the Oregon-California border (the Siskiyou/Klamath Mountains), the southern Oregon coast can be 10–20 °F warmer than the rest of the Northwest, with high temperatures soaring into the 80s °F even in midwinter. Not surprisingly, the local chamber of commerce advertises this area as the “banana belt” of the Northwest. Other Northwest locations are famous for their extreme cold. Mazama and Winthrop, Washington, located in a deep valley protruding into the northern Cascades, are often the coldest locations in the state, both setting the all-time record low for Washington of –48 °F on December 30, 1968. Even colder temperatures can occur within the frigid valleys of the uplands of eastern Oregon, where Ukiah and Seneca cooled to –54 °F during the winter of 1933. The all-time record for annual snowfall in the world is held by the Mount Baker Ski Area, where 1,140 inches fell during the 1998–99 winter season, breaking the previous world record (1,122 inches) at Mount Rainier. The snow was so plentiful that year that skiing had to be suspended until the ski lifts were dug out.
The Northwest is also home to what might be called weather curiosities. Air streaming over the Northwest mountains can sometimes create wavelike clouds that resemble hovering flying saucers; in fact, such an apparition set off the UFO craze in 1947. Changes in air temperature above Puget Sound often cause optical effects in which ferry boats and other marine vessels appear to be flying above the water and shorelines seem thrust high into the skies. More ominously, the combination of strong winds and arid conditions east of the Cascades can produce terrible dust storms that decrease visibility to near zero and cause multicar accidents. The eruption of Mount Saint Helens in 1980 covered vast areas of the Northwest with darkness, with the ash cloud acting as an insulator that kept temperatures virtually constant for over twelve hours across much of eastern Washington. And the foggiest location in the continental United States is found near the outlet of the Columbia River at Cape Disappointment, where the typical year brings 106 days of dense fog with a visibility of a quarter mile or less.
Serious misconceptions about Northwest weather abound and many are put to rest in these pages. Probably the most repeated unsubstantiated claim is that Seattle receives more rain than virtually anywhere else in the continental United States. Not true. With an average annual precipitation of roughly 37 inches, Seattle’s rainfall is handily beaten by New York City (47 inches), Miami (56 inches), and many other locations across the eastern, central, and southern portions of the country. Another canard is that the Northwest is wet year-round. The truth is that Northwest precipitation is concentrated in relatively few months from November through February and that our summers are among the driest in the nation—even including the desert Southwest. Finally, some assert that Northwest mountains make weather prediction difficult; as explained later, the mountains have the opposite effect, improving forecast skill and giving Northwest forecasters advantages over their eastern colleagues.
1.3. High-resolution computer predictions of Northwest weather are now greatly improving forecasts. This graphic shows a thirty-six-hour forecast of precipitation over Washington State using a state-of-the-art computer-forecasting model. The values shown are for the three hours ending at 4:00 AM on January 15, 2006, and are in hundredths of an inch, with blue and dark green indicating the heaviest precipitation. Note the rain shadow to the northeast of the Olympic Peninsula and the heavy rainfall over the southwestern side of the Olympic Mountains and the western slopes of the Cascades. Terrain contours and wind flags are also shown.
Northwest meteorologists are often the brunt of local humor, and it is not unusual to hear people muse that dice would be a more reliable forecast guide. But the truth is that forecasts are getting better. Making use of new technologies—such as weather radar, satellite imagery, and high-resolution computer weather simulations—meteorologists have unraveled many of the details of Northwest weather, and forecasting skill has increased substantially (figure 1.3). While in decades past, major windstorms like the Columbus Day Storm of 1962 were poorly predicted, many of the recent great blows, such as the Inauguration Day Storm of 1993 or the Hanukkah Eve Storm of 2006, were forecast accurately days in advance. Something has changed, and this book describes the evolving technologies that have made improved predictions possible.
With its dependence on melting snow as a source of water and hydroelectric power during the summer and early fall, the Pacific Northwest may be particularly sensitive to the effects of global warming. Although the mountains and complex land-water contrasts of the Northwest make prediction of its future climate challenging, recent scientific advances are slowly revealing the region’s future. Some of these revelations are surprising, including an increase in springtime clouds west of the Cascades and local warming “hot spots.” As described in this book, the effects of global warming will vary greatly across the region, with warming weakened near the coast and enhanced on mountain slopes.
Both poorly understood and forecast until recently, the complex meteorology of the Northwest has been the subject of intense scrutiny by local weather scientists since the late 1970s. Making use of these insights, this book describes the weather of the region stretching from southern British Columbia to the California border, and from the western slopes of the Rockies to the Pacific Ocean. The goal is to provide a description of Northwest weather that is both accessible to a layperson and scientifically accurate.

2

THE BASICS OF PACIFIC NORTHWEST WEATHER

IF ONE COULD USE A SINGLE PHRASE TO DESCRIBE PACIFIC NORTHWEST weather, “wet and mild” would be a start, but not a particularly exact one. Although the region west of the Cascade crest is considered “wet” by many, it enjoys some of the driest summers in the nation and receives less annual precipitation than much of the eastern United States. East of the Cascades, where arid conditions dominate, “wet” is certainly not an apt description, and east-side temperature extremes, ranging from –48 to 119 °F, makes “mild” a misnomer at times. Northwest weather and climate are dominated by two main elements: (1) the vast Pacific Ocean to the west and (2) the region’s mountain ranges that block and deflect low-level air. Together, these factors explain many of the dominant and fascinating aspects of the region’s weather. The ocean moderates the air temperatures year-round and serves as a source of moisture, and the mountains modify precipitation patterns and prevent the entrance of wintertime cold air from the continental interior.

WHY IS PACIFIC NORTHWEST WEATHER GENERALLY MILD?

The Pacific Northwest is located in the northern hemisphere midlatitudes, a zone stretching from approximately 30° to 60° north where winds generally blow from west to east. This eastward movement of air is usually not uniform in strength, but is typically strongest in a relatively long, narrow current a few hundred miles across and a few miles deep, known as the jet stream. Usually centered 5–8 miles above the surface, jet-stream winds often reach 100 to 200 miles per hour during the winter. Weather systems, such as the low-pressure systems that bring rain and wind, tend to follow the jet stream, and thus...

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