Why Is the Pacific Northwest So Rainy?

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iStock

The Pacific Northwest is home to some of the most breathtaking scenery in the United States, but that greenery comes at a cost. It rains in the Pacific Northwest, and it rains a lot. Despite the gorgeous landscapes and cosmopolitan cities, the western parts of Washington and Oregon get a bad rap for how gray and dismal they can be. But why exactly does it rain so much in the Pacific Northwest?

The reason gloomy weather is so common boils down to prevailing weather patterns and the unique terrain that makes this part of the world so gorgeous. This stretch of land between Portland, Oregon, and Seattle, Washington, usually finds itself directly under the track of the jet stream. The jet stream is a fast-moving river of air that encircles the Northern Hemisphere right around the latitude of the U.S.-Canadian border.

Whenever the jet stream swoops to the south, creating a trough, it can generate low-pressure systems at the surface that produce heavy rain and high winds. These troughs and resulting low-pressure systems often intensify in the Gulf of Alaska and over the Pacific Ocean off the coast of Washington, allowing large storms to crash into the coast with ease. Most of these storms are run-of-the-mill rainmakers, lasting for a day or two before moving on, but some of them can be enormous and cause significant wind damage and flooding.

While the photogenic low-pressure systems that swirl into the coast are the driving force behind the Pacific Northwest’s seemingly endless rains, it’s the region’s terrain that locks in those dismal weather conditions. When moist winds blow inland with an approaching storm, the high terrain of the Cascade Range forces the moist air to rise into the atmosphere, enhancing the thick clouds and steady rainfall.

Rain clouds over Pacific Northwest
Cloudy skies over the Pacific Northwest on May 16, 2017.

Even though it rains quite a bit along the northwestern coast, actual rainfall totals in the Pacific Northwest vary wildly from place to place due to changes in elevation. Some spots at high elevation right along the coast or along the Cascade Range can see more than 10 feet of precipitation in a single year, accounting for both rain and the equivalent amount of liquid in snowfall.

Seattle, Washington, and Portland, Oregon, each only see around 36 inches of rain every year, which hardly makes these bustling metro areas the wettest in the country. Compare their rainfall totals to New York City’s Central Park, which measures nearly 50 inches of rain every year, and Mobile, Alabama, commonly one of the wettest cities in the United States; its proximity to the Gulf of Mexico gives it nearly 66 inches of rain every year.

The rain that does fall in Seattle and Portland, though, falls over a longer period of time. Between 1981 and 2010, both cities saw a little more than 150 days with measurable precipitation per year, compared to about 122 rainy days in New York City and just 115 in Mobile. This accounts for the Northwest’s reputation as the gloomiest part of the country—but makes for spectacularly green landscapes when the skies clear out.

Denver is About to Experience Summer and Winter Temperatures Within 24 Hours

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iStock.com/mphotoi

In a story tailor-made for exhaustive Weather Channel coverage, Denver, Colorado is about to experience one of the more bizarre weather shifts in recent memory. After an expected Tuesday high of 80°F, residents can anticipate a dramatic shift down to 32°F by midday Wednesday, with an initial half-inch of snow accumulation increasing to up to 7 inches by Wednesday night.

Put another way: Citizens who need to make sure they hydrate in the warm temperatures Tuesday will have to bring out the parkas the following day.

The Denver Post reports that the warm air coming ahead of the cold can result in a clash of air masses, prompting areas of low pressure that can create forceful and damaging weather conditions. The storm could bring winds of up to 60 miles per hour and possibly even cause power outages. Snow accumulation should dissipate by the weekend, when temperatures are expected to climb back into the 60s.

The high temperature record for April 9 in Denver is 81°F, set in 1977.

[h/t The Denver Post]

What Is a Bomb Cyclone?

Maddie Meyer/Getty Images
Maddie Meyer/Getty Images

The phrase bomb cyclone has re-entered the news this week as parts of the central U.S. face severe weather. Mountain and Midwestern states, including Colorado, Nebraska, Wyoming, and South Dakota, all fall in the path of a winter storm expected to deliver tornadoes, hail, heavy snow, flooding, and hurricane-force winds on Wednesday, March 13 into Thursday. It seems appropriate for a storm that strong to have bomb in its name, but the word actually refers to a meteorological phenomenon and not the cyclone's explosive intensity.

According to The Denver Post, the bomb in bomb cyclone stands for bombogenesis. Bombogenesis occurs when a non-tropical storm experiences at least a 24 millibar (the unit used to measure barometric pressure) drop within 24 hours. Low pressure makes for intense storms, so a bomb cyclone is a system that's built up a significant amount strength in a short length of time.

This type of storm usually depends on the ocean or another large body of water for its power. During the winter, the relatively warm air coming off the ocean and the cold air above land can collide to create a sharp drop in atmospheric pressure. Also known as a winter hurricane, this effect has produced some of the worst snowstorms to ever hit the U.S.

The fact that this latest bomb cyclone has formed nowhere near the coast makes it even more remarkable. Rather, a warm, subtropical air mass and a cold, Arctic air mass crossed paths, creating the perfect conditions for a rare bombogenesis over the Rockies and Great Plains states.

Central U.S. residents in the bomb cyclone's path have taken great precautions ahead of the storm. Over 1000 flights have been canceled for Wednesday and schools throughout Colorado have closed.

[h/t The Denver Post]

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