Potent atmospheric river racing toward the Pacific Northwest
A late-season atmospheric river is aiming at the Pacific Northwest and British Columbia, Canada, and when it reaches land, AccuWeather meteorologists say it could cause significant flooding concerns.
Much of western Washington and southwestern British Columbia can't seem to escape winter's grip as storms continue to roll in from the Pacific Ocean. Many locations had two to three times their normal rainfall in May and most have nearly fulfilled their entire monthly rainfall for all of June in only the first week of the month.
Storms over the northern Pacific Ocean typically don't frequent the region by this point of the season, experts say. However, more storms are lining up and have the potential to produce a fire hose effect of heavy rain in the northwestern United States and southwestern Canada later this week. These same areas were slammed by an even larger atmospheric river which triggered a deadly and disastrous flood last November.
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In Seattle, 3.82 inches of rain fell in May, compared to the monthly average of 1.88 inches. During the first eight days of June, 1.03 inches of rain fell; about 1.45 inches typically falls during the whole month in the city. It has been a similar story in Vancouver, British Columbia, with 3.45 inches (87.6 mm) falling in May or 2.2 times that of normal and 1.24 inches (31.5 mm) pouring down from June 1-7, which is already 70% of the average for all of June.
Even though the rain amounts this spring have not been astronomical, it is the persistence of the storms from the Pacific that usually pack their bags and leave for the summer season that remains a problem. The period from late May through the first part of September is considered the dry season in the region with only sporadic rainfall that generally amounts to about an inch of rain per month for the coastal areas with slightly higher amounts for the western slopes of the mountains.
Last spring, dry conditions helped set the stage for all-time record heat in parts of the region. This spring, the ground is plenty moist and, in some areas, it can set the stage for a different problem.
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With the absence of moisture-laden storm systems during an average summer season, warmer air typically leads to a gradual meltdown of the snowpack at intermediate elevations from about 4,000 to 8,000 feet.
However, when storms with drenching rain continue into the first part of the summer, there is the potential for the existing snow cover to liquefy and lead to a rapid runoff that can lead to dangerous and damaging flash flooding.
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This image, captured on Thursday morning, June 9, 2022, shows storms more typical of the early spring or late winter lining up over the northern Pacific Ocean. |
Over the past several days, AccuWeather meteorologists have been tracking an atmospheric river capable of causing that very issue. There is a series of potent storms and a plume of moisture that covers thousands of miles from waters east of Asia to the south of Alaska's Aleutian Islands, according to AccuWeather Senior Meteorologist Mike Doll.
That plume containing warm, moist air and heavy rain is taking aim at southwestern British Columbia and western Washington, and its impacts are expected to be felt primarily from Thursday to early Friday.
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The rain is likely to arrive in multiple waves into this weekend, according to AccuWeather Meteorologist Alyssa Smithmyer.
"It is the dose just prior to the end of the week that can lead to major problems in terms of rapid snowmelt at intermediate elevations and quick flooding of the streams and rivers that flow through and out of the mountains down below," Smithmyer said.
A general 1-2 inches of rain will fall near sea level, but 2-4 inches of rain is likely with an AccuWeather Local StormMax™ of 5 inches over the west-facing slopes of the mountains. This amount of rain would not be enough to bring major flooding problems by itself, but there is plenty of snow piled up in the mountains.
Forecasters say the snowpack is currently being measured in yards at intermediate and high elevations in the region.
"The concern is that the temperature of the snowpack at intermediate elevations is close to the freezing mark and ready to melt," AccuWeather Senior Storm Warning Meteorologist William Clark said.
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That snowpack has many inches of water locked up within. When the heavy rain hits the snowpack that is on the verge of melting, it can thaw rapidly and have the effect of doubling or tripling the rainfall amounts.
There is the potential for some roads and neighborhoods to be inundated with rapidly rising water in this setup. In locations where flash flooding occurs at night, the situation can become especially dangerous.
Some secondary roads and bridges could be washed out, forecasters say, and mud and other debris could block some roadways. Motorists that travel through areas that are prone to flooding in the winter and spring during heavy rain events should anticipate problems and be prepared to seek an alternative route.
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Repairs continued at the Highway 1 Malahat Tunnel Hill, British Columbia, washout, as of May 31, 2022. (Photo British Columbia Transportation) |
Last November, an atmospheric river hit British Columbia and part of Washington with rounds of torrential rainfall. In some locations, more than 20 inches of rain fell and produced significant damage on a number of highways. In British Columbia, repair projects are still ongoing and crews could face reconstruction delays from the atmospheric river. At one point last fall, all major roads and rail lines leading to Vancouver were closed during the storms, and operations at the third busiest port in North America ground to a halt.
One person was killed as a result of the storms in the Canadian province, according to The Associated Press.
In Washington and Oregon, damage from the November 2021 floods totaled several billion dollars. In Sumas, Washington, located about 115 miles north of Seattle, 75% of all the homes sustained flood damage.
A second round of rain will swing in from Friday night to Saturday, depending on the speed of a trailing storm. That storm does not appear to be as potent as Thursday's event. However, some moisture may dip farther to the south with the second storm this weekend and could bring some showers to parts of Northern California.
While the storm train will bring an overabundance of moisture to the Northwest through the weekend, substantial rain is not forecast to dip southward into drought-stricken California and much of the southwestern U.S. in the pattern, according to AccuWeather meteorologists.
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