Atmospheric river brings heavy rain, snow and wind to the US Northwest
Atmospheric rivers are streams of moisture that are typically thickest in the mid-levels of the atmosphere, often extending for thousands of miles from the deep tropics to the mid-latitudes. In this case, some of the moisture can be traced back about 8,000 miles to near Guam and the Northern Mariana Islands.
A corridor of moisture swirls between a high pressure system between Hawaii and the West Coast and a low in the Gulf of Alaska. Like interlocking gears, a pair of weather systems are capable of pulling in a thread of moisture that will aim directly for the Pacific Northwest.
Seattle was already in a flooded westerly flow as of early Friday, and will be until dry air to the north arrives overnight Saturday morning. The city’s flood watch remains in place through Saturday, but occasional rainfall could linger into Monday beneath the core of an approaching low.
There are numerous rivers in Western Washington under flood warnings for heavy rains cause them to swell over their banks.
A turbulent weather pattern in the Pacific Northwest suddenly appeared.
Between July and mid-October, Seattle was abnormally dry with little measurable precipitation. Less than three weeks ago, the temperature soared to nearly 90his sky was filled with smoke and it had some of the worst air quality in the world. But since then, wet weather has set in, clearing the air and dumping nearly 3 inches of rain.
In Portland, the worst weather will be Friday night, when 1 to 2 inches is possible. Although the atmospheric river will weaken by Saturday afternoon, more rain and snow is expected on Monday. Some showers could linger around Tuesday under the core of a cold low, at which point snow levels are expected to drop to 600 feet above sea level.
Much worse conditions are expected in the mountains — both in the Coast Range and in the Cascades. This is because atmospheric rivers carry most of their moisture about a mile above the ground. As this saturated air is forced upwards, the moisture in it cools, condenses and leads to more precipitation. In Washington, the Quinault in the Coast Range could see 6 to 8 inches total, with 10 to 15 inches at Paradise in the Cascades and 6 to 8 inches at Snoqualmie Pass.
Between 6,500 and 8,000 feet in elevation, this heavy precipitation will change to snow, with up to three feet near some of the highest mountain peaks.
Mountains also move closer to the jet stream, creating stronger winds. Peaks higher than 5,000 feet could see gusts in the 50 to 60 mph range.
In Oregon, one can expect much the same. Mount Hood could see wind gusts in excess of 70 mph.
Some of that Pacific moisture will reach the Columbia River Basin and the Northern Rockies; a winter storm warning and high wind warnings cover much of northeastern Oregon, Idaho and even northwestern Wyoming. Up to 2 feet of snow is possible in the Teton Range. Across Montana, the threat is mostly for strong winds, with gusts of 40 to 60 mph through Saturday afternoon.
Through Sunday, the atmospheric river will slide further south, bringing 12-hour snowfall to the Sierra Nevada. Two or more feet of snow can fall during that span at the highest elevations.
While the weather forecast is cold and stormy in the West, calm and unseasonably warm conditions will dominate the eastern US, where dozens of record high temperatures are forecast to be set through Monday.
Jason Samenow contributed to this report.
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