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Brewing storm to sock Pacific Northwest, Upper Midwest

The storm will unload heavy precipitation from Washington state to Northern California and could bring a blizzard to the Dakotas

American (GFS) model shows atmospheric river or zone of high atmospheric moisture bombarding northern California Monday night. (WeatherBell)
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A potent storm system in the northeastern Pacific will sling a plume of deep tropical moisture into Washington, Oregon and California, bringing heavy rain, wind and accumulating snow in the Cascades and northern Sierras. Then it will become a major weathermaker for the Northern Plains and Upper Midwest, potentially producing high winds and heavy snow around Veterans Day.

The storm’s first act comes Monday night into Tuesday, when it will drag an atmospheric river, a narrow swath of exceptionally moist air that produces heavy precipitation, into the Pacific Northwest. Drought-plagued Northern California, blasted by an even stronger atmospheric river two weeks ago, will be hit again, chipping away at its long-standing water deficit.

After slamming the Northwest, the storm will glide across southern Canada, grazing the northern Rockies with snow Tuesday into Wednesday. Between Wednesday night and Friday, it will gain strength as it dives into the Northern Plains and Upper Midwest, blasting parts of northern and western Minnesota and the Dakotas with wind-whipped snow and bringing whiteout conditions to some.

Farther south across the Plains and toward the East Coast, a line of downpours will form along a cold front dragged east by the system, with a soaking rain likely by Friday into Saturday.

Act 1: West Coast soaker

The catalyst for the heavy precipitation in the Pacific Northwest is an intensifying zone of low pressure several hundred miles west of the Washington-British Columbia border. This storm will strengthen fast enough through Tuesday morning to qualify as a meteorological “bomb,” or “bomb cyclone.”

Counterclockwise winds around the storm will yank ashore a ribbon of moisture that will translate to heavy rain and appreciable mountain snows from western Washington to Northern California. The Center for Western Weather and Water Extremes is projecting that this atmospheric river will reach Level 3 on its 0 to 5 scale for these events.

The core of the atmospheric river will aim at Northern California on Monday night and Tuesday. San Francisco is expecting one to two inches of rain. The Bay Area and much of northwestern California are also under a wind advisory for gusts up to 45 mph at low elevations and 55 mph in the mountains.

Since atmospheric rivers transport most of their moisture at the mid-levels of the atmosphere. That means the greatest precipitation totals will be found on the western, or windward, side of mountains, where moisture-rich air will be forced up the higher peaks and condense. Three to four inches of rain could fall along the coastal mountain ranges.

In the higher elevations of the Sierra Nevada range, generally above 7,500 feet, winter weather advisories are in effect for four to 15 inches of snow.

Heavy rain and mountain snow are also anticipated in western Oregon and Washington. While Seattle may see only an inch or two of rain, the northern Cascades may get up to a foot of precipitation (rain or the equivalent in melted snow) by the time mid-November rolls around. Mount Baker in Washington received nearly two feet of snow over the weekend.

Act 2: Possible blizzard over Northern Plains

After the atmospheric river lands in the Pacific Northwest and California, its parent storm will lose some steam near the U.S.-Canadian border late Tuesday, but its remnant moisture will bring some snow and low-elevation rain showers to the northern Rockies.

But by Wednesday, the storm will reorganize and intensify as it dives into the Northern Plains. Plummeting temperatures on its back side will combine with wraparound moisture, leading to a snowstorm that could bring blizzards and challenging driving conditions to parts of northern and western Minnesota and the Dakotas.

A steep pressure gradient, or change in air pressure with distance, will make for strong west to west-northwesterly winds gusting upward of 50 mph over the Dakotas on Thursday night. Those strong winds topping 40 mph will expand southeast over the Central Plains and Corn Belt during the day on Friday.

Moisture will be limited in the wake of the system, but enough may linger to brew light to moderate snow across the Northern Plains and Upper Midwest, with heavy amounts in northeastern North Dakota and northern Minnesota. While snow totals may be modest for the areas impacted, the presence of strong winds could lead to whiteout conditions and near-zero visibilities for a time on Friday.

Details are still being sorted out, but “confidence of some measurable snow is quite high,” according to the National Weather Service office in Grand Forks, N.D.

Some models, like the American GFS model, plaster the northern half of Minnesota with amounts flirting with a foot. The European model, meanwhile, depicts about half that. Not until Wednesday can a more accurate prediction be made.

In Minneapolis, temperatures may be too borderline to foster much in the way of accumulating snow on Veterans Day and into early Friday, but a few snow showers are possible if precipitation rates become heavy enough briefly during the day on Friday. That said, the National Weather Service notes that “any accumulations would be light if any at all.”

Farther south, the cold front extending from the storm center will be a rain producer. As of noontime Thursday, the front should be draped from Chicago to Memphis to Baton Rouge. On Friday, the front will swing toward the Appalachians.

A 6- to-10-hour window of light to briefly moderate rain is likely, totaling a half-inch to an inch in spots. Eastern Pennsylvania, New York state and New England may see a bit more, as a secondary wave of low pressure develops along the front off the New England coastline.

The rainfall will combine with gusting winds of 25 to 40 mph in the Great Lakes and Tennessee Valley out of the south ahead of the low, as well as on the back side of the system as cool air filters in from the northwest.

Jason Samenow contributed to this report.

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