Atmospheric river hitting West with rain, snow; yet another storm on the way

The second of three atmospheric rivers is battering the West, bringing prolific mountain snows and soaking rains in the lower elevations and driving the risk of flooding and avalanches. Yet another in the recent series of storms will approach Northern California late in the week.
More than 2 feet of snow has already fallen in the highest peaks of the Sierra Nevada, and an additional 5 to 8 feet could fall in the highest summits. Snow could fall at rates near 2 inches per hour, which could render Interstate 80 near Truckee and Donner Pass in California impassible.
“Travel will be very difficult to impossible,” the National Weather Service wrote. “Slick travel conditions are expected, with chain controls likely.”
The heavy mountain snows are set to spread across much of the West Coast and Columbia River Basin. Winter storm warnings blanket the Sierra Nevada, the southern half of Oregon, much of Idaho, Montana and even northwestern Wyoming.
There’s also a flood watch in effect for the coast of central California, including the San Francisco Bay Area, as well as the Sacramento Valley. Rains over the past several days have saturated the ground, priming it for additional flood concerns in the days ahead.
The heaviest rains will fall in the Santa Cruz Mountains and Santa Lucia Range.
How much has fallen since Friday?
Waves of heavy rain from the first of the storms began moving through the region late last week. When those have passed, some locations will have experienced more than a week of rain.
So, far, about 1 to 3 inches has fallen in the San Francisco Bay Area since Friday, with 3 to 6 inches across the rest of Northern California. The mountains have seen as much 10 to 12 inches of rain.
Some totals from Weather Service weather stations since Friday include:
- 10.14 inches in Forbestown, California (in the Sierra Nevada foothills).
- 6.89 inches in Redding, California.
- 3 inches in Lake Tahoe, California.
- 2.35 inches in Eureka, California.
- 1.47 inches in Napa, California.
- 1.08 inches in downtown San Francisco.
Snow levels were quite high in the Northern California region, which kept accumulating snowfall at or above resort level through much of the weekend. Some of the highest elevations picked up about 2 feet of snow. Snow levels are expected to come down a bit over the week as the atmospheric rivers and related warmth shift southward.
To the north, mostly light snow fell around Seattle, although it coated roads and caused up to a couple inches of accumulation in the hardest-hit spots. It was the first snow of the season for many in that region.
Atmospheric rivers carry the bulk of their moisture at the mid-levels of the atmosphere. That’s why they are comparatively unremarkable in lower elevations – like in Sacramento, where only an inch or two of rain is expected – but severe in the higher terrain. Mountains literally poke into and focus the atmosphere’s moisture, causing exceptional precipitation totals, particularly above 3,000 or 4,000 feet elevation.
The atmospheric river is being driven by a low pressure system midway between Hawaii and the West Coast, as well as a secondary low-pressure system offshore of the Pacific Northwest. Both are swirling counterclockwise, sweeping robust moisture along from the central tropical Pacific and slingshotting it nearly 3,000 miles to the northeast.
The ongoing atmospheric river will be mostly aimed at Northern California and southern Oregon through late Tuesday or early Wednesday, at which point it will begin to sweep southward down the coast while pinching off from its tropical connection. That will cause precipitation to abruptly shut off west to east on Wednesday – reminiscent of what happens when you kink a garden hose.
However, yet another atmospheric river is expected to target Northern California by Thursday – the grand finale in a trio of moisture-loaded storms.