Atmospheric rivers bring wet weather to the San Francisco area and the West Coast

Weather alerts — for heavy rain in the lowlands and snow and wind in the mountains — cover most of central and northern California. Coastal and central Oregon and Washington are largely under flood watches and high wind warnings, with winter storm warnings in the Cascades and Columbia River Basin.

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Atmospheric rivers are ranked on a scale of 1 to 5 based on how much moisture they transport from the tropics to the mid-latitudes. The Center for Western Weather and Water Extremes has ranked the current atmospheric river event as a level 4 out of 5 — which corresponds to “extreme” moisture transport.

The atmospheric river can carry more than 220 million pounds of water across the sky over the Pacific coast every second—providing abundant fuel for rain.

“[A] a strong storm [is] currently moving into the region,” the National Weather Service in Sacramento wrote in its online discussion of the forecast. “A weaker series of systems, but of longer duration, moves in early Wednesday evening and continues through the weekend.”

The continuing atmospheric river

A tongue of heavy moisture currently stretches from central California, just south of the Bay Area, to about 1,000 miles north of Hawaii. The core of this moist cloud is about 200 to 300 miles wide and pointed due east.

This highway of moisture is entrained and pulled eastward by a counterclockwise spinning bowling ball of low pressure moving ashore in the Pacific Northwest. Some of the moisture swirls into this low pressure system and, with the cool air aloft, contributes to rain in the lowlands and snow in the higher elevations.

Seattle and surrounding areas are under a flood watch through Wednesday morning. Approximately an inch of rain has fallen so far, and another quarter to half inch is likely to come. This is in addition to the 3.09 inches that fell between Christmas Eve and December 26.

“Ground conditions are already saturated from previous storms. Additional rain on Tuesday could cause some rivers to flood later Tuesday into Wednesday morning,” the weather service said.

Even as the rain eases over the next 24 hours, the winds will be howling. They will increase significantly from the southwest on Tuesday afternoon, peaking around the evening before easing off during the second half of the night. The National Weather Service warns that gusts up to 55 mph are possible.

These strong winds will pile up water against the shoreline, prompting a coastal flood warning for Seattle and much of northwest Washington.

“Flooding of lots, parks and roads with only isolated road closures [is] expected,” the weather service wrote.

In the Cascades, up to 60 inches of snow is likely in the highest, though unpopulated, peaks. However, winter weather advisories are in effect for the mountains, where snow levels can drop to 2,500 feet. Communities above that line could see a mix of snow and ice, with some possibly getting up to a foot of snow accumulation.

Farther south in Oregon, Salem, Eugene and the Willamette Valley are under high wind warnings. Beaches, headlands and other open areas may experience gusts in excess of 70 mph, although most inland areas will see gusts of 55 to 60 mph.

Golden State is seeing its own bad weather. Flood warnings extend from Santa Rosa through the Bay Area and south to the Salinas Valley. San Francisco had a 24-hour rainfall total of 1.78 inches as of 9 a.m. local time and could see another half inch to an inch. A similar event on December 10-11 dropped 1.84 inches in San Francisco.

Wind advisories and gust warnings are in effect for the central and northern San Joaquin Valley, but the worst conditions, with gusts over 100 mph, will be felt in the Sierra Nevada. Widespread 3 to 4 feet of snow fell there. Winter storm warnings cover the mountains, with possible snowfall rates of 4 inches per hour.

So far, Interstate 80 above Donner Summit was in good enough condition that chain controls — when highway conditions require vehicles to have chains — were lifted, but snow levels could drop below 6,000 feet in the evening hours. This will again put the pass at risk of heavy snow.

The effects of the atmospheric river will weaken overnight as the atmospheric river drifts south and thins.

Some improvement is likely Wednesday and Thursday in the west, but there are signs of another atmospheric river event on the way. The inciting low pressure system, visible as a swirl in water vapor images, is currently sliding east to south of the Aleutian Islands in Alaska and will pull another band of moisture ashore as we head into the weekend.