First Storm Was a 'Bomb'; It's Mostly Moved On; More Rain Soon

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Commuters slog through downpour at the Mountain View Caltrain station Tuesday night.  (Miranda Leitsinger/KQED)

The highlights:

  • Storm was a record-breaking "bomb"
  • Storm arrived a little earlier than expected
  • Rain totals somewhat higher than forecast
  • Sierra highways a mess
  • Widespread PG&E power outages, especially on North Coast and Santa Cruz County
  • Coming next: a more potent storm this weekend?

The details:

So, that happened: As forecast, we got an adult dose of wintry weather that put an abrupt end to our long, parched autumn.

What we did not quite imagine, despite the forecast, was the storm's sudden, break-down-the-door ferocity, marked by high winds, heavy rain and copious mountain snowfall. And the good news — good, if you're thinking about our longer-term water needs and not traveling over the coming weekend — is that forecasts show the wet weather resuming this weekend and continuing into next week. A few specifics about what just passed and what's to come:

A "bomb cyclone" that broke records: The National Weather Service says the storm that dove into California Tuesday had the lowest atmospheric pressure ever recorded in the state: 28.69 inches, or 973.4 millibars, as recorded at Crescent City, on the coast near the Oregon border.

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Those extremely low numbers were the product of "explosive cyclogenesis," also often referred to as "bombogenesis" — a process in which a low pressure area or storm center intensifies very rapidly. The effect is what's called a tight pressure gradient, or an abrupt change in pressure between areas that are relatively close together. And the result is an increased potential for very strong winds and other intense storm conditions, such as periods of heavy precipitation.

What did it look like in Crescent City, where the "bomb cyclone" was at its most intense? Take a look:

And here's a view of what was happening in the atmosphere as that mess on the ground transpired:

Storm arrived early, rained hard, moved through quickly: Most forecasts called for the Tuesday storm to reach the area around San Francisco and Oakland about 6 p.m. But umbrellas began sprouting up by midafternoon as a cold front approached. The front moved through the city just after 6 p.m., triggering brief but impressive downpours that caused widespread roadway flooding.

A similar scenario occurred throughout the region, with the steady rain that had preceded the front turning into short-lived cloudbursts, then giving way to clearing skies and occasional showers — which are expected to continue into early Thursday.

Here's the height of the storm as captured by KQED's Miranda Leitsinger in Mountain View:

The highest 24-hour rain totals in the region as of 1 p.m. Wednesday were spread across western Marin County, where Mill Valley has recorded 2.05 inches, and the Santa Cruz Mountains, where Scott Creek reported 1.97 inches. Dozens of locations across the region topped an inch of rain, including downtown San Francisco and San Francisco International Airport (1.14), Hayward Airport (1.13) and Oakland International Airport (1.01).

Driving across the mountains? Not a lot of fun: Winter storm warnings are in effect through Thanksgiving afternoon for most mountain highways in Northern and Central California and through early Friday for higher-elevation routes in Southern California.

Traffic has been intermittently halted on Interstate 80, U.S. 50 and Highway 88, the main routes to Lake Tahoe and other resorts in the central Sierra Nevada. But as of midafternoon Wednesday, passenger vehicles were moving, slowly, across those routes, with chains or snow tires required for long stretches.

For the most up-to-date information, check the Caltrans road condition page or the agency's live traffic camera network.

A traffic camera image showing vehicles moving east on U.S. 50 toward Echo Summit and South Lake Tahoe. (Caltrans)

Lights out again: The onset of heavy rain likely marks the end of this year's fire season and eliminates the need for further wildfire-safety blackouts by PG&E. But the storm's rambunctious arrival on the coast, with high winds that took down trees and power lines and caused other damage, left more than 80,000 customers without power Tuesday night.

Hardest hit was Humboldt County, where at least 20,000 customers lost power starting early Tuesday afternoon. Thousands remained without electricity going into a second night Wednesday. The utility was forecasting that some areas, mostly around the cities of Eureka and Arcata, may not have lights restored until Thanksgiving evening. In the interior areas of the county, on the Hoopa Reservation and along the Highway 299 corridor near Willow Creek, PG&E has no estimated time for restoring power.

To the south of the Bay Area, more than 11,000 customers lost their lights in Santa Cruz County and another 8,000 were without power in Monterey County.

In one high-profile Bay Area electricity failure, much of Oakland International Airport lost power about 6:30 p.m. Tuesday. That disrupted everything: flight arrivals and departures, security lines, baggage service and traffic outside the terminals. Airport representatives said Tuesday night they were uncertain was caused the outage, which took about two hours to resolve.

Next up — a wet weekend: Forecast models show rain arriving in the Bay Area (and snow in the mountains) sometime Saturday afternoon and continuing into Monday. Some parts of the Central California coast — it's too early to say exactly where — will be hit with heavy rains with moisture, courtesy of an atmospheric river reaching back to near Hawaii.

The current models point that atmospheric river at the Big Sur coast and areas to the south, where higher elevations could get 5 to 7 inches or more of rain. The storm could also prompt Caltrans to preemptively close Highway 1, something the agency began doing after frequent slides, a bridge failure and other damage that occurred during the extremely wet winter of 2016-17.

Like our Tuesday-Wednesday storm, the incoming storm will bring snow and difficult or hazardous conditions to mountain areas throughout the state. Homeward-bound holiday travelers, get ready for another challenge.