A model simulation of strong winds at the mid levels of the atmosphere fueled by a raging jet stream screaming in from the northwest. (WeatherBell.com)

An “unprecedented” bomb cyclone roared ashore in southwest Oregon and Northern California on Tuesday evening, bringing powerful winds and smashing all-time air pressure records.

Marching ashore with winds clocked at hurricane force, the storm that pounded the Pacific Coast intensified at nearly twice the rate of a “bomb cyclone.” Sustained 85 mph winds were measured Tuesday afternoon at Cape Blanco, Ore., with a gust to 106 mph.


An eye-like feature appears on satellite as the tightly-wound “bomb cyclone” approached the U.S. Pacific Coast. (RAMMB/CIRA)

The winds

The extreme winds appear to have been most potent at the leading edge of a “warm seclusion.” In essence, the storm got so wrapped up that it pinched off a lobe of warm air that became detached from the main warm sector of the system.

You can see that in the model animation below. For all intents and purposes, red represents warmth, and blue demarcates cooler air. What’s really being depicted are “thickness” measures of an air column. Warmer air expands, making a volume of air taller, or thicker. That’s shown here as a bubble of red detached from the rest of the warm air.


A high-resolution model simulation showing textbook warm seclusion as the storm moved ashore near the California/Oregon border. (WeatherBell.com)

It’s not unusual for a narrow nose of hurricane-force winds to form along the bent-back warm front encircling the lonesome pocket of warmth.

Meanwhile, a potential “sting jet,” another zone of intense winds, may have formed on the southwest side of the system.

Elsewhere, Crescent City, Calif., gusted to 69 mph. Squaw Peak in Oregon hit 90 mph at 6:26 p.m. Tuesday as the core of the storm arrived. Weed, Calif., saw a 70 mph gust, while Sixes, Ore., maxed out at 75 mph. Elsewhere, a slew of locations saw winds well above 60 mph.

As chilly, stable air surged southward behind the storm in the “dry slot,” a several-hundred-mile-long rope cloud formed ahead of the encroaching air mass.

The air pressure

The air pressure dropped 43 millibars in 24 hours — almost double the 24 millibars in 24 hours needed to classify a storm as a meteorological bomb. The faster the air pressure drops, the more air drawn into the storm, and the stronger the winds. What happened Tuesday falls under the realm of extreme rapid intensification.

But it wasn’t just how quickly the storm’s pressure plummeted. In Crescent City, Calif., an air pressure reading of 973.4 millibars was measured, breaking all-time records in California as the lowest sea-level pressure ever observed. That same low air pressure is what you’d get on an average day if you went to the top of a 90-floor building. Crescent City also saw gusts to 69 mph.


A plot of wind speed/gust and air pressure captured by a buoy off the coast of Crescent City, Calif. (NOAA)

A buoy off the coast recorded the staggering drop in pressure, heralded by damaging winds. Note how the strongest winds accompany the most rapid changes in pressure.

Also pay close attention to what the winds do as the pressure bottoms out — they briefly slacken. That came as an eye-like feature passed overhead, the wind speed falling to only 12 mph before abruptly cresting again near 60 mph.

The precipitation

Now, the story shifts from wind and pressure to heavy precipitation. Virtually the entire Sierra Nevada is blanketed beneath winter storm warnings, with strong jet stream dynamics and plentiful moisture expected to vomit snow measured in feet.


The National Weather Service in Hanford, Calif., is forecasting isolated pockets of more than 4 feet of snow to fall in the Sierra Nevada. (NWS)

The National Weather Service is forecasting more than four feet through Friday in Lodgepole, Calif. Yosemite Valley is engulfed by the 34- to 50-inch zone, while Mammoth Lakes will see a comfortable two feet.

Closer to sea level, it’s all rain — but a good amount of it. In the Central Valley, between 0.5 and 1.0 inches are expected, but amounts may approach an inch and a half closer to the coast through Friday.

Even a few thunderstorms — a rarity in California — are possible Wednesday and Thursday, especially over the western San Joaquin Valley and West Side Hills to the coast. Any thunderstorms that do gel could bring gusty winds, small hail, and even offshore waterspouts. Small hail was already observed overnight in Eureka, Calif.

The same jet stream dive that helped spawn the bomb cyclone will move into the Desert Southwest on Wednesday night and Thursday. Santa Fe and Albuquerque, N.M., are both under the gun for three to six inches of snow.

Much of the Arizona desert is under a flash flood watch as well.

The winter storm should start to wind down in most areas by Friday, but it will bring more snow to the Intermountain West and Four Corners region before triggering a new storm over the Plains this weekend.