How California storm’s ‘rapid intensification’ led to extreme impacts — and will it happen again?Gerry Díaz March 22, 2023 Updated: March 22, 2023 4:55 p.m. Comments
Gerry DíazMarch 22, 2023
 The eye of one storm headed north of San Francisco toward the Marin Headlands, while the second eye near Santa Cruz fizzled away on Tuesday afternoon.
College of DuPage WeatherA rare storm system with two eyewalls spinning around San Francisco and Santa Cruz churned powerful winds, downpours and damage across the Bay Area on Tuesday, along with a brief thunderstorm and tornado warnings along the California coast. The low-pressure system rapidly evolved into a storm that caused unusual springtime severe weather.
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It culminated in sea level pressure that fell to 985 millibars, the lowest level ever recorded at San Francisco International Airport in the month of March.
This uncommon, but not unprecedented, low-pressure system’s pressure dropped so quickly that its center quickly grew an eye. The storm kept intensifying and splintered into two centers of circulation. The centers grew their own eyes and began to dance around each other — a Fujiwhara Effect unfolding over Monterey Bay. Both eyes then spun out in different directions: One made landfall over Santa Cruz and the other over San Francisco. Residents across the Bay Area noticed sudden sunny skies and warm, muggy air as the eyes hovered over the region as severe weather was unfolding.
Cyclonic behaviorThe rapid development of Tuesday’s low-pressure system into a storm led to pressure readings at Monterey Bay falling to criteria that meet the definition of a “meteorological bomb,” meaning the pressure at the center of the storm — based on a National Weather Service buoy — fell by 24 millibars in just under 24 hours at that buoy.
This led to speculation that the storm reached bomb cyclone status based on one station’s readings. What is significant is that the storm’s cyclogenesis — falling pressure readings and rapid intensification — resulted in likely record-breaking winds.
 Damage to boats and docks are seen at Jack London aquatic center on Wednesday in Oakland.
Lea Suzuki/The Chronicle“Today’s cyclone was notable in that it intensified the most as it came ashore, which is not typical on the U.S. West Coast; most ‘meteorological bombs’ in the Pacific weaken before reaching the coast,” wrote Shawn Milrad, associate professor at Embry-Riddle Aeronautical University, in a message. “It also developed one or two ‘eye-like’ features, which is pretty unusual in these systems over water that cold.”
The storm quickly grew an eye as it spun toward the Big Sur coast. But it struggled to keep up with its own growth, leading to the center breaking off into two parts. Meteorologists watched on radar as two areas of circulation formed just off the Monterey Peninsula. The Fujiwhara Effect — when two circulations spin around each other — was in full effect by midmorning over Monterey Bay.
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The two landfallsAs the eyes of the two centers of the storm spun around each other, one was blown toward northern Monterey Bay, making landfall near Santa Cruz.
This led to an afternoon of gusty winds across Monterey Bay and the Santa Cruz Mountains. The eyewall of the storm ushered in rounds of 70 to 80 mph gusts at the summits of the mountains, surpassing forecasts by over 10 mph. Heavy rainfall then raised concerns about more flooding in regions of Santa Cruz County, like Felton, Ben Lomond and Scotts Valley, where soils are already saturated from the past few storms. Trees came down as saturation unmoored their roots, leading to widespread road closures.
The second eye of the storm then spun north, making landfall in the Peninsula and flowing over San Francisco.
Residents on the Peninsula saw an uptick in winds as the eyewall slammed into San Mateo County, eventually hovering over Sutro Tower. Winds on the Embarcadero surpassed 75 mph, while gusts across the city hit 55 to 66 mph. These winds were roughly 25 mph stronger than the previous forecast, largely because of the eye breakoff earlier in the day.
The one weather model that did pick up on the breakoff was the high-resolution rapid refresh, which hinted at two eyes forming over the course of the day.
Once the eyes trailed off, winds and rain steadily fell across the Bay Area in the evening. The unusual March storm was coming to a close.
Is this the new normal?This storm’s record-breaking sea level pressure readings and ferocity will be studied in academic and operational spaces across the weather industry. While it’s not unheard of to have storms like this one rapidly intensify, it is unusual to see it occur just a few miles off the shoreline.
In most instances, the centers of winter storms either stay out at sea or make landfall in remote stretches of the Oregon and Washington coast, keeping the strongest winds away from populated areas. But as Northern California experienced this past winter with rounds of storms and January’s bomb cyclone, it does happen.
“ ‘Meteorological bombs’ are not new and happen quite frequently each winter in the western North Pacific between Japan and Alaska as well as the North Atlantic between New England and Iceland. Having one occur in coastal California, while not unprecedented, is rare,” Milrad wrote.
But to see the low-pressure system evolve into a storm next to the coast and to then have a Fujiwhara Effect is extremely unusual.
The unusual weather may persist for another week or two, as the European and American models once again suggest that a couple of low-pressure systems off the coast of Hawaii are tracking east toward California. Given the constant flow of moisture between the two states throughout winter and early spring, wet and windy weather could return to the Bay Area early next week.
Reach Gerry Díaz: gerry.diaz@sfchronicle.com; Twitter: @geravitywave |
