Meteorologists and their weather models have not come to any agreement about whether Tuesday's incoming storm will meet the criteria of a "bomb cyclone," but suffice it to say it will be really windy and wet.
There is a huge area of low pressure out over the Pacific Ocean, carrying with it wind, rain, and potential thunderstorms, and it is scheduled to hit the Northern California coast with a vengence by 7 pm.
A "bomb cyclone" occurs when a storm’s central pressure drops at least 24 millibars within 24 hours, also known as bombogenesis — and the lower the central pressure, the stronger that storm can become.
Per the Chronicle's meteorology team, models are showing that we could still get more moderate storm impacts, with winds up to 50 mph in San Francisco — still a very significant storm. Or, things could get more intense, with winds 60 mph and up, and northern counties like Mendocino and Del Norte potentially seeing destructive gusts of 80 mph, equivalent to a Category 1 hurricane.
Ready...set...
— NWS Bay Area 🌉 (@NWSBayArea) December 23, 2025
It's almost go time. Low pressure about 700 miles SW of San Francisco is poised to provide some heavy weather to the Bay Area/Central Coast tonight. Wind, thunderstorms, possible tornado, and rain.
Prep today! pic.twitter.com/pZtCRcPP5X
SFO was already seeing 30-minute average delays on outgoing flights Tuesday afternoon, and 80-minute delays for incoming flights with a ground-delay program in place, due to high winds.
Those delays could escalate into cancellations as the day wears on, and Christmas Eve travel could be a real challenge.
As with many winter storm systems like this, the heaviest rain totals are forecast for the coastal mountains of the North Bay, Santa Cruz, and Big Sur. And with this system, the most extreme downpours are expected to hit the Los Angeles area, with San Francisco only at "slight risk" of excessive rainfall — but this was based on models on Monday.
A rare High Risk has been issued:
— National Weather Service (@NWS) December 22, 2025
"The potential for high hourly rainfall totals
will accentuate runoff over soil that is relatively dry and or over recent burn scar areas. This will pose a significant threat of flash flooding, landslides, rock falls and mud slides. The greater… pic.twitter.com/vdHmVapAXe
San Francisco and most of the Bay Area are under a flood watch through Friday night. The Chronicle has this interactive map showing the most flood-prone neighborhoods in the region.
Additionally, there is a slight risk for tornadoes tonight near the coast, with the National Weather Service giving the Bay Area and Central Coast a 2% chance.
The wind and rain are expected to stick around the Christmas in San Francisco, with Boxing Day (Friday) seeing just moderate rain.
Photo by Misael Silvera
