Petaluma, CA 94954 Weather

Key Messages

Updated at 801 PM PDT Tue Oct 20 2026

- Cooling trend continues through the end of the week

- Beneficial light rain showers expected Wednesday through Thursday

- Thunderstorms possible Wednesday afternoon into Thursday morning across Central Coast

- Warm and dry next weekend

Update

Issued at 801 PM PDT Tue Oct 20 2026

It is the calm before the storm. Weather admirers are anxiously waiting for the first rain to reach the surface. The most recent 12Z and 00Z KOAK soundings observed precipitable water values nearly double from 0.44 inches yesterday to 0.84 inches today. Mid-level moisture is increasing, though dry air at the surface remains. As a result, today's cloud buffet consisted of cirrostratus and altocumulus castellanus with virga (rain that evaporates before reaching the ground) at the cloud base. Soon enough this moisture will saturate the lower levels setting the stage for the first rain showers to develop overnight.

Navarrete

Short Term

Issued at 104 PM PDT Tue Oct 20 2026 (This evening through Wednesday night)

Near term - mid and upper level clouds continue to stream over the forecast area this afternoon. There have been a few pockets of sunshine, but temperatures are still seasonably cool. Highs this afternoon will top out in the 60s and 70s. We have a few spots interior Central Coast with a forecast to reach close to 80, but that may be tricky given temps are holding in the 60s currently.

The much talked about storm system is knocking on our door step. Afternoon visible satellite imagery shows a solid circulation well offshore. That's the surface low getting its energy from a robust upper level jet rounding the base of the trough. We've actually begun to pick a few lightning strikes near the center of the surface low. Between the surface low and the coastline is another swath of clouds. This particular location is where the digging northern jet is phasing with the southern jet. In other words, solid lift. KMUX radar continues to pick up bands of shower activity associated with this region of lift and moisture. So what's it look like this evening through Wednesday? The upper level jet dynamics will continue to fuel the cut-off low at 500mb and the occluded surface low. Both of these features will inch their way eastward between now and early Wednesday. As such, a few scout/leading edge showers will be possible across the region. Still some dry air in the mid levels so it will take some time to get the precip to hit the ground. Where it becomes more interesting dynamically is early Wednesday. As the low moves toward the coast stronger flow aloft takes aim at the Central Coast with a jet max showing up from 700-500mb leading to deeper layer shear. The increasing shear also lines up with some higher lapse rates and decent CAPE (Cali standards). The other wildcard at play are the SSTs in that region. Current SPORT data shows values in the upper 60s or 8-10 degrees above normal. One would think that will add some extra fuel. Given the setup, dynamics, and pattern recognition some low-topped supercells are within the realm of possibilities. Rotating cells could spawn a waterspout that may make landfall. The threat for thunderstorms have been extended northward to Santa Cruz, but the most likely area for waterspouts remains off Big Sur. SPC has now included the Big Sur Coast in a 2-4% chc of tornadoes. Peak convective threat remains from 12Z Wed through 03-06Z Thursday. Outside of this threat and this region, elsewhere around the CWA will see rain shower throughout the day.

Timber and Plaskett burn area concerns - given the convective threat there are definitely concerns for debris flows. Given the low conf and more cellular nature will not issue any flash flood watch at this time. Persons along the Big Sur Coast should still be prepared. Needless to say, we'll watching this situation closely.

Convective threat end Wednesday night into Thursday, but lingering showers remain.

Long Term

Issued at 204 PM PDT Tue Oct 20 2026 (Thursday through next Monday)

The cut-off low transitions to an open wave on Thursday as it moves eastward. While the best dynamics may have eased, lingering showers remains through the day Thursday. Rainfall totals: remain the same for most of the Bay Area 0.05-0.25", while Central Coast into Santa Cruz has increased 0.25-0.75" locally up to 1.25" Santa Lucia.

By Friday the open wave/trough will be east of the area with upstream ridging moving in. The ridging will usher in a warming and drying trend for the upcoming weekend. The pattern over the weekend into next week has some omega blocking vibes, but upstream lows keep trying to knock into the ridge. Dry through the rest of the extended.

Totally fantasy land at this point, but longer range guidance keeps the pattern active with wet weather returning toward the end of the month just in time for Halloween. Stay tuned...