Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

412 am PDT Mon Oct 25 2021


Rain this morning will gradually taper to midday showers as the atmospheric river shifts south of the region. Dry weather returns all areas tonight into Tuesday morning. A weak system will bring some light rain chances to the North Bay Tuesday while the rest of the region stays dry. High pressure builds Wednesday through Friday with a return to seasonably mild and dry weather.


As of 4:12 AM PDT Monday, Its been a memorable past 24 hours for the Bay Area as the long talked about atmospheric river rolled through the region. We literally have gone from fire/drought conditions to flooding in one storm cycle. Some of the rainfall totals are staggering with our ever reliable observer in Kentfield reporting over 11 inches of rain on Sunday at the base of Mt Tamalpais. It'll take a bit to reliably check all of our data but 24 daily record of 4.02 inches for downtown SF smashed the previous October daily rainfall record of 2.48 inches back on Oct 13th 2009. It looks like yesterday was the 4th wettest day ever for downtown SF where records go back to the Gold Rush years. The 4.02 inches of precip looks to be the 4th wettest day ever according to our records. In an odd coincidence SFO reported 4.02 inches yesterday as well with the old daily record 0.74 back in 1998. The list goes on and on but even usually dry spots like Livermore reported 3.57 inches on Sunday. The Mt Tamalpais gage has a 2 day total of 16.29 inches and 26.92 inches this month alone! San Jose 2 day total 1.79 inches, Oakland 4.27 inches and Ben Lomond 8.93 inches.

The AR is now centered over Monterey county and the gauges in the Santa Lucia range are tipping rapidly with totals of 3-4.5 inches over the last 6 hours. We currently have a flash flood warning in place for the Dolan Fire burn scar with concerns about rock and debris rolling onto Highway 1. Will see what daylight brings.

We are seeing some rain redevelop across the North Bay at this hour as some of the cooler/unstable air starts to filter in behind the AR. Expect rain to continue through 7-8 am before decreasing rapidly in the North Bay. Several of the creeks and streams remain in flood stage with local flood products still in place across Sonoma county. Even at this hour Mt Tam is still reporting 0.41 hourly rain rates with several locations in the East Bay hills and Santa Cruz mountains still reporting rates around 0.20 inch per hour. In general though as we approach daybreak the heaviest rains will be confined to the coastal hills of Monterey county. As the AR drifts south most of the steady rain should move out of Monterey county before noon. A few showers possible this afternoon but nothing of consequence expected as things stabilize pretty quickly.

Dry conditions tonight but then a fast moving system approaches overnight with increasing clouds. This will lead to a chance of some light rain for the North Bay on Tuesday but amounts look very light on the order of a tenth or two. The rest of the Bay Area looks to remain dry south of the Golden Gate on Tuesday.

High pressure builds Wednesday through Friday with a noted warming and drying trend as highs warm back into the 70s most locations. No more AR events on the horizon and things look dry into the weekend as a more seasonable pattern returns.


As of 10:40 PM PDT Sunday, For the 06z TAFs. The Atmospheric River continues to stream RA/+RA across the terminals tonight along with MVFR/IFR cigs and low vis at most of the terminals. Coastal and North Bay terminals (e.g. STS/LVK/HAF/WVI/MRY) report vis between 1 to 3 SM, while OVC15-30 cigs hover over most of the inland terminals. The strongest +RA showers are still on track to affect MRY/SNS through 8-10Z this morning, while lingering -RA/VCSH will continue to affect the rest of the terminals through the rest of the morning. Winds behind the FROPA have quickly dropped off, so are on track for light winds at the Bay Area terminals along with LLWS 35-45 kts gradually diminishing by noon. Strong winds 20-35kts will still linger over the Central Coast terminals until just after 10Z as the narrow cold frontal rainband eventually exits the region. The rest of the TAF period will be dominated by -RA/VCSH along with light surface winds from dawn through the end of the period, with skies FEW- SCT060.

Vicinity of KSFO, MVFR/IFR conditions with +RA and LLWS. Breezy to light SW/W winds overnight. LLWS will begin to diminished after 09Z. -RA/VCSH will continue through 18Z Monday, with skies clearing out by Monday 00Z. Breezy W winds Monday afternoon with gusts up to 25kts, before gradually dropping off towards the end of the TAF period.

KSFO Bridge Approach, Similar to KSFO.

Monterey Bay, Cigs will remain low ahead of the heavier showers with MVFR (possibly IFR/LIFR) conditions then persisting through at least the early to mid morning hours. Gusty southerly winds also expected with gusts 25-35 kt expected through 9-10Z. Stronger winds aloft are also expected at 40-50 kt thus resulting in some LLWS through at least 12Z. Winds will then gradually diminish as the RA along the narrow cold frontal rainband clips the Central Coast. Lighter winds and diminishing LLWS conditions by dawn, with light winds through the remainder of the TAF period.


As of 02:43 AM PDT Monday, Winds over the waters have reduced to 20 to 30 miles per hour with some occasional gusts up to 40 miles per hour this morning. These strong winds will generate very steep swell that result in hazardous conditions, especially for smaller vessels. Winds will continue to diminish into the early to mid-morning hours, becoming calm by the afternoon. A large west to northwest swell train with swell heights of 16 to 24 feet at 16 to 18 seconds moves through the morning. Swell heights to peak this afternoon and bring hazardous conditions to the surf zone and near harbor entrances with breaking waves of 20 to 30 feet.


As of 3:00 AM PDT Monday, A strong, early season storm system will continue to increase wave energy into early Tuesday. This wave energy will transition through the coastal peak today. Swell of 16 to 24 feet at 16 to 18 seconds is forecast to arrive with the swell train as it peaks this afternoon and results in a number of coastal hazards. These hazards include large breaking waves of 20 to 30 feet (higher at favored break points), increased risk of strong longshore and rip currents, increased risk of coastal erosion and minor coastal flooding, and enhanced coastal run up concerns due to the summer beach profiles in place. Due to the early arrival of these large waves, many beaches are still transitioning from their summer beach profiles and lack the features and steepness to resist larger wave run up on coasts. This means that more of this wave energy will have a chance to move onto the beach and overtake individuals, potentially injuring them, or pulling them into the cold ocean. Each year, people die at the coast due to these or similar ocean conditions. A high surf warning for the entire coast has been issued for this threat and is in effect through 11AM Tuesday, with the highest risks once again at west to northwest facing beaches.

Watches, Warnings, Advisories

Today, high surf warning, caz006-505-509-529-530 sca, pt arena to pt reyes 0-10 nm sca, pt reyes to pigeon pt 0-10 nm sca, pigeon pt to pt pinos 0-10 nm sca, pt pinos to pt piedras blancas 0-10 nm sca, pt arena to pigeon pt 10-60 nm sca, pigeon pt to pt piedras blancas 10-60 nm sca, mry bay

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