Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

855 pm PDT Fri Mar 23 2018


A cold front will spread rain from north to south across much of the region tonight and Saturday morning. Additional shower activity is forecast Saturday late afternoon and evening with isolated showers possibly lingering into Sunday. Dry weather is expected to return on Monday and continue through next week. A warming trend is forecast for the first half of the week.


As of 8:55 PM PDT Friday, Satellite, radar, and observations indicate that the cold front approaching from the northwest is currently near Point Arena. Based on the latest HRRR model, expect the cold front to spread rain across the North Bay between about 10 pm and 2 am, then south across the rest of the SF Bay Area between 2 am and 5 am Saturday, and finally through the Monterey Bay Area between 5 am and 8 am. Several lightning strikes have been detected along the northern California coast north of Point Arena this evening, so a slight chance of thunderstorms have been added to the North Bay for later this evening.

All of the models indicate little or no precipitation following the front on Saturday until the cold upper low approaches late in the day. Thus, most of Saturday may end up being dry under partly cloudy skies. The evening forecast update included a reduction in shower chances for most of Saturday.

Shower chances will increase late Saturday afternoon and evening as the cold upper trough approaches the northern California coast. Based on the 00Z models, shower chances will spread into our forecast area from late Saturday afternoon into Saturday night. But rainfall then is not expected to be as widespread as with the cold front moving through tonight. There is also a slight chance of thunderstorms late Saturday afternoon and early evening, mainly across the North Bay where instability will be greatest.

Isolated showers may linger into Sunday, mainly near the coast in the morning, and then along the eastern portion of our forecast area in the afternoon. But for the most part Sunday is expected to be a dry and cool day.

This incoming system will not have a whole lot of moisture to work with and so rain totals from tonight through the weekend are forecast to generally be a half inch or less, with highest amounts in the North Bay. Some southern inland areas may see little, if any, rainfall.

Snow levels will start off around 3500 feet tonight, but then drop as low as 2500 feet by Saturday and Saturday night. Since the bulk of the precipitation is expected to fall with the cold front tonight and prior to the arrival of the coldest air, only light snowfall accumulation is expected on the higher peaks.

Models continue to indicate dry weather through next week, along with a warming trend, as a ridge of high pressure develops offshore and then slowly moves eastward.


As of 04:39 PM PDT Friday, for 00Z TAFs. Currently VFR conditions at all sites with scattered clouds and occasional showers in the North Bay. A cold front is still expected to move through late tonight bringing another round of steady showers. As the front approaches expect cigs to begin to lower back to MVFR. Shower chances will end early tomorrow morning once the front has passed and VFR conditions should once again prevail.

Westerly winds will remain breezy through the evening at around 15 kt before taking a more southerly turn and somewhat decreasing.

Vicinity of KSFO, VFR anticipated through about 03z or so, with FEW/SCT clouds between 3,000 to 5,000 ft AGL. Cigs expected to lower to MVFR this evening, ahead of a cold front that is forecast to pass through the terminal around 08-11z tonight. Models still suggest a narrow band of rain as the cold front sweeps through. Left a few hours of light rain in the tafs. Breezy southwesterly winds expected to continue into tonight.

SFO Bridge Approach, Similar to KSFO.

Monterey Bay Terminals, VFR conditions through tonight. Cigs will lower after midnight tonight, ahead of an approaching cold front forecast to pass through early Saturday morning. Expect wet runways from light rain as the front passes. Breezy westerly winds will continue into the evening.


As of 8:16 PM PDT Friday, Lightning strikes have been observed off the Northern California coast near Fort Bragg. There's a slight chances of thunderstorms across the northern waters over the next few hours. Light to moderate west to southwest winds will prevail as a cold front makes its way through the coastal waters late tonight. Light rain is possible along the front, and winds will turn northwesterly in its wake. A second cold front is expected Saturday evening into Sunday morning. Northwest swell will increase slightly overnight tonight.

Watches, Warnings, Advisories

Tonight, sca, pt arena to pt reyes 0-10 nm sca, pt arena to pigeon pt 10-60 nm sca, SF Bay from 9 pm until 3 am sca, pt reyes to pigeon pt 0-10 nm from 9 pm until 3 am

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