Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

904 am PST Fri Jan 24 2020


Another weak front will pass over the region tonight with some sprinkles possible for the North Bay but mainly dry conditions overall. A warm front will bring some light rain chances to the North Bay late Saturday, spreading south across the rest of the Bay Area Saturday night into early Sunday. Rain amounts will be light overall with a drying trend Sunday afternoon through early next week as temperatures trend to above normal.


As of 9:04 AM PST Friday, A weak impulse moving across the North Bay has touched off some showers there, but overall, only a few hundredths of an inch have been picked up in Sonoma and Marin counties. Otherwise, some dense fog is the main weather story this morning. Visibilities in the south and East Bay Valleys have been fluctuating between a few miles to less than 1/4 mile. Some improvement is being indicated at San Jose Airport, now reporting 1 3/4 miles. Salinas and Palo Alto are both reporting 1/4 mile or less. This fog layer is very shallow, and is expected to clear by late morning giving way to mostly sunny skies in most areas. Clouds will increase across the north as the weak system approaches the coast. This is expected to bring light rain to mainly just the North Bay, but only light amounts.

High pressure will briefly rebuild over the area allowing for another round of night/morning fog followed by mild afternoon temperatures on Saturday with highs in the 60s. The ridge will weaken later in the day as a weak front approaches. Light rain chances will return to the North Bay late in the day Saturday. Rain chances increase and eventually spread southward Saturday night through Sunday. Rainfall amounts over the weekend will generally be a few hundredths to a tenth or two. Highest rainfall amounts will be over the North Bay and up to nearly three quarters of an inch there.

Another weak boundary will bring yet another chance for light rain across the North Bay late Monday into Tuesday. Dry weather is then expected for the rest of the week and possibly beyond as a blocking ridge pattern locks in place over the west.


As of 03:45 AM PST Friday, For 12Z TAFs. Some overnight clearing occurred during the peak of the night, allowing for patchy dense fog and low stratus decks to form near KLVK/KMRY/KSNS. Latest satellite imagery shows a weak boundary introducing low to mid level clouds in the North Bay with KSTS reporting BKN decks between 2000 - 4500 ft. Expect to see this boundary shift southward through the early morning hours and impact the Bay Area terminals with MVFR cigs as well. Reductions in visibility to 5-6sm and VCSH may accompany this boundary as it moves from the North Bay to immediate Bay Area shorelines. Look for clearing in the wake of this boundary mid to late afternoon to early evening before another weak boundary introduces another round of low to mid level moisture Friday night. Winds generally light and terrain/diurnal driven.

Vicinity of KSFO, Visibility will be reduced to 5-6sm range due to BR thru the early morning hours as weak boundary arrives from the north. In addition, SCT/BKN ceilings from 1000-4000 ft are possible as the boundary moves over between 14-19Z. Winds light.

SFO Bridge Approach, Similar to KSFO.

Monterey Bay Terminals, Satellite shows rapid development of low ceilings (400 feet AGL) from the Monterey Peninsula through the Northern Salinas valley, with OVC004 at KMRY and SCT004 at KSNS as of 345AM LT. Expect to see these deck persist through at least 15Z this morning with reductions in visibility to accompany the low ceilings. Patchy fog is also not out of the question at these terminals. The boundary will weaken significantly by the time it pushes into the Central Coast area, but expect to see additional mid to high level clouds arriving from the north later in the TAF period.


As of 8:48 AM PST Friday, Generally light winds today with southerly winds over the northern waters and north to northwest winds over the southern waters. Locally breezy winds are expected along the Big Sur Coast today south of Point Sur. Winds will then become west to southwest late Saturday and into Sunday as the next system approaches. A long period west to northwest swell will move through the waters peaking today and tomorrow. This will create conditions hazardous to small crafts. Additional long period northwest swells will arrive this weekend and early next week.

Watches, Warnings, Advisories

Today, sca, pt arena to pt reyes 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, pt reyes to pigeon pt 0-10 nm from 9 am

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