Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

944 pm PDT Sun aug 1 2021


Deep marine layer persists along the coast with seasonable temperatures through Monday. High pressure rebuilds over Southern California Tuesday with inland warming while the marine layer continues along the coast. Coastal troughing returns later in the week.


As of 09:31 PM PDT Sunday, No updates to the forecast package this evening. The marine layer is rather deep around 2600 ft and we haven't have much coastal stratus south of San Francisco through the coast of Monterey County. Latest satellite imagery does show that the stratus is starting to spread southward so low clouds will likely impact that section of the coastline during the night hours. Highs today were generally in the low 70s in coastal areas while inland temps ranged mid 70s to mid 80s. The Parkfield and Pinnacles hot spots finally dipped below 100 coming in at 98 and 97, respectively.

Previous Discussion

As of 02:45 PM PDT Sunday, Visible satellite shows low clouds have cleared back to the coast. Even seeing a dry slot developing over the coastal waters from the Central Coast up through the SF Peninsula. Temperatures are similar to slightly cooler than at this time yesterday, owing to a somewhat deeper marine layer with greater extent of morning cloud cover. Overall, conditions will not change much today and Monday as we find ourselves positioned between a high- amplitude monsoonal ridge over the Intermountain West and a deep 500 mb trough offshore with a parent low in the Gulf of Alaska. This setup will ensure a persistent marine layer near 2000 ft and onshore flow near the surface.

Some subtle warming can be expected Tuesday, mainly inland, as high pressure becomes focused over the Great Basin and SoCal. However, the trough stationed offshore will help moderate temperatures, especially at the coast. As the trough approaches the NorCal coast Wednesday, temps will cool back to seasonal norms across the CWA. High pressure will slide under the base of the trough Thurs/Fri bringing renewed warming. Another subtle cooling trend arrives next weekend with the aforementioned Gulf of Alaska low entering the PacNW with weak troughing developing over CA. Overall, a benign pattern for the week with only minor daily fluctuations.

Latest deterministic runs support a strong ridge forming over the eastern Pacific the second week of August, bringing the potential for a heat-up. Still a lot of ensemble uncertainty this far out, but worth mentioning.


As of 5:15 PM PDT Sunday, For the 00z TAFs. Currently VFR with stratus lingering off shore. Onshore winds are breezy, but will diminish after sunset this evening. The marine layer is around 2000ft this afternoon, which should allow a decent push back inland. MVFR/IFR ceilings should fill in around 03Z for KMRY/KSNS, and around 05-06Z for KSTS/KAPC/KOAK. KSFO will see skies fill in later, around 10Z. Some models keep skies VFR through the period for KLVK/KSJC. There is still some uncertainty if stratus will reach that far inland. Might add TEMPO after 00Z guidance comes in. Skies will lift and clear around 17Z for most sites, with 18-19Z for KMRY/KSNS. Onshore winds become breezy again Monday afternoon.

Vicinity of KSFO, VFR skies through 9Z today, with onshore winds diminishing after sunset. Expecting MVFR ceilings building in after 9Z. Skies should lift and clear around 16-17Z. Onshore winds build back Monday afternoon.

KSFO Bridge Approach, Similar to KSFO.

Monterey Bay, VFR conditions this afternoon with light onshore winds. Skies should start to fill in (MVFR/IFR) around 03Z this evening. IFR conditions will prevail through 18-19Z Monday. Light onshore winds Monday afternoon.


As of 04:45 PM PDT Sunday, Light winds will prevail across the coastal waters through tomorrow. Expect locally breezy winds in the afternoon and evening through the Golden Gate gap and northward to the Delta today and Monday. Northwest winds will gradually increase throughout the rest of the week. Short period northwest waves continue to dominate the sea state at 7 to 9 seconds along with a moderate period southerly swell. A weak west to northwest swell will also arrive this week.

Watches, Warnings, Advisories

Tonight, none.

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