Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

447 am PDT Fri Jun 22 2018


A robust warming trend is forecast through Saturday afternoon as high pressure builds over the region. This will lead to hot conditions on Saturday with moderate to locally high heat risks across mainly the interior. Additionally, offshore winds will develop and result in heightened fire weather concerns to portions of the region on Saturday. Cooling is then expected on Sunday with increased onshore flow.


As of 04:23 AM PDT Friday, A marine layer of around 1,000 feet in depth remains in place this morning. This has resulted in low clouds and patchy fog from the San Francisco Coast down along the Big Sur Coast and into the Salinas Valley. In addition, some low clouds can be seen developing over Marin County at this hour. Meanwhile, temperatures are generally in the 50s region-wide. These conditions will likely improve through the morning as low clouds retreat to the coast in response to building high pressure aloft. A warming trend will also continue today, especially across the interior and the marine presence becomes compressed. Look fore widespread 90s across the interior this afternoon with 80s around the San Fransisco Shoreline and upper 60s to 70s near the coast.

The warmest day of the week is forecast to occur on Saturday as offshore flow develops over the North Bay and East Bay Hills/Mountains. Widespread 90s are likely with many of the warmest inland areas reaching to between 100 and 105 deg F. Given the hot daytime temperatures and mild overnight conditions in response to the offshore flow, the risk for heat related illnesses will increase tonight into Saturday. Thus, a Heat Advisory has been issued for inland areas as well as in the hills/mountains on Saturday. Coastal areas will also warm to above seasonal averages, however weak onshore flow should limit overall heat risks.

Breezy to locally gusty offshore winds are forecast to be strongest in the North Bay and East Bay Hills/Mountains on Saturday. Northerly winds of 15 to 30 mph with gusts up to or exceeding 40 mph will be possible across these areas and will combine with humidity values in the upper single digits to middle teens to create increased fire weather concerns. As a result, a Fire Weather Watch remains in effect from Saturday into Sunday for the aforementioned areas. Be mindful of these fire weather conditions and avoid any activities that may lead to fire starts, such as lawn or automotive work, driving motorized vehicles on dry grass, lighting campfires or fireworks, or discarding hot or burning objects such as lighter coils or cigarettes.

Onshore flow is then forecast to return on Sunday as a southerly surge develops first along the Big Sur Coast and spreads northward through the afternoon and evening. This should cool conditions near the coast and locally inland as onshore winds increase. With this said, inland areas will likely remain warm to hot through Sunday afternoon before the marine airmass advects further inland. Further cooling is then likely region-wide on Monday with temperatures dropping to near seasonal averages. Seasonably conditions are likely next week with the presence of a more established marine layer and onshore flow.


As of 4:47 AM PDT Friday, For 12z TAFs. High pressure aloft will continue to compress/erode the marine layer through the day. Latest GOES-16 fog product shows mainly clear skies around the region, except for stratus affecting the San Francisco/San Mateo coasts, as well as the Monterey Bay coast/Salinas Valley. LIFR/IFR cigs will continue affecting Monterey Bay terminals through about 15z before mixing out and becoming VFR. Elsewhere, VFR will prevail through the day and expected to continue through the TAF period. Weak onshore flow today will turn more offshore by early Saturday. Light to moderate west/northwest winds this afternoon.

Vicinity of KSFO, VFR expected through the forecast period. Breezy onshore winds this afternoon, sustained around 20 kt with higher gusts possible.

SFO Bridge Approach, Similar to SFO.

Monterey Bay Terminals, LIFR/IFR cigs & vsby will affect KMRY and KSNS through 14-15z before VFR. VFR expected to dominate through the rest of the forecast period, but there could be a couple of hours of LIFR cigs/vsby at KMRY around sunrise Saturday. Included this in TAF for now, but with low confidence on this occurring. Generally light onshore winds this afternoon, around 10 kt.


As of 2:43 AM PDT Friday, A strong area of high pressure over the Eastern Pacific will maintain moderate to strong northerly winds across the coastal waters through late Saturday night. The strongest winds will be located over the northern outer waters, north of Pigeon Point. These sustained strong winds will produce relatively large and steep winds waves on top of a building northwesterly swell with moderate periods. Winds will decrease Sunday as a southerly surge moves up the coast, but remain breezy along the inner coastal waters.

Watches, Warnings, Advisories

Today, sca, pigeon pt to pt pinos 0-10 nm from 12 pm sca, pt arena to pigeon pt 10-60 nm sca, pt arena to pt reyes 0-10 nm sca, pigeon pt to pt piedras blancas 10-60 nm sca, pt pinos to pt piedras blancas 0-10 nm sca, pt reyes to pigeon pt 0-10 nm from 4 pm

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