Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

931 pm PDT Tue Jun 15 2021


Clear skies tonight with hot temperatures expected the next three days. Widespread 90s to triple digit temperatures will be likely across the interior Wednesday, Thursday and potentially into Friday. Meanwhile, onshore flow will limit heat impacts along the coastline and around the bays. Gradual cooling is then likely this weekend and into early next week.


As of 09:30 PM PDT Tuesday, Temperatures got into the upper 70s and low 80s this evening as the warming trend begins slowly today. Many areas were two to four degrees warmer than their high temperature yesterday, while some isolated locations were as much as six to eight degrees warmer. Temperatures will drop into the upper 50s tonight, with low-to-mid 60s in higher elevations. No big changes to the forecast of the previous discussion. Models still have the high over the desert southwest expanding bringing the robust warming to the region tomorrow forecasting the hottest day to be on Thursday.

Skies remain mostly clear, despite high level clouds beginning to move into Southern Monterey County as the first bit of subtropical moisture of 2021 moves northward along the ridge's western flank toward the Central Coast. This elevated moisture will continue to track northward as the previous discussion mention which could influence the exact temperature values achieved. It will also be monitored due to the slight uptick in instability along with it. Both the GFS and NAM have modified Total Totals at 500 mb above 25, with a minor increase in MUCAPE between 700-500 mb. However, overall the instability is too low, and even the highest values of instability are displaced from the largest plumes of moisture reducing the chance of any real threat for a lightning strike. Despite being monitored, it is expected to just gently pass through the region, and assist in the potential for some spectacular sunsets. Any elevated fire concerns that are present are simply from how dry the vegetation is, so caution is still advised and it is encouraged that the public stay vigilant to not be that spark that starts one.

Beyond the heat, the ridge begins to break down on Friday, but will be strong enough to bring hot temperatures similar to what is being forecast for tomorrow, Wednesday. Saturday is forecast to have slightly cooler temperatures, and on Sunday a return to much calmer temps in the 70s and 80s. So in summary: hot and dry, yet cooler at the coast. Welcome to summer in the Bay Area.

Previous Discussion

As of 01:55 PM PDT Tuesday, Heat. GOES-WEST Geocolor Imagery this afternoon shows clear skies all across California with the exception of some elevated smoke over SoCal and a few convective pockets over far northern portions of the state. The clear skies over Northern and Central California, in conjunction with the building mid-level ridge over the 4-Corners Region, have supported rapid diurnal heating across the interior portions of the North and East Bay, along with the 101 corridor between San Jose and Gilroy and interior San Benito and Monterey Counties. The interior as a whole is running between 5-10 degrees hotter than this time 24 hours ago, while RH values have tanked in response to a compressed marine boundary layer that is only evident along the immediate coast and portions of the SF Bay shoreline and coastal valleys. Winds have also remained light but have observed a few breezy spots beneath the marine layer as onshore flow kicks up in response to the developing temperature gradient between the coast and interior. Light offshore winds have also been observed at our higher peaks this afternoons. All of the pre-described conditions this afternoon will be amplified over the next few days. No matter how you slice it, we are on track for the heat event.

Looking at a warm night across the entire region, with the building ridge influence and hot 850mb temps aloft impeding diurnal cooling up on the hills and and mountains. Latest NAM-12km and local WRF solutions are all signaling at the hot air mass from the Desert Southwest rapidly expanding into Southern California this evening before eventually funneling in to Central and Northern California overnight. Another way to think about it is that just above us there will be an airmass with 25+ C temps gradually seeping those hotter temps down to the surface. And while the marine layer will greatly help in ventilating the coast and near-coast interface with much cooler temperatures overnight and through the remainder of this heat event, it should be noted that the coast will feel muggy over the next few days. More on that in the following section. As for overnight lows, looking at warm starts in the 60s F across the far interior (Concord, Napa, et cetera) while the coast will start its Wednesday morning in the upper-50s F.

The hot air mass will continue to seep into the Bay Area Wednesday. The added diurnal heating from the Sun on Wednesday afternoon will propel interior temperatures into the 90s and low 100s F, while the marine layer will help to bring in onshore flow (the sea breeze) into portions of the South Bay and interior valleys, keeping them in the upper 80s to low 90s F. As for the coast, are expecting windy conditions owing to the strong temperature gradient, which will help to keep the coastal and bay interfaces at a cooler 65-75 degree F range. Nonetheless, the hot air mass will only be a couple thousand feet overhead. Thus, elevated portions of the East Bay foothills and SF (e.g. twin peaks) have the potential to reach the low 80s F owing to the hot air seepage. Winds in the interior will remain more limited, but weak offshore flow, in concert with low RH values, do pose some elevated fire weather concerns. Nonetheless, the lack of stronger surface winds will prevent widespread critical fire weather conditions. As a reminder, a heat advisory will be in place for much of the interior starting at 11AM Wednesday morning.

Previously mentioned that the coast will be largely spared from heat event. While temps will remain MUCH cooler here than just a few miles inland, should note that sea surface temps are currently running 3 degrees warmer than this time last month (57F vs 54F), largely due to a decrease in upwelling along the California Current. This caveat is one of the reasons why it's felt slightly warmer than in previous weeks along the coast, but the reason it has also felt muggy is largely to do with the added moisture plumes that have nudged the coast. The latest hi-res/short-range solutions pick up on some of the weak moisture signatures for Wednesday evening/Thursday morning just off the coast as well. All of this is to say that solutions are picking up on the influence of the near-surface thermal low in the Central Valley. Because of this feature, air along the southern California Coast has the potential to rapidly build northward. For those who have read these discussions in depth over the years, you know that this means that the potential will be there for a southerly surge to develop. The latest GFS deterministic solution hints at a surge developing late Wednesday night and into Thursday morning. This surge would bring much cooler air northward in response to the VERY hot air mass over the Central Valley. As a result, are looking at the potential for some marine stratus to redevelop and build into the coast during the day on Thursday, injecting cooler air past the Golden Gate and into the Delta. There is also the potential for some high-based cloud cover to roll in as the surge moves northward during the day on Thursday, which could limit some of the diurnal heating during the day across the interior and affect some of our forecast afternoon highs. Should the surge not make it north before Thursday afternoon, will easily reach the upper 90s to low 100s F across the interior, with some of our hottest locations approaching 110 F. Expect sharp 30-35 temperature within just a few mere miles of separation between the coast and the near-shoreline (e.g.Oakland vs Oakland hills, Santa Cruz vs Santa Cruz Mountains, et cetera). Despite the timing of the potential surge and the influence from some of the potential high- based clouds during the day, will still observe sharp temperature gradients during the day on Thursday. The poor overnight temperature recoveries and continuation of hot temperatures on Thursday have prompted the excessive heat warning for much of the interior, while the Santa Cruz Mountains and portions of the interior South Bay, San Benito, and Monterey Counties get added on to a heat advisory. For the latest on our heat products (advisories/warnings) be sure to go to our website and/or social media.

GFS, Canadian, and EC ensembles are all indicating a continuation of onshore flow and a weakening mid-level ridge starting Friday, signaling the beginning of a welcomed cooling trend. Nonetheless, the lag between the weakening ridge and the thermal low at the surface will result in a continuation of moderate to high heat risk across the interior through much of Friday, with widespread 90s to low 100s F still forecast for the interior. Thus, the Heat Advisory and Excessive Heat Warning will remain in effect through Friday evening. Temperatures look to gradually cool this upcoming weekend as onshore flow spreads further inland and the ridge aloft weakens. Cooling will continue through the weekend, with interior temps gradually returning to seasonal (70s and 80s F) by early next week.


As of 5:13 PM PDT Tuesday, For the 00z TAFs. High pressure has brought dry air and clear skies across the region. The only clouds observed on satellite are over SoCal. These will bring Few-Sct high clouds to our region overnight. Low clouds may develop along the coast overnight but should not impact most terminals, aside from possibly KHAF and KMRY, where patchy Cigs may form. Otherwise, VFR conditions prevail through the TAF period. Breezy and gusty onshore winds will continue through the evening, especially near KSFO. Lighter winds expected tomorrow, with onshore breezes near the coast.

Vicinity of KSFO, VFR through the TAF period with a few low clouds possible around the bay overnight. WNW Winds have been increasing through the afternoon, and are currently sustained around 25 kt with gusts near 30 kt. May see gusts up to 35 kt through the evening, then subsiding gradually overnight. Lighter winds expected tomorrow afternoon, though still breezy in the afternoon.

SFO Bridge Approach, Similar to KSFO.

Monterey Bay Terminals, VFR prevailing through the TAF period. Possible low clouds developing overnight around the peninsula but low confidence in cigs near KMRY. Breezy NW winds will weaken overnight and resume Wednesday afternoon.


As of 09:06 PM PDT Tuesday, Gusty northwest winds continue overnight across the waters with gale force gusts along the Big Sur coast south of Point Sur. Near gale force gusts are expected across the outer waters. These winds will generate steep northwest waves resulting in hazardous seas conditions, particularly for smaller vessels. Winds will ease slightly on Wednesday but remain strong to moderately breezy over the northern and outer waters and along the Big Sur coast most of the week. Seas remain mainly wind driven with a lighter southerly swell.

Watches, Warnings, Advisories

Tonight, sca, pt reyes to pigeon pt 0-10 nm sca, pigeon pt to pt pinos 0-10 nm sca, pt arena to pt reyes 0-10 nm sca, pt arena to pigeon pt 10-60 nm sca, pigeon pt to pt piedras blancas 10-60 nm glw, pt pinos to pt piedras blancas 0-10 nm until 3 am sca, mry bay sca, SF Bay until 3 am

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