textproduct: San Francisco Bay Area

KEY MESSAGES

Updated at 1226 PM PDT Thu Jul 16 2026

- The marine layer returns today

- Cooler and more humid conditions through the weekend

UPDATE

Issued at 913 AM PDT Fri Jul 17 2026

As mentioned in the short term discussion below, the marine layer is back at around 1,200 feet in depth. As such, low level clouds reached deep into the Salinas Valley, the North Bay Valleys, and through the Golden Gate into San Francisco and Oakland early this morning. These clouds are retreating back to the coast and will result in sunny skies this afternoon across the interior. No updates to the ongoing forecast are needed at this time.

RGass

SHORT TERM

Issued at 1250 AM PDT Fri Jul 17 2026 (Today and tonight)

The marine layer is back! That doesn't necessarily mean it's filled with clouds yet, but the latest weather balloon confirmed a 1,200 feet deep layer of cool, moist air below a strong temperature inversion and much drier air aloft. In fact if you were to hike Loma Prieta right now, the trail head temperature and relative humidity is 55F/95%, while the summit is 71F/11%. Stratus is forming in this shallow cool layer, currently extending from San Francisco to the Santa Cruz county line, with separate patches over Marin, the East Bay and the Monterey Peninsula. This coverage will expand through the pre-dawn hours with good radiational cooling and moisture transport from the ocean.

Buoy observations can tell us a lot about marine layer stratus and fog potential. We're looking for 4 indicators. First we want to see the air temperature slightly higher than the SST. This indicates a stable inversion as there is a huge source of cooling available for the air temperature to drop towards the SST. Second is for the dew point to be higher than the SST. This one might not seem as obvious as the other factors, so let's linger on it for a minute. The dew point is one way to measure the amount of water vapor (H2O in gas form) in the air, and specifically tells us how cold the temperature has to drop for condensation (liquid H2O, dew, fog, clouds) to form. At night the air temperature is almost always going to trend towards the SST over the water. So if the SST is cooler than the dew point, the air temperature will drop towards the SST, hitting the dew point on the way down. The only way to continue cooling below the dew point is to first lower the dew point via condensation. The third indicator is a small dew point depression. That's simply the air temperature minus the dew point. If they are within 2 or 3 degrees, it doesn't take much cooling for the air to become saturated. The final indicator is moderate wind speeds. We want enough to advect the air over the cooler water, but not too much to mix in the drier air above the marine layer.

So let's look at the buoys using our 4 indicators:

1) Air temp > SST

2) Dew point > SST

3) Dew point depression < 3F

4) Winds 10-25 kts

Monterey (46042) is reporting an air temp of 57.2F, SST of 54.7F, dew point of 54.1F, and sustained winds of 20 knots. So that hits indicator 1 and 4, but not quite 2 or 3. That's still pretty good since the dew point is only 1/2 degree below the SST and the dew point depression is just outside the threshold at 3.1 F. Let' s look at another example. Half Moon Bay (46012) has an air temp of 57.4F, dew point of 54.5F, SST of 57.7F, and winds of 15 knots. With a higher SST, that only hits indicator 3 and 4. Based on these observations the stratus deck should continue to expand, but it will take some time.

With the reforming marine layer, temperatures will be noticeably cooler today than recent memory. The coast will be stuck in the 60s. Inland and high elevations area will remain warm and dry, but they too will get some relief from the cooler onshore wind.

LONG TERM

Issued at 1250 AM PDT Fri Jul 17 2026 (Saturday through Thursday)

The pattern is now pretty stable. At 500 mb there is a trough off the coast and a strong ridge over the central US. There is really no indication that will change anytime soon. Everything will be pretty standard for this time of year with marine layer clouds, onshore winds, and near normal temperatures. The one interesting part of the forecast continues to be Tropical Storm Elida. This system will soon become a hurricane in the Eastern Pacific and move NNE into open water through the weekend. While it will certainly fall apart after hitting the much cooler waters north of 25N, the residual moisture is very likely to get absorbed in our persistent mid-level SW flow and transported over the Bay Area. Expect to see some mid to high level clouds Sunday - Tuesday. Despite the moisture, it still doesn't look favorable for any thunderstorm activity. The GFS sounding over SFO shows a max 700-500 mb lapse rate of 7.2 and 0 MUCAPE. Even with the moisture, that's not going to cut it for convection. These will more likely be harmless stratiform clouds that make for a colorful sunset. We would need an upper level cold system to move in from the NW at the same time as this moisture arrived to trigger a major thunderstorm event. This won't be the last storm of the season, though. El Nino has a strong positive impact of Eastern Pacific hurricane activity and models suggest there will be a conga line of tropical cyclones over the next couple weeks.

AVIATION

(12Z TAFS) Issued at 424 AM PDT Fri Jul 17 2026

Currently a mixed bag of IFR-VFR. Patchy stratus continues to expand with low confidence on it reaching the interior terminals of LVK, SJC, and STS this morning. The 1,200 foot deep marine layer will deepen tonight to near 1,500 feet, allowing for the inland intrusion of stratus. Moderate to high confidence on all terminals getting an MVFR ceiling tonight. Coastal terminals and STS may be locally lower at IFR. Diurnal winds will prevail.

Vicinity of SFO...Currently MVFR with westerly flow. Conditions will begin to improve by 16Z with high confidence in VFR by the afternoon. High confidence on the return of the MVFR ceiling tonight.

SFO Bridge Approach...Low clouds below FL045 will remain confined to OAK and SFO and northward this morning. Tonight, a deeper marine layer will allow the San Mateo Bridge Approach to fill in.

Monterey Bay Terminals...Currently IFR with southwesterly flow at MRY and IFR with northwesterly flow at SNS. Stratus will clear out late morning (SNS) to early afternoon (MRY) and remain close to the coast today. High confidence in ceilings on the cusp of IFR/MVFR returning to the terminals tonight.

MARINE

(Today through Wednesday) Issued at 913 AM PDT Fri Jul 17 2026

Moderate northwesterly breezes will ease through this afternoon and evening. Small craft advisory conditions will gradually taper off this afternoon and evening across most areas with exception to the waters north of Point Reyes and along the immediate coast south of Point Pinos. Moderate to rough seas will persist through today. Conditions improve Saturday as seas abate and northwesterly winds diminish.

MTR WATCHES/WARNINGS/ADVISORIES

CA...None. PZ...Small Craft Advisory until 9 PM PDT this evening for Pt Arena to Pt Reyes 0-10 nm.

Small Craft Advisory until 3 AM PDT Saturday for Pt Arena to Pt Reyes 10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm.

Small Craft Advisory until 3 PM PDT this afternoon for Pigeon Pt to Pt Pinos 10-60 NM.