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

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

(06Z TAFS) Issued at 956 PM PDT Thu Jul 16 2026

VFR with stratus developing offshore. HAF is currently the only site where stratus has redeveloped but stratus is expected to expand and move inland overnight. Pushed the timing of stratus arrival to MRY/SNS back slightly as stratus is developing more slowly than models initially suggested. Confidence is highest that stratus will reach HAF, MRY, and SNS overnight with diminishing confidence that stratus will reach STS, APC, OAK, or SFO. For now, left stratus in the TAF for those airports with any potential development occurring much later tonight. Initial model guidance suggests an early return of stratus along the coast tomorrow night. Otherwise, diurnally breezy winds continue but winds should be slightly weaker during the afternoon/evening tomorrow than they were today.

Vicinity of SFO...VFR with low confidence that MVFR-IFR conditions will develop overnight. Stratus is attempting to redevelop along the coast but has not managed to push into the SF Bay yet. Model guidance has overestimated stratus development this evening which is decreasing confidence in the extent of stratus coverage overnight. The HRRR continues to indicate SFO and the SF Bay could remain stratus free tonight while OAK sees periods of overcast conditions. For now, left MVFR CIGs in the TAF but confidence is low.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR becoming IFR-LIFR overnight. Stratus is arriving later than initially expected so have pushed arrival times back slightly. Current thinking is that stratus will initially be IFR before lowering to LIFR during the early morning hours. There is some potential for decreases in visibility and fog after 12Z. Confidence is moderate that SNS will clear during the afternoon while MRY's potential clearing is questionable. Model guidance shows stratus lingering along the coast for much of the day which may keep overcast conditions at MRY through the entire TAF period. Kept the MRY TAF slightly more optimistic with a brief period of afternoon clearing before an early evening stratus return but this may need to be amended.

MARINE

(Tonight through next Wednesday) Issued at 956 PM PDT Thu Jul 16 2026

Moderate to strong northwesterly winds gradually diminish through the day on Friday. Moderate to rough seas and hazardous conditions for small crafts persist through Friday across the coastal waters. Conditions improve Saturday as seas abate and northwesterly breezes diminish.

MTR WATCHES/WARNINGS/ADVISORIES

CA...None. PZ...Small Craft Advisory from 3 PM this afternoon to 9 PM PDT this evening for Mry Bay.

Small Craft Advisory until 9 PM PDT this evening for Pigeon Pt to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm.

Small Craft Advisory until 9 AM PDT this morning for Pt Reyes to Pigeon Pt 0-10 nm.

Small Craft Advisory until 3 AM PDT Saturday for Pt Arena to Pt Reyes 10-60 NM.