Marine Weather and Tide Forecast for Castro Valley, CA

UPDATE
Issued at 950 AM PDT Tue Aug 18 2026

Water vapor imagery continues to show a trough of low pressure over the Eastern Pacific, with multiple areas of low pressure rotating around this trough. One of this areas of low pressure is retrograding a bit, which is allowing the high pressure system over the Desert Southwest to expand more over California today.
That said, the upper level flow over California remains out of the southwest, and we continue to see high clouds streaming through this flow into Central California. These clouds are not going to limit heating today, and given the building ridge, expect inland areas to warm 3 to 5 degrees today over yesterday.

Meanwhile, did once again see a good marine layer push this morning, despite it being slightly shallower, around 1500 feet. We also some drizzle along the immediate coast in San Mateo and Monterey County this morning, with 0.01 inch reported. Given the marine layer being slightly shallower, it is clearing more quickly this morning. However, expect it to once again to linger along the coast today, with some areas struggling to see the sun.
Therefore, expect coastal areas to remain at or below normal for this time of year, while the inland areas see the warmer conditions.

One other note is that we did see significant nocturnal drying in the higher elevations last night. In fact, we had widespread fair to moderate with areas of poor recoveries, especially down in the Santa Lucia, where several sites struggled to recover to 30 percent. Expect these overnight recovery conditions to continue to worsen tonight, as the ridge takes hold, and the marine layer compresses even more. Palmer

SHORT TERM
Issued at 1145 PM PDT Mon Aug 17 2026 (Today and tonight)

Mid/upper level clouds continue to stream from southwest to northeast across our area with mild temperatures expected once again overnight into early Tuesday morning. The mild start to the day will help bump temperatures at least a few degrees (away from the marine layer) higher from Mondays values as the influence from upper level ridging over the Four Corners nudges west. H85 temperatures will be around 23-24C by peak heating which is in the 90th percentile for this time of year. With that, the warmest temperatures will be at higher elevations, especially the East Bay Hills and Santa Lucias, including higher portions of the Timber Fire. Fortunately, light winds continue to be a mitigating factor to more rapid spreading of the fire.

LONG TERM
Issued at 1145 PM PDT Mon Aug 17 2026 (Wednesday through Monday)

Warmer temperatures persist Wednesday under the influence of the aforementioned ridging aloft, however the troughing over the EPAC pushes back east and slightly north by Thursday with a sharpening trough axis well offshore. The cool down for Thursday through the remainder of the extended forecast will be subtle and gradual.
There are at least two tropical waves that will pass well to our south, the first on Thursday followed by a more organized wave with tropical development possible by the end of the weekend into the beginning of next week. We'll be keeping an eye on any midlevel moisture advection north from either of the two waves.

AVIATION
(18Z TAFS)
Issued at 1049 AM PDT Tue Aug 18 2026

Seeing typical retreat of stratus to the coastline this morning with most interior sites returning to VFR through the afternoon.
Onshore winds are forecast to increase late this morning and persist through the early evening before diminishing after sunset.
Going with a persistence forecast for this evening and into Wednesday morning. Thus, moderate to high confidence for IFR to LIFR conditions to return to the coast by mid-to-late evening and then spreading across the Bay Area terminals late evening into early Wednesday morning. Expecting stratus to retreat to the coastline by mid-to-late Wednesday morning.

Vicinity of SFO.. MVFR. Stratus is forecast to clear by between 18Z- 19Z and give way to VFR conditions through late evening. In addition, moderate to fresh west-northwestery breeze is forecast this afternoon and early evening. High confidence for MVFR and moderate to high confidence for IFR returning late this evening and persisting through late morning on Wednesday.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Mix of LIFR to IFR conditions currently.
VFR conditions briefly return this afternoon before IFR to LIFR conditions redevelop early this evening into Wednesday morning.
Gentle breeze is forecast this afternoon before diminishing after sunset.

MARINE
(Today through Sunday)
Issued at 950 AM PDT Tue Aug 18 2026

A fresh to strong northwest breeze will develop across the outer waters through the day and extend into the inner waters tonight into Wednesday. Winds then remain elevated through the end of the forecast period. Locally stronger winds are possible in the coastal jet regions north of Point Reyes and along the Big Sur coast. Seas build to 7 to 9 feet Tuesday and Wednesday before subsiding late week.

MTR WATCHES/WARNINGS/ADVISORIES
CA...None.
PZ...Small Craft Advisory until 3 PM PDT Wednesday for Pt Arena to Pt Reyes 10-60 NM.