Marine Weather and Tide Forecast for San Rafael, CA

SHORT TERM
Issued at 725 AM PDT Tue Apr 21 2026 (This afternoon through Wednesday)

We have pretty good thunderstorms chances today. The surface cold front has passed through the cwa, ending the continuous rain we had last night. Our attention now shifts to the post-frontal environment. Scattered showers are expected through the day with a a 30-40% chance of thunderstorms around midday. The 12Z sounding featured high shear, but low instability. Specifically the 0-6 km shear was measured at 82 kts, but the surface based CAPE was only 12 J/kg. While that wouldn't support anything other than gusty winds, the best environment is not here yet. Cold fronts tend to stack back (westward) with height, meaning while the surface front has moved through, cold air aloft is still filtering in. The 500 mb temperature was measured at -21C on the 12Z sounding, but is expected to drop to -26C by 18Z. That rapid cooling of the upper levels changes the instability profile drastically. The latest HRRR model sounding for Watsonville brings the CAPE up to 713 J/kg at 17Z (10 AM). This instability is followed by a post-frontal trough that will serve as a trigger to release the potential energy as upward vertical motion. We are already starting to see this main band of thunderstorms develop roughly 130 miles SW of Big Sur, moving NE at around 20-25 mph.

While the instability is rapidly increasing, the shear is decreasing. The same model sounding for Watsonville only has 8 kts of 0-1 km shear, and the 0-6 km shear drops from 78 kts at 12Z to 43 kts by 17Z. So this morning will be a transition from low CAPE, high shear, to high CAPE, low shear. While there may be a sweet spot in the interim, by late morning it looks like any thunderstorms will be the garden variety. We are not expecting any severe criteria including tornadoes, damaging wind, or large hail. The biggest hazards will be lightning strikes and heavy downpours. These storms will also bring small hail and gusty, erratic winds.

As far as timing goes, after scattered showers this morning, high resolution models are highlighting the 11AM - 3PM window for the main band of showers and thunderstorms to pass through, with more isolated coverage through the rest of the day. Coverage looks pretty widespread across the cwa, with the best chances anywhere south of the Golden Gate.

LONG TERM
Issued at 725 AM PDT Tue Apr 21 2026 (Wednesday night through next Monday)

The drying trend will continue into Wednesday, although there could still be a lingering shower or two in the morning. By the afternoon it will be a nice, cool, sunny day with highs in the mid 60s. A very weak ridge will build on Thursday and Friday increasing the temperature a bit, but this will quickly be overtaken by another trough moving in from the Eastern Pacific.
This next feature brings a slight chance of light rain through the weekend, but it won't be anything like what we're going through yesterday and today. The disturbed weather and slight rain chances will continue through at least the first half of next week.

AVIATION
(12Z TAFS)
Issued at 421 AM PDT Tue Apr 21 2026

Multiple rounds of light to moderate showers will continue to move across our area through the morning hours into the afternoon.
There will be breaks at individual terminals at times between rounds. North Bay terminals should be the first to see widespread showers come to an end, especially KSTS by mid-afternoon. KAPC may be far enough south to see shower activity extend into late afternoon. There continues to be a 30-40% chance of thunderstorms after 8AM this morning through the afternoon hours. Expect gusty erratic winds and moderate to heavy rain with the strongest showers/thunderstorms.

Vicinity of SFO.. MVFR to VFR is expected to prevail through the TAF period with multiple rounds of scattered showers continuing through the morning and afternoon, with some breaks in shower activity at times. There is a chance of isolated thunderstorms as early as 8AM this morning through late morning into the afternoon.
Expect gusty erratic winds and moderate to heavy rain with the strongest showers/thunderstorms.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR is expected to prevail with periods of MVFR as several rounds of showers move across the peninsula this morning and afternoon. There's a chance of isolated thunderstorms late this morning into the afternoon, expect gusty and erratic winds and moderate to heavy rainfall with the strongest showers/thunderstorms. MVFR ceilings are possible later this evening into the overnight hours.

MARINE
(Today through Sunday)
Issued at 421 AM PDT Tue Apr 21 2026

Moderate to fresh westerly breeze will develop today. Scattered light showers will increase in coverage later this morning into the afternoon. Some of these showers will be strong with a 20-30% chance of thunderstorms. Once the weather breaks by late Wednesday afternoon, winds will become moderate out of the northwest, then increase to strong out of the northwest on Thursday.

MTR WATCHES/WARNINGS/ADVISORIES
CA...None.
PZ...Small Craft Advisory until 9 PM PDT this evening for Mry Bay- Pigeon Pt to Pt Pinos 0-10 nm.

Small Craft Advisory until 3 PM PDT this afternoon for Pt Pinos to Pt Piedras Blancas 0-10 nm.