Marine Weather and Tide Forecast for Colma, CA

SHORT TERM
Issued at 1214 AM PDT Tue Apr 21 2026 (Today and tonight)

As of midnight tonight, the main frontal rain band continues to make its way into the interior regions of Monterey and San benito Counties, with the steady rainfall extending across the majority of the San Francisco Bay Area, the Monterey Bay region, and the Santa Lucia range. The back edge of the steady rainfall has passed through the western half of Sonoma County and the Point Reyes area, behind which some scattered showers have developed offshore. The steady rain band should continue to pass through overnight, and should move east of the SF Bay Area and Monterey Bay regions by the morning commute. Low temperatures tonight range from the middle 40s to the middle 50s, while today's highs range from the upper 50s to the middle 60s in the lower elevations, down to the middle 40s to lower 50s in the higher elevations.

The main forecast issue today is the chance for convective activity across the Bay Area and Central Coast. To discuss this, let's take a look at the ingredients for convection, namely instability, moisture, and lift. At the 200-250 mb level (around 35000-40000 feet above sea level), the polar and subtropical jet streams are phasing, meaning that they are aligned so that their troughs are roughly placed along the same line of longitude. This enhances the upper- level low and enhances its cold pool, and this system will pass over the region, while scattering skies behind the frontal rain band will enhance surface warming, both effects destabilizing the atmosphere for the afternoon and evening hours. Most unstable CAPE values from the models are peaking around 500 J/kg, near the 99th percentile value for pretty much any time of year in the long-term records for the OAK radiosondes. Some moisture will continue to be advected into the region through the morning on the back side of the front, but most of it will be left behind from the passing of the front earlier in the day. That leaves lift, and there are a couple of sources for lift across the region. First, and most intuitively, the wind can force air to condense and form clouds or storms when it flows across our mountain ranges (we call this "orographic" lift). The second source requires a fair bit of explanation. Back at the 200-250 mb level we see a jet streak, or some hints of one, passing over the state around the upper level low. Some of the model output hints that we may very well see the left exit region of the jet streak, where due to the dynamics of the atmospheric flow, divergent flow aloft will induce convergent flow at the surface, thus promoting atmospheric lift across the region in question.

Chances for thunderstorms top out around 30 to 40% across the region late this morning into the early evening, while K-Index values of around 25-28 degrees Celsius suggest a possibility for isolated to widely scattered thunderstorms. If thunderstorms do develop, lightning, locally heavy rainfall, and small hail are the primary threats. Not expecting waterspouts or weak tornadoes with this system. Model output values for the 0-6 km shear look impressive, but most of that shear is confined to the 3-6 km layer, whereas you would want higher shear between 0-3 km to note a higher risk of rotating cells. We will keep monitoring the forecast evolution through the day. The SPC continues to mention a general risk of thunderstorms across the region, but is likewise backing off of any severe thunderstorm chances.

LONG TERM
Issued at 1214 AM PDT Tue Apr 21 2026 (Wednesday through Monday)

Some lingering showers may hang on through Wednesday afternoon as a shortwave trails behind the upper level trough as it departs into the Intermountain West. As the frontal system passes, temperatures will warm slightly into a rather dry latter part of the week, close to or slightly below the seasonal averages as highs in the lower elevations range from the lower 60s to lower 70s. Ensemble model cluster analysis points to continued troughing across the state which will help modulate the warming trend through the 7-day outlook. On Saturday, another passing trough will bring a chance for a passing shower to the region, and although the ensemble members continue to show more scattered precipitation total forecasts, most members continue to show some rainfall coming into the region. Model output is even putting rain chances in the early part of next week and extending beyond that, but it is still too early to say anything definitive at this point in the forecast.

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 from 9 AM this morning to 9 PM PDT this evening for Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm.

Small Craft Advisory from 9 AM this morning to 3 PM PDT this afternoon for Pt Pinos to Pt Piedras Blancas 0-10 nm.