Marine Weather and Tide Forecast for Portola Valley, CA

UPDATE
Issued at 1002 AM PDT Sat Oct 17 2026

The forecast largely remains on track this morning with no updates anticipated at this time. That said, KMRY (Monterey Regional Airport)
is already 84 deg F as weak downslope winds resulted in rapid warming this morning. Thus, coastal locations may very well be warmer than currently forecast before onshore winds return by late morning and into the afternoon.

RGass

SHORT TERM
Issued at 1053 PM PDT Fri Oct 16 2026 (Tonight through Sunday)

Clear skies continue this evening, but the offshore winds have calmed substantially. The SFO-WMC gradient peaked at -13.7 mb Friday morning and has since weakened to -7.8 mb. 24 hours ago Mt.
St. Helena was reporting NNE winds of 44 mph. Now it's NW at 5.
The 500 mb ridge over northern California is weakening. The 00Z balloon measured the 500 mb height at 5910m, down from 5930m at 12Z. That's still above the 90th percentile for this time of year, but the trend will accelerate through the day Saturday as the Channel Islands cut-off low gets absorbed back into the jet stream, bringing more zonal flow to the upper levels.

At the surface this means the return of onshore winds and the start of a cooling trend. Max temperatures Saturday are expected to be a couple degrees cooler than Friday. It will still be a warm, sunny day for everyone. The bigger change is Sunday. A new deep low pressure system approaches from the west. This system will bring back the marine layer and drop the max temperatures by another 10-15 degrees. It will also bring another round of wind, but from the west this time. Our current forecast shows gusts up to 50 mph through Altamont Pass Sunday night. That's the outlier.
Most places will have gusts in the 15-25 mph range.

LONG TERM
Issued at 1053 PM PDT Fri Oct 16 2026 (Sunday night through next Friday)

A brief period of zonal flow aloft gets interrupted on Monday as an upper-level trough pushes southward along the California coast.
Ahead of the low pressure system, onshore winds increase across the Bay Area and Central Coast with a deepening marine layer, resulting in cooling to near seasonal temperatures. The trough becomes a cut- off low by Tuesday and lingers off the Central Californian coast before gradually propagating inland through Southern California Wednesday evening into Thursday. The system's position allows for the reinforcement of cold air advection across the Bay Area and Monterey Bay regions through the end of next week.

In terms of precipitation chances, light rain showers are possible Tuesday through Thursday across the Bay Area and Monterey Bay regions. The coastline and southernmost extent of the Central Coast have a 40-60% probability of light rain meanwhile the North, East, and South Bay have a 20-40% chance. In addition to rain, there is potential for thunderstorm development during the aforementioned time period. The three main ingredients required for thunderstorm development are instability, lift, and moisture. Given the complex terrain within the region, we can guarantee some degree of orographic lifting as the system moves inland. Both the European and GFS ensemble members are hinting at precipitable water values around the 75th percentile for this time of year, so we can also check moisture off our thunderstorm ingredient list. Finally, we have instability. European ensemble members are exhibiting 500 mb temperatures around -15 to -20 degrees Celsius, which falls below the 10th percentile for this time of year. Meanwhile, the 850 mb temperatures are around the seasonal normal of 9 to 12 degrees Celsius. Atmospheric instability may arise from the mixing of air masses, particularly when a cold air mass sits on top of a warm air mass. Since cold air is more dense than warm air, it will sink to the surface while forcing the ascent of warm air. As warm air rises, it cools adiabatically and condenses, resulting in latent heat release, which further warms the surrounding air allowing for increased vertical mixing. This process is dependent on other thermodynamic factors, however the structure of our cut-off low supports an unstable system due to vertical mixing.

Despite all of the above, there is still high uncertainty regarding the rainfall and thunderstorm potential as it is dependent on the exact trajectory of the cut-off low and the alignment of various elements. Be sure to check back on the forecast for updated information.

AVIATION
(18Z TAFS)
Issued at 1002 AM PDT Sat Oct 17 2026

Currently VFR. Moderate confidence on LIFR conditions returning to the coastal terminals of HAF, MRY, and SNS tonight. The atmosphere will need to undergo a lot of change over the next 18 hours for this to happen coming off of hot and dry conditions with no marine layer. Upstream observations show some stratus and fog, but a pattern change with an increase in wind and mixing will likely make it more difficult for the marine layer to reform.
Diurnal winds will prevail. Haze will reduce slant range visibilities.

Vicinity of SFO...Currently VFR with northerly flow. High confidence in VFR through the TAF period. Northerly flow will back through the morning to become westerly with the sea breeze this afternoon.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently VFR with southwesterly flow at MRY and VFR with southeasterly flow at SNS. Moderate confidence on LIFR conditions returning to the terminals tonight. Diurnal winds will prevail.

MARINE
(Today through Thursday)
Issued at 1002 AM PDT Sat Oct 17 2026

Moderate to fresh northwesterly breezes will prevail through Monday with strong breezes across the northern outer waters.
Widespread hazardous conditions will spread across the coastal waters as a result Sunday afternoon, likely continuing into Monday. Moderate seas will prevail across the inner waters with rough seas building across the outer waters Saturday through Monday, rebuilding again Tuesday. Light rain is expected Monday night through Wednesday.

FIRE WEATHER
Issued at 1053 PM PDT Fri Oct 16 2026

While the offshore winds have calmed substantially, hot and dry conditions will continue through Saturday. Max temperatures will be in the mid 80s to low 90s with RH dropping into the the teens for nearly all areas away from the immediate coast. A significant cool-down arrives Sunday through Monday with light rain possible mid-week.

KR

BEACHES
Issued at 1002 AM PDT Sat Oct 17 2026

Hazardous beach conditions will be present at Pacific Coast beaches through Sunday and likely into next week. Long period southerly/southwesterly swell will bring an increased risk for sneaker waves and strong rip currents with breaking waves of 7 to 12 feet. Remain off jetties, piers, rocks, and other waterside infrastructure as waves can sweep across the shoreline without warning, pulling people into the sea. Remain out of the water to avoid hazardous surf with strong rip currents and never turn your back on the ocean!

Sarment

MTR WATCHES/WARNINGS/ADVISORIES
CA...Beach Hazards Statement through Sunday evening for CAZ006-505- 509-529-530.

PZ...Small Craft Advisory from 3 AM to 9 PM PDT Sunday for Pt Arena to Pt Reyes 0-10 nm.

Small Craft Advisory from 3 PM to 9 PM PDT Sunday for Pigeon Pt to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.

Small Craft Advisory until 9 PM PDT Sunday for Pt Arena to Pt Reyes 10-60 NM.