Marine Weather and Tide Forecast for Portola Valley, CA

SHORT TERM
(Today and tonight)
Issued at 1203 AM PDT Wed Apr 30 2025

The threat for fog returns later this morning. The marine layer is currently around 1,000 feet according to area profilers. With some light dry offshore wind in the North Bay, the best chance for fog is along the San Francisco Peninsula coast and Monterey Bay region, including Big Sur and the Salinas Valley. A few upper level clouds and some wind could limit radiation fog, but even so, the typical foothill locations will almost certainly be dealing with reduced visibility in the marine layer cloud deck. Slightly elevated coastal areas such as Corallitos and Hurricane Point have the best chance.

A weak short wave trough is moving from north to south across California today. This system will bring slightly colder air to the upper levels while the surface temperature remains warm. With relatively high moisture (PW around 0.8") and conditionally unstable lapse rates, there is a chance for some isolated convection today. In fact the HRRR, NAM, and ECMWF deterministic model soundings for Santa Rosa all agree that the MUCAPE will exceed 1,000 J/kg this afternoon. On the other hand, some weak offshore winds will keep the low-level moisture suppressed and there will be a bit of convective inhibition below the LCL. Most likely if this convection is realized, it will only create some puffy cumulus clouds. There is very little support in the guidance for thunderstorms, or even rain, but there is still a non-zero chance based on the instability and relatively high integrated moisture.

LONG TERM
(Thursday through Tuesday)
Issued at 1203 AM PDT Wed Apr 30 2025

By Thursday the weak disturbance and associated instability will move into southern California. This will allow the ridge to rebuild over northern and central California, bringing us another day of warm inland temperatures and cool marine layer conditions along the coast. Temperatures will start to fall a bit on Friday as a much more robust trough approaches the coast. This system will support a fast moving cold front at the surface late Friday or early Saturday, and could bring some pre-frontal rain. The models have almost universally backed off on the rain chances, but we still have PoPs around 15%. While we expect overcast skies and much cooler temperatures, the biggest story with this system is wind. The trough will spawn a cut-off low that moves into Southern California by Saturday. The gradient between the low and higher pressure over the Eastern Pacific will support 50-70 mph winds at 500 mb. As the atmosphere becomes well mixed Saturday afternoon, some of this momentum will transfer down to the surface where NW winds of 15-30 mph, and gusts as high as 50 mph are expected. The strongest winds will be along the coast and in higher terrain.

By Sunday the low will start to move inland towards the desert Southwest. We will still be dealing with lingering strong winds, but it should be an improvement over Saturday. Early next week a ridge will start to rebuild, decreasing the wind speed and causing the temperatures will gradually recover. By mid-week we should be back to the nice seasonal weather we've had this week.

AVIATION
(06Z TAFS)
Issued at 957 PM PDT Tue Apr 29 2025

The marine layer depth varies from 700 to 1100 feet. Coastal stratus and fog /MVFR-IFR-LIFR/ will continue tonight and Wednesday morning with local inland intrusions of stratus and fog due to onshore winds. Sea surface temps 49F to 54F are running in good contrast to lower level temperatures in the 60s, both Fort Ord and Point Sur show a well developed lower level temperature inversion. Adding complexity to the forecast, a 500 mb trough (with near late April normal height and temp) arrives tonight and Wednesday with a reflection of weak cyclonic circulation developing at 850 mb and 925 mb early Wednesday within the lower level temperature inversion/lower level thermal ridging. Light offshore winds aloft North Bay will sustain the lower level warmth and stability, while onshore winds to the south will slowly erode the lower level warmth and stability, deepening the marine layer but perhaps not fully eroding the lower level warmth Wednesday.

Best chance for afternoon convection will be inland, however there may be enough residual lower level warmth i.e. stability and/or surface cooling depending on extent of inland marine air intrusion limiting surface convective temperature. Based on NAM, HRRR convective parameters and global model QPF and perceptible water there will be potential for a few inland showers/downpours; at this time not expecting any impacts to our forecast area airport terminals.

Vicinity of SFO...VFR, onshore wind through the period. Low confidence MVFR ceiling Wednesday morning, if the marine layer deepens there's a better chance of a stratus ceiling.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...IFR-LIFR due to stratus and fog, however with the development of a weak cyclonic circulation in the lower levels beginning early Wednesday morning, onshore winds through a greater depth as well as weak cooling will deepen the marine layer in this area. May need to adjust TAFs to reflect IFR gradually lifting to MVFR early Wednesday, with subsequent mixing out to VFR likely by late morning or early afternoon.

MARINE
(Tonight through next Monday)
Issued at 952 PM PDT Tue Apr 29 2025

Northerly flow will persist thanks to high pressure anchored to the west. Winds will be locally stronger, especially over the outer waters leading to hazardous conditions for smaller craft through Wednesday. Moderate seas and moderate northwesterly breezes will prevail Thursday and Friday. Strong northwesterly breezes and rough seas return Saturday to the outer waters.

MTR WATCHES/WARNINGS/ADVISORIES
CA...None.
PZ...Small Craft Advisory until 9 AM PDT this morning for Pt Arena to Pt Reyes 0-10 nm.

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

Small Craft Advisory until 3 PM PDT this afternoon for Pigeon Pt to Pt Pinos 10-60 NM.