Marine Weather and Tide Forecast for Portola Valley, CA

UPDATE
Issued at 822 AM PDT Sat May 17 2025

Damp start to the day for portions of the Bay Area, mainly along the coast and coastal mountains. Solid low level moisture through 2500-3000 feet brought drizzle to those areas. One spotter in Carmel Valley even described it as light rain this morning.
Automated rain gages tipped a few hundredths over the last 6 to 12 hours. The low level moisture was captured nicely on both the KOAK sounding and Ft Ord profiler. Other item of note this morning are the winds, especially the East Bay gaps/passes with gusts 40-60 mph. Why so windy? Two words: onshore gradient. The onshore pressure gradient between KSFO and KSAC is currently 4.6 mb. For context, any value over 3 mb is considered decent with values approaching 6 mb is extreme.

Current forecast captures both the drizzle and windy conditions.
No update needed.

For the rest of the day - the onshore gradient is forecast to ease as the N-S gradient increases. As such, gusty westerly winds will becoming more northerly. As noted below, gusts up to 50 mph will be possible. Morning visible satellite is already showing the N-S gradient kicking in as clouds are rapidly thinning over the N Bay.
Expect some afternoon sunshine, but breezy to gusty conditions.

MM



SHORT TERM
(Today and tonight)
Issued at 300 AM PDT Sat May 17 2025

Wind gusts increasing through this morning as a potent upper low swings through and digs into the Great Basin. As of writing, the trough axis is situated just offshore of the NorCal/Oregon border and will continue digging SE today thanks to a robust 130 kt upstream jet. Surface low pressure developing ahead of the trough axis will promote a very strong onshore pressure gradient today, which will help get the gusty conditions going this morning. As the trough axis moves inland, strong winds will be reinforced throughout the column as the jet moves overhead. This will kick off a roughly 30 hour period of robust upper level support and the best window for gusts of up to 50 mph along ridgetops and the immediate coastline. Held off on a Wind Advisory given the lack of spatial extent of expected wind speeds that would meet criteria.
Areas away from the coast can still expect gusts of 35-40 mph.

LONG TERM
(Sunday through Friday)
Issued at 300 AM PDT Sat May 17 2025

These windy conditions persist through the weekend until the upper level support finally moves out of the picture by Monday afternoon. However, with wind direction changing every so slightly more northerly overnight Sunday into Monday morning, we are messaging a brief period of elevated fire weather concerns for interior areas of the North Bay and East Bay. Conditions are not critical, but gusty conditions along with dry daytime RH 15-25% on Monday will certainly lead to some fire weather worries regarding dry grasses. Beyond Monday, temperatures warm above average as ridging dominates the pattern the rest of the week.

AVIATION
(12Z TAFS)
Issued at 424 AM PDT Sat May 17 2025

Currently a mix of IFR-VFR at the terminals. High clouds from a passing upper-level shortwave trough will continue to drift southeastward throughout the day. A dry cold front will sweep across the region from northwest to southeast through this afternoon - this will provide lift for drizzle as well as mix the marine layer out. As such, after VFR is achieved this afternoon (outside of the Monterey Bay terminals), it is expected to prevail. Strong and gusty onshore winds are expected through the TAF period with widespread gusts of 20-30 knots.

Vicinity of SFO...Currently IFR with westerly flow. High confidence in VFR prevailing by this afternoon. Westerly winds will strengthen through the morning with gusts in excess of 35 knots expected this afternoon and evening.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently IFR with southerly flow at MRY and IFR with westerly flow at SNS. Westerly winds will prevail through the TAF period. While HRRR and NBM probabilities suggest that it is likely that sub-VFR ceilings will develop tonight, I am not completely sold, here's why. As the dry cold front moves through it will mix out the marine layer. That in tandem with northerly (drying) winds succeeding it should in theory lead to drier conditions that are not conducive for sub-VFR conditions. I think the limiting factor will be that the Monterey Bay is geographically sheltered from northerly winds that are forecast this afternoon, and thus will maintain a moist northwesterly flow that may be just enough for MVFR ceilings to develop.

MARINE
(Today through Thursday)
Issued at 300 AM PDT Sat May 17 2025

Hazardous marine conditions will be relentless through Wednesday.
Widespread strong to near gale northwesterly breezes with gale force gusts are expected today. Winds will become northerly tonight, remaining strong to near gale with gale force gusts through Wednesday. Rough to very rough seas are expected through Wednesday.

MTR WATCHES/WARNINGS/ADVISORIES
CA...None.
PZ...Small Craft Advisory until 3 AM PDT Sunday for Mry Bay-SF Bay N of Bay Bridge.

Gale Warning until 9 PM PDT Sunday for Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM-Pt Reyes to Pigeon Pt 0-10 nm.

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

Gale Warning from 3 PM to 9 PM PDT Sunday for Pigeon Pt to Pt Pinos 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm.

Small Craft Advisory until 9 PM PDT this evening for Pigeon Pt to Pt Pinos 10-60 NM.

Gale Warning from 9 PM this evening to 9 PM PDT Sunday for Pigeon Pt to Pt Pinos 10-60 NM.