Marine Weather and Tide Forecast for Piedmont, CA

UPDATE
Issued at 911 AM PDT Mon May 19 2025

Welcome to another work week. Not a bad way to start, sunny skies and warmer temperatures. Webcams and satellite imagery both show clear skies over the region with a few clouds over the outer coastal waters. One potential spoiler for sensible weather today will be the winds. If you've been thinking, 'It sure has been windy lately,' you're right-it has been breezy to gusty since last week. Today will be no different with light to moderate winds this morning, but at the northerly pressure gradient (SFO-ACV)
tightens this afternoon winds ramp up as well.

No update for the morning.

Key Messages for the week -Windy off and on -Elevated fire weather concerns due to winds -Warm up through midweek and then a cooldown by next weekend

MM

SHORT TERM
(Today and tonight)
Issued at 154 AM PDT Mon May 19 2025

High pressure over the Eastern Pacific Ocean and a reinforcing upper- level trough over the Four Corners will be the driver of our weather today. High pressure in the Pacific Northwest and low pressure in the Desert Southwest will continue to support a surface pressure gradient that is conducive for northerly (drying) flow as pressure flows from high to low and wind follows suit in the isobars. Wind and subsequently elevated fire weather conditions from wind and relative humidity are the focus of the short term forecast. Wind gusts of 40-50 mph can be expected along the immediate coastline, over the waters, and in the higher terrain with 25-35 mph expected elsewhere. Elevated fire weather conditions are for dry fuels (primarily grasses and other finer/flashier fuels) alongside low relative humidity and strong and gusty winds that will help spread a fire if one were to develop.

LONG TERM
(Tuesday through Sunday)
Issued at 154 AM PDT Mon May 19 2025

As the surface pressure gradient relaxes Tuesday, northwesterly flow will return, resulting in better overnight relative humidity recovery and cooler conditions. Wednesday will be the warmest day of the week with temperatures up to 10 degrees above normal away from the immediate coast. This will translate to widespread minor HeatRisk away from the immediate coast and moderate HeatRisk in the East Bay Interior Valleys. Global ensemble clusters dictate that it is likely that heights will fall Thursday and that an upper-level ridge is somewhere across the West by Sunday. While there is some uncertainty in the longwave pattern Thursday through Saturday, no impactful weather or rainfall is expected through the long term forecast. Oakland Museum (OAMC1) and San Francisco Downtown (SFOC1) will put their daily minimum temperature records in jeopardy this week. Oakland Museum (OAMC1): Wednesday (5/21), Saturday (5/24), and Sunday (5/25) have previous records of 49 degrees set in 1986, 2011, and 1980 respectively, the official forecast is 51, 52, and 51 degrees respectively. San Francisco Downtown (SFOC1): Friday (5/23) has a previous record of 47 degrees set in 2010, the official forecast is 50 degrees.

AVIATION
(18Z TAFS)
Issued at 1059 AM PDT Mon May 19 2025

VFR conditions prevail across almost the entirety of the region, the exception being KHAF where sea spray has kept visibility in the MVFR range. Expect this to continue through much of the period, with stratus off to our west gradually coming closer to the coast. Expect coastal communities likely see stratus begin either late this evening or early in the overnight hours. The sign is favorable for at least MVFR cigs along the coast, including KHAF and the Monterey Bay region, though there is a smaller signal for IFR cigs. We me see stratus seep into through the San Bruno Gap, which could make it to KOAK and KSJC. Opted to hint at that possibility rather than bite at a more pessimistic forecast. Any stratus looks to mix out or retreat by mid to late morning.

Outside of stratus, the other concern will be the increasing gusty northerly to westerly winds depending on the location. These should start to pick up over the next couple of hours, with gusts ranging from 20-30kt. Gusts should gradually taper during the evening hours, with winds falling to around 10kt or less for most sites between 7Z and 12Z.

Vicinity of SFO...Fairly quiet weather this morning so far, with VFR conditions prevailing. West to northwest winds will be increasing over the next few hours, with 30-40kt gusts expected later this afternoon and evening. Confidence was high enough to bump gusts at KSFO up to 35kt, though 20Z might still be a bit too bullish with this earlier start time. An Airport Weather Warning is in place from 23Z to 07Z tonight, which reflects the higher confidence in timing.
May up the gusts in the TAF at the next amendment to reflect 38kt winds given some of the guidance. Winds do look to ease after 7Z.
The question then becomes, the status of the stratus. Will it make it to SFO or perhaps be confined to western side of San Mateo County? Opted to trend with a more optimistic forecast and go with SCT MVFR cigs rather than BKN. This may need to be adjusted if the next round of guidances shows a stronger surge and more favorable pressure gradient from SFO to SAC. Medium confidence in the forecast.

SFO Bridge Approach...Similar to SFO. Webcams around the Bay show some haze, though reporting stations put visibility at P6SM.

Monterey Bay Terminals...Benign weather is currently ongoing over the region, with VFR conditions prevailing. Northwest to west winds will begin increasing over the next few hours, with gusts up to 20- 25kt possible through the early evening. Offshore, stratus looms and will eventually slither its' way towards the coast. This will bring MVFR conditions through the overnight hours, with CIGS likely eroding between 12-16Z.



MARINE
(Today through Saturday)
Issued at 439 AM PDT Mon May 19 2025

Widespread strong northerly breezes with gale force gusts are expected over the inner and outer waters through Tuesday.
Hazardous marine conditions will persist Wednesday through Friday with strong northwesterly breezes with near gale force gusts and rough to very rough seas.

MTR WATCHES/WARNINGS/ADVISORIES
CA...None.
PZ...Small Craft Advisory from 3 PM this afternoon to 3 AM PDT Tuesday for SF Bay N of Bay Bridge.

Small Craft Advisory until 9 PM PDT Tuesday for Mry Bay.

Gale Warning until 9 PM PDT Tuesday for Pigeon Pt to Pt Pinos 0- 10 nm-Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 0- 10 nm-Pt Arena to Pt Reyes 10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.