Marine Weather and Tide Forecast for Piedmont, CA

UPDATE
Issued at 749 PM PDT Thu Apr 17 2025

Satellite imagery shows some increases and decreases in the stratus this evening. Some models show a more aggressive push inland with offshore/northerly winds starting a little later.
This could impact the temperature forecast for tomorrow if it pans out as clouds could linger past mid to late morning.

SHORT TERM
(This evening through Friday)
Issued at 228 PM PDT Thu Apr 17 2025

A fairly deep marine layer allowed for widespread stratus to extend throughout the Bay Area and into the Central Coast. Satellite shows cloud cover dissipating over portions of the North Bay but much of the region remains socked in. The slower dissipation of cloud cover may result in today's high temperatures struggling to reach their forecasted highs. For example, the forecast high for San Jose today is 68 degrees with the airport reporting 63 degrees as of 1PM. This is true of other sites with SFO reporting 56 degrees as of 1PM and a forecast high of 64 degrees or LVK reporting 60 degrees as of 1PM and a forecast high of 68 degrees. The deciding factor will be how long it takes for stratus to clear this afternoon with peak heating generally occurring between 3PM-4PM. Stay tuned for tonight's update to see how temperatures faired.

Upper level troughing will continue to progress eastward with upper level ridging to build over the West Coast tonight into tomorrow. As high pressure builds, the marine layer will start to compress with WRF guidance showing it compressing to 1000-1500 ft overnight. This will limit the inland spread of stratus tonight. This morning, the inland intrusion of stratus brought with it much more moist air which resulted in widespread drizzle to light rain across the coast and elevated terrain where orographic uplift was favored. Drizzle is not expected to be as widespread tonight into tomorrow morning with guidance showing drier conditions offshore and keeping higher PWAT values (0.5"-0.7") generally in the Monterey Bay Region. For now, kept tomorrow mornings drizzle chances confined to the Monterey Bay Region but may need to expand drizzle chances northward along the coastline.

LONG TERM
(Friday night through next Wednesday)
Issued at 228 PM PDT Thu Apr 17 2025

A warming trend is expected to kick off Friday with temperatures becoming more seasonal to slightly above normal across the interior as high pressure builds. Temperatures will warm into the low to mid 70s on Friday and Saturday with temperatures to build into the mid to upper 70s Sunday into Tuesday. As temperatures build, minor HeatRisk will spread across urban areas of the Bay Area and Central Coast Friday through Tuesday. Minor HeatRisk generally only impacts individuals who are extremely sensitive to heat or without adequate cooling/hydration. While temperatures are forecast to peak in the mid to upper 70s, remember to listen to your body and drink plenty of water/take breaks as needed while outdoors as we transition from the cooler seasons to the warmer seasons.

Offshore winds are expected to develop Friday as high pressure builds across the West Coast and upper level troughing deepens over the Intermountain West. Upper level support still looks minimal with the 500mb and 200mb jet streams centered to our east over the California-Nevada border and the 850 mb jet stream centered offshore over the ocean. The lack of upper support will weaken offshore flow with offshore winds to peak between 20-30 mph across the elevated terrain (primarily North Bay Interior Mountains and East Bay Hills). Probabilistic guidance shows a low (<10%) chance for gusts between 30 to 35 mph across far northern Napa and Sonoma Counties on Friday. We will see some warming and drying across the region this week but fire concerns remain minimal with fuels still decently moist from this past winter. Offshore winds will diminish and shift onshore again overnight Friday into Saturday as upper level ridging builds over the West Coast and upper level troughing moves into the Central United States.

Heading into next Wednesday the forecast becomes a little more uncertain. Deterministic ECMWF guidance shows an upper level trough moving into the West Coast mid to late next week. In terms of probabilistic cluster guidance, models are not in agreement as to how deep this trough will be or where it will set up. The current forecast shows temperatures cooling back into the upper 60s to low 70s starting Wednesday, but, this cooling trend is dependent on where the trough sets up and how deep it is.

AVIATION
(06Z TAFS)
Issued at 1033 PM PDT Thu Apr 17 2025

Scattered MVFR stratus remains over parts of the Bay Area with more extensive stratus coverage in the Central Coast. Stratus evolution has been difficult to forecast today, and model indications are that the difficulty will continue into the overnight and morning periods.
The current forecast calls for MVFR stratus to expand across the Bay Area tonight, with IFR-LIFR stratus possible at the immediate coast.
Stratus should dissipate through Friday morning, with low to moderate confidence on the exact time. Along the immediate coast, models disagree on whether there will be periods of clear skies or whether the terminals will be socked in through the TAF period.
Breezy onshore flow will dominate Friday afternoon and evening. A compressing marine layer should result in less extensive, but lower ceiling, stratus Friday night into Saturday morning.

Vicinity of SFO... The terminal should remain VFR through the evening hours and MVFR ceilings will return near midnight. Current forecast has the clouds scattering out early Friday afternoon with breezy west-northwest winds. Low confidence for any stratus development over the terminal Friday night, but will monitor conditions given the forecasting challenges over the last couple of days.

MARINE
(Tonight through next Wednesday)
Issued at 1033 PM PDT Thu Apr 17 2025

Moderate to fresh northerly breeze across the outer coastal waters with light and variable winds closer to the coast. As the winds diminish, rough seas are subsiding now through Friday before another strong breeze rebuilds seas this weekend into early next week.

MTR WATCHES/WARNINGS/ADVISORIES
CA...None.
PZ...Small Craft Advisory from 9 PM Friday to 9 AM PDT Saturday for Pt Arena to Pt Reyes 10-60 NM.