Marine Weather and Tide Forecast for Castro Valley, CA

SHORT TERM
(Today and tonight)
Issued at 111 AM PDT Wed May 21 2025

Conditions are warmer and drier than they were this time last night, but only in the higher terrain. Why? Well, last night there was a shallow marine layer that impacted coastal areas while tonight the region is clear and winds are calm, allowing for maximal radiational cooling. A low amplitude upper-level shortwave ridge will trek across the region today, bringing the warmest conditions of the week. Temperatures will be 10 degrees above normal with widespread minor HeatRisk and areas of moderate HeatRisk for the Interior Bay Area and Central Coast. Elevated fire weather conditions will continue through tomorrow for the Interior Bay Area and Central Coast as dry conditions (poor daytime relative humidity of 15-25%, overnight relative humidity recovery of 30-50% in the higher terrain) and moderate to strong onshore winds continue. The threat is primarily in the finer, flashier fuels of brush and grass as recent offshore flow and warm conditions continue to cure them.

LONG TERM
(Thursday through Tuesday)
Issued at 111 AM PDT Wed May 21 2025

The lack of a longwave pattern over the region is evident in the extended forecast. An upper-level shortwave trough will skirt the region Thursday, causing temperatures to return to seasonal normals.
A second, deeper upper-level shortwave trough will dig into the region Friday, bringing the coolest conditions of the week. An upper- level longwave ridge builds into the West Saturday which will bring temperatures up to 5 degrees above normal. Global ensemble clusters show an Omega Block, the combination of two cutoff lows with one blocking high sandwiched between them, developing by Sunday. By nature, this will lead to uncertainty and low confidence in the progression of the longwave pattern into early next week as these features have very weak forcing and thus can be persistent.

AVIATION
(06Z TAFS)
Issued at 951 PM PDT Tue May 20 2025

The marine layer depth to the lower level temperature inversion varies from approx 700 feet to 1400 feet. Drier to much drier air is in the atmospheric mix within the gusty northwest winds along the coast, resulting in VFR. Gusty northwest winds are otherwise resulting in haze/hazy conditions due to sea spray/salt condensation nuclei lofted into the marine layer.

The northerly ACV-SFO pressure gradient is 7.4 mb, the onshore gradient SFO-SAC is 2.7 mb. Dry air tonight and Wednesday will have a good chance of continuously mixing into the marine layer i.e. the boundary layer helping to keep VFR going, except the immediate coast may see a patch or two of stratus and/or fog late at night and/or early morning. Otherwise VFR is forecast Wednesday.
Gusty west to northwest winds developing Wednesday afternoon and evening.

Vicinity of SFO...VFR. West wind 5 to 15 knots tonight and Wednesday morning. West wind with gusts increasing to near 30 knots Wednesday afternoon and evening.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Moderate to high confidence VFR continues through late evening, then low to moderate confidence VFR forecast late tonight and Wednesday morning. Plenty of dry air is arriving from the north at and above the marine layer temperature inversion, since dry air is molecularly heavier than water vapor the current near term weather pattern may just help extend VFR here along the coastline through late tonight and Wednesday morning. Otherwise VFR Wednesday. West winds 5 to 10 knots tonight and Wednesday morning, increasing to 10 to 20 knots Wednesday afternoon and early evening.

MARINE
(Tonight through next Monday)
Issued at 909 PM PDT Tue May 20 2025

Strong northwesterly winds and hazardous seas will continue over the coastal waters tonight through late week. Westerly winds will become gusty over the bays each afternoon and evening. Winds will diminish and seas will subside gradually into the upcoming weekend.

MTR WATCHES/WARNINGS/ADVISORIES
CA...None.
PZ...Small Craft Advisory from 3 PM this afternoon to 9 PM PDT this evening for Mry Bay.

Gale Warning until 3 AM PDT early this morning for Pigeon Pt to Pt Pinos 0-10 nm-Pt Arena to Pt Reyes 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.

Small Craft Advisory until 3 AM PDT Thursday for Pigeon Pt to Pt Pinos 0-10 nm-Pt Arena to Pt Reyes 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.

Gale Warning until 3 PM PDT this afternoon for Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 10-60 NM.

Small Craft Advisory from 3 PM this afternoon to 3 AM PDT Thursday for Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 10-60 NM.