Marine Weather and Tide Forecast for Diablo, CA

SHORT TERM
Issued at 1114 AM PDT Sun Sep 20 2026 (This afternoon through Monday)

A cut-off low at 500 mb has allowed the marine layer to deepen (currently 2,000 ft) and ushered in a blanket of high clouds above the eroding stratus. The radar is actually showing pretty decent elevated returns over the ocean, and I just switched the scanning strategy into "precip mode". While there is some precip in the upper atmosphere, the air between these clouds and the marine layer is very dry. Most, if not all, of these rain drops will evaporate before reaching the surface. The combination of these cloud decks and a cooler upper atmosphere is keeping max temperatures notably cooler than yesterday. The coast will be stuck in the 60s all day, with inland ares broadly in the 70s this afternoon. The low will continue to approach from the west, moving overhead by Monday morning. This will allow the marine layer to deepen further and brings a good chance for drizzle Monday morning. Temperatures will be very similar Monday before a warming trend starts Tuesday
Before we get to that

Could we get surprised by high based thunderstorms today? Probably not, but it's worth keeping an eye on. As the cut-off low approaches from the west, the associated clouds moving in from the SW look better than expected. Cloud tops are around 30,000 feet, and there are even some 20-30 dBz radar returns around 15,000'.
Those numbers don't scream thunderstorms at all, but there is an outside chance that this system could become more energetic as it approaches the Central Coast over the next few hours. The 18Z HRRR shows a distinct blob of 500 mb vorticity moving into Monterey County around 1 PM this afternoon before pushing north.
This HRRR also shows a signal across Monterey and San Benito County in the lightning flash rate product.

On the other hand, there is virtually no LightningCast activity, and the reflectivity at -10C and -20C is far too weak to support the amount of ice particles needed for thunderstorms to develop at the moment. With ample shear associated with the cut-off low (45 kts 0-6 km), CAPE is the limiting factor. There is a conditionally unstable layer with lapse rates around 7 C/km, but most of the moisture is above that. As the low approaches, the moisture will lower and that may open the door. Again there is a very small chance of lightning, but if it does happen, it's unlikely to be accompanied by any rain. This poses an obvious fire weather concern as dry lightning can cause new ignitions.

LONG TERM
Issued at 1114 AM PDT Sun Sep 20 2026 (Monday night through next Saturday)

The low will continue to slide east, and we should be firmly on the back side by Tuesday morning. This will allow a low amplitude ridge to build over the next few days. This pattern change will result in a warming trend while compressing the marine layer. 850 mb temperatures are expected to climb above 20 C Wednesday - Thursday, but the trend has been a little cooler since yesterday's forecast. The widespread moderate HeatRisk is now mostly confined to Thursday. It used to look like Wednesday and Thursday. While it will certainly be warm through the middle of the week, if the recent trend continues we may not need a Heat Advisory.

Heat Advisories are somewhat common in late September, but they typically come with offshore winds and a longer duration event.
This is a quick warm-up with continued onshore winds and some, albeit shallow, marine layer surviving along the coast.

The pattern changes again by Friday as another trough approaches the Pacific Northwest. This will reverse our warming trend, and more seasonal temperatures are expected through next weekend.

AVIATION
(00Z TAFS)
Issued at 504 PM PDT Sun Sep 20 2026

Satellite shows mid to high level clouds and KMUX shows a few high based showers, otherwise VFR prevails except for areas of coastal stratus /MVFR-IFR/. Onshore winds will bring stratus and patchy drizzle /MVFR-IFR-LIFR/ inland tonight and Monday morning.
Conditions improve to MVFR-VFR near the coast Monday afternoon, VFR prevails inland. Stratus and a few patches of light drizzle redevelop Monday night and early Tuesday morning.

Vicinity of SFO...VFR and a west wind 10 to 20 knots during the evening, west wind diminishes to near 10 knots tonight and Monday morning. MVFR stratus ceiling forecast 04z-19z Monday. Stratus /MVFR/ redevelops Monday evening and night. West wind 15 knots Monday afternoon to mid evening.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Stratus /MVFR-IFR-LIFR/ moves inland tonight. Patchy light drizzle tonight and Monday morning. Conditions improve to MVFR-VFR by late Monday morning and afternoon. Stratus /MVFR-IFR/ redevelops Monday night. Onshore winds 5 to 15 knots.

MARINE
(Tonight through next Friday)
Issued at 455 PM PDT Sun Sep 20 2026

An upper level area of low pressure will keep pressure gradients generally light across the southern waters tonight into Monday. As the low moves inland surface pressure gradients will increase over the northern outer waters with increasing northwest winds for the offshore waters from Point Arena southward towards Point Reyes resulting in building short period seas. Lighter wind and seas continue across the southern waters through midweek as high pressure rebuilds.

FIRE WEATHER
Issued at 416 AM PDT Sun Sep 20 2026

Near to slightly below normal temperatures, diurnal winds, a deep marine layer, and a chance for drizzle are on tap for Sunday and Monday. Warming and drying trend kicks off Tuesday with temperatures up to 15 degrees above normal by Thursday. Areas of moderate HeatRisk are expected as a result Wednesday and Thursday. The marine layer will compress below 1,000 feet during this warm-up with nocturnal drying expected above it. Diurnal winds will prevail; however, they will be weaker than the preceding days.

MTR WATCHES/WARNINGS/ADVISORIES
CA...None.
PZ...Small Craft Advisory until 9 PM PDT this evening for Pigeon Pt to Pt Pinos 0-10 nm.

Small Craft Advisory until 3 AM PDT Monday for Pigeon Pt to Pt Pinos 10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm.

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