Marine Weather and Tide Forecast for Napa, CA

SHORT TERM
Issued at 218 PM PDT Tue Aug 18 2026 (This evening through Wednesday night)

A 500hPa of high pressure anchored over the desert southwest continues to nose into our service area today yielding near normal temperatures for most areas. The exception are area ridges where temps are running a few degrees above this time yesterday. The more interesting part is the relative humidity. RH values at elevation are anywhere from 5 to 20 percent below this time yesterday. For example, the Hastings RAWS (Santa Lucia Range - Monterey County) has an RH of 14% as of 1:11 PM today. At the same time on Monday the RH was 38%. That's a 24% drop in RH in 24 hours. This is to state that the upper ridge has compressed the marine layer which is allowing for significant drying at elevation. These patterns continue into the night as well and can drop RH into single digits during nocturnal drying events. As the ridge pushes in a touch more tonight expect slightly warmer temps and drier conditions above the marine layer. While the NBM seems to handling the bulk of the near term well, areas above about a 1000 feet were warmed and dried slightly using a blend of NBM and short range high resolution models such as HRRR, RAP, and NAM. For our populated areas conditions tonight through Wednesday night will be a near copy of the previous 24 hours. The coastal areas will see some low clouds and some fog. There may even be some light drizzle in areas like Half Moon Bay, Santa Cruz, and Santa Rosa in pre-sunrise AM hours of Wednesday.



LONG TERM
Issued at 218 PM PDT Tue Aug 18 2026 (Thursday through next Monday)

An upper level area of low pressure has been quasi-stationary over the NE Pacific for the last few day. By the time we get into late week, the upper low is expected to transit Eastward into the Pacific Northwest, pulling an upper trough into the Bay Area.
Looking at the WPC Clusters, confidence is high this trough will impact the Bay Area and Central Coast with an onshore push of marine air and a deepening marine layer. This will cause clouds to hang on a little longer in the morning hours and redevelop a little earlier in the late afternoon. The deepening marine layer will also bring higher humidity to areas of elevation. However, surface winds will also kick in with breezy conditions at the coast, in coastal gaps like the Petaluma gap, and over East Bay saddles like Altamont Pass and the Delta. This cuts two ways. One being higher moisture content is good in helping to suppress fire ignitions. Two being the breezy conditions on fully cured fine fuels. If something gets going into the lighter to moderate fuels, it could run with the onshore winds. From a purely sensible weather standpoint, it will simply be pleasant weather all week.

The final piece the address in the medium term is mid level moisture advecting over the area late in the week. Model are all over the map with this potential. While the probability is there, confidence remains low as to where the highest PWAT will advect towards. Whenever we see a pattern like this the first thought is high based convection. Looking out as far as possible with higher resolution models, moisture and instability do look to be present, in some current model runs. The limiting factor is cooling aloft.
Like we have seen in other similar situations this summer, the lacking ingredient is cooling aloft. This would really turn over the mid layers and could provide deeper convection. However, at this time cooling aloft is not in the cards. As it stands right now we have moisture spreading in Thursday night into Friday (and beyond) with marginally supportive lapse rates during times of highest moisture advection. Even if the vertical development could get going, tapping into the zone to exchange charges and produce lightning has less than a 1% chance at this time. I just don't see it happening. That said, we'll need to watch it over the coming days as we get more data. The most we would likely see are some high based convective showers that produce little to no measurable precipitation at the ground. The only other item to watch for during any convective shower activity would be locally gusty winds, especially in the mountains where it will be closer to the base of the storms.

AVIATION
(18Z TAFS)
Issued at 1049 AM PDT Tue Aug 18 2026

Seeing typical retreat of stratus to the coastline this morning with most interior sites returning to VFR through the afternoon.
Onshore winds are forecast to increase late this morning and persist through the early evening before diminishing after sunset.
Going with a persistence forecast for this evening and into Wednesday morning. Thus, moderate to high confidence for IFR to LIFR conditions to return to the coast by mid-to-late evening and then spreading across the Bay Area terminals late evening into early Wednesday morning. Expecting stratus to retreat to the coastline by mid-to-late Wednesday morning.

Vicinity of SFO.. MVFR. Stratus is forecast to clear by between 18Z- 19Z and give way to VFR conditions through late evening. In addition, moderate to fresh west-northwestery breeze is forecast this afternoon and early evening. High confidence for MVFR and moderate to high confidence for IFR returning late this evening and persisting through late morning on Wednesday.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Mix of LIFR to IFR conditions currently.
VFR conditions briefly return this afternoon before IFR to LIFR conditions redevelop early this evening into Wednesday morning.
Gentle breeze is forecast this afternoon before diminishing after sunset.

MARINE
(Today through Sunday)
Issued at 950 AM PDT Tue Aug 18 2026

A fresh to strong northwest breeze will develop across the outer waters through the day and extend into the inner waters tonight into Wednesday. Winds then remain elevated through the end of the forecast period. Locally stronger winds are possible in the coastal jet regions north of Point Reyes and along the Big Sur coast. Seas build to 7 to 9 feet Tuesday and Wednesday before subsiding late week.

MTR WATCHES/WARNINGS/ADVISORIES
CA...None.
PZ...Small Craft Advisory from 3 PM Wednesday to 3 AM PDT Thursday for Pt Arena to Pt Reyes 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm.

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