Issued at 957 AM PDT Fri Aug 28 2026
Have increased rain shower chances to include the San Francisco Bay Area and the North Bay through this afternoon as precipitation (albeit light) is reaching the surface. The greatest rainfall over the past 6 hours has been across the Central Coast with up to 0.05" at Chews Ridge (just above 5000 feet in elevation). There still is a chance for thunderstorms across the Central Coast as there is some instability aloft, however the 12Z KOAK sounding reported only 3 J/kg of MUCAPE with the KVBG (Vandenberg AFB) reported 459 J/kg MUCAPE. The threat diminishes this afternoon and moreso by late evening as the remnant moisture from Tropical Storm Iselle shift to the east. Thus, the Red Flag Warning remains in effect through 3 PM this afternoon for much of the Central Coast and the Santa Cruz Mountains. For more, please see the Short Term discussion below.
RGass
, Issued at 1041 PM PDT Thu Aug 27 2026 (Tonight through Saturday)
As the remnant moisture from Tropical Storm Iselle moves over the region we must analyze the risk for high-based thunderstorms. This type of thunderstorm forms high in the atmosphere above a dry layer of air. Most of the rain evaporates in the dry layer, but lightning has no trouble getting to the ground. This dry lightning is a well-known wildfire ignition source, so let's take a close look. The 00Z sounding measured a PWAT of 1.24", with 307 J/kg of MUCAPE. The best lapse rate was 850-500 mb at 7.6 C/km, with the moisture all above 700 mb. As this moisture gradually moves lower in the atmosphere, the instability will increase. That, combined with subtle lift from the trough and terrain could trigger some thunderstorms today.
Models are not doing a good job analyzing the (307 J/kg of MUCAPE) instability measured on the 00Z weather balloon sounding. For example the 00Z ECMWF model initialization at OAK has 0 MUCAPE. The lapse rates and precipitable water are in the ballpark, but not lined up as well as reality. It's the same story with the HRRR and NAM. Specifically the models aren't resolving the moisture at the base of the unstable layer. As such, we've increased the probability of thunderstorms to 15% across the Santa Cruz Mountains and most of Monterey County from 5AM to 2PM.
The two big questions are will this instability be realized, and will the rain reach the ground. The instability piece looks better as the night goes on. Now that the balloon data is getting into the models, they are gradually getting more interesting. The 06Z HRRR is just starting to come in and the lightning flash density has trended up. It's now showing a potential peak of 1 flash every 5 mins across parts of Monterey County, with more scattered activity across the rest of the Bay Area. The 00Z ECMWF has a similar trend across the Central Coast, but still doesn't show much in the Bay Area.
One factor working against convection is the timing. While the upper level support peaks in the mid to late morning, peak surface heating will lag behind by several hours. If these lined up, there would be some more help from surface based instability. On the other hand, if the storms preform well in the morning, the increasing surface temperatures may continue to support convection that keeps some activity going longer into the afternoon than currently expected.
As the 500 mb trough axis approaches Saturday it will usher in a much drier air mass. This will end the threat of thunderstorms, while ushering in some gusty onshore winds and cooler temperatures this weekend.
..issued at 1041 PM PDT Thu Aug 27 2026 (Saturday night through next Thursday)
The marine layer will gradually reform and deepen through the weekend. An active pattern will bring several short waves into the Pacific Northwest this week, keeping the Bay Area on the cool and breezy side. There is even a chance for some light rain midweek.
(18z TAFS) Issued at 1120 AM PDT Fri Aug 28 2026
VFR across all terminals, except for HAF where it will remain IFR until the early afternoon. Gentle to moderate onshore winds increase this afternoon and ease overnight. High level clouds associated with a moisture plume persist into the afternoon and gradually decrease this evening. Drizzle/light rain is being measured across the region which is expected to continue into the early afternoon as well.
As the moisture remnants continue on their eastward track out of our region, the marine layer will begin to build back to its usual pattern. There is currently low confidence on how widespread and organized it will be tonight as the stratus feed looks patchy on satellite to our northwest. Rather than locking in IFR to MVFR ceilings, the TAF hints at the possibility with SCT decks. High confidence for coastal sites to develop IFR ceilings later this TAF period, though the exact timing is uncertain.
Vicinity of SFO, Moderate northwest winds persist through the afternoon with some light rain nearby the terminals. Onshore winds ease through the evening with low confidence for MVFR ceilings to develop around midnight.
SFO Bridge Approach, Similar to SFO.
Monterey Bay Terminals, VFR with gentle northwest breezes through the day. The potential for light rain near the airports persists this afternoon and gradually diminishes by sunset. Overnight, winds become more west to southwest, but remain light. Medium confidence for IFR ceilings to develop with an arrival around midnight.
(today through Wednesday) Issued at 957 AM PDT Fri Aug 28 2026
Fresh to strong northwest breezes persist through the weekend and diminish to moderate by the beginning of the week. A long period southerly swell peaks on Saturday around 17 seconds and gradually subsides by the end of the weekend. Moderate wind driven seas persist into next week.
Ca, Red Flag Warning until 3 PM PDT this afternoon for CAZ512-516- 517-528>530.
PZ, Small Craft Advisory from 9 AM Saturday to 3 AM PDT Sunday for Mry Bay.
Small Craft Advisory until 9 AM PDT Sunday for 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.
Small Craft Advisory until 3 AM PDT Sunday for Pigeon Pt to Pt Pinos 0-10 nm.
