textproduct: San Francisco Bay Area

KEY MESSAGES

Updated at 1104 PM PDT Sun Jul 12 2026

- Monsoon moisture brings a slight chance for thunderstorms through Tuesday

- Minor coastal flooding continues through Thursday morning due to large tidal swings

- Warmer temperatures through Wednesday

SHORT TERM

Issued at 1104 PM PDT Sun Jul 12 2026 (Tonight through Tuesday)

You may have noticed some colorful sunrises and sunsets over the last two days. Those are being caused by monsoon moisture pushing up from the south bringing us mid level clouds. Even if it's overcast, there is enough room underneath the clouds (about 10,000 feet) for the setting sun to illuminate the bottom of these altostratus and altocumulus clouds and create the beautiful sunsets. It also helps that the low-level marine layer clouds, which typically only sit about 1,000 feet above the surface and famously ruin sunsets, were not nearly as widespread as we saw last week. So that explains the sunset, but what about the monsoon?

While the word invokes many different feelings, "monsoon" actually describes a seasonal wind direction reversal. The North American Monsoon primarily impacts NW Mexico, Arizona, and New Mexico as the high summer temperatures in the desert induce thermal low pressure that brings in moist air primarily from the Gulf of California. This moisture, combined with the instability driven by high surface temperatures, often generates thunderstorms over the desert states in the Summer. In fact, Tuscon gets 52% of its rain from July-September, while we are lucky to get anything. Every once in a while the weather pattern aligns in such a way to bring some of this monsoon moisture to the Bay Area. In this case it's driven by a very strong high pressure system over the Rockies. This system is so strong that Salt Lake City just set their all time record high temperature of 109F. Since winds flow clock-wise around a high, this system is pushing the monsoon moisture from Arizona in a circular pattern towards Southern California and then northward to the Bay Area. This final push of southerly winds is enhanced by a low pressure trough over the Eastern Pacific. As the moisture moves along this conveyer belt it also moves higher in the atmosphere (humid air is less dense than dry air). This explains where the uncommon mid level clouds are coming from.

While this moisture was mostly harmless today, there is a slight chance for thunderstorms through Monday. If that happens it could bring significant impacts because the high cloud base and much drier lower atmosphere means that most of the rain would evaporate before it hits the ground. When lightning strikes without much rain, wildfires can start. Thunderstorms need 3 ingredients: instability, moisture and a lifting mechanism. We have moisture, check that off the list. The instability looks pretty meager. The 00Z weather balloon from Vandenberg found the steepest lapse rates to be -7.3 C/km between 850 and 500 mb. That's not terrible, but most of the moisture is above 600mb, where it's more like -6.6 C/km, which is pretty bad. To add to the issues, there's not really an obvious trigger either. Surface heating is the typical trigger for monsoon thunderstorms in the desert, but these would be high based thunderstorms that wouldn't really feel that impact. More likely is nocturnal cloud top radiational cooling, combined with cloud base heating from surface radiation. While surface cooling makes things more stable, cloud top cooling does the opposite. If the top of the cloud gets colder and the bottom gets warmer, the instability increases due to the steeper lapse rate. The HRRR soundings also suggest that in the morning the the moisture plume will move further down towards the -10 to -20 C sweet spot where the combination of supercooled water and ice maximize particle collisions and cloud charging potential. So the best chance is probably early Monday morning. Even then it's a long shot. We're talking like somewhere between 5 and 10%. Don't be surprised if you see another good sunrise and even a few drops of rain, however. Otherwise Monday will feel pretty much like Sunday. Maybe a couple degrees warmer.

LONG TERM

Issued at 1104 PM PDT Sun Jul 12 2026 (Tuesday night through next Sunday)

The slight high based thunderstorm threat continues through at least Tuesday morning. There is some evidence that a short wave will move through that could make things more interesting, but the moisture also starts to clear by then, so we're still talking about single digit percentages. The bigger story is the heat. As the moisture clears out, so do the clouds. The full sunshine will bring temperatures into the mid 90s for several of our hotter cities, with mid 70s in the more coastal locations. That's about 5-10 degrees above normal. Strong onshore winds will arrive in the afternoon and help cool the coast quickly. Wednesday will be very similar to Tuesday, just without the thunderstorm threat. By Thursday the Eastern Pacific trough that helped pump the moisture earlier in the week will arrive and bring a weak cold front, kicking off a late week cooling trend.

A quick note on the tropics. As the El Nino Advisory continues, the National Hurricane Center is currently tracking 5 disturbances in the Central and Eastern Pacific. While these systems rarely impact California directly, the residual moisture can bring impacts similar to the monsoon with high based thunderstorms. Refer to Tropical Storm Fausto, the 2020 lightning outbreak and subsequent wildfires for a good example of this. Additionally, the ocean swell generated from these storms can bring hazardous beach conditions. While there is no immediate threat, there is a high chance for an active hurricane season in the Pacific and we'll be watching closely for these impacts.

AVIATION

(12Z TAFS) Issued at 448 AM PDT Mon Jul 13 2026

VFR is expected to prevail through the forecast period with the exception of KHAF that may toggle between VFR and IFR/LIFR through mid-morning. Slight chance of low end MVFR cigs early Wednesday morning for terminals on the immediate coast.

Vicinity of SFO...VFR, with breezy onshore flow this afternoon and slight chance of low end MVFR cigs early Wednesday morning.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR is expected to prevail through the TAF period with a slight chance of low end MVFR ceilings early Wednesday morning.

MARINE

(Today through Saturday) Issued at 448 AM PDT Mon Jul 13 2026

Moderate to fresh north-northwest winds across the northern outer waters continue with hazardous conditions, along with moderate northwest swell. Northerly winds over the coastal and outer waters become fresh to strong mid to late week, with near gale force gusts. Expect building seas in response to the increasing winds and long period southerly swell late week into next weekend.

FIRE WEATHER

Issued at 448 AM PDT Mon Jul 13 2026

Warming temperatures this will cause fuel moistures to plummet for the first half of the week. At the same time we are tracking monsoonal moisture moving across the area and any threat of dry thunderstorms today and Tuesday. While dry thunderstorms have a 10% or less chance of development, any strike can start a fire. Not to mention, in and around active thunderstorms, winds can become gusty and erratic. For more information on the thunderstorm threat, see discussion above.

MTR WATCHES/WARNINGS/ADVISORIES

CA...Coastal Flood Advisory until 4 AM PDT Thursday for CAZ006-506- 508.

Coastal Flood Advisory until 4 AM PDT Wednesday for CAZ505-509- 529-530.

PZ...Small Craft Advisory from 3 PM Tuesday to 3 PM PDT Wednesday for Pt Arena to Pt Reyes 0-10 nm.

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