textproduct: San Francisco Bay Area

KEY MESSAGES

Updated at 154 PM PDT Sun Jul 19 2026

- High based showers through Tuesday.

- Breezy onshore winds return for the second half of the week.

- Temperatures will hover around normal through the week.

SHORT TERM

Issued at 105 AM PDT Tue Jul 21 2026 (Today and tonight)

Isolated to widely scattered high based showers mainly continue over the coastal waters and the Bay Area. A few reports via social media and webcams Monday showed patchy light sprinkles resulting in a mix of trace reports and a couple isolated 0.01". Almost all of the forecast area has remained dry. The precipitable water (pw) on late Monday afternoon's Oakland upper air sounding was 1.42", the max moving average for July 20. Earlier over the weekend Saturday evening's pw was 0.50", 10th percentile. Deep southerly winds currently transporting remnant tropical moisture into the forecast area is largely distributing the moisture between 15,000 feet and 26,000 feet. Between 15,000 feet and the ground, the layer is relatively drier, likely resulting in some evaporation of rain drops before reaching the ground.

No lightning has been observed so far over our forecast area, including no lightning activity since Sunday evening over the Pacific all the way out across the remnants of Elida. An ASCAT wind pass at 0511z (10:11 pm PDT Monday) showed the remnant surface low pressure circulation ~ 500 miles southwest of Monterey with winds 15 to 30 knots with a few higher gusts to 35 knots. The surface low center will slowly fill and weaken while moving northward. The area out ahead of the low has pw upwards of 1.50" to 1.60" which is not far from the July daily maximum pw (1.74") on the period record since 1948. This pw arrives today and forecast soundings show this will continue to be largely distributed in the middle to upper troposphere. The highest mid to upper level pw will be in our area when there is weak positive vorticity at and above the 500 mb level this afternoon/evening otherwise negative vorticity will be present in the 700-500 mb layer. Given model trends, a lack of lightning seen so far, temporary weak positive vorticity at and above 500 mb and weak convergence at 250 mb later today, it's a persistence forecast today. The atmosphere is very complex though, could it still convect and produce an isolated lightning strike? Yes. Stay tuned to the latest updates.

00z GFS and ECMWF forecast the 700-500 mb layer drying out tonight. However, elevated pw will remain from the surface to lower levels.

The recent increase in pw (water vapor) will largely slow radiational cooling through morning, possibly slowing how warm it gets today too as well, though lower level temperatures around the 925 mb level are forecast to increase a few degrees Celsius today. Daytime highs may get above normal today, varying from the 70s/80 coastside to the 80s bayside and 90s/100 far inland.

LONG TERM

Issued at 105 AM PDT Tue Jul 21 2026 (Wednesday through Monday)

As mentioned in the short term discussion, elevated pw will remain from the surface to the lower levels tonight. This will carry over through Wednesday. This is tropical moisture that'll keep surface dewpoint temperatures in the upper 50s/60s Wednesday. Dewpoints in this range will make it feel muggy Wednesday while the mid to upper levels will be drier. If low clouds return by early Wednesday, patchy light measurable drizzle may develop in the tropical air mass. Weak cool air advection, onshore winds and gentle upsloping may help with the development of patchy drizzle.

Daytime onshore breezes return mid to late week. It'll otherwise be dry, warm to hot inland and coolest at the coast and bays later in the week. The high pressure system over the interior west will gradually shift southward then slowly move to the Four Corners by the weekend.

AVIATION

(06Z TAFS) Issued at 923 PM PDT Mon Jul 20 2026

Mid to high level clouds are interrupting normal radiative process for wides spread stratus. Low confidence that stratus may develop during the overnight through sunrise Tuesday morning. Expecting mainly VFR through the TAF period for majority of the terminals with the exception of KHAF which is on the coastline. Be aware of the potential for elevated showers to produce locally gusty and erratic winds in their vicinity through the forecast period.

Vicinity of SFO...VFR expected to prevail through the TAF period with a slight chance of IFR ceilings developing late in the overnight through sunrise Tuesday. Be aware of the potential for elevated showers to produce locally gusty and erratic winds in their vicinity through the forecast period.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR is expected to prevail with a slight chance of IFR ceilings developing overnight before sunrise or shortly after. If ceilings don't develop, expect VFR through the remainder of the forecast period. Be aware of the potential for elevated showers to produce locally gusty and erratic winds in their vicinity through the forecast period.

MARINE

(Today through Saturday) Issued at 1049 AM PDT Mon Jul 20 2026

Light to locally moderate northwesterly winds will prevail through midweek, becoming moderate Thursday into Friday. Slight to moderate seas will prevail. Tropical cyclone activity in the Eastern Pacific Ocean will spur several rounds of moderate period southerly swells; hazardous conditions from mixed seas may result. Instability from the remnants of the tropical cyclone will also bring a slight chance of convection that may bring showers as well as gusty and erratic outflow winds that would be result in hazardous conditions to small craft.

FIRE WEATHER

Issued at 154 PM PDT Mon Jul 19 2026

No significant impacts expected as sub-tropical moisture moves over the Bay Area and Central Coast. There has been some discussion of dry thunderstorms, but as of this afternoon models are not indicating an environment that would support their development. At this point I would give dry thunderstorms a probability of less than 1% this week. What should be expected is an isolated shower here or there with little to no accumulation. The marine layer will remain fairly well mixed out through Tuesday afternoon, but should start to build back in late Tuesday and Tuesday night. Expect it to reach around 1000 feet by midweek. Strong onshore winds will develop each afternoon for the second half of the week. They typical places will gusts like they do every summer afternoon.

MTR WATCHES/WARNINGS/ADVISORIES

CA...None. PZ...None.