textproduct: San Francisco Bay Area

KEY MESSAGES

Updated at 148 PM PDT Sun Jul 19 2026

- Near or slightly below normal temperatures and higher humidity through the first half of the upcoming week.

- The marine layer and mid to upper level cloud cover will continue to help keep temperatures cooler under building high pressure.

- Approximately 5% chance of elevated showers with isolated thunderstorms Tuesday afternoon through Wednesday evening.

SHORT TERM

Issued at 158 AM PDT Mon Jul 20 2026 (Today and tonight)

A 581 decameter geopotential height low remains located 500 miles west of San Francisco. The southern branch of the polar/subtropical jet stream is just forward of this low with jet stream winds to our southwest through northeast. These jet stream winds have helped quickly transport high and mid level clouds away from TS Elida in the last 24 hours or so. GOES-West Geostationary Lightning Mapper showed lightning approx 600 miles southwest of Point Conception earlier Sunday, but since Sunday evening it's been quiet. This lightning activity was closer to the northward moving and weakening center of TS Elida.

KMUX has occasionally shown widely scattered high based showers over our forecast area, but there have been no measurable rain or lightning reports so far. A band of light high based rain is developing over the coastal waters as well. We're in a little bit of unknown territory here with mainly minor energy transformations taking place, complexities that occur whenever tropical moisture travels northward and interacts with the mid-high latitude synoptic pattern. At the moment, the high based showers/band of light rain correspond to an area of weaker 500 mb positive vorticity that is forecast to continue moving northward of our forecast area along with the aforementioned upper level jet stream. So in a way, the combination of parameters and the steady diffusing of mid and high level clouds (water vapor) with minimal to no organization so far appear to be self limiting, leaning the pattern toward stability, keeping convective potential at a minimum.

Cloud cover will be a mix of stratus and mid and high level clouds today. The sun may break out over the southern interior today, possibly some sunny breaks farther north to the South Bay as well. Daytime highs today will be about the same it was on Sunday, varying from the mid to upper 60s along the coast to the 70s bayside to the 80s and 90s well inland. Tonight lows will cool to the 50s except in the 60s in the hills and mountains.

LONG TERM

Issued at 158 AM PDT Mon Jul 20 2026 (Tuesday through Sunday)

With generally decreasing 500 mb positive vorticity Monday this will help keep convective potential at a minimum with exception of the North Bay Monday afternoon to early evening when there may be a brief uptick in positive vorticity. However, even the 00z NAM output for this timeframe shows minimal convective potential along with convective inhibition. The HRRR, RRFS and GFS show essentially no convective potential.

For Tuesday, 500 mb positive vorticity is essentially non-existant including in the 00z NAM cross sections. Residual mid to high level clouds may still be around, but there'll likely won't be a lifting mechanism left to destabilize an atmospheric layer or air parcels. The westerlies, wind shear and drier air is destructive to tropical origin convective development, but there can be the exception to this when a steep east to west moisture gradient helps temporarily destabilize a lifted layer or air parcels. Given model trends and further dissipation of TS Elida, this is unlikely Tuesday when the tail end of the moisture gradient moves across the forecast area. Stay tuned to the latest updates.

Otherwise it'll be summertime dry weather, warm to hot inland and coolest at the coast and bays mid to late week. The high pressure system over the interior west will gradually shift southward then slowly move to the Four Corners by the weekend.

AVIATION

(12Z TAFS) Issued at 436 AM PDT Mon Jul 20 2026

Currently a mixed bag of LIFR-VFR. Patchy stratus with mid-level clouds streaming in from the southwest is making for a difficult forecast. Low confidence on stratus expanding this morning with high confidence in LIFR-IFR conditions continuing at the coast with the help of coastal drizzle. Elevated rain showers moving in from the southwest will bring virga to the region. Widespread VFR and diurnal winds will prevail by this afternoon. High confidence in the return of IFR conditions to coastal terminals tonight with low confidence elsewhere due to the shallow nature of the marine layer.

Vicinity of SFO...Currently VFR with northwesterly flow. Confidence is decreasing in the terminal getting a ceiling this morning based on current conditions and camera observations. If a ceiling does develop, it'll likely be IFR and clear by 16Z. Low confidence on the return of an IFR ceiling tonight.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently LIFR with westerly flow at MRY and VFR and calm at SNS. Since SNS had an IFR ceiling earlier in the night, they have a TEMPO through 16Z for it returning. Stratus will retreat to the coast by late morning with brief VFR expected at the terminals this afternoon. Moderate (SNS) to high (MRY) confidence on IFR ceilings returning tonight.

MARINE

(Today through Saturday) Issued at 436 AM PDT Mon Jul 20 2026

Moderate northwesterly breezes will prevail through Wednesday, increasing to become moderate to fresh 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 swell; hazardous conditions from mixed seas may result. The same activity will yield 5% chances for elevated rain showers through Wednesday. The hazards would be gusty and erratic outflow winds that would be capable of generating steep seas.

FIRE WEATHER

Issued at 148 PM PDT Sun Jul 19 2026

Near to slightly cooler than normal temperatures will continue through the week with typical diurnal intrusion of the marine layer each night and retreat to the coast each morning as onshore flow persists. However, above 1500-2000ft we'll see relatively weak thermal belting the first half of the week with continued warm overnights and poor RH recovery at higher elevations.

Guidance is trending toward an increasing potential for elevated showers and isolated thunderstorms Tuesday afternoon through Wednesday evening. Chances are below 5% in the official forecast but may increase in subsequent updates tonight and Monday. The primary concern is dry lightning strikes across the North Bay, with lesser chances farther south into the Bay Area.

MTR WATCHES/WARNINGS/ADVISORIES

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