Palo Alto weather in September

Locals like to say September is the real summer in Palo Alto. The numbers back them up. It is the warmest month on average, the most likely month for a heat wave and the month when the all-time record was set. It is also when the coastal fog finally loosens its grip, so San Francisco and Half Moon Bay get their best weather of the year. Nights are cool and the days begin to shorten fast.

All temperature and rain figures here are NOAA's 1991 to 2020 climate normals for the Palo Alto station (USC00046646), located on the flats at about 25 feet. NOAA marks the station's values "representative" because its record has gaps, so do not read too much into a few tenths of a degree. Sun times are calculated for central Palo Alto for September 2027 and match published tables within a minute. They describe a typical September, not a forecast. The full year is on our weather page.

September by the numbers

NOAA's guide has no long term Half Moon Bay sea temperature. The ocean figure is its average for the buoy about 18 nautical miles west of San Francisco, which peaks in September.

Heat: the month to watch

September's average high of 79.6°F edges out July's 79.4°F. The bigger story is the extremes. The Palo Alto station averages 3.7 days at 90°F or hotter in September, the most of any month. It also averages 12.8 days at 80°F or more. On September 6, 2022, during a long statewide heat wave, the city hit 108°F, its all-time record.

Why September? The coastal fog that cools the region all summer weakens as the season turns. At the same time, high pressure over the interior and the first offshore winds of fall can push hot, dry air from inland toward the coast. When that happens the sea breeze stalls and the whole Bay Area heats at once.

September also has the biggest daily temperature swing of the year. The average gap between high and low is about 26.6 degrees, so a 53°F dawn and an 80°F afternoon on the same day is normal. Many older homes in Palo Alto have no air conditioning. During heat advisories the city names its five libraries as cooling centers. Our heat waves guide covers the details.

Rain and daylight

Rain is rare. September averages 0.07 inches and the median is zero. About half a day in a typical September brings measurable rain. A rare early storm or tropical moisture from the south can bring a brief shower or thunderstorm.

The days shorten quickly. Sunrise moves from about 6:39 a.m. on September 1 to about 7:03 a.m. on September 30. Sunset moves from about 7:38 p.m. to about 6:53 p.m. Daylight drops from about 12 hours 59 minutes to 11 hours 50 minutes, dropping below 12 hours around September 26, a few days after the equinox on September 22. Stanford posts September hours of 6:30 a.m. to 7 p.m. for the Dish.

Fog, smoke and air quality

Fog

The marine layer thins in September. Coastal upwelling weakens, the ocean warms and the pressure pattern that drives the sea breeze relaxes. San Francisco and the coast get more sunny days. Gray mornings in Palo Alto become uncommon.

Smoke

September falls squarely in wildfire smoke season. On September 9, 2020, smoke from several large fires sat high above the marine layer and turned the Bay Area sky dark orange for most of the day. Smoke episodes vary hugely from year to year. Many Septembers have none. When smoke arrives, the AirNow Fire and Smoke Map gives street-level readings. Our air quality guide explains clean rooms and masks.

Ozone and fire weather

Hot, still September days are prime ozone days. The Air District can call summer Spare the Air alerts. Offshore winds and low humidity raise fire danger in the hills. The National Weather Service issues Red Flag Warnings on the worst days. Stanford can close the Dish during them.

Microclimates in September

In September the bay's cooling effect is weaker than in midsummer, but it still shows. Moffett Field, on the shoreline, averages a high of 77.7°F and a low of 57.3°F, against 79.6°F and 53.0°F at the Palo Alto station. San Jose averages 80.7°F, a degree warmer than Palo Alto.

On a heat wave day, the differences across town grow. The foothills and the Stanford campus can run several degrees hotter than the Baylands, where any hint of a sea breeze arrives first. Exposed trails at the Dish and Pearson-Arastradero are best before 10 a.m.

The ridge behaves differently in fall. During offshore wind events, Skyline Boulevard can stay warm and dry all night while cold air settles on the flats below. That warm, windy ridgetop air is part of why fire officials watch the hills so closely in September and October.

How September compares with San Francisco and San Jose

September is San Francisco's warmest month. Downtown averages a high of 70.2°F as the fog thins, still nearly ten degrees cooler than Palo Alto's 79.6°F but well inside the summer gap of 11 to 13 degrees. San Jose runs slightly warmer than Palo Alto by day and about four degrees warmer at night.

That makes September the best month for a day trip to the city. A light jacket usually does the job for the evening. Rain is negligible everywhere, with San Francisco at 0.10 inches and Palo Alto and San Jose at 0.07.

The coast in September

The open ocean reaches its warmest in September. NOAA's buoy west of San Francisco averages 58.3°F and the Monterey buoy 59.5°F. There is no published long term Half Moon Bay average, but the coast often has its clearest, calmest weather of the year in September and October. Surf is still cold enough for a full wetsuit. Inside the bay, NOAA's Redwood City reading averages 70.7°F, just past its August peak.

A sunny September Saturday on the coast is a local favorite. Our Half Moon Bay day trip guide and beaches guide cover where to go.

What to pack for September

• Summer clothes for afternoons near 80°F, plus your lightest clothes for heat waves.
• A sweater for mornings in the low 50s.
• A light jacket for San Francisco or the coast in the evening.
• Sun protection. A brimmed hat and sunscreen.
• N95 masks in case of smoke, if you are sensitive.

What September weather is good for

Warm days and a clearer coast make September one of the best months of the year. Our September trip guide covers move-in, football and other events. Weather-driven ideas:

1. Coast days. September and October bring the coast's best weather.
2. Early or shaded hikes. On hot days, choose a redwood or creekside trail from our shaded hikes guide.
3. Warm evenings outdoors. Patios downtown stay comfortable later than in summer, because the evening sea breeze is weaker.
4. A San Francisco day. The city is at its warmest. See our San Francisco guide.

September is also busy on campus, with move-in and fall sports. If your dates are firm, compare Palo Alto hotels early.

Frequently asked questions