San Francisco Rainfall Totals: 2026 Hydrological Guide And Historical Climate Data
Disambiguation Note: This guide references official meteorological data recorded at the Downtown San Francisco weather station (SFDT), currently located at the United States Mint. This dataset is distinct from measurements taken at San Francisco International Airport (SFO), which is located 13 miles south in San Mateo County and experiences a different microclimate.
Understanding San Francisco rainfall totals requires navigating a complex matrix of microclimates, oceanic influences, and Pacific storm tracks. For municipal engineers, environmental scientists, urban planners, and local residents, tracking precipitation in this region goes far beyond checking a standard weather app.
This comprehensive technical guide details San Francisco's hydrological architecture, historical precipitation benchmarks, meteorological drivers, and current data for the 2025–2026 water year.
Understanding the San Francisco Water Year: Metrics and Standards
Meteorologists and hydrologists in California do not track rainfall by the traditional calendar year. Instead, they utilize the California Water Year, which runs from October 1 to September 30 of the following year. This system aligns directly with the state's Mediterranean climate cycle, capturing the entirety of the winter wet season within a single continuous reporting period.
[October 1] ================= WET SEASON ================= [September 30] (Peak Precipitation: Dec - Mar)
The baseline metric used to evaluate any given season is the 30-Year Climatological Normal, updated every decade by the National Oceanic and Atmospheric Administration (NOAA).
- Current Baseline Normal (1991–2020): 22.89 inches (581.4 mm) of rain per water year.
- Historical Average (1849–2020): 21.78 inches.
- Measurement Location (SFDT): The official San Francisco station has recorded data continuously since 1849, making it one of the longest-running operational weather records in the western United States.
Historically, the weather station has migrated across several downtown locations, including the Merchants Exchange Building, the Federal Office Building, and its contemporary home at the U.S. Mint. Despite these minor geographical shifts, the dataset remains highly robust.
Historical Benchmarks: San Francisco's Wettest and Driest Years on Record
San Francisco's climate is characterized by extreme interannual variability. Driven by fluctuations in the Pacific jet stream and global ocean temperatures, the city can swing from severe drought to historic deluge within a single season.
The following table outlines the extreme precipitation benchmarks recorded at the Downtown San Francisco station since records began in 1849, compared against the current climatological normal.
| Water Year | Total Precipitation (Inches) | Percentage of Normal (22.89") | Hydrological Significance & Climate Drivers |
|---|---|---|---|
| 1861–1862 | 49.27 | 215.2% | The Great Flood of 1862; characterized by a relentless sequence of warm, tropical atmospheric rivers. |
| 1997–1998 | 47.22 | 206.3% | Wettest water year of the modern instrument era; driven by one of the strongest El Niño events on record. |
| 1982–1983 | 38.17 | 166.7% | Historic El Niño season featuring severe coastal erosion and localized urban flooding. |
| 2022–2023 | 33.93 | 148.2% | Marked by an extraordinary sequence of 12 atmospheric rivers between late December and March. |
| 1991–2020 Normal | 22.89 | 100.0% | Current climatological baseline established by NOAA. |
| 1923–1924 | 9.17 | 40.1% | Severe early-century drought that accelerated regional water infrastructure development. |
| 1976–1977 | 8.01 | 35.0% | Driest water year in recorded San Francisco history; initiated a major state-wide water conservation era. |
24-hour rain totals for San Francisco Bay Area
The 2025–2026 Hydrological Season: Current Totals and Climatological Context
The 2025–2026 water year in San Francisco has developed under the influence of a transitioning equatorial Pacific. Following a period of neutral conditions, a weak La Niña pattern emerged in late autumn, exerting a distinct influence on the western storm track.
Typically, La Niña winters split the jet stream, often steering the wettest storms north into the Pacific Northwest while leaving Southern California dry. However, the Bay Area acts as a transitional zone. During weak La Niña years, the regional outcome depends heavily on the behavior of high-pressure ridges off the coast of California.
For the 2025–2026 water year, the cumulative precipitation totals at the Downtown station have tracked near the historical median, punctuated by highly concentrated atmospheric river events in mid-winter. The distribution of precipitation highlights the stark reality of California's "all-or-nothing" climate: a significant percentage of the yearly total fell during a few intense multi-day storms, rather than being distributed evenly across the wet season.
The Science of Bay Area Precipitation: Atmospheric Rivers and ENSO Cycles
To interpret San Francisco's rainfall totals, one must understand the large-scale atmospheric engines that dictate West Coast hydrology. Two primary phenomena control how much rain falls on the city: the El Niño-Southern Oscillation (ENSO) and Atmospheric Rivers (ARs).
The El Niño-Southern Oscillation (ENSO)
ENSO is a periodic fluctuation in sea surface temperatures and atmospheric pressure across the equatorial Pacific.
- El Niño Phases: Warm ocean anomalies shift the subtropical jet stream southward, frequently positioning it directly over California. This pathway acts as a conveyor belt for moisture-rich storms, often resulting in above-average winter rainfall totals for San Francisco.
- La Niña Phases: Cooler equatorial waters push the jet stream northward. While this typically results in drier conditions for Southern California, San Francisco's geographic position at roughly 37.7° N latitude sits right on the boundary. Consequently, La Niña years can result in highly unpredictable totals, ranging from severe dry spells to normal accumulation.
Atmospheric Rivers: The West Coast's Water Towers
Atmospheric rivers are narrow corridors of concentrated moisture transport in the atmosphere, often referred to colloquially as the "Pineapple Express" when they originate near Hawaii.
Atmospheric River Scale and Hydrological Impact
AR-1 and AR-2 (Beneficial): These moderate storms provide critical reservoir replenishment and soil moisture without causing widespread infrastructure damage. They are vital for maintaining the region's water security.
AR-3 (Transitional): These systems represent a mix of beneficial water supply and hazardous flooding risks. They require close monitoring by emergency management officials.
AR-4 and AR-5 (Hazardous): These powerful storms deliver extreme rainfall intensity, often overwhelming municipal storm drains, triggering mudslides on steep coastal terrain, and causing local rivers to breach their banks.
The Microclimate Effect: Why SFDT and SFO Differ
A common point of confusion is the variance between San Francisco Downtown (SFDT) and San Francisco International Airport (SFO) rainfall reports.
San Francisco’s complex topography—specifically the presence of Twin Peaks and Mount Davidson—acts as a physical barrier to incoming maritime weather systems. SFO lies in a partial rain shadow created by the Santa Cruz Mountains to the west and the San Bruno Gap.
Furthermore, the downtown station sits further north, exposing it more directly to storms entering through the Golden Gate gap. It is common for the Downtown station to record 2 to 4 inches more rainfall per season than SFO.
Actionable Guide: How to Track Real-Time San Francisco Precipitation Data
For professionals requiring precise, real-time rainfall data for engineering, construction, environmental compliance, or agricultural planning, relying on commercial consumer apps is insufficient. Follow this structured process to access official, certified meteorological records.
Step 1: Utilize the National Weather Service (NWS) Daily Precipitation Reports
The NWS San Francisco Bay Area office publishes daily hydrometeorological products.
- Navigate to the official NWS San Francisco portal.
- Access the Daily Rainfall and Climatological Report (CLI) for station SFDT.
- This report provides 24-hour precipitation totals, monthly accumulation, and departure-from-normal metrics.
Step 2: Access the California Nevada River Forecast Center (CNRFC)
For interactive mapping and regional context:
- Open the CNRFC Interactive Precipitation Map.
- Filter by the "San Francisco Bay" hydrologic basin.
- This tool allows you to view observed precipitation totals over 1-hour, 6-hour, 24-hour, and multi-day windows, utilizing calibrated radar data verified by ground rain gauges.
Step 3: Download Historical Datasets via NOAA’s National Centers for Environmental Information (NCEI)
If you require historical data for statistical analysis, legal cases, or environmental impact reports:
- Visit the NOAA NCEI Climate Data Online (CDO) portal.
- Search for station ID USW00023272 (San Francisco Downtown).
- Select your desired date range to download verified Quality-Controlled Local Climatological Data (LCD) in CSV or PDF formats.
Frequently Asked Questions About San Francisco Rainfall
What is the average annual rainfall for San Francisco?
The official average annual rainfall for San Francisco is 22.89 inches (581.4 mm), based on the NOAA 30-year climatological normal measured from 1991 to 2020. This average is compiled using the California Water Year cycle from October 1 to September 30.
When does the rainy season start and end in San Francisco?
San Francisco’s rainy season primarily spans from November through April, with peak precipitation typically occurring between December and February. On average, more than 80% of the city’s annual rainfall occurs during this five-month window, while summers are dry due to the presence of the North Pacific High-Pressure system.
What is the wettest single day in San Francisco history?
The wettest calendar day on record for downtown San Francisco occurred on November 25, 1861, when the city recorded 5.54 inches of rain in a 24-hour period. In the modern era, the second-wettest day occurred on December 31, 2022, when an atmospheric river dropped 5.46 inches of rain, causing widespread urban flooding.
Does it ever snow in San Francisco?
Snow is extraordinarily rare in San Francisco due to the moderating influence of the Pacific Ocean. However, measurable snow has occurred historically, most notably on February 5, 1887, when 3.7 inches fell downtown, and more recently on February 5, 1976, when a brief dusting accumulated on the city's highest peaks, such as Twin Peaks and Mount Davidson.
How do atmospheric rivers impact San Francisco's water supply?
Atmospheric rivers are critical to the region's water supply, accounting for 30% to 50% of California's annual precipitation. While they can cause severe local flooding, they are essential for filling municipal reservoirs, replenishing groundwater basins, and building the Sierra Nevada snowpack, which supplies drinking water to the Bay Area via the Hetch Hetchy aqueduct.
Navigating Changing Hydrological Realities
As regional climate patterns evolve, managing stormwater and monitoring local precipitation trends has become a cornerstone of Bay Area infrastructure planning. Whether you are managing a municipal job site, evaluating real estate risks, or tracking the local water supply, relying on validated data from NOAA and the National Weather Service ensures decision-making is grounded in scientific accuracy. Keep this guide bookmarked to track historical context and current developments throughout the 2025–2026 water year.
