Access live NWS precipitation data from KSFO overlaid with Kalshi market pricing to identify mispriced rain contracts in one of California's most distinctive microclimates.
San Francisco operates under a Mediterranean climate (Köppen Csb) characterized by pronounced wet and dry seasons, with precipitation almost entirely confined to the winter months. The city's location on a peninsula surrounded by the Pacific Ocean and San Francisco Bay creates unique meteorological conditions, including persistent summer fog that delivers moisture without measurable rainfall. Elevation changes across the city—from sea level to 934 feet at Twin Peaks—produce significant precipitation gradients, though KSFO at the airport captures the official measurement for contract settlement.
The station records an average of 23.65 inches of precipitation annually, with January ranking as the wettest month at 4.45 inches and July receiving effectively zero measurable precipitation (0.00 inches). The rainy season runs from November through March, accounting for 85% of annual totals, while the six-month period from May through October contributes less than 1 inch combined. San Francisco averages 73 days per year with measurable precipitation (≥0.01 inches), with December seeing rain on an average of 11 days despite shorter daylight hours.
These stark seasonal boundaries create highly predictable baseline conditions that make San Francisco contracts attractive for directional positioning during summer months, when zero-precipitation outcomes approach certainty. Winter markets present greater complexity, as atmospheric river events can deliver 3-5 inches in 24-hour windows, creating binary outcomes that challenge even sophisticated traders. The Mediterranean pattern produces market dynamics unlike humid continental or subtropical climates, with liquidity concentrating heavily around seasonal transition periods in October-November and March-April when forecast uncertainty peaks.
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View Dashboard →San Francisco precipitation becomes predictable 7-10 days in advance during winter storm sequences, when the GFS and ECMWF models track atmospheric river positioning with reasonable accuracy. Markets typically see highest liquidity 3-5 days before potential precipitation events as medium-range ensembles converge, though the 12z NAM runs often introduce volatility by modeling orographic enhancement differently than global models. Summer contracts (June-August) trade at steep discounts to zero-precipitation outcomes, occasionally creating arbitrage opportunities when contracts price in >5% rain probability despite climatological precedent showing <1% historical frequency.
The dominant edge case in San Francisco trading involves atmospheric river forecasts that shift 50-100 miles north or south in final model runs, often 12-24 hours before precipitation onset. The airport's bayside location means systems tracking slightly offshore frequently produce heavy precipitation in coastal hills while leaving KSFO with trace amounts that fail settlement thresholds. Traders who anchor to city-wide rain observations without confirming KSFO-specific totals systematically overestimate contract values during marginal setups. Winter 2023 demonstrated this pattern repeatedly, with downtown locations recording 0.15-0.25 inches while KSFO measured 0.00-0.01 inches on five separate occasions.
Another trap involves trading spring contracts (March-April) based solely on seasonal climatology without accounting for ENSO phase. La Niña springs frequently extend dry conditions through April, while El Niño patterns can maintain active storm tracks into May. The 00z GFS runs issued 5-7 days out show the highest skill for multi-day precipitation totals, as they incorporate the most complete Pacific buoy data. Traders who react to 18z runs often chase model noise rather than signal, particularly when mesoscale convective elements appear in Bay Area forecasts—a phenomenon that rarely produces measurable KSFO precipitation despite impressive radar presentation.
KSFO operates an ASOS (Automated Surface Observing System) station located at San Francisco International Airport at 13 feet elevation, approximately 0.5 miles from the bay shoreline. The tipping-bucket rain gauge reports at one-minute intervals, with data aggregated into hourly METAR observations and daily CLI (Climate Summary) reports that serve as the official settlement source for Kalshi contracts. The gauge measures precipitation in 0.01-inch increments, meaning any accumulation between 0.001 and 0.009 inches registers as trace precipitation, recorded as 0.00 inches in official totals and failing to meet the standard ≥0.01-inch settlement threshold. Kalshi contracts explicitly reference the daily CLI report issued by the Weather Forecast Office in Monterey (WFO MTR), typically published between 7-9 AM Pacific Time for the previous calendar day's totals. Traders should note that KSFO's bayside microclimate frequently records lower totals than elevated or western neighborhoods—downtown locations at 200-300 feet elevation often receive 15-20% more annual precipitation than the airport. For contract settlement purposes, only the KSFO gauge reading matters, regardless of precipitation observed elsewhere in the city. Quality control procedures occasionally flag erroneous readings due to gauge maintenance or calibration issues, which can delay CLI publication and contract settlement by 24-48 hours, though such occurrences average fewer than three times annually.
KSFOMarch through May marks the transition from wet to dry seasons, with March averaging 2.95 inches but May dropping to 0.53 inches. Late-season atmospheric rivers occasionally deliver significant April precipitation during El Niño years, creating trading opportunities when markets underestimate lingering storm track activity. Fog frequency increases in May, though this moisture rarely produces measurable precipitation at KSFO.
June through August represents the driest period, with all three months averaging 0.00-0.05 inches combined. The marine layer produces persistent fog that blankets coastal areas most mornings, but measurable precipitation is extraordinarily rare—KSFO records zero precipitation on 99% of summer days over the 30-year climate normal. Summer contracts consistently trade near maximum discount to precipitation outcomes.
September and October extend summer drought conditions with minimal precipitation, averaging 0.24 and 1.13 inches respectively. November marks the return of Pacific storm systems, averaging 2.49 inches as the jet stream shifts southward. This transition period creates the year's highest forecast uncertainty, with first significant storms often arriving 2-3 weeks earlier or later than climatological averages, producing volatile market conditions.
December through February constitutes the core rainy season, delivering 11.8 inches (50% of annual total) across approximately 30 precipitation days. Atmospheric river events account for most accumulation, with individual storms capable of producing 3-5 inches in 24-48 hours. Extended dry periods lasting 10-14 days between systems are common, creating distinct trading windows. January precipitation totals vary dramatically year-to-year, ranging from under 1 inch to over 10 inches depending on atmospheric river frequency.
San Francisco receives an average of 23.65 inches of precipitation annually at KSFO, with 85% falling between November and March. The city averages 73 days per year with measurable precipitation (≥0.01 inches), though this varies significantly by neighborhood due to elevation differences.
January is the wettest month, averaging 4.45 inches of precipitation, followed closely by December at 4.13 inches and February at 3.76 inches. These three months account for over half of San Francisco's annual precipitation total.
San Francisco receives effectively zero measurable precipitation from June through August, with July and August each averaging 0.00 inches. The famous summer fog delivers moisture but rarely produces rainfall meeting the 0.01-inch measurement threshold required for Kalshi contract settlement.
Kalshi offers binary contracts on whether KSFO will record ≥0.01 inches of precipitation on specific dates or within defined brackets. Contracts settle based on the official NWS CLI report from KSFO, with yes contracts paying $1 if the threshold is met and $0 if not. Traders can enter or exit positions any time before market close.
Liquidity peaks during seasonal transition periods in October-November and March-April when forecast uncertainty is highest, and during atmospheric river events when 3-5 day forecasts show potential for significant precipitation. Summer markets trade at low volumes due to near-certain zero-precipitation outcomes.
Atmospheric rivers are narrow corridors of concentrated moisture that deliver 30-50% of West Coast annual precipitation in just a few events. For San Francisco, these systems create high-stakes binary outcomes, as slight track variations of 50-100 miles determine whether KSFO records heavy precipitation or remains dry, producing rapid price swings in Kalshi markets.
KSFO sits at 13 feet elevation on the bay side of the peninsula, while downtown and western neighborhoods reach 200-900 feet elevation. Orographic lift enhances precipitation at higher elevations, and proximity to the Pacific increases rainfall on the western slopes. Only KSFO measurements count for Kalshi settlement, regardless of city-wide conditions.
Winter storm systems become predictable 7-10 days in advance when atmospheric river patterns are established, with highest forecast skill at 3-5 days out. Summer precipitation is predictable months in advance due to climatological certainty of dry conditions. Spring forecasts carry the highest uncertainty due to transitional weather patterns.
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