San Francisco's May weather typically ranges from 55 to 70 degrees Fahrenheit, making 49°F or colder an unusually cold scenario for this time of year. The threshold of 49°F is notably rare in late May, historically occurring only during uncommon cold-air outbreaks. This market resolves based on the National Weather Service's official high temperature reading for San Francisco on May 18, 2026, providing a definitive, publicly verifiable outcome that eliminates interpretation disputes. The current 1% odds reflect an extremely low probability that the Bay Area will experience such unseasonable cold, suggesting traders believe warming trends will firmly persist through mid-May. A high of 49°F or below would require an unusual cold snap or atmospheric disruption—perhaps an anomalous marine layer, a rare low-pressure system pushing cool air onshore, or an unexpected weather pattern reversal. The market's tight pricing indicates strong consensus around a milder outcome. Traders monitoring this contract are essentially assessing the risk of spring-season temperature outliers versus the baseline expectation. Given timing and historical precedent, most market participants have confidently priced in a high above 50°F.
What factors could move this market?
San Francisco's microclimate during May is dominated by the Pacific Ocean's moderating influence, which typically suppresses extreme temperature swings. Historical data from the last two decades shows that May highs in San Francisco average around 67°F, with the coldest readings typically falling between 50 and 55°F on rare occasions when marine fog persists unusually late or a weak low-pressure system tracks through Northern California. A high of 49°F or colder would place May 18 among the coldest May days on record for the city—a 3-to-4 standard deviation event from the mean. Such a scenario would require a convergence of unfavorable atmospheric conditions: a strong high-pressure system over the interior forcing cold air toward the coast, persistent marine layer without break, or an early-season weather system delivering cooler-than-normal air masses. Examining May records from the past 30 years, only 2–3 days per May season dip below 50°F in San Francisco, and sub-49°F readings are exceedingly rare. The current 1% odds reflect this statistical rarity and the market's assessment that no such system is forecast for May 18.
Factors pushing toward YES (colder outcome) include: an unexpected strengthening of marine fog layers driven by coastal upwelling, an anomalous low-pressure system or cold front pushing inland from the Pacific, or a sudden reversal in the warming trend that has characterized recent weeks. Weather models would typically show the signal 7–10 days in advance, and with only two days until resolution, the absence of such model guidance strongly favors the warmer scenario. Historical cold-snap events in May have often been preceded by noticeable forecast signals; the lack of a current cold-event warning from the National Weather Service further reduces the probability of the YES outcome.
Factors pushing toward NO (warmer outcome) include: the seasonal trend toward warming temperatures as May progresses, the typical dominance of high pressure during this time of year, and the ocean's moderating influence, which becomes more pronounced as it warms. May in San Francisco is characterized by reliable, mild conditions with highs commonly reaching 65–72°F. The current 1% pricing suggests traders believe the baseline forecast models indicating temperatures in the 60–70°F range, and they assign minimal tail-risk probability to a cold outlier.
The 1% odds represent near-zero conviction in the YES outcome, reflecting the market's view that a high of 49°F or below is effectively a statistical impossibility for May 18 given current conditions, historical patterns, and near-term weather forecasts. This extreme divergence between YES and NO odds underscores how rare and unlikely such a weather scenario is perceived to be. The tiny liquidity ($2,704) and low trading volume ($5 in 24 hours) suggest this is a niche recreational market with limited institutional participation, typical of weather derivatives at the daily granularity level.
What are traders watching for?
National Weather Service official high-temperature reading for San Francisco on May 18, 2026—the definitive resolution criteria
Marine layer persistence and coastal upwelling strength; any unusual fog patterns suppressing daytime temperatures
Movement of low-pressure systems or cold fronts from the Pacific; any unexpected weather reversals toward cooler air
Real-time weather model updates May 17–18; any shift from typical 60–72°F May patterns toward unseasonably cold forecasts
How does this market resolve?
This market resolves based on the National Weather Service's official high temperature reading for San Francisco on May 18, 2026. YES wins if the high is 49°F or below; NO wins if it exceeds 49°F.
Polymarket Trade is an independent third-party interface to the Polymarket CLOB prediction market exchange on Polygon — not affiliated with Polymarket, Inc. Prediction markets aggregate trader expectations into real-time probability estimates. Every market question resolves YES or NO based on a specific event outcome; traders buy shares of the side they believe will resolve positively. Prices range 0¢ (certain no) to 100¢ (certain yes) and naturally reflect the crowd-implied probability of YES. Polymarket Trade is non-custodial — your funds never leave your wallet. Open the full interactive page linked above to place orders, see order book depth, and execute a trade.
