San Francisco typically experiences mild spring weather in mid-May, with average daily highs around 60-65°F. A high of 47°F or below would represent a substantial cold anomaly for the Bay Area during this season, requiring an unusual weather system to push temperatures well below normal. At just 1% YES odds, the market is pricing in an extremely low probability of such a cold snap occurring on May 19. This reflects both historical weather patterns and current meteorological forecasts for the region. The market uses official National Weather Service data for San Francisco to determine resolution, ensuring objective and verifiable outcomes. Traders have demonstrated near-unanimous conviction that normal spring conditions will prevail on this date, leaving essentially no disagreement visible in the market structure. The market closes at midnight UTC on May 19, providing a precise 48-hour window to track actual temperature outcomes. Such heavily skewed markets are common in seasonal weather prediction, where long-term climatological patterns create strong baseline expectations.
Deep dive — what moves this market
San Francisco's microclimate is shaped by its position on the Peninsula, with the Pacific Ocean providing a thermoregulating influence that keeps temperatures relatively moderate year-round. In mid-May, the region typically transitions into its early summer weather pattern, characterized by stronger afternoon heating and onshore flow that limits maximum temperatures. Historical data from the National Weather Service shows that temperatures at or below 47°F in late May are exceptionally rare, occurring perhaps once every several years, if at all. The last significant May cold snap in San Francisco was in 2011, when a late-season system brought unusually cold air. To reach 47°F or below on May 19, San Francisco would require an unusual atmospheric setup: a strong high-pressure system to the north pushing cold air southward, combined with cloud cover that prevents daytime warming. Such systems become increasingly uncommon as spring progresses toward summer. The jet stream pattern would need to dip southward into California, steering a cold air mass directly into the Bay Area. Current medium-range forecasts show no indication of such a system developing. The warming trend that typically accelerates in mid-May would actively work against cold development. Conversely, normal May conditions—or even warmer-than-average conditions—would easily keep highs well above 47°F. San Francisco averages highs in the low 60s during May, and readings in the upper 60s or even low 70s are not uncommon on clear days. The ocean-moderated climate rarely produces extreme cold anomalies. Even unusual May weather systems typically produce overcast conditions and onshore winds that moderate temperatures rather than depress them dramatically. The 1% YES odds reflect both the climatological rarity of such an event and the lack of any current meteorological signal pointing toward cold conditions. Traders pricing the market at this extreme are essentially saying San Francisco will experience normal-to-warm spring weather, as expected for mid-May. This is a low-volatility market where the outcome is driven almost entirely by whether baseline expectations hold.
What traders watch for
May 18-19 overnight minimum temperatures and morning readings establish whether cold air mass can persist into daytime hours.
National Weather Service May 17 forecast update: monitor for any unusual cold system mention or sub-50°F high guidance.
Upper-level jet stream position on May 19: northern placement favors normal spring warmth; southward dip increases cold risk.
Morning marine layer persistence and onshore wind strength: thick fog and westerlies typically push temperatures higher.
How does this market resolve?
The market resolves YES if San Francisco's official National Weather Service high temperature on May 19, 2026 is 47°F or below; otherwise NO. Resolution occurs at midnight UTC on May 19 based on final verified NWS data.
Prediction markets aggregate trader expectations into real-time probability estimates. On Polymarket Trade, 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. This page summarizes the market state for readers arriving from search; for live trading (place orders, see order book depth, execute a trade) open the full interactive page linked above.
