San Francisco High Temperature on May 31: Can It Hit 70°F?

San Francisco’s weather is notoriously hard to predict even 24 hours out. The market for May 31’s high temperature is sitting at a coin flip, with 70°F or higher priced at roughly even odds. That even split tells you something: traders are genuinely split on whether the Bay Area fog machine stays quiet long enough to push temperatures into the low seventies.

The market question asks for the highest temperature recorded in San Francisco on May 31, 2026. The 70°F or higher outcome is priced at $0.52, implying a 51.5% probability. The field of alternatives runs from 51°F or below all the way up to the primary threshold. The market resolves at noon on May 31, 2026, with $11,530 in total volume traded.

How the San Francisco Temperature Contract Works

This market resolves based on the highest temperature recorded in San Francisco on May 31, 2026. Traders are choosing from a bracket of outcomes spanning roughly 20 degrees of range.

• YES (70°F or higher) is priced at $0.52, implying a 51.5% probability that San Francisco reaches or exceeds the threshold.
• NO covers all outcomes below 70°F, including the brackets at 68-69°F, 66-67°F, 64-65°F, and lower.

The NO side pays out if San Francisco’s daily high lands anywhere in the twelve temperature brackets below 70°F. Late May is not the heart of San Francisco’s warmer season. The city’s marine layer and afternoon fog patterns often cap afternoon highs in the mid-to-upper sixties, even when inland valleys are baking. A persistent marine influence on May 31 pushes the reading toward the 64-67°F range, which is where the bulk of alternative outcomes cluster.

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Momentum and Market Signals: Sharp Drop, Thin Volume

The momentum composite here is a red flag. The 70°F outcome has dropped 11% in the past hour and 16.5% over 24 hours, despite a trend score of 70.55. That combination, a high trend score alongside steep short-term declines, suggests a market that spiked on early optimism and is now correcting as traders reassess the atmospheric setup for May 31.

Total volume is $11,530, with $8,010 of that trading in the last 24 hours. Liquidity sits at $86,588, which is relatively deep for a market this size. The thin volume means a single moderately sized trade can move the price sharply. Here’s what the measurements are telling us: the market opened at $0.45, ran up to $0.74 at its peak, and has been selling off since. That price arc reflects traders chasing a warm signal and then retreating.

• The 70°F outcome dropped 11% in one hour, driven by reassessment of the marine layer forecast for May 31.
• The 24-hour decline of 16.5% erases much of a prior rally, suggesting early buyers are trimming positions.
• Total volume of $11,530 is thin enough that any fresh weather data could move the price by five to ten percentage points quickly.
• Liquidity at $86,588 is healthy relative to volume, meaning the order book can absorb moderate trades without slippage.
• The trend score of 70.55 reflects recent directional activity but lags the current hourly sell-off.

Lines Analysis: What the Atmospheric Setup Says

San Francisco’s late May climatology is the core variable here. The city’s mean daily high in late May runs between 62°F and 66°F, with warm outliers pushing into the low seventies when the marine layer retreats and an offshore flow develops. The 70°F threshold is achievable but requires a specific setup: dry continental air, light winds, and a break in the coastal stratus pattern. That is not the default state for the last day of May.

The data doesn’t care about the politics, and in this case it doesn’t particularly favor the primary outcome either. The bracket structure of this market matters. Even if San Francisco misses 70°F narrowly, landing in the 68-69°F range still counts as NO for the primary outcome. That means the YES side needs a clean break above the threshold, not just a warmer-than-average afternoon. The probability of hitting exactly the right atmospheric window narrows the effective YES condition further.

• A National Weather Service forecast showing a high-pressure ridge building over Northern California before May 31 would push the YES probability sharply higher.
• Any marine layer advisory or fog forecast issued for the San Francisco Bay Area on May 30 would favor the NO side and the lower temperature brackets.
• Wind direction data from SFO is the most reliable real-time signal: northerly or easterly winds favor warming, westerly winds favor cooling.
• The 68-69°F bracket is likely the single most important alternative outcome. A near-miss would redistribute value toward that bracket at the expense of both YES and the lower ranges.

Total volume of $11,530 is modest. The data slightly favors the NO side at current prices, given San Francisco’s climatological tendency to hold below 70°F in late May and the recent downward price momentum. But the market is pricing genuine uncertainty, not a settled scientific question.

LINES VERDICT

SLIGHT NO LEAN

San Francisco’s late-May marine layer and the steep recent sell-off in the 70°F outcome both point away from YES. The climatological baseline and the price momentum are aligned on the same side.

What the market says: At 51.5%, the market treats this as a genuine coin flip with a slight lean toward YES. Given thin volume and a sharp recent decline, that number is volatile and could shift several points on any updated weather forecast before the May 31 resolution deadline.

Key unknown: The single most important input is the National Weather Service high-temperature forecast for San Francisco on May 31. Any forecast update issued on the morning of May 31 showing a high at or above 70°F would reprice the YES outcome significantly higher.