SF May 21 High Temp: Will 68-69°F Win?

San Francisco’s weather on May 21 is one of the most compressed betting markets you’ll see. Traders are pricing the 68-69°F band at 27.5% probability, making it the single most likely outcome across eleven possible temperature ranges. That’s not a confident bet. That’s the market admitting it genuinely does not know where the thermometer lands tomorrow.

The 68-69°F outcome sits at $0.28 YES and $0.73 NO. Total market volume is $4,508, with $4,467 of that trading in the last 24 hours. This market effectively opened today. Thin liquidity at $7,864 means a single moderate-sized bet could move this price sharply before the 2026-05-21 12:00:00 resolution.

How the Contract Resolves: Temperature Bands and the 68-69°F Threshold

This market resolves YES if San Francisco’s highest recorded temperature on May 21, 2026 falls between 68°F and 69°F inclusive. Resolution source is Polymarket’s market resolution mechanism, which typically draws on official weather station data. The contract closes at noon on May 21, so only the morning peak temperature window matters.

• YES ($0.28, 27.5%): The high temperature on May 21 lands at 68°F or 69°F.
• NO ($0.73, 72.5%): The high temperature falls outside that two-degree band, anywhere from 57°F or below up through 76°F or higher.

The NO side wins in nine out of ten possible temperature ranges. San Francisco’s May climate is famously erratic. Fog suppression, marine layer timing, and offshore wind patterns can push a daily high from 58°F to 74°F within the same week. A miss on the 68-69°F band does not require dramatic conditions. It just requires the temperature landing one degree in either direction.

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Momentum and Market Signals: A Sharp Move on Thin Volume

The momentum composite here combines an 8.0% one-hour gain, a 13.0% 24-hour gain, and a trend score of 51.08. That combination points to genuine buying pressure into this specific band, most likely driven by a weather model update or forecast narrowing toward the 68-69°F range for tomorrow afternoon.

That said, $4,508 in total volume is extremely thin. The $4,467 in 24-hour volume tells you almost all of this activity is new. At this liquidity level, the 13.0% price gain could reflect a handful of trades rather than broad trader consensus. Here’s what the measurements are telling us: the market is live and moving, but the signal is fragile.

Key Factors

• The 1-hour price change of +8.0% and 24-hour change of +13.0% together suggest a recent forecast update narrowed the expected high toward the 68-69°F range.
• Total volume of $4,508 is well below $1 million, meaning this price can move dramatically on a single trade or a new forecast model run.
• San Francisco’s May 21 historical highs cluster between 60°F and 72°F, giving the 68-69°F band a plausible but far from dominant share of the probability space.
• The resolution window closes at noon on May 21, meaning only pre-noon temperatures determine the outcome if a marine layer burns off late in the afternoon, it may not count.
• Eleven outcome bands split the probability. Even the leading band holds less than 30%. The market is pricing genuine forecast uncertainty, not scientific consensus.

Lines Analysis: What the San Francisco Forecast Actually Shows

The 68-69°F range is the modal forecast band right now. That means current weather models are pointing here more than anywhere else. San Francisco’s typical May pattern involves a marine layer that burns off by midday, pushing temperatures into the mid-to-upper 60s in neighborhoods away from the coast. The Inner Sunset stays cooler. The Mission and Downtown run warmer. Which station determines resolution matters.

The conditions that push the outcome outside 68-69°F are not exotic. A stronger marine layer keeps the high in the 62-65°F range. A clear, dry offshore flow pushes it toward 72°F or above. Both scenarios happen regularly in late May. The 72.5% NO probability reflects exactly that: the two-degree target is narrow, and San Francisco’s microclimate variability is real.

Signals to Monitor

• National Weather Service San Francisco Area Forecast Discussion updates overnight May 20 into May 21 will be the clearest leading indicator for this contract.
• Marine layer depth reported at SFO early May 21 morning directly predicts whether the high stays below 66°F or breaks toward 70°F and above.
• Wind direction at the Bay is decisive. Westerly flow keeps temperatures suppressed. Easterly or offshore flow accelerates warming past 70°F.
• Any forecast revision moving the expected high to 70°F or above would shift probability mass away from 68-69°F and toward the next band up.
• The noon resolution cutoff means traders should watch morning forecast models, not afternoon updates.

At $4,508 in total volume, this market is pricing uncertainty, not science. The data slightly favors the 68-69°F band right now, which is why YES has risen to 27.5%. But nine other bands are still in play, and San Francisco’s weather has a long history of humbling confident forecasters.

LINES VERDICT

Narrow Band, Real Uncertainty

The 68-69°F band is the current modal forecast for San Francisco on May 21, which explains the recent price move. But at 27.5% probability, the market is saying it gets this right less than one in three times, and that honesty is correct given how many bands can absorb the actual reading.

What the market says: 27.5% probability means traders see 68-69°F as the most likely single outcome but still expect the high to land somewhere else nearly three out of four times. With resolution at 2026-05-21 12:00:00 and volume below $5,000, this price remains highly sensitive to any overnight forecast update.

Key unknown: The National Weather Service overnight forecast discussion for the San Francisco Bay Area on May 21 is the single event that could reprice this contract before resolution. A forecast nudge toward 70°F or a stronger marine layer signal would immediately shift probability mass out of the 68-69°F band.

Scientific Context: San Francisco’s May Temperature Distribution

San Francisco’s May daily high temperatures historically span a wide range. Days in the low 60s and days in the low 70s both occur regularly within the same week. The city’s microclimate geography, driven by the proximity of the Pacific, the Bay, and the coastal hills, produces high day-to-day variability that makes any two-degree temperature band a low-probability target even when forecasts are reasonably confident. The data doesn’t care about the politics of any particular forecast model. It just produces a distribution that is genuinely wide in May, and that distribution is exactly what this eleven-band market is attempting to price.

FAQ

• What does 27.5% probability mean here? It means traders currently believe there is roughly a one-in-four chance San Francisco’s May 21 high lands between 68°F and 69°F. Ten other temperature bands share the remaining probability.
• What does the NO contract represent? The NO contract at $0.73 reflects a 72.5% probability that the daily high falls outside the 68-69°F range, either cooler or warmer. Nine out of ten outcome bands resolve NO for this specific contract.
• What data would move the price most? A National Weather Service forecast update showing a clear high near 70°F or a confirmed strong marine layer forecast below 66°F would both shift probability mass out of this band and move the price sharply.
• When does this market resolve? The contract resolves at 2026-05-21 12:00:00. Only temperatures recorded before noon on May 21 in San Francisco determine the outcome.
• Is this market liquid enough to trust? Total volume is $4,508 and liquidity is $7,864. Both are well below $1 million. The current price reflects a small number of trades and can move dramatically on a single new bet or forecast update.

This analysis reflects market conditions as of 2026-05-20 14:17:29. Prediction market probabilities are volatile and shift as new data and regulatory decisions emerge, especially as the 2026-05-21 12:00:00 resolution date approaches. Lines.com does not accept bets or provide financial or gambling advice. All market outcomes are uncertain.