Tokyo May 31 Low: Can 21°C Hold at 46%?

Tokyo’s overnight temperature market for May 31 is moving fast. The 21°C outcome has surged nearly 27% in 24 hours, climbing from a mid-market position to 46% implied probability. That kind of momentum in a short-dated weather contract usually means one thing: fresh forecast data is landing, and traders are repositioning around it.

The market question asks: what will the lowest temperature recorded in Tokyo be on May 31? The 21°C outcome currently trades at $0.46 YES and $0.54 NO, with the market closing at noon on May 31, 2026. Total volume sits at $4,451, with $3,217 of that trading in the last 24 hours.

How the 21°C Contract Works

This is a single-outcome market. YES pays if official measurements confirm Tokyo’s minimum temperature on May 31 lands at exactly 21°C. Resolution follows the designated source for that day’s low reading. Every other temperature bucket, from 15°C or below up to 25°C or higher, competes for the remaining probability mass.

• YES ($0.46): Tokyo’s official May 31 low registers exactly 21°C.
• NO ($0.54): The low registers at any other temperature, including adjacent buckets like 20°C or 22°C.

The NO side wins whenever the overnight low drifts even one degree in either direction. Tokyo’s late-May temperatures sit in a transition zone. The city’s historical lows in late May range roughly from the high teens to the low-to-mid twenties depending on the synoptic pattern. A persistent southerly flow pushes lows upward toward 22°C or 23°C. A clearing cold front drags them back toward 19°C or 20°C. That uncertainty is exactly what the market is pricing right now.

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Momentum and Market Signals

The momentum composite here is unusually strong for a micro-duration weather contract. A 16.5% gain in one hour combined with a 26.5% gain over 24 hours and a trend score near 75 points to a sharp directional shift. The most likely driver is updated numerical weather prediction output, particularly the Japan Meteorological Agency’s short-range forecasts or the European Centre ensemble guidance for the Tokyo area overnight into May 31. When forecasters cluster near a specific temperature, traders follow.

Total volume of $4,451 is thin. The 24h volume of $3,217 represents most of the market’s lifetime activity, which tells you this market activated late. Liquidity at $21,945 is deeper than the volume suggests, meaning the order book can absorb movement, but a single large trade could still reprice the contract sharply. At this volume level, the 46% price reflects active trader conviction, not broad market consensus.

• The 1h and 24h price changes together signal a rapid convergence around the 21°C forecast band, likely driven by updated short-range model output for Tokyo overnight May 30 to May 31.
• Trend score of 74.95 ranks this among the higher-momentum short-dated weather contracts currently active on the platform.
• Liquidity exceeds volume by nearly five to one, which is unusual and suggests market makers have staged deep books anticipating late-money positioning before the noon resolution.
• The 54% NO price reflects the single-bucket structure: even if 21°C is the modal forecast, the probability mass is spread across eleven outcomes, making any single bucket structurally capped.
• Thin total volume means this price can move sharply on any new data or a single motivated trader entering before the market closes.

Lines Analysis: What the Temperature Data Is Saying

Here’s what the measurements are telling us. Late May in Tokyo sits in the pre-rainy season transition. The Baiu front, Japan’s early summer rain band, typically begins influencing the Kanto region in early to mid-June. In the days immediately before frontal arrival, southerly maritime air tends to hold overnight lows in the 20°C to 23°C range. If current synoptic patterns show a ridge of high pressure over the region, the 21°C to 22°C band becomes the modal outcome. The market’s move toward 21°C is consistent with that pattern.

What makes the NO side real is the precision required. A minimum of 20°C or 22°C, each just one degree away, each carries its own market probability. The Japan Meteorological Agency’s automated surface observing network measures Tokyo’s official low, and forecast uncertainty at 24 to 48 hours is typically plus or minus 2°C for minimum temperatures. That spread covers at least four separate outcome buckets. The data doesn’t care about the politics of which bucket wins. The atmosphere resolves this with a single number.

• Japan Meteorological Agency short-range forecast updates, expected in the hours before the market closes, are the single highest-impact data release for this contract.
• European Centre for Medium-Range Weather Forecasts ensemble output for the Tokyo area overnight May 30 to May 31 would shift probability mass if it diverges from JMA guidance.
• Any significant cloud cover change over the Kanto Plain affects radiative cooling overnight and can push the minimum temperature up or down by 1°C to 2°C.
• Urban heat island effects in central Tokyo tend to keep official lows 1°C to 2°C above surrounding suburban stations, a factor already baked into JMA’s Tokyo station readings.

The market is pricing uncertainty, not science. With $4,451 in total volume, the 46% implied probability reflects the best available forecast information from traders watching short-range model output. The adjacent 20°C and 22°C buckets likely carry meaningful probability as well. The data favors 21°C as the modal outcome right now, but the single-degree resolution precision means the NO side has structural staying power regardless of the forecast.

LINES VERDICT

LEAN YES, LOW CONFIDENCE

The momentum into 21°C is real and driven by fresh forecast data pointing to that temperature band overnight May 31. But the single-bucket structure of this market and the inherent uncertainty of minimum temperature forecasts at this range keep NO structurally competitive.

What the market says: At 46% implied probability, the market treats 21°C as the modal outcome but not a certainty. With resolution in under 30 hours, any forecast revision could reprice this contract sharply in either direction.

Key unknown: The Japan Meteorological Agency’s next short-range forecast update for Tokyo overnight May 30 to May 31 is the single data point that would move this contract most decisively before the noon resolution.