Seattle May 21 High Temp: Will It Hit 74°F?

Tomorrow’s forecast for Seattle is already moving markets. The 74°F-or-higher outcome has surged to 61% implied probability, with a momentum composite that jumped nearly 30 points in a single hour. That kind of single-session move doesn’t happen without a meaningful catalyst, and in short-duration weather markets, that catalyst is almost always the latest model run.

This contract resolves at 2026-05-21 12:00:00, less than 30 hours from now. The market is pricing uncertainty, not science, and with this resolution window this tight, every new forecast update carries real weight.

How the Seattle Temperature Contract Works

This market asks a simple question: what will the highest temperature in Seattle be on May 21, 2026? YES here means the daily high reaches 74°F or above. The alternative outcomes span a wide range, from 55°F or below all the way up to 74°F or higher. Resolution depends on official observed temperature data for Seattle on that date.

• 74°F or higher (YES): priced at 0.61, implying a 61% probability.
• All other outcomes (collectively NO): priced at 0.39, implying a 39% probability spread across nine alternative temperature bands from 55°F or below up to 72-73°F.

The NO side doesn’t require Seattle to stay cold. It just needs the official high to land anywhere below 74°F. That’s a wide window. A reading of 73°F, which sounds close, pays NO. The 72-73°F band alone covers an outcome many forecasters would consider plausible given late-May Pacific Northwest variability. Any incoming marine layer, cloud persistence through the afternoon, or onshore flow would keep the high below that 74°F threshold and split the probability across those lower bands.

Momentum and Market Signals

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The momentum composite combines the 1-hour price change of positive 29.5%, a 24-hour trend score of 72.21, and a volume spike driven almost entirely by a single session. That combination points to one thing: a shift in the overnight or morning model consensus toward a warmer solution for May 21. In short-horizon weather markets, this pattern typically follows an updated National Weather Service forecast or a GFS/European model run that pushes projected highs upward.

Total volume stands at $2,913, with $2,882 of that arriving in the last 24 hours. Liquidity sits at $9,628. Both figures are well below $1 million, which means this is a thin market. Thin liquidity means price can move sharply on new data, and with a 30-point hourly swing already on the books, that volatility is already happening. A single updated forecast or a surge in one-sided trading could push this contract to 70% or drop it back toward 50% before resolution.

• The 1-hour surge of positive 29.5% aligns with the timing of a morning model update cycle, the most likely driver of this move.
• A 24-hour volume figure of $2,882 represents nearly all trading activity, suggesting this market was quiet until a forecast change triggered buying.
• Liquidity at $9,628 is thin enough that a mid-size position could shift the displayed probability by several percentage points.
• Open interest at $0 indicates no unresolved positions carried from prior sessions, consistent with a market that reopened or repriced sharply today.
• The trend score of 72.21 reflects sustained directional pressure toward the 74°F outcome, not a temporary spike.

Lines Analysis: Seattle’s May 21 High

The National Weather Service’s Seattle forecast office publishes high-temperature guidance updated multiple times daily. Here’s what the measurements are telling us: late May in Seattle can deliver warm days when a high-pressure ridge builds over the region, pushing marine influence offshore and allowing afternoon temperatures to reach the mid-70s. The 74°F threshold is not extreme for Seattle in late May, but it does require the right synoptic setup to hold through the afternoon peak heating window.

What makes the NO side real is Seattle’s exposure to marine air. The city sits close enough to Puget Sound and the Pacific that a shift in surface winds can cap afternoon highs five to eight degrees below what inland or model forecasts suggest. A 73°F high would resolve this contract to the 72-73°F band. That outcome is not a longshot. The 39% collective probability on the NO side reflects genuine model spread, not a dismissible tail risk.

• National Weather Service Seattle updates: any forecast revision pushing the expected high above or below 74°F will reprice this contract immediately.
• GFS and European model afternoon runs: the 12Z and 18Z model cycles on May 20 will be the last high-resolution guidance before resolution.
• Surface observation at Seattle-Tacoma International Airport: this is the most likely resolution data source, and afternoon cloud cover there can suppress readings by several degrees.
• Marine push timing: if onshore flow arrives before the afternoon peak, the 74°F threshold becomes harder to clear even if morning forecasts looked warm.

At $2,913 in total volume, this is not a market with deep institutional conviction. The data currently favor the 74°F-or-higher outcome, but the margin is not wide, and the forecast window is short enough that a single model update could reprice everything before 2026-05-21 12:00:00.

LINES VERDICT

Warm Lean, Thin Market

The current forecast and momentum signal favor Seattle clearing 74°F on May 21, but the margin is narrow and the market is too thinly traded to treat the 61% as a settled conclusion.

What the market says: At 61%, the market assigns a slight majority probability to the 74°F-or-higher outcome, with genuine uncertainty reflected in a 39% collective NO. With resolution in under 30 hours, expect sharp price moves on any updated NWS forecast before 2026-05-21 12:00:00.

Key unknown: The afternoon GFS and European model runs on May 20 are the single most important inputs before resolution. If those runs cool the projected high below 73°F, this market reprices fast.

Scientific Context

Seattle’s late-May climatology shows average highs in the low-to-mid 60s, making a 74°F reading roughly eight to ten degrees above the seasonal norm. That gap is achievable under a strong inland high-pressure system but requires offshore flow to suppress the marine influence. The Pacific Northwest has recorded several warm spells in recent years associated with ridge patterns, and the 2024 and 2025 spring seasons both included above-average temperature episodes in late May. The current year’s pattern, if consistent with that trend, would support the warm outcome. But individual day forecasts carry enough uncertainty at the 24-hour mark that the 39% NO probability is not a mistake. It reflects real meteorological variability at this threshold.

Frequently Asked Questions

• What does 61% probability mean here? It means the market currently assigns a 61-in-100 chance that Seattle’s official high temperature on May 21, 2026 reaches 74°F or above. Probabilities shift as new forecast data arrive.
• What pays out on NO? Any of the nine alternative temperature bands below 74°F resolve as winning outcomes for traders who hold those specific positions. A reading of 73°F, for example, pays the 72-73°F band, not the 74°F-or-higher outcome.
• What data event would move this price most? An updated National Weather Service forecast for Seattle on May 20, particularly the afternoon model runs, is the single most likely catalyst for a price shift before resolution.
• When does this contract resolve? Resolution is set for 2026-05-21 12:00:00, based on the official observed high temperature for Seattle on May 21, 2026.
• Is the volume reliable? Total volume of $2,913 and liquidity of $9,628 are well below $1 million. This is a thin market. Price can move sharply on small trades or new forecast data, and the displayed probability may not reflect broad market consensus.

This analysis reflects market conditions as of 2026-05-20 06:15:06. 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.