Track NWS KSEA observations and Kalshi market pricing for Pacific Northwest precipitation. Identify edge in convergence zone events, extended dry summer windows, and atmospheric river systems.
Seattle sits in the Puget Sound Convergence Zone under a temperate oceanic climate (Köppen Cfb), where Pacific frontal systems collide with the Olympic and Cascade mountain ranges. The city's position between these topographic barriers creates pronounced orographic lift on westerly flows and sheltering effects that produce microclimates within the metro area. Marine air from the Pacific Ocean moderates temperatures year-round while delivering persistent stratiform precipitation from October through May. The wettest months are November through January, each averaging 5.5 to 6.0 inches, while July and August combined deliver less than 1.5 inches. Annual precipitation at KSEA totals 37.7 inches distributed across approximately 150 days, though "rainy days" frequently feature light drizzle rather than heavy downpours. June through September constitute a pronounced dry season driven by the northward migration of the Pacific High, creating the longest predictable precipitation drought in any major U.S. city outside the Southwest. These sharp seasonal transitions create high-conviction trading windows on Kalshi. The dry summer allows traders to short precipitation contracts from late June through early September with historical confidence, while the transition months of October and May produce elevated volatility as atmospheric river events can deliver 2-3 inches in 24 hours or miss entirely based on jet stream positioning. The convergence zone—where post-frontal northwesterly flow splits around the Olympics and reconverges over Puget Sound—generates localized heavy precipitation that KSEA instruments capture inconsistently, creating basis risk between city-wide rainfall and official station measurements.
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View Dashboard →Pacific Northwest precipitation operates on a 5-7 day predictability horizon for frontal timing but only 24-48 hours for precise accumulation totals at KSEA, as slight shifts in front trajectories determine whether systems track north into British Columbia or stall over Puget Sound. The 00z and 12z GFS and ECMWF model runs drive the majority of price discovery, with Kalshi markets typically showing widest spreads during the 06z window when traders await updated ensemble data. October and November contracts see the highest liquidity as the transition from dry summer to wet season creates binary outcomes: either atmospheric rivers establish the autumn pattern early, or high pressure extensions delay measurable rain into mid-month. Traders consistently underestimate the persistence of June gloom, when low clouds and mist create the impression of rain without triggering the 0.01-inch settlement threshold. The Puget Sound Convergence Zone represents the single largest trap in Seattle trading—post-frontal showers can dump 0.50 inches in Everett or Renton while KSEA records trace amounts, leading to contract losses despite widespread regional precipitation. Flash transitions also occur when the Pacific High breaks down in late September: a single atmospheric river can deliver a month's worth of rain in 48 hours, flipping "No" positions instantly. Winter markets carry lower edge because synoptic-scale patterns are well-resolved five days out, compressing premiums on binary contracts. The most profitable trades exploit the sharp summer dry season: shorting July and August precipitation contracts offers consistent returns, though traders must exit before Labor Day when early-season troughs occasionally penetrate the weakening high-pressure ridge.
KSEA operates an Automated Surface Observing System (ASOS) with a tipping-bucket rain gauge that reports one-minute interval observations, feeding into hourly METAR reports and the daily Climate Summary (CLI) report that Kalshi uses for official contract settlement. The gauge measures precipitation in 0.01-inch increments, and the daily CLI total reflects the 24-hour accumulation from midnight to midnight local time. Trace precipitation—moisture insufficient to tip the bucket and register as 0.01 inches—appears as "T" in CLI reports and does not satisfy Kalshi contract settlement criteria for measurable rain. This distinction matters frequently in Seattle, where marine drizzle and mist can persist for hours without accumulating to the 0.01-inch threshold. During convergence zone events, KSEA's location south of the typical convergence axis means the station sometimes undersamples localized heavy cells that form 10-15 miles north near Everett, creating divergence between regional radar estimates and official settlement data. The ASOS rain gauge is calibrated monthly and heated to measure liquid-equivalent precipitation during mixed-phase events, though heavy wet snow occasionally clogs the funnel and causes temporary underreporting until maintenance crews clear the instrument.
KSEAMarch through May transitions from wet to dry as the Pacific High builds northward. March still averages 3.7 inches with frequent frontal passages, while May drops to 1.9 inches. Late-season atmospheric rivers in April can deliver surprise accumulations of 1-2 inches in 24 hours, creating volatility in contracts as traders anticipate the summer dry season too early.
June through August represents the most predictable trading window, with the Pacific High suppressing frontal activity and limiting precipitation to occasional weak troughs. July averages just 0.7 inches across the entire month. Marine stratus and morning drizzle rarely accumulate to measurable amounts, making short positions on precipitation contracts statistically favorable, though traders must exit before early September when the high begins to weaken.
September through November marks the sharpest transition in Seattle's climate calendar. September still averages only 1.5 inches as the Pacific High lingers, but October jumps to 3.3 inches and November to 6.0 inches as atmospheric rivers re-establish. The timing of the first significant autumn storm creates wide Kalshi spreads and high liquidity, with early-month contracts offering the most edge as traders position for pattern changes.
December through February delivers consistent frontal precipitation with monthly averages between 5.0 and 5.6 inches. Synoptic-scale systems are well-resolved in medium-range models, compressing premiums on binary contracts. Convergence zone events and Puget Sound snow-level rain/snow transitions create the primary sources of uncertainty. Atmospheric rivers during El Niño winters can produce 3+ inch daily totals, though these events are typically telegraphed 3-5 days in advance.
Seattle receives 37.7 inches of precipitation annually at KSEA, distributed across approximately 150 days. November through January are the wettest months with 5.5 to 6.0 inches each, while July and August combined total less than 1.5 inches during the pronounced dry season.
Kalshi contracts settle based on the official daily Climate Summary (CLI) report from KSEA's ASOS station. Measurable precipitation requires at least 0.01 inches recorded in the 24-hour period from midnight to midnight Pacific Time. Trace amounts do not count toward contract settlement.
July and August offer the highest-conviction short opportunities, as the Pacific High creates a pronounced dry season with fewer than 0.75 inches per month historically. June and September carry higher risk due to transition dynamics, when late-season fronts or early autumn systems can break through the ridge.
The convergence zone forms when post-frontal northwesterly flow splits around the Olympic Mountains and reconverges over Puget Sound, creating localized heavy precipitation. KSEA sits south of the typical convergence axis, so the station sometimes records trace amounts while areas 10-15 miles north receive 0.50+ inches, creating basis risk for traders.
Frontal system timing is predictable 5-7 days out, but precise accumulation totals at KSEA are reliable only 24-48 hours in advance. Slight shifts in atmospheric river trajectories determine whether systems stall over Puget Sound or track north into British Columbia, affecting settlement outcomes significantly.
No. Marine stratus and mist create persistent cloud cover and light drizzle that often fail to reach the 0.01-inch threshold required for measurable precipitation. Traders shorting precipitation must account for this: cloudy conditions do not guarantee contract settlement in Seattle's favor.
Atmospheric rivers are narrow corridors of concentrated moisture from the tropical Pacific that deliver 30-50% of Seattle's annual precipitation in just a few events. A single atmospheric river can produce 2-3 inches in 24 hours, creating sharp price movements in Kalshi markets and flipping low-probability contracts when systems arrive unexpectedly during transition seasons.
October and November see the highest trading volume as the transition from dry summer to wet season creates binary outcomes with wider spreads. The uncertainty around atmospheric river timing and the breakdown of the Pacific High generate edge opportunities that attract the most market participants.
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