**An upper-level low tracking from the Gulf of Alaska southward along the British Columbia coast into Washington coastal waters is the dominant driver of trader positioning for Seattle’s September 4 overnight low.** This feature is advecting cooler, unstable marine air into the region, with National Weather Service guidance highlighting mid-40s to lower-50s lows Thursday night and similar or slightly moderated values into Friday amid lingering showers and possible afternoon thunderstorms. Model consensus and recent runs show overnight temperatures most likely settling in the low-to-mid 50s (near or slightly below the 54.9 °F climatological normal for September 4 at Sea-Tac), but the precise minimum hinges on variables such as cloud-cover duration, wind speeds, and the low’s exact inland placement. Clearer breaks or lighter winds would enhance radiational cooling and support readings near or below 50 °F, while persistent marine stratus or stronger flow would anchor minima higher in the mid-50s. The closely matched market-implied probabilities around 45 °F or below versus 52–53 °F reflect this narrow but meaningful sensitivity: even a 2–3 °F shift from the current forecast envelope can separate those bins, and traders are monitoring the next NWS model cycles and local mesonet observations for confirmation ahead of the overnight period.