Seattle's September precipitation averages 1.6 inches, with the rainy season typically ramping up later in the month as Pacific storm tracks strengthen. As of mid-September 2026, totals stand near 1 inch amid a persistent upper-level ridge and thermal trough favoring subsidence and onshore flow that limits moisture. Short-term forecasts from NOAA and ensemble models indicate continued mostly dry conditions with low rain probabilities through late September, though weak systems could deliver light showers near the border or coast. This setup supports trader consensus around 1-1.5 inches while leaving room for below-average totals if ridging holds or slight increases if troughing develops, consistent with El Niño-influenced drier Northwest patterns.