Forecasts from the National Weather Service and multiple models indicate Seattle's August 21 high will likely fall in the upper 70s to low 80s, with increasing afternoon cloud cover from an approaching upper-level low suppressing peak temperatures below recent warm readings near 85°F. This setup creates the tight spread among leading outcomes, as slight differences in cloud arrival timing, northwest winds, and marine influence produce the 2–4°F variations that separate the 78–83°F bins. Traders weigh official guidance showing mid-70s to lower-80s highs against historical August normals around 75°F, with the market consensus reflecting genuine uncertainty in exact maximum amid the transitional pattern rather than a single dominant model solution.