**Upper-level ridging building over the Pacific Northwest on August 23–24 is the dominant driver**, promoting northerly flow that limits the usual marine influence from Puget Sound and supports afternoon highs centered in the mid-to-upper 70s. Official National Weather Service guidance and multiple model runs converge on 75–79°F for Seattle on the 24th under mostly sunny skies, with patchy smoke possible from regional wildfires but minimal direct impact on peak temperatures. This aligns with the market’s leading bins (78–79°F at 42.5%, 80–81°F at 22.5%, 76–77°F at 21.5%), reflecting trader consensus around the ridge’s moderate warming effect ahead of an approaching trough that will increase onshore flow and cool conditions by midweek. Historical August averages near 76°F and the absence of extreme heat signals further anchor probabilities away from the tails. Minor forecast spread in model timing or local wind shifts explains the secondary weight on 80–81°F versus 76–77°F.