Trader consensus on Polymarket assigns a 42.5% implied probability to a Seattle high of 56-57°F on April 17, driven by the National Weather Service forecast of mostly cloudy skies with highs near 56°F and a chance of rain, amid persistent onshore flow delivering cool marine air. A recent frontal passage over the past 48 hours has entrenched this cooler pattern following early April warmth exceeding 70°F, with low-level stratus clouds and light precipitation potential suppressing daytime heating. Ensemble model runs from GFS and ECMWF show tight clustering in the mid-50s, though minor spread exists on afternoon clearing possibilities. Watch for 12z model updates today and initial observations tomorrow, as marine layer depth could nudge outcomes toward 54-55°F (28.5%) or slightly higher.