Traders assign near-certainty to a 98-99°F high in Dallas on September 13 because National Weather Service model guidance and surface observations from the prior 48 hours showed a stable high-pressure ridge dominating north Texas, with light southerly flow, clear skies, and dew points supporting only modest warm advection. This aligned with mid-September climatology, where Dallas averages near 90°F and recent warm anomalies lacked the strength for a 100°F+ spike. The slim 0.1% odds on 100-101°F reflect the low likelihood of forecast busts from unmodeled downslope effects or rapid ridge shifts, though final Dallas-Fort Worth Airport readings could still adjust the official maximum within normal verification tolerances.