**Persistent warm, humid conditions under light southeast winds are keeping overnight lows elevated well above the mid-September climatological average of around 67°F.** Multiple forecast models from sources like the National Weather Service and private providers converge on a daily minimum in the mid-70s Fahrenheit for September 18, 2026, with most guidance centering on 75–78°F. This reflects an ongoing heat wave that has already made September on pace to be Austin’s warmest on record, sustained by a strong subtropical ridge and high dew points near 73°F that suppress radiative cooling. **The tightly clustered market probabilities across the 70–77°F buckets capture genuine uncertainty in the precise timing and magnitude of the overnight minimum.** Small differences in cloud cover, wind speed, or local urban heat island effects near official observation sites (such as Camp Mabry or Austin-Bergstrom) can shift the exact low by a degree or two. With no major synoptic changes or cold-air advection expected before sunrise, the distribution of trader-implied odds aligns with the narrow spread in ensemble forecasts rather than any single outlier scenario. Updated model runs overnight will provide the next key data point for refining these probabilities.