**Trader sentiment for the lowest temperature in São Paulo on September 22 reflects substantial uncertainty in short-range forecast guidance, with market-implied odds clustered near 40% for both 14°C or below and 24°C or higher.** Recent model runs show a transition from warm, northerly flow on September 21 to an approaching frontal system bringing clouds, showers, and a wind shift to westerly or southerly directions by the 22nd. This setup introduces variability in overnight radiative cooling and boundary-layer mixing. Stronger cold-air advection or clearer skies after evening rain could allow surface temperatures to drop toward or below the seasonal average low near 12–16°C, supporting the lower outcome. Conversely, persistent cloud cover, higher dew points from moisture, or delayed frontal passage would limit cooling and keep the minimum closer to 18–20°C or above. Official agency guidance and ensemble spreads highlight modest but meaningful differences in timing and intensity of precipitation, which directly influence the diurnal temperature range. With resolution imminent, the next model updates and real-time observations will likely narrow the range of plausible outcomes.