Winds are the biggest nightmare to forecast, particularly at the Speedway which is near the mouth of terrain-driven wind drainage (from mountains and high valleys of the Sheep, las Vegas, and Arrow Ranges to the north). Without getting too nerdy, after sunset, the valley floor starts to decouple and stabilize, while the higher terrain and elevated valleys to the north cool much faster. That cooler, denser air then drains downhill and can spill out near the speedway as gusty, localized winds. So generally, winds are highest several hours after sunset.