'Unrelenting' heatwave fueled in part by high overnight temperatures Overnight temperatures play a big role in the current heatwave. With no time of the day to escape the heat, some Treasure Valley residents are concerned. Author: Andrew Baertlein Updated: 7:01 PM MDT July 8, 2021 BOISE, Idaho — Boise just experienced nine straight days of temperatures over 100 degrees. While it’s making headlines, the National Weather Service is more interested in gauging this heat wave by looking at the coldest part of the day. “A lot of time that overnight low will be the more important thing,” NWS Science and Operations Officer Tim Barker said. "It's unrelenting. Multiple days of high heat will eventually bring up the lows."