"You can see that across the entire United States, including Alaska, there is more of a chance that temperatures will be above normal," Dan Collins, a meteorologist and seasonal forecaster with the NOAA Climate Prediction Center-Operational Prediction Branch, told Live Science.
Collins cites multiple reasons for the temperatures being higher than usual. One is atmospheric ridging over the northern-central US. High-pressure systems cause elevated temperatures.
Additionally, long-term climate trends signal higher temperatures. The NOAA Climate Prediction Center notes the "most recent decade is somewhat warmer than the previous three decades." Moreover, this June was 1.4 degrees warmer than the 20th-century average and was the third warmest June on record. (2015 and 2016 were the only warmer Junes, with El Niño contributing to those elevated numbers.)