Mark Albright, from the Dept. of Atmospheric Sciences at the University of Washington writes:

Many of us saw the exaggerated headlines about the “massive heat wave” gripping the USA over the past weekend of 20-21 July 2019:
https://www.cbsnews.com/live-news/heat-wave-2019-extreme-heat-advisory-warning-deaths-latest-weather-forecast-us-nyc-2019-07-20/

Well , there is an explanation, it’s called mid-summer.

Climatologically speaking, the two warmest days of the year were this past weekend: July 20 and 21. 

Nationwide, Saturday averaged +0.8 F above climatology while Sunday averaged -0.2 F below climatology based on data from the Climate Reference Network:
https://atmos.uw.edu/marka/crn.2016/usa48/201907.usa48.txt

On Saturday morning, two sites in the Pacific Northwest dropped to below freezing, Stanley ID with 29 F and Meacham OR with 31 F. 

And on Sunday morning (21 July 2019) the temperature dropped to 38 F at Great Falls MT, the coldest July temperature of the past twenty years!