SPOKANE, Wash. — Tuesday and Wednesday's Heat Advisory marks the first alert of this kind to be issued by the National Weather Service in Spokane this year. Widespread high temperatures ranging between 95 and 105 both days prompted the advisory.
Over the past five year, NWS Spokane issued on average two a year. In 2015 it was four, 2016 saw none, 2017 had two, and 2018 had three. It's important to note that this is the number of alerts issued, and not the total days. For example, this advisory in effect now is one advisory accounting for two days...
It's easy to correlate that hotter and drier summers in 2015 and 2018 to the extra Heat Advisories needed.
But what makes Heat Advisories unique in Washington and the west coast is that it's entirely dependent on temperatures. All of the central and eastern U.S. add humidity to the equation, and thus the "Heat Index" is looked at for reaching Heat Advisory criteria than just the temperature.