A strengthening upper-level ridge
of high pressure is the culprit - or the benefactor, depending on your view of extremely hot weather - for the continued high temperatures.
"An upper-level ridge
of high pressure - a really strong one - has built up over us, moving basically from the southeast Nevada desert," said meteorologist Wanda Likens at the Weather Service office in Portland.
The explanation from the National Weather Service in Portland is that an upper-level ridge
of high pressure is building just off the Pacific Northwest coast, and is combining with a surface-level trough of low pressure near the coast to create a warm air mass.
Their research showed that in the month of July, as the number of days with tornadoes occurring has decreased (from 11-20 tornado days per July in the 1990s to 6-13 days in the early 2010s), those Julys that had fewer tornado days also usually had stronger upper-level ridges
across the northwest and north-central United States.