West Coast residents could be forgiven for rubbing the sweat out of their eyes and staring in disbelief at both their thermometers and their calendars this week: 100 degrees? In June?

A heat wave has punctured any expectations of a mild spring, with millions of people sweltering through blistering temperatures in cities like Las Vegas (102 degrees), Sacramento (103) and Phoenix (109).

Temperatures at San Francisco’s airport surged into the triple digits on Monday, a record for this time of year, the National Weather Service said. Other milestones included a new daily high of 97 degrees in Portland, Ore., and 106 degrees in Modesto, Calif., both set on Tuesday.

The scorching weather has plagued firefighters battling dozens of wildfires, caused major transit delays and forced many to ask just what is going on?