The National Weather Service forecasts that the upper-level high pressure system responsible for the historic stretch of heat that broiled the Northwest over recent days will slide eastward along the U.S.-Canada border near northern Idaho today and eventually weaken by Thursday. As a result, the core of the extreme heat will be found across interior Washington and Oregon through midweek, with temperatures soaring into the 100s and 110s across this region. Residents are advised to stay in air conditioned buildings, drink plenty of water, and avoid strenuous outdoor activities.