What's up with the weather in the Pacific Northwest?

500 hPa contours for Monday, 5:00 p.m.
From Cliff Mass blog referenced in text
Note the N-S alignment of the contours
as discussed in the text. The low is the red
area offshore of the northwest; the high
is the blue region centered over Idaho. This
system has been migrating from east to west
over the past week.

    CliffMass.blogspot.com" and again refer you to it regarding the nation's weather--particularly the weather in the Northwest. Mass is a professor at the University of Washington in Seattle; here is his home page. His posts of Wednesday and Friday, June 26-28, discuss the upcoming heat wave in detail. Note: this heat wave follows a cold and wet spring, and is only a week before the infamous July 4 date when "summer comes to the Northwest."
 I have several times referred to one of my favorite blogs "
     Briefly, the whole west coast is heating up, with the highest temperatures off the coast in the interiors of Washington and Oregon, with somewhat lower, but still high temperatures, in eastern California and western Nevada (Death Valley could be interesting). Last week (which was wet and cool here) there was a VERY deep low (italicization same as Mass) over the eastern Pacific, and a high-pressure ridge over the Rockies and western Plains. This setup (at the 500 hPa height) allowed moist air to flow in from the southwest. Yesterday (Friday) the low center and the high pressure ridge moved westward, the high-pressure ridge intensified, and the Northwest dried out. By Monday, the high pressure will intensify and will be located west of the Rockies. The low and high regions are getting "squashed" from east to west, aligning the pressure contours in a north-south configuration. Since the winds follow the contours, warm air will flow into the Northwest from the south. This pattern will continue into Tuesday, bringing dry hot air up from the south into the Northwest. The highest temperatures will be east of the Cascades, but they will not be record breaking. To have new records in western Washington, the ridge would have to be further westward than it currently is, essentially sitting on top of Seattle. Mass predicts that on Monday there is a chance of a record high at SeaTac airport; the current high for that date is 87 F.

CAPE index for Monday and Tuesday
From the Cliff Mass blog referenced in the text

     I have now moved to the Northwest from the Midwest, thinking that I left thunderstorms behind me. WRONG! In a very unusual situation for the Northwest, it is possible that there could be some severe thunderstorms in the region, largely along the Cascades in Oregon. There is a measure of thunderstorm potential called CAPE (Convective Available Potential Energy). Typical high values of the CAPE are a few hundred in this region; it is projected to reach 2500-3000 Monday and Tuesday. However, countering the tendency for massive convection that would cause thunderstorms, is a sinking motion of the high pressure ridge and a lack of strong upward convective motion that would release the energy. Mass says, however, that "the models would not have to be very wrong for something very interesting and very severe to happen. Need to watch this."