Before we dive into some of the details of what can be expected over the next seven to ten days, let's have a little review of the summer season thus far. The figure below is a daily mean composite of the 500 MB height pattern from July 1st through August 17th.
So, how has this pattern affected our local weather in July through the first half of August? Well temperatures have been hot. Temperatures at the Spokane International Airport have reached 90 degrees in 26 of the 48 days during this period and the average high temperature was 89 degrees. In comparison, the average number of 90 degree days Spokane will see at the airport is 14 and the average high temperature is 84.2 over this same period dating back to 1881. That means we have seen almost double the number of 90 degree days than what is typically observed. July and August are typically dry across the Northwest, but the current water year (beginning and ending on October 1st) has been below normal across the Inland Northwest; through August 17th, Wenatchee is 3.06 inches below its normal value of 7.58 inches, Spokane is 3.83 inches below its normal value of 15.63 inches and Lewiston is 2.80 inches below its normal value of 11.32 inches just to name a few locations. This deficit has directly contributed to the current drought status in Washington state (see figure below).
Now that we have summarized a little bit of what has happened through the summer so far this year, it is time to look at what the rest of the summer has in store for us...well we will at least look at the next ten days. It appears as though the ridge over the Four Corners area is going to be suppressed and actually slide eastward a bit. This is going to allow a long wave trough to dig across the Rocky Mountains with a ridge of high pressure building into the Gulf of Alaska. This is in direct contrast with what we have seen throughout this summer. The next series of images is a model forecast off of the GFS, which was ran at 4:00 PM PDT on Monday. Displayed is the 500 MB heights starting with Tuesday afternoon and continuing out at 48 hour increments through Thursday afternoon August 28th.
Notice that the western U.S. is under a trough of lower pressure through at least early next week. High pressure over the Four Corners area becomes suppressed and moves more over the southeast portion of the U.S. Meanwhile, high pressure builds and strengthens over the eastern Pacific. The image below is a rough sketch of the mean 500 MB pattern that can be expected the rest of this week and how temperatures will compare to normal.