The issue is that our nemesis, an upper level trough of low pressure, will be approaching and greatly influence our region Sunday and Monday. Here are a series of 500 hPa (about 18,000 ft) upper level maps.
Friday morning (at 5 AM)....a low to our northwest.
Saturday morning, it slowly easing southeastward.
What a contrast to last year, when ridging (high pressure) was the rule.
With a low hanging around, temperatures will be 5-10F below norming (upper 60s).
Precipitation? Nothing on Saturday and early Sunday before early afternoon will be dry. Thus, the Seafair festivities will generally be fine. But later in the afternoon, the combination of upward motion from the trough and surface heating will result in the formation of convective showers (including thunderstorms), as suggested by the forecast precipitation for the 3-hours ending 5 PM on Sunday. The greatest threats will be over the coastal zone and NE Washington.
As the upper low moves by, the showers will rotate northward into western WA. The 48-h totals ending 5 AM Tuesday are impressive along the coast and over southern BC and Alberta, with some areas getting more than an inch (pink and black colors).
The big worry? lightning. The models are suggesting lots of instability on Sunday and Monday and thunderstorms can be expected. So there may be a lot of smoldering fires ready to rev up with warmth and wind next week.