For the last several years, the Pacific Northwest has been fortunate during the July 4th weekend, with sunny warm weather. Some folks called it the "new normal."
Well, the "old normal" is back, with the traditional clouds and light showers.
The key issue is that the upper level pattern has been stuck for the past several weeks in a configuration with a trough over the Northwest, in contrast to the persistent ridge of last year. To illustrate, here is the forecast 500 hPa (roughly 18,000 ft) heights for 8 PM today (Saturday). A distinct trough is approaching British Columbia. An unusually strong jet stream is approaching our region (the gradient in the lines is proportional to wind speed)
The trough Saturday night and Sunday AM will be strong enough to bring showers to the coast and northwest WA, with a few sprinkles getting as far south as Tacoma.
What about Monday and fireworks time? Here is the upper level map for 5 PM Monday. Still general troughines over our region, but the main axis has pushed past us. Models suggest partly clouds skies, but dry on Monday evening, so we will probably have decent fireworks watching conditions. But don't expect a heat wave: high temperatures will probably only rise to around 70F on Monday.