Have you ever looked at one eather forecast one day, seen it as forecasting rain, then seeing the forecast switch to sun, only to have it switch back to rain? That is what is happening in the models right now, and it is very, very hard to make accurate forecasts when models can't decide what path to take with a certain storm. It makes it particularly hard if all the models are doing this, which is what they have been doing.
Thankfully, they seem to be converging on a solution of sun Thursday and Friday, with light rain returning Saturday night and Sunday. This is September, so the sunshine won't be your July sunshine, and the storm won't be your typical November storm. Things will generally remain quiet and benign around here as summer transitions into fall. Fall transitioning into winter brings the big storms, and winter transitioning into spring often brings instability and heavy showers, but summer to fall (and spring to summer) are both fairly quiet as far as weather phenomena go.
Our long range forecast shows this low pressure trough hanging around for a while, directing cool air from the northern Pacific right in our neck of the woods from a westerly direction. The mountains, especially the Cascades, will likely pick up some decent orographic (topographically enhanced) precipitation, but the Seattle area will likely get rain shadowed. It depends on how much onshore flow there is, but we could also see a PSCZ form, although it is hard to predict convergence zones one day in advance, much less 5 days. But if this scenario shown in the models were to come true, I would expect a convergence zone of some sort, likely near the eastern entrance to the Strait of Juan de Fuca. Also, I expect we will be seeing a lot of these upper-level lows (although this is a very weak one) dropping down from the Gulf of Alaska and directing a cool westerly flow into the Cascades. This type of setup is not conductive to a lot of rain in Seattle (shadowed by the Olympics) or lowland snow but snow levels are usually around 1000 feet in the winter with these types of setups and snow can really pile up in the Cascades. I remember one such event in early February 2008 where Snoqualmie Pass got 7 feet in 4 days. 2007-2008 was a La Nina year much like the upcoming one, and Snoqualmie Pass got 50 feet of snow (as opposed to maybe 20 feet last year). I'm excited for it and I hope you are too.
Here is the latest update on the SSTs in the tropical Pacific. As you can see, there is a wide swath of cooler-than-average temperatures. This swath is only expected to grow with time, and La Nina itself will really start to have an impact on our weather after Christmas, although it will certainly have an impact before then as well.