Saturday night into Sunday expect to see a heavier slug of moisture and precipitation enter the scene as positive vorticity advection from a shortwave trough will provide synoptic-scale lifting across Western Washington. Without going into too much of the math, vorticity is a measure of spin (large-scale in this case, think 100s of miles wide). Through some fun fluid dynamics math, advecting or increasing the spin over an area will cause upward motions in the atmosphere. This process helps squeeze rain and snow out of airmasses in a somewhat similar capactity to how air forced over mountains will rise and produce more rain or snow. When we speak of "troughs", "shortwaves", "disturbance", or low pressure system, we often times see this vorticity increase --> more rising air --> more precipitation process unfold.

Medium-Term (Monday-Wednesday)

Though they differ slightly in the timing of individual storms, both the European and American (GEFS) model ensembles have a clear cooler trend starting Sunday through at least Thursday. By next Thursday, we will likely see freezing levels drop back below our lowest pass levels (namely Snoqualmie Pass). Precipitation totals during this Mon-Wed period vary depending on the modeling system you look at. The Euro is quite excited about an atmospheric river roughly next Wednesday while the GFS is not. As a result, the Euro has quite a bit more precipitation. Time will tell who will prevail. Either way, I expect scattered precipitation here and there throughout next week which likely will result in some decent accumulation by the following weekend.

🧙🔮🧙‍♂️ Reading the crystal ball - 7+ day outlook

We're starting to see some agreement between ensemble systems favoring strong and persistent high pressure for several days somewhere around 13 January. Both the ECMWF and GEFS agree with this timing, though shifts in its location east/west or north/south will make a significant difference in what that means for skiing and riding in Washington. The image below shows this.