This morning the low center and associated clouds/snow has moved south of us...with snow still falling over Oregon (see image). Low temps last night dropped into the low 20s in Seattle and in the teens in the cooler hinterlands. If you think this is cold...head to the upper plains where low records have been broken (such as the -15F at Denver). As noted in earlier blogs, this cold will be around for a while. But first lets talk snow.
An upper level disturbance...known as an upper level short wave trough in the business...will move southward out of Alaska and BC late Tuesday and Wednesday morning (see upper level map). This is close to the canonical situation for NW snow. Right now it looks like snow will begin falling over our area early Wed morning (perhaps very late Tuesday) into early Wed afternoon. As this point, it looks like amounts will be modest, roughly 1-4 inches, with some scattered higher amounts. To get really major snow we need a low developing off of the SW Washington coast and this is not being predicted (see map). The reason this is such a favorable pattern is that the low draws cold air in the from the north, while rotating Pacific moisture up and over it. But at this point, the low is too far inland for such heavy snow. So be ready for a modest snow event.
But the real threat is on Sunday. A far stronger weather system is forecast to approach the area...and with cold air in place we could have a major snow event that would make Wednesday look small. But this far out this is still a lot of uncertainty.... But if you need to buy car chains...I would do it.