Take a look at this picture from a hail storm that passed over I-405 in the Mill Creek area near 216th Street SE Tuesday afternoon. While that looks like snow, it's actually a whole lot of hail that fell over a very short period of time.
A similar situation played out in Skagit County a few hours later, where a hail storm crossed I-5 and was the apparent cause of a several-car accident along Conway hill.
A very cold trough of low pressure is now sliding through Western Washington in the wake of Monday's windstorm. How cold? Aside from temperatures struggling to break 50 here on the ground, temperatures are very, very cold in the upper atmosphere.
In fact, readings from about 18,000 feet up were around -36 Celsius Monday -- the second coldest reading of the "winter" season and colder than anything observed in January and February.
So combine very cold air with some low pressure spinning around and you have the recipe for a very active afternoon and evening. Sure enough, showers and thunderstorms have popped up and will be roaming around through the early night hours.
We've already had reports of hail and lightning in the greater Mill Creek area (obvious from the photo above), and hail reports have come in from another cell that passed through Lacey. I suspect we'll see plenty more of similar reports as the night wears on and the main center of the low drifts through the Puget Sound area.