An eruption, even one where the aviation color code goes to red, doesn't necessarily mean that flights will be affected. Winds out of the east will blow the ash away from Anchorage, affecting perhaps Bethelians and Nomites (Gnomes?) but completely unaffecting flights to and from SEA.

Therefore, check the National Weather Service's volcanic ash forecast page, updated about an hour after each eruption which will let you know if you're safe. The graphic model at the bottom shows the predicted ash cloud movement based on forecasted winds aloft.

To be among the first to know, subscribe to the Alaska Volcano Observatory's Twitter feed (and enable "device updates" to receive text message alerts). While you're at it, you might as well subscribe to the Alaska Airlines Twitter feed--it's run by one guy at AS, so it's not quite real-time, but it occasionally provides useful information about flight operations.