Yeah, my POI's initials are CW.
My POI said formal invitations will be sent later on this week! Their interview weekend is January 31st - February 2nd.
I got a personal email from my POI informally inviting me to Northwestern's interview weekend. But I had also just done a preliminary skype interview with my POI the week before.
I don't know if all of the POIs at Northwestern do preliminary skype interviews. So there may be a mass email that will be sent out as well.