I just received one at 7:00 AM EDT to my secret alert email address, that is only known by WU and is tied to my ancient WU account. Tearing the headers apart, it originated in the Amazon AWS cloud and passed SPF verification as an authorized sender of @weather.com emails.
My guess is that TWC is trying to import WU user accounts into TWC's web site, in an attempt to coax WU users to use weather.com. As they import batches of users, these "new TWC user" messages are automatically generated.
Reading between the lines, it probably means that WU's days are numbered.....