It is virtually impossible to accurately ID a specific cultivar without a DNA analysis (many are far too similar in appearance) but we can come close :-)) Given your location and the fact that it has bloomed reliably for you in your colder climate, I'd guess it is a serrata cultivar and Lanarth White is a reasonable guess.

I would have said Twist and Shout as it seems to be very cold hardy but that wasn't introduced to the commercial market until 2008.

btw, coloring of the sterile florets (the large petaled flowers) can be extremely variable for most serratas, ranging from a very pale pink or blue (nearly white) to more intensely colored as well as pure white. The fertile flowers colors can also be somewhat variable/dependent on soil pH.

I concur with serrata! Reminds me of GRAYSWOOD in particular. It's been commonly sold at just about all big box retailers I can think of for quite some time.

You've had a lot better luck with yours blooming than I have (late freezes and such). In fact, I mine was just beginning to reach 5x5' so I just moved it away from an Alice oakleaf into a brightly shaded corner where it should hold up better.