I'll kick myself for not remembering its name when I hear what this is
SOLVED: Seattle plant ID
Camellia japonica maybe 'High hat' or 'Elegans Splendor'. The Washington Park Arboretum has a great collection and they might be able to give you a more clear cultivar name.
Seriously. How did I not remember that? I plead winter-addled brain. Thanks for the info.