At some point, a flash mob stops being a flash mob and starts being regular old public performers. But who cares what they’re called when they can pull off a stellar routine?The crowd of about 600 singers who walked into Nordstrom’s flagship store in downtown Seattle on Saturday and broke out into Handel’s “Hallelujah Chorus” were not quite everyday joes, but instead members of more than 70 local choirs. The dude standing above the crowd directing was Seattle Symphony Music Director Gerard Schwarz and the gal on the piano was the symphony’s Kimberly Russ.It also wasn’t quite spontaneous or secret as the event had been put on by Nordstrom itself and plenty of folks who’d got wind of the performance were there at the store, camera phones raised before the choir even got started.But whatever. The singers showcased some serious pipes and with less than two weeks until Christmas, seeing them in action with a classic holiday number should be a prerequisite for kicking off the holidays.