In the mod/stronger cases, it's usually hard to get a +PNA in a La Nina. We actually managed it for the first 2 months of last winter despite Nina being moderate....but then again, those SSTAs in the N PAC looked more like an El Nino.
But even popping a +PNA for like 1 month during a La Nina can be pretty helpful....Jan 2011 and Jan 2009 are both +PNA months within a sea of otherwise -PNA pattern and they really gave the eastern US a sustained wintry period. Jan 2011's +PNA was well-timed too as it really popped big once the 1/12/11 storm reversed the big NAO block....so instead of a relaxation that comes with the NAO going more neutral/positive, we stayed cold and snowy with the +PNA ridge out west.