In the west a lot of our weather is effected by the EPO/PNA phase. I don' t know much about EPO but I know when it is a + phase you get a 'raging' jet that blocks cold air from coming down. -PNA I think brings on more NW events.

Lately neither cannot go negative for very long though I think the PNA is negative but the atmospheric setup is having a hard time recoupling and the GFS weather model is crap about it.