I've mentioned this before but the increased melting from Greenland is likely reinforcing this pattern. That dumps cold water formerly locked up in glaciers into the Newfoundland area, and promotes the troughs getting stuck near there. The ridge that pops up over the trough in Greenland promotes more melting, especially this time of year when much more heat is available (and more heat in general due to global warming). It seems that this year with the west based -NAO in late spring is causing potentially a record melt season there. It won't always be the case, but it's like rolling dice with two or three times the possibility of getting snake eyes.
I know it's not the climate change thread but it's not hard to see the connection to the last 2-3 months' continuous pattern.