Looks like a polar vortex or at least a wavy jet stream dipping to the south.

http://thinkprogress.org/climate/2014/07/14/3459809/polar-vortex-missed-the-point/

It will be unseasonably chilly in the eastern part of the United
States this week, due to a peculiar weather pattern that’s causing deep waves in
the jet stream. One of those big waves is bringing cool air down from the
northeast Pacific and the Arctic. This will cause nighttime temperatures to be,
on average, in the 50s or 60s on Tuesday and Wednesday.

“We’ve got this cool air coming down over the eastern half of the country, and
that’s gonna just be kind of nice,” said Jennifer Francis, an atmospheric
scientist and research professor at Rutgers University’s Institute of Marine and
Coastal Sciences. “But along the east coast, we’re looking at storms and floods.
On the west coast, we’re looking at heat and fires. And it’s all part of this
jet stream pattern.”