A strong disturbance with cold air aloft will move over the northern Rockies and then the northern Plains tomorrow and Thursday. Severe thunderstorms will develop and the chance for tornadoes will increase with that system. After leaving the northern Plains, the main energy from that disturbance will head toward Ontario and Quebec. The GFS appears to take most of the energy north of New England this weekend.

However, the European has a different look. It strengthens the ridge over or just east of the middle of the Mississippi Valley on Saturday. Downstream, this would force the eastern Canadian jet stream to turn right to cause a west-northwesterly flow to head into central and northern New England. This kind of setup can lead to the eruption of nasty thunderstorms as far south as New York state and Pennsylvania in the middle of the weekend. Until we can tell which solution (or combination of solutions) turns out to be right, we won't have a lot of confidence about where and when thunderstorms will break out at the northern rim of the hot air mass during the holiday weekend. The map below shows the solution that would bring thunderstorms farther south.