Tuesday 9 a.m.
As a major ridge aloft builds over the Ohio Valley and Middle Atlantic states late this week and holds for the weekend, heat wave conditions are likely to develop. The upper-air pattern definitely points in this direction, but the computer forecasts counsel caution about how hot it gets from the central Great Lakes into New England. You can see this inconsistency in the video. The upper-air forecast maps show a classic heat wave situation, but the surface maps suggest areas of showers and thunderstorms can limit the heating somewhat.
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.
The NWS Storm Prediction Center issued this outlook for today and tomorrow: A preliminary area of showers may advance from the Carolinas as far as Southeastern Pennsylvania and Southern New Jersey by tonight.
For the almost 24 hours between 10 a.m. ET Tuesday and 9:20 a.m. ET today, here is a the lightning recap. The dry pattern from the Midwest will now advance across New York and New England.
Cooling aloft and heating moist air closer to the ground should trigger strong thunderstorms from eastern New York and much of New England southwest through parts of the Middle Atlantic states.
The front will move into a region with high humidity as it approaches the I95 corridor tomorrow. This is the basis for SPC's forecast of thunderstorms approaching severe limits tomorrow.
Tropical Storm Colin is caught in the southern stream while the northern stream is helping to send unseasonably cool air out of central Canada.
Then, as the cold front arrives, there may be violent thunderstorms. This map shows the early morning SPC assessment of the severe weather risk on Sunday: