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.
...with almost 16 inches of rain in Myrtle Beach, South Carolina, and more than 20 inches around Charleston. You don't find amounts like that anywhere in the historic record for this area. This picture shows the radar-estimated rainfall over South Carolina between Friday afternoon and mid morning today:
This map shows where Hurricane Joaquin was just before 8 a.m. ET. You can also see the stripe of clouds centered just of the Middle and North Atlantic coasts.
There are competing forces acting on it, and each move it makes will place it under different influences. This has made it very difficult for computer models and meteorologists to judge where it will actually go. This is reflected in the track model collection on this map:
In assessing the final impact of the storm system coming into the East, there are three main components. First is the cold front coming across the Appalachians tonight in a very rich moisture field with ...
On this map, the cold front that will eventually move through the Northeast is in the far northwest corner of the picture. There are areas of showers moving northeastward well ahead of the front, but the steadiest rain is not likely until the cool air moves in and the front stalls.
The Midwest and Northeast are in the latitude zone where winds are primarily from the west. The direct opposite is the case today, as seen on this pressure analysis. The easterly flow brings in moisture from the Atlantic.