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.
This upper-air forecast map for next Saturday shows the flow that would foster a warming trend later in the week and for next weekend.
We always look back at our previous forecasts to try to learn from episodes in which we believe we could have done better. The following satellite picture shows dry weather today in just the area that originally looked like it would have more rain.
In other words, while late summers in Phoenix have gotten wetter during the last few years, Boston has become drier. Is there anything more momentous or general that we can say about this?
This enhanced infrared satellite picture shows the cold front in the Northeast and the moisture wrapping around Odille on the southwest part of the map.
Across the Central and Northern states, thunderstorms are less common at this time of year than in late spring and summer. One area that has had more thunderstorms than usual recently is across the Desert Southwest.
Last week, I mentioned that longer range computer models were suggesting a major warmup by next weekend. More recent runs have backed off on the that idea. However, there is extreme uncertainty beyond the next 7-10 days. This can be seen by looking at the following map.