Violent, flooding thunderstorms ripped through much of the area between Philadelphia and New York City last evening, and the same areas could be hit again later today and this evening. The showers dwindled as they moved into eastern New England. That area is likely to receive its heaviest rain late tonight and tomorrow.
Meanwhile, cool and mostly dry air will send temperatures down to the 50s tonight from Wheeling, Illinois, to Wheeling, West Virginia. As the storms move offshore, it will turn less humid in the Middle and North Atlantic states, but the amount of cooling will be much less than was the case across the Midwest and Great Lakes.
Late this week and during the weekend, it appears that a piece of the cool upper-air trough that brought in the cooler weather will break off from the main upper air current. Once that happens, the disturbance can mill around for days, causing clouds and spotty showers in seemingly random places. At the same time, the main current will run from the central and northern Rockies all the way to south-central Canada, warming up that area quite a bit. By Saturday, daytime temperatures at Bismarck, North Dakota, will be soaring into the 90s while places like Pittsburgh and Philadelphia have readings almost 10 degrees lower.
This video has the forecast through the weekend as well of a time-lapse movie showing a squall line arriving at my house in central Pennsylvania on Sunday evening.
This map shows the scenario of hot air spreading across the northern Plains and north of the Great Lakes, while a pocket of cool air aloft gets stranded over the Middle Atlantic region.
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.
n the forecast office, we often track cold fronts with pressure maps like these. The examples are from 4AM and 10 AM today. You can see that the northern part of the front is moving more quickly than the southern end. The arrival of the front signals the start of the cooling trend that is spreading east.
The upper air flow over the East is from the west-southwest now, and some of the moisture associated with the cold front can be traced back to tropical storm activity off the Mexican west coast last week. The same moist air mass set the stage for recent flash floods in Arizona. By early next week, the upper-air flow will be coming to the Northeast region from well up in western Canada, as seen on this forecast map forecast map for next Tuesday:
As chilly air pours into the northern Plains and backs into the Rockies, snow can break out at higher elevations. Gillette, in northeastern Wyoming at an elevation of just over 4,000 feet, could have a close shave with snow on Thursday. This map shows the setup.
The main U.S. computer model shows very cool air coming into the Northeast this coming weekend. However, the same model shows a complete reversal by the following weekend.