Tropical downpours likely in Northeast US this weekend
Other than a few scattered passing thunderstorms on Wednesday, much of the Northeast US has been devoid of rain over the past several weeks. This is about to change, perhaps dramatically so in some spots. On Sunday, a cold front and associated potent shortwave trough with plenty of vorticity will run into a well established Southeast Ridge, and the remnants of Tropical Storm Julia — two great moisture sources. This combination of energy, dynamics, and moisture — as well as the fact that the ridge and the mid-level flow are parallel to the frontal boundary — will lead to the potential for slow-moving tropical downpours, some of which will train over each other and lead to flash flooding.
As we move towards Autumn, atmospheric troughs can deepen and amplify more than during the warmer summer season. This, combined with tropical moisture and dynamics can lead to impressive meteorological results. This particular shortwave trough is digging from the Great Lakes all the way down to the Tennessee Valley, and is even tilting neutrally or negatively on some guidance. This acts to slow down the atmospheric flow even more, leading to a prolonged fetch of moisture, lift, convergence, and thus heavy downpours.