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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.

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Synopsis: Autumn air in the Northeast US late week

A powerful cold front and thermal gradient, associated with a northern stream atmospheric disturbance, will move toward the area later this week. Along with some showers and storms, the front will bring a much cooler and drier airmass, with origins in Canada, allowing temperatures to fall fairly dramatically on Thursday and Friday. This will come in stark contrast to the overall warm and moist regime which our area has been settled in for quite some time now.

The front will move through later Wednesday, without too much fanfare. Lingering through Wednesday night, a few showers and storms will be possible, but overall limited atmospheric instability will preclude any threat for organized thunderstorms or hazardous weather. Instead, the main story will be a wind shift and temperature drop, as very cool air aloft rapidly shifts into the region and winds turn west-northwesterly.

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Premium Weekly Outlook: Unseasonable warmth bookended by tastes of fall

Another relatively calm and dry week looks to be in-store, as we’ll start off with a taste of fall, moderate to unseasonably warm temperatures, and then again experience a taste of fall. A cold front that moved through on Saturday night has completely cleared the area, giving way to a broad area of high pressure across the region. This has helped dropped humidity substantially, bringing us very comfortable temperatures — we started in the 50s in many locations —  as well as a lovely crisp morning. This high pressure will be in-control for today, with light winds, mostly sunny skies, and temperatures rising into the upper 70s to around 80. The weak wind flow may allow for some seabreezes to develop, but without a big difference in temperature between the land and ocean, these seabreezes should not be too strong, nor penetrate too far inland.

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ENSO state in limbo as La Nina struggles

Much fuss has been made over the past several months in regards to the development of La Nina this year, coming on the heels of one of the strongest El Nino’s on record. There is a propensity for these things to occur, after all, and a significant cooling of Nino-region pacific temperatures led many to believe that a La Nina was not only on its way — but could be moderate or strong by the time cold season arrived in the Northern Hemisphere. Those ideas will not come to fruition.

The lack of La Nina development has, instead, been notable — with a significant lack of depth to any cooler sea surface temperature anomalies. Trade winds aren’t cooperating (we’ll get into that more later) and tropical forcing seems to favor this pattern continuing through the next few months. It appears likely, now, that La Nina will never truly get off the ground. Forecast models have responded, with monthly and seasonal data now pointing to a Neutral (La Nada) pattern through at least the first half of North America’s calendar Winter.

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