Debby's remnants to bring wave of rain to New England late this week

Tropical Storm Debby remains over the waters off the southeast coast line. The storm will move inland and begin to accelerate up the east coast later Thursday into Friday. As Debby moves up the coast, it will interact with a trough moving out of the Great Lakes. This is a prime scenario for western portions of New England to receive a period of tropical-like downpours. Where these downpours set up, and the magnitude of them will come down to a few other factors.

Photo byNew England Storm Center

The overall timing of the storm has trended faster. It initially looked like this would be a long duration event stretching from Thursday night and potentially lasting into Sunday. Now, it is looking more likely that the storm will get caught in the jet stream and pushed away much more quickly. The bulk of the storm is looking to occur from Friday afternoon through Saturday morning.

ICON showing potential weather during the height of the storm early Saturday morning. This timing, and where the heaviest rain is shown could still wobble:

Photo byICON/Tropical Tidbits

Yesterday, the track of Debby began to shift to the west and north. This trend has continued this morning, with National Hurricane Center's official forecast track now taking Debby through northern Vermont. While Debby will be absorbed into the trough coming out of the Great Lakes, the track remains important as the heaviest of the rainfall will likely occur just to the west of it with the frontal boundary providing the focus for the heaviest rain.

The general westward shift in the track has started to change the overall outlook of rain totals. First and foremost, this track trend would place the highest rainfall amounts over upstate New York. This would also place an initial shield of rain that could break out ahead of Debby's arrival on Friday to the north of New England. These trends may spare recently hard-hit areas, like the Northeast Kingdom, from the worst of the rains, though this is not locked in.

The heavier downpours would only push east as the trough's cold front (coming out of the Great Lakes) pushes east. The one thing that has become certain is that western New England will see the highest rainfall totals with amounts dropping off heading east.

Photo byNational Hurricane Center

Current rainfall amounts look to range from around a half an inch to upwards of 3+ inches. The lowest amounts will likely be near the coastal plain with amounts increasing heading west. Again, the heaviest of the rain is trending toward falling to the west of New England. With that said, there is still a range in track possibilities and the axis of heavier rain is still subject to shifting. Where exactly the heaviest rain band sets up is also to be determined.

Weather Prediction Center forecast total rainfall through Sunday. This is not set in stone:

Photo byWeather Prediction Center

We mentioned that a wobble remains possible in where the heaviest rain sets up due to the range in potential tracks of Debby. While a majority of ensemble members now have the storm passing through western New England, some continue to paint a picture where the storm passes farther to the south and east in New England, mainly the GEFS (Global Ensemble Forecast System). So, while the trend (and more reliable models) favors the westward shift, an eastward rain axis can't be ruled out quite yet.

Global hurricane model ensembles (1st image) vs GEFS (2nd image):

Photo bytropicaltidbits.com

Photo bytropicaltidbits.com

The threat for flash flooding remains heightened over western New England, particularly Vermont. The overall amount of rain has not really changed despite the trend toward a faster exit of the storm. This means the total rainfall will fall in a shorter period of time. This is because the more westward track could promote more thunderstorm development over New England, leading to instances of brief, but heavy downpours.

Due to this potential, and the fact that much of Vermont remains sensitive to rainfall after recent flooding events (mainly northern Vermont), the Weather Prediction Center continues to have most of Vermont in the "moderate" category (level 3 of 4) for excessive rain. This setup could also lead to river flooding, but the magnitude of this (and what rivers would be most affected) is hard to gauge right now.

Current Weather Prediction excessive rainfall outlook for Friday morning through Saturday morning. The red zone could shift west if westward trends continue:

Photo byWeather Prediction Center

With the westward shift, it opens the door for the potential of more thunderstorm activity, as mentioned before. This could increase the severe potential, mainly for southern New England. The tropical setup combined with Debby's re-curving could lead to instances of rotating storms, but this remains a low risk with heavy rain remaining a much bigger concern.

To close, there remains much uncertainty with what may happen with this storm. We understand this can be frustrating, for residents and forecasters alike, especially when forecasts start shifting around (which it could shift over the next 24-36 hours). The reason for the uncertainty with this storm is due to the interaction between the incoming trough and Debby's remnants. This setup is relatable to what occurs in the winter with snowstorms. As we are all well aware, snowfall forecasts can shift and move around quite a bit.

In the winter, we often talk about the uncertainty regarding interaction between a northern stream and southern stream system, and how much (or little) the two phase together. This is basically the incoming setup with the Great Lakes trough acting as the northern stream and Debby acting as the southern stream. With it being August and talking about a post-tropical system, the uncertainty falls in where the heaviest band of rain will set up rather than the heaviest band of snow.