Debby's remnants arrive in New England: Impacts, timing

Rain from the remnants of Debby have begun to move through New England this morning. An initial shield of rain has passed through northern New England, bringing 1-2 inches of rain to northern Vermont. After an afternoon lull, the main round of rain will push through in the evening. A flood warning has been issued for portions of the Northeast Kingdom through mid-morning.

Photo byNew England Storm Center

ALERTS

Photo byNew England Storm Center

TIMING

The initial batch of steadier, more widespread rainfall associated with a warm front has begun to shift north of New England this morning as the front lifts north. Scattered showers will continue through the morning hours before becoming more isolated this afternoon. Many areas of New England will be mostly dry this afternoon, but pop-up thunderstorms will be possible.

HRRR showing potential weather early this afternoon:

Photo byHRRR/Tropical Tidbits

Heading toward the evening, the main batch of rainfall will begin to move into western New England. This will come as Debby's remnants interact with a cold front that will be sweeping across the region. With very high moisture-content in the atmosphere, this line of rain and thunderstorms will be able to produce torrential downpours at times, with the greatest chance across the northern third of New England.

HRRR showing potential weather this evening (1st image) and around midnight (2nd image):

Photo byHRRR/Tropical Tidbits

Photo byHRRR/Tropical Tidbits

Following trends of a hasty departure, most of New England will clear out of the rainfall before sunrise Saturday. Some lingering showers may be present across eastern areas in the morning, but rapid clearing is expected behind the frontal passage for the day Saturday.

HRRR showing potential weather around sunrise Saturday:

Photo byHRRR/Tropical Tidbits

RAINFALL

Where the center of Debby tracks is very important to overall rain totals in western New England and New York. The heaviest of the rain will likely fall to the north and west of Debby's circulation as it interacts with the incoming trough to the west. Debby's center will likely track just to the west of New England, perhaps passing through northwest Vermont. This will keep the highest totals across southern Quebec and upstate New York.

While Debby is no longer tropical, the storm will still be bringing a tropical air mass to New England. The atmosphere will become very moisture-rich this evening into the overnight hours as a strong southerly flow develops. This will allow for torrential downpours to develop within the line of rainfall and storms. Overall, 1-3 inches of rain is expected across the northern third of New England with amounts dropping off heading south and east, farther away from Debby's center of circulation.

Photo byNew England Storm Center

A strong southeasterly low-level jet will allow for upslope enhancement of rainfall across the eastern slopes of the White and Green Mountains, potentially adding to rain totals in those areas. These slopes will be most likely to see three or more inches of rain today and tonight.

FLOODING POTENTIAL

With the potential for torrential downpours this evening into the overnight hours, a flood watch has been posted for northern Vermont and New Hampshire. These areas stand the best chance to see the highest totals as upslope enhancement remains a threat. These areas could also briefly see storm training as the cold front pushes eastward and rainfall moves northeastward.

The greatest threat for flooding with Debby as a whole will be across upstate New York and southern Quebec, where higher rainfall totals up to 5+ inches will be possible. Still, the rain coming to western New England will likely be enough to set off scattered flash floods through early Saturday morning. This threat drops heading south and east as the line of rain and storms gradually weakens.

Photo byNew England Storm Center

The threshold for flash flooding to occur remains on the lower end of the spectrum after recent rains for northern Vermont and New Hampshire. Currently, much of western New England has a threshold of 2 to 2.5 inches in six hours to create flash floods. Three hour thresholds are around 1.5 to 2 inches, but it will be harder for rainfall rates to reach this threshold.

Photo byNortheast River Forecast Center

The threat for flash flooding will drop off quickly as you move eastward in New England, but a line of heavy downpours and thunderstorms early Saturday morning could lead to isolated instances of poor drainage flooding across eastern New Hampshire, eastern Massachusetts and Maine.

As for River flooding, the highest concerns are in New York, but Vermont will likely see some rivers rise. Any river flooding issues that develop may linger through Saturday, well after the rain has stopped falling. Central and northeast Vermont streams and rivers remain above average heading into this event. The White Mountains may see some smaller rivers jump their banks, but the large rivers are not expected to flood at this time.

SEVERE WEATHER

While heavy rain remains by and large the greatest impact from this system, there will be a chance for some strong to severe storms on the east side. With New England being placed in the warm sector of the storm, some instability will be able to develop ahead of the line of storms expected to move through southern and central New England.

The greatest chance for a couple isolated severe storms will be across western New England. The line of rainfall and thunderstorms will be weakening as it pushes eastward, lowering the chances for stronger storms the farther east you are. The greater severe thunderstorm threat overall will be to the west of New England where storms will likely fire up earlier in the afternoon.

Photo byStorm Prediction Center

These tropical-like setups do favor brief, spin-up tornadoes, so this will be something to watch as the line of storms moves through southern and central New England in the evening. It should be noted that these tropical-like tornadoes tend to be very brief and weak. This is not much of a threat, but the potential is there and worth bringing up.

These kinds of tropical-like setups typically see a strong amount of shear. Couple this sufficient instability building in the warm sector and plenty of moisture to help fuel storms, and you have the potential for some rotation. There are limiting factors, and this threat is very low, but it's worth bringing up.

Storm Prediction Center tornado outlook for today:

Photo byStorm Prediction Center

WIND

Gusty southerly winds are possible as a low-level jet of 55-65 mph crosses the region ahead of the cold front. There will be an inversion in place, and not all of this will mix to the ground. With that said, gusts of 40-50mph are possible along the south coast and across western Vermont. This has prompted a low-end wind advisory for these areas.