Debby Remnants to Bring Heavy Rain to New England: Impacts, Timing

Tropical Storm Debby has made a second landfall in South Carolina this morning. 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, eventually merging with that system's cold front. This will bring a period of heavy rain to western New England, with lesser amounts and less time in the rain heading farther east and south in New England.

Photo byNew England Storm Center

TIMING

As Debby moves up the east coast, a warm front will form. This will help create an increasingly strong southerly flow as the front moves through in the morning, increasing moisture in the atmosphere and allowing scattered showers to break out Friday morning.

GFS showing potential weather Friday morning:

Photo byGFS/Tropical Tidbits

The warm front is expected to intensify, but the bulk of the heavy rain from this process is expected to fall north of the Canadian border at this time. Scattered showers will likely continue throughout the day Friday as New England will be within the warm sector of the system, though more dry hours are expected over wet hours.

GFS showing potential weather Friday afternoon. You can see a swath of heavy rain north of the border:

Photo byGFS/Tropical Tidbits

The rainfall will ramp up for western New England heading into Friday evening as Debby moves through New York. The heaviest rain may be confined to New York until we start heading for the overnight hours. The late night hours of Friday and early morning hours of Saturday continue to look like the bulk of the storm for western New England. For areas farther east, a more narrow band of showers and storms will likely move through Saturday morning as the cold front from the trough pushes through the area.

GFS showing potential weather Friday evening (1st image) and early Saturday morning (2nd image):

Photo byGFS/Tropical Tidbits

Photo byGFS/Tropical Tidbits

Heading into Saturday morning, the heaviest rain threat will shift into northern Maine as the cold front continues to push east. Once the front passes, rapid clearing is now expected and much of New England (with maybe the exception of eastern Maine) will likely see dry conditions for the afternoon.

GFS showing potential weather Saturday morning, after sunrise:

Photo byGFS/Tropical Tidbits

RAINFALL

Where the center of Debby tracks is very important to overall rain totals in western New England and New York. A lot of times, you'll hear about how the track of the center is not important during tropical systems, but remember Debby will be post-tropical. 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.

Over the past 36 to 48 hours, Debby has been trending westward. These trends have now moderated with little changes in Debby's forecast track over the past 12 hours. The track continues to look like it will pass through New York into northern Vermont. This would place the highest rain totals across New York and Canada.

Photo byNational Hurricane Center

With that said, a period of heavy rain will move through the northern third of New England, likely bringing 1-3 inches of rain. The highest potential for rainfall in New England will be across northern Vermont. The White Mountains may also see higher totals due to upslope enhancement. Amounts will drop off the farther south and east you go as these areas will be on the east side of the track.

Photo byNew England Storm Center

As frustrating as it is, there remains space for a wobble in the track and rainfall amounts to shift. This comes as some ensemble members (particularly from the GFS) are still showing a more easterly track. This would pull the higher rainfall amounts farther to the east. As of now, the official National Hurricane Center track remains the go-to and would keep the highest amounts north and west. The bottom line is that trends in the track need to continue to be watched closely and the track as the storm moves in will need to be watched.

FLASH FLOODING

With the highest confidence in high rain totals to the west of New England, a flood watch has been issued for much of upstate New York. With both confidence and rainfall amounts beginning to drop off heading into Vermont, flood watches have not been posted as of Thursday morning, but could come later today depending on today's trends.

The Weather Prediction Center is maintaining most of Vermont in the "moderate" category (level 3 of 4) for excessive rainfall. This is being maintained due to the potential for eastward wobbles in the track as well as the fact that much of Vermont has very wet soils from recent rains. This area may be refined and trimmed in an update later today. The below map shows this current update.

Photo byNew England Storm Center

The threshold for flash flooding to occur remains on the lower end of the spectrum after recent rains. 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 to reach this threshold with expected rainfall rates.

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.

RIVER FLOODING

The extent of river flooding will be dependent on the final track of the storm. A track farther east would enhance river flooding potential. As of now, the highest concerns for River flooding 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.

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. 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. This threat is low and very limited, but can't be ruled out. These tropical-like environments can favor very brief and weak spin-up tornadoes and strong straight-line winds gusts.

Photo byNew England Storm Center