The cold front that dipped into New England yesterday is now very slowly moving through southern New England. After focusing a shield of rain across southern New England, the boundary will continue to sag southward, likely settling just south of New England. After this morning activity, a general drying trend should take over for the afternoon, though most of the day will remain mostly cloudy. Heading into the overnight hours tonight, an area of low pressure will traverse the stalled front, bringing another round of rain.
Rain will likely break out this evening along the immediate south coast before filling in across all of southern and central New England during the first part of the night. Rounds of rain will continue to move through the area all night long into Wednesday morning. Periods of heavy rain are possible across southern New England. A very sharp cutoff in rainfall is likely heading into central Vermont, central New Hampshire and Maine.
HRRR showing potential weather late this evening (1st image) and early Wednesday morning (2nd image):
With moisture remaining elevated just north of the cold front, sporadic downpours will be likely across southern New England, possibly into southern Vermont and New Hampshire, through the night. The greatest chance for multiple downpours will be south of the Mass Pike. Here stands the best chance of seeing over an inch of rain. The biggest uncertainty is where exactly the boundary sets up. Should it stall just a bit further north, the higher rain totals would be pushed north as well.
Overall, the flooding threat is looking to be isolated and localized. The greater risk for flooding issues will be to New England's southwest, over New York City, New Jersey and into Pennsylvania. Still, minor and localized flooding will be possible across southern and central New England, especially if the frontal boundary slams on the brakes a little sooner. A flood watch is in effect for Connecticut's southern counties.
After this system, there will be a break in the action from Wednesday afternoon through Thursday as high pressure builds in from the north. Thereafter, attention turns to Tropical Storm Debby. After stalling out (and bringing an exorbitant amount of rain to the southeast), Debby will move up the east coast and pass New England for the weekend.
The overall setup for Thursday night through the weekend will involve three things: Debby, a trough in the Great Lakes, and the frontal boundary that will continue to be to the south of New England. How these three items interact will ultimately determine where heavy rain falls and how much of it falls late this week.
This storm could be a "predecessor rain event", which is an event where the rainfall breaks out well ahead of the main system's arrival. This would occur due to the storm's interaction with the frontal boundary. This would allow rain to break out for Friday despite Debby's current forecast passage on Saturday.
GFS model run showing the potential for showers to break out well ahead of Debby, creating a longer duration rain event. This is not to show expected weather at any given time as a wide range of timing and track still exists. This is just to show the potential for rain to break out well ahead of Debby:
As Debby moves up the coast, the storm will undergo the transition to a post-tropical cyclone. This will occur as it interacts with a system moving out of the Great Lakes. As this happens, an axis of heavy rain will likely set up somewhere over New England. Where this sets up remains rather low in confidence. With Debby's remnants interacting a trough to the west, the heaviest rainfall will likely set up to the north and west (of the left side) of Debby's eventual track.
As of now, this would favor more interior locations for southern New England rather than eastern/coastal areas. For northern New England, this could favor southern Vermont, New Hampshire and into interior Maine. Again, this is nowhere near locked in at this point, it's just the current trend, which will be adjusted as Debby's track is adjusted.
Other than where the heaviest rain sets up, the other big question will be how much. While the exact magnitude of rainfall can't really be gone into yet, the potential will be there for a swath of 2-4 inches of rain in New England. This would fall from Friday through Saturday, making it a longer duration event.
This event could bring about both flash and river flooding where the heaviest rain sets up. The Weather Prediction Center currently paints all of New England with the exception of northern Maine in the "slight" category (level 2 of 4) for excessive rainfall on Friday and Saturday. This will be refined as confidence increases on where the heaviest rain will set up.
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