Flood watches issued for southern New England ahead of next system

A stationary front is currently draped across New England. This boundary will act as the main source for forcing showers and thunderstorms as a disturbance rides along this front into southern New England. With a moisture-rich environment remaining in place, areas near this frontal boundary will have the chance to see heavier downpours tonight into Saturday.

Photo byNew England Storm Center

Similar to the Wednesday night event, the placement of where the frontal boundary will set up Saturday morning will be important in determining where the heaviest rainfall is most likely. To be clear, we're not expecting the same level of devastating flooding that occurred in parts of northern New England to repeat for southern New England. This boundary will slowly shift from the coast back inland through the day today.

While it's difficult to pinpoint exactly where the band heaviest of downpours will set up, it will be most likely near and to the southeast of the frontal boundary. This will likely place it across southern New England and possibly into southern Vermont and New Hampshire. This has prompted a flood watch to be issued for most of southern New England.

Photo byNew England Storm Center

Before the main downpours and storms arrive for Saturday morning, some scattered activity is possible this afternoon across New England. Coverage will likely be limited with more areas not seeing a storm today than areas that will. Storms are not expected to become strong to severe. While there is plenty of moisture in the atmosphere to work with, shear will be very weak or even non-existent. This will make it difficult for any storm to strengthen or last very long.

HRRR showing potential weather around mid-afternoon today:

Photo byHRRR/Tropical Tidbits

The main rainfall from the incoming system won’t likely begin to break out across western New England until after midnight. At this time, scattered showers will begin to move across western Massachusetts, southern Vermont and southern New Hampshire. As the morning goes on, these showers will continue to move eastward and fill in, creating a band of torrential downpours across southern New England.

WRF-ARW showing potential weather around 3am (1st image) and around mid-morning Saturday (2nd image):

Photo byWRF-ARW/Tropical Tidbits

Photo byWRF-ARW/Tropical Tidbits

Unlike this past Wednesday night in northern New England, where there was a widespread band of heavy rain moving across the region, the downpours will generally be more scattered during this event for southern New England. This means rainfall amounts will likely be very varied from community to community.

Areas that get into the heaviest banding and see multiple downpours could see 2-4+ inches. While all of southern New England is at risk for these rainfall amounts, there will also likely be areas that see less than an inch. This really shows the scattered nature of the showers. Areas that do see multiple inches of rain will be at risk for flash flooding as rainfall rates will be able to become intense and exceed the flash flood threshold, which currently stands at 2.5 to 3.5 inches of rain in six hours, which will be possible with this system. 

Photo byNew England Storm Center

These heavy rainfall rates will come as the atmosphere is primed for torrential downpours over southern New England. Precipitable water (PWAT) values (the amount of water vapor in the atmosphere that could become precipitation) will be close to 2.5 inches for southern New England. This indicates an extremely moisture-rich environment that is capable of producing torrential rainfall rates.

GFS showing potential PWAT values at 2+ inches for much of southern New England Saturday morning. You can see the plume of moisture extending across most of the east coast. Flood watches currently extend down to South Carolina:

Photo byGFS/Tropical Tidbits

The heaviest and most widespread rain potential will be on Saturday morning, however, scattered showers, storms and downpours will likely hang around through the day. It won’t be a total all-day washout, but raindrops will be lurking around the region for much of the day.

WRF-ARW showing potential weather Saturday afternoon:

Photo byWRF-ARW/Tropical Tidbits

Heading into next week, a renewed heat wave looks to be in store as the persistent ridge of high pressure over the Atlantic Ocean remains in place. This will continue to pump above average heat and humidity up the east coast amid a general southwest flow. Heat will begin to rebuild on Sunday before peaking Monday and Tuesday. 

Sunday will be a mainly dry day, but daily thunderstorm chances will be in the picture for New England thereafter. Heading beyond midweek, relief from both heat and humidity may be in store as guidance is in pretty good agreement about a cold front crossing the region. This front looks to be more impactful than previous ones this month in regards to dropping New England back to seasonal averages (80s over 90s) and lowering the oppressive humidity. 

Weather map for Thursday morning (July 18th) showing New England post-cold front with an expansive high pressure to the west, ushering a potentially cooler, drier northwest flow:

Photo byWeather Prediction Center