Scattered Storms, Downpours Possible Across Southern New England Today

Today, the frontal boundary that was hung up across northernmost New England will continue to push through New England in earnest. This will move the chance for more widespread unsettled weather farther south in New England compared to Monday. A period of scattered showers and storms will be likely across southern and central areas as the front passes. An area of low pressure will move along the boundary this afternoon and evening, providing the basis for scattered storms and the potential for torrential downpours.

As of this morning, the frontal boundary remains over the mountains of northern New England, but it will be diving southward this afternoon and evening. As it moves into southern New England, it will slow down and become nearly stationary once again. This will provide the forcing for scattered showers and storms to break out along the boundary beginning later this afternoon and lasting through the evening and into the night. With the boundary stalling once again, repeated rounds of showers will be possible.

Northernmost New England will likely be the driest as the atmosphere will need some recovery time after the more active Monday and Monday night. This front will be slow-moving, and the air mass won't be entirely replaced, so it will be warm and remain humid, but not to the levels of Sunday and Monday for northern areas.

Below: Potential weather late this afternoon (1st image) and early tonight (2nd image):

There is basically a tropical air mass over New England that is on the south side of the cold front. Dew points will be well into the 70s across southern New England today. Drier air will be filtering into northern New England, on the north side of the front, this afternoon and evening. Multiple factors are lining up to produce scattered torrential rainfall. Rainfall rates of 1-2 inches an hour will be possible within these storms. This will lead to the potential for flash flooding. A flood watch has been issued for southern Massachusetts, Connecticut and Rhode Island.

The remnants of Tropical Storm Chantal passing south of New England will help pump this tropical air mass into southern New England today. The atmosphere is already very moisture rich, with precipitable water values (PWAT) at 2 to 2.5 inches, which is highly elevated for the time of year. Precipitable water value measures the amount of water vapor in the atmosphere, which could be turned into precipitation if acted upon by a forcing mechanism (which it will be today). With PWATs over 2 inches, the atmosphere will be primed for heavy downpours.

There will be multiple forcing mechanisms in place across southern New England today. The most prominent will be the frontal boundary dropping southward. On top of this boundary, an area of low pressure will ride along the front later this afternoon and evening. A sea-breeze boundary may also set up across southeast Massachusetts and Rhode Island. These will all help to ring out the moisture in the atmosphere in the form of downpours.

Below: Precipitable water values this afternoon showing 2+ inches over southern and eastern New England. You can see the very high moisture from the remnants of Tropical Storm Chantal east of Cape Cod as well:

The current threshold for flash flooding to occur within an hour currently sits at 2-2.5 inches for most of southern and central New England. With 1-2 inch an hour rainfall rates within storms, it may be difficult for one individual storm to produce flooding.

With that said, a stalled frontal boundary with very high moisture overhead and an area of low pressure, the potential for slow moving and training storms will be possible. The threshold for flash flooding to occur within 3 hours currently sits at 2.5-3.5 inches. Areas that see multiple rounds of storms, or training storms, will certainly be able to reach this number, which is why a flood watch has been issued.

With all that said, storms will be scattered today, so not everyone will get a storm, and not all storms will produce torrential rainfall. A major flooding situation is not expected today and any flooding that develops will likely be localized rather than widespread. There's a marginal risk of strong to severe storms as instability and moisture will be high, however, there are significant limiting factors as well in the form of weak shear and meager lapse rates). This will keep the coverage of severe storms on the low end.

After Tuesday, the frontal boundary will remain to New England's south for the rest of the week. This will provide the basis for continued unsettled weather with daily scattered shower and storm chances in the afternoon. Forcing will be weak, but afternoon heating and destabilization may allow for diurnally driven unsettled weather in the afternoon. Wednesday through Friday will likely feature many more dry hours than wet hours, however, given the weak forcing. Temperatures will be much cooler as well.