Overnight Saturday night, a disturbance will be moving from the southwest to the northeast providing a chance for showers and thunderstorms. Coupled with the disturbance moving through, upper level divergence will be overhead in Southern New England, especially near Cape Cod, but also in interior Massachusetts and Southern New Hampshire where a low level jet is also located. The upper level divergence provides large scale lift, which can aide in the development of heavy rain. Also a rise to 1.5 to 1.75 inches of precipitable water occurs during that time showing the increase in moisture in the atmosphere at that time. Further adding to the event, an increase in upward motion signified by convective energy (CAPE) begins just after sunset on Saturday and continues into Sunday afternoon. Another ingredient present for this time frame is shear, which rises to between 30 and 40 kts Saturday night and into the first half of Sunday, but mostly resides in the mid layers of the atmosphere. This means that convection may be elevated allowing for heavy rain, wind and lightning as the main threats. All of these combine to allow for a chance for thunder across southern portions of New England Saturday night and into Sunday. The passage of the disturbance around midday means that most of the thunder should occur before midday, but some instability and low level moisture may allow for a few more storms before sunset Sunday, especially in the interior.
The next chance for thunder comes on Monday night into early Tuesday in the Northeast when a cold front moves through. The air mass comes in from Canada and changes the humid warm weather with high temperatures in the 90s and dew points in the mid to upper 60s to temperatures in the mid 80s and dew points in the upper 50s. This change in low level warmth and moisture coupled with a center for storms (the front) often translates into storms. New York is forecast to have the highest convective energy before sunset on Monday. After sunset, the convective energy decreases, but mid-level lapse rates in the 6 to 7 degree range may be able to compensate while combined with mid-level shear carrying the left over pieces of thunderstorms from New York into the Northeast. Once again, large scale lift and plenty of precipitable water means heavy rain may be a threat, especially since storms may be in the dissipating stage after sunset. Often, this dissipating stage brings heavy wind and frequent lightning as the main threats as well, especially as the stronger low level winds move in.
Thereafter, as mentioned earlier, the Northeast has cooler temperatures and drier air come in for Wednesday as a Canadian air mass moves in. Low level temperatures will be below normal and will allow for morning temperatures around a refreshing 60 degrees.