On Sunday, an upper-level trough will dig into the Great Lakes region. This will spawn a surface low pressure system which will drive eastward through Ohio and Pennsylvania, lifting a warm front toward New England. The system itself will travel through New England on Monday. This will bring rounds of showers and widespread rainfall to New England Sunday and Monday.
The system’s warm front will likely begin to spread showers into New England as early as Sunday morning. Showers on Sunday during the daylight hours will be most likely across northern Massachusetts and points north, excluding northern Maine. Forcing for showers will be greatest just to the north of the warm front.
Exactly when showers begin to arrive will be determined by how quickly the more moist air can spread into New England, Dry air and surface high pressure will try to eat away at the showers initially. Showers should continue to fill in through the afternoon and become more widespread by the evening as the dry air gets eroded. Sunday will not be a washout for most.