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 Sunday morning. After a morning round of showers, more isolated activity will be likely across New England heading into the early afternoon. Forcing for showers will be greatest just to the north of the warm front. Areas south of this front (mainly south of the Mass Pike) will see little by way of showers after the morning round.
How widespread showers are in the morning and early afternoon 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. The southern cutoff in steadier rain in the evening will be determined by the location of the warm front.