Dry spell after Wednesday's rain; a little cool with frost and freeze threat later this week but big warmup and a taste of summer is expected next week

Plain-language summary:

Light to moderate rain will affect the region Tuesday night into Wednesday, with a narrow band of heavier rainfall just south of the U.S./Canada border. An extended dry spell is likely afterward as a storm misses to the south this weekend. It turns a little cool with frost and freeze threat later this week, but a big warmup and a taste of summer is expected next week, all with abundant sunshine on most days. Low afternoon humidity, light winds, and large day-to-night temperature swings are expected during this stretch.

Meteorological discussion:

No snow to talk about this time. A low pressure passing to the south will bring rain Tuesday night into Wednesday, especially south of the U.S./Canada border. The clouds and rain, along with southeasterly flow off the still cool Atlantic Ocean east of the Green Mountains and cool northerly flow to the northwest, will cause cooler than average temperatures on Wednesday. For most locations, this rain will be lighter than the rainfall events in April.

Source: PivotalWeather

However, there will be a narrow area, likely from Lake Ontario through northern New York into northern Vermont and northern New Hampshire, that will receive heavier rainfall due to enhanced low-mid level frontogenesis and a bit of instability aloft, with the warm moist air rising above the cooler low-level air. A few elevated thunderstorms are possible, though severe thunderstorms are very unlikely.

Source: PivotalWeather

Afterwards, it appears that we will a flip in the weather pattern, with an extended dry, (mostly) sunny spell. This is supported by the North Atlantic Oscillation (NAO) returning back to neutral after being negative last week, marking the end of the cool, Greenland blocking pattern and persistent trough in eastern North America. Meanwhile, the Pacific North American (PNA) oscillation is going negative after being neutral to positive for the past week, leading to a developing western trough/eastern ridge pattern.

Source: NCEP CPC

One more upper-level trough and a cool Canadian surface high pressure will move in behind the storm for Thursday into this weekend, keeping temperatures below average again, but not as cool as last week. The surface high pressure will actually stay just to the north initially as a storm associated with the southern branch or subtropical jet stream passes into the mid-Atlantic U.S. That storm will bring a soaking rain, breezy easterly winds, and unseasonably cool temperatures just to its north. However, almost all models suggest that it will go too far south to bring more than mid-high level clouds to southern areas and probably not even that north of the U.S./Canada border. As such, there will be enough sunshine for temperatures to rise into at least the 50s F (low-mid teens C) during the daytime despite the high pressure to the north providing cool, dry northerly flow. However, nights will be chilly with a frost and freeze threat away from urban areas and larger bodies of water, especially from far northern New England on northward, closer to the high pressure center, where the sky will be the clearest with the least wind.

Source: TropicalTidBits

After the storm misses to the south, the entire eastern U.S. will be dominated by high pressure with the cool northerly flow disappearing, allowing the initially cool air mass to quickly warm through the strong May sun and clear skies. Since the initial Canadian air mass is dry and there aren't a lot of leaves on the trees yet to transpire moisture into the air, the air mass will continue to be dry as it warms up, making for very low afternoon humidity and large temperature swings from day to night. Fire threat will be limited at least initially by the moist ground from the wet April and the light winds associated with the high pressure. 

By the middle of next week, both operational and ensemble models have a strong signal for unseasonably warm (perhaps even hot) and still sunny weather. At this time of year, with the strong sun angle and little foliage to block the sun, this type of pattern can lead to afternoon temperatures exceeding 86F (30C), a big change from the relative lack of true warmth recently. That said, due to the dry air, clear skies, and light winds, nights will still be relatively cool until the high pressure slowly moves offshore later next week, leading to stronger southerly winds and more moisture from the Gulf of Mexico. The overall pattern will be slow-moving, with a Rex block potentially forming in the eastern U.S., leading to an extended period of warm and dry weather.

Source: PivotalWeather

This expected weather pattern is still a week away though, so it could still bust a little, especially in Maine where the warmth could be relatively easily interrupted by backdoor cold fronts bringing cooler air from eastern Quebec, or a southeasterly flow off the still cool Atlantic Ocean, both of which are common in the spring. There is higher confidence of impressive warmth to the west, as is typical for spring.