While most of the area is currently seeing either light precipitation or dry conditions, that will be changing as we move through the day today. A front will be approaching from the southwest, and tropical moisture will be flowing northward ahead of the front. This combination will result in bands of moderate to heavy rain this afternoon and evening.
This steadier/heavier rain will arrive in NH just after noon before moving into Maine during the early afternoon hours. Before then, light precipitation will continue to pose a mixed precipitation/ice threat away from the coast.
As we move towards the evening, any remaining near-surface cold air will retreat farther towards the mountains, and bands of moderate-heavy rain will be rotating through from SW to NE. The rain won’t be intense enough to cause any major flooding issues, but low-lying areas typically vulnerable to urban/small stream flooding may see water for a brief time this evening. There may be some thunderstorms embedded within the final round of heavy rain right along the front itself. This line should move through NH around 8, arriving in Portland around 10.
Temps today will be unpleasantly chilly as low-level cold struggles to depart. Look for highs ranging from the low 30’s in the mountains to the mid 50’s in Southern NH. There will be a benefit to this cold air though, which is that it will mitigate the threat posed by strong winds as a low level jet moves overhead this evening. When temps warm with height (as is forecast tonight due to low level cold below 3,000 feet becoming trapped by mountains, which don’t extend with much regularity above 3,000 feet), the atmosphere is vertically stable, and air doesn’t move very far up or down. You need downward “momentum transport” from the jet layer at 5-7,000 feet to the surface to get strong winds in a setup like this, and with the exception of the immediate coastline east of Route 1 (where warmer surface temps will dampen the rate of warming with height), we’re not going to get much if any of that momentum transport tonight. This means that while winds will be breezy, I don’t expect any major power outage issues west of Route 1.
Rain will move northeast of the area around midnight with drying conditions expected behind the front.