We use
http://www.weather.com/
They have an hour by hour forecast that looks out 48 hours as well that gives some good details. For hike planning we compare the forecast for Twin Mountain, Lincon, and Mt Washington to get a gut feel on what to expect. Not always acurate, but one time the hour by hour indicated going form cloudy to sunny at 10 AM. We were on Guyout at 10:05 and the clouds simply went away and we were left will bright blue skies!
Even a blind squirrel sometimes finds the nut...
There is a good side and bad side to the hour by hour on weather.com/accuweather.com, as well as weather.gov click on mountains feature.
The good side...they are there, and they generally make some allowances for mountains being there.
The bad news, they are completely done by computers off of one model and one model run. No human intervention or oversite at all. When they initallize right, and are in a pattern that plays well to a models known biases, they can hit. But they can be very wrong. I wouldn't use these for anything more than general planning...like seeing if a storm is coming, or if it trends colder.
The higher summits forecasts provided by the Observatory, Eye on the Sky in VT, and the higher summits forecast by NWS are the best way to go. They have trained mets who know the mountains making the forecasts. And while the official forecast doesn't go beyond 36 hours for any of these products, they often elude to trends beyond in the discussions.
When I worked at MWO, we tested our skills against longer distance forecasts for the higher summits. Skill level dropped off significantly beyond 36 hours with respect to timing of in and out of clouds, as well as wind speeds (some of the bigger issues for mountaineers in these hills.)
Simply, the mountains of New England are animals of their own breed, and they are difficult to forecast for beyond the general. They very much mold an air mass to their liking and intent. I've been burned many times, but in general, through 36 hours, they do pretty good.
Beyond that, use the other sites, but please be skeptical and understand the product and the inherent limitations...
One last tip...the Observatory updates an .mp3 discussion on their website that goes out 36 hours from the afternoon, a product that isn't published anywhere else. It's typically updated by 5PM...well worth a listen!