Some stray notes I took down in planning trips. I wish I had done this for all my trips.
I can test to see if water is salty, using a TDS meter, or just a having a taste and spitting it out. The TDS meter can help you determine if a water source is fresh, brackish, or salt water.
Most water running into the ocean is going to be fresh water unless the tide is high and the stream is being pushed back by the tide. Then the water will be brackish for a ways upstream, and the water will revert to fresh when the tide goes out. At least it will if the stream or river has a fairly good flow and it not just barely moving. If it is barely moving it may be brackish for quite a ways upstream. Brodie Brickey on Quora
You can find out if the tide is going in or going out, much like you can get a weather forecast.
I used satellite view, to spot small streams emptying into the ocean. Hopefully they won't be all dried up.
To avoid especially cold or rainy days, or just to take a recovery day, there are hotels and hostels all over the place. Supermarkets and restaurants as well. At least on Google Maps, I won't be more than 10 miles without a hotel or restaurant to get more water. There is a stretch of 20 miles without a hotel under $50. They all cater to the tourist industry, which at the moment is practically non-existent due to winter and COVID-19. Although it isn't my desire, I could do this trip without a tent, if I was assured everything was open.
I am trying to find weather conditions, and I think this Mount Washington Observatory may be somewhat higher altitude: https://www.mountwashington.org/experience-the-weather/mount-washington-weather.aspx
Temperature change with elevation: http://www.onthesnow.com/news/a/15157/ask-a-weatherman--how-does-elevation-affect-temperature-
Not seeing a lot of snow, where we are going, based on doing a search for “New Hampshire” on this site the says it has relatively live satellite photos (Landsat Live):
Found site linked from http://www.citylab.com/design/2015/03/roam-the-earth-in-real-time/388252/
This site says that the weather station is NHFN1: http://www.weatherforyou.com/reports/index.php?pands=franconia+notch+state+park%2Cnew+hampshire
On the following map, you can turn on “Terrain” by hovering over “Map”: https://mesonet.agron.iastate.edu/sites/site.php?station=NHFN1&network=NH_RWIS
There I see, that the first hut is about 1000 feet above the station, or 3.3 degrees F cooler.
I later learned that the following app, if you put in the longitude and latitude coordinates, gives you the weather forecast at that very location, including altitude. Pretty awesome. https://play.google.com/store/apps/details?id=com.nstudio.weatherhere.free
Mosquitoes won't be as bad as later in the summer, but I think they are out already in late May. They don't bite below 50 degrees, or so I read at one site.
Black Flies we should worry about, according to Whiteblaze and Farmers Almanac. I'm going to see if I have a head net, just in case.
I'll be using clothes I've treated with permethrin.
Post backpack report (May 28-29-30): Nothing bit us. Didn't see any mosquitoes or flies!