Based on the thread here - much of the evidence is anecdotal. Things like - "It was really cold when I did the TransAm." Or - "It was over 100 most of the time when I rode cross-country." Well, 2009 had one of the coolest summers in a long time and 1997 one of the hottest. And on any one day in any one month in any one year, there can be a wide range.
But looking at average highs, lows, and precip can give you core knowledge from which to work. Summer temp ranges are far great in the West than the East. It might be 95 in both Montana and Ohio, but it will only go down to 70 in Ohio while it may drop to 40 in Montana. Not to mention that the humidity will be much worse in Ohio. There may be one day in the summer where the humidity in Cincinnati is lower than the humidity in Helena, but it still does not change the basic fact that Montana has far lower summer humidity.
The biggest weather misconception that most easterners have about the West has to do with June. In the Northern Rockies you can still get snow well into June. It comes and goes fast, but a day or two can be nasty. Plus, there is still a lot of snow on the ground in June at higher elevations. Especially if it has been a snowy winter, it takes a long time for the snow to melt out - particularly on back roads and forest roads that are not plowed.
Climate normals are just that - long-term averages. However, I can assure you that over a multi-year period, you have a much better chance of having good weather if you start a tour in early May on the West coast in Sausalito than in Astoria.