Despite the great diffeence in elevation between the Appalachians on the east coast and the higher ranges of the west, the weather is remarkably similar on the high summits in both places. The main difference is between north and south, that is, between northern states like Montana or Maine and southern ones such as Arizona or Georgia. All serious peaks, though, get pretty snowy and cold in winter, and warm and sunny in summer, no matter if they are in Southern California or Glacier National Park.
However, please bear in mind that the following paragraphs are intended only as the broadest outline of what to expect in any given month in the mountains. Mountain weather is as changeable as it is fierce, and you should be prepared for cold, wind, rain, and possibly snow no matter when you go.
Winter in the mountains, although brutal, can be the most rewarding season of all for those equipped to hande it's unique challenges. Winter ofter comes early up high, maybe as early as September in the high Northern Rockies, by November or so in most places. All major ski areas are open by Thanksgiving, and it usually is snowing reliably all the way until at least late March. Places such as the High Sierra and Utah's Wasatch Range can get over 40 feet of snow in a winter, while even the high peaks in sunbelt states like Arizona and Georgia get seriously snowed in.
99 percent of winter recreation in the mountains is in the form of skiing at downhill ski resorts, but more and more people are discovering the wintry mountain backcountry. Braving avalache danger, hypothermia, blizzards, and unplowed access roads, usually on cross-country or backcountry skis, the adventurous few discover a starkly beautiful white world of peaks that they often have all to themselves.
Spring is in many ways the worst season to be in the mountians. All the heavy snowfall of winter turns into melting snow in April and May, creating what is often called "mud season". Travelling on ground that is a mixture of solid ground, snow, and mud can be exasperating, especially when the snow is too deep to walk on without postholing, while not continuous enough to ski or snowshoe on. Also, the streams during spring runoff are usually wide and swollen, making crossings more treacherous, and all this water and new warmth is a boon to the new insect population, eager to prey on humans once again.
In the higher elevations skiing is still possible in April and May, or even into June, but by mid-June in most ranges it's pretty much summer, with the snow gone (or long gone) and hiker traffic increasing. In the high mountains during the height of spring recreational use is practically non-existent, except for a few hard-core ski bums. This is great for solitude, but the miserable terrain conditions make it great for little else.
Summer is high season in the mountains. Except for the glacier clad peaks of the Cascade Range, several others in the Northern Rockies, and a few random snowbanks here or there, U.S. mountains are entirely snow-free in July and August. June has a general tendency to be somewhat springish, while September is likewise fallish. In any of these four months, though, it can actually get extremely hot, as anyone who has slogged uphill under the broiling sun can attest. Rain is pretty much always frequent, and the daily afternoon thunderstoms of the west are legendary, but periods of good weather are encountered quite often. Even the wet and stormy ranges of New England and the Pacific Northwest sometimes see several warm, sunny days in a row.
Predictably, summer sees, by far, the most outdoorsmen thronging to the mountians, and sunny August weekends are about the most crowded popular hiking areas ever get.
Fall is in many ways the best season to be in the mountains. The increasing coolness can be a huge relief after the heat and (in the east) humidity of summer, and, unlike in spring, there is generally no snow on the ground. Rain and thunderstorms diminish a little in the fall, and the huge populations of insects largely disappear. Of course, the trees in their autumn splendor are stunning, and the reduced foliage even makes views better in the densely forested east. A final advantage is that the crowds of summer are also much reduced, since the kids are back in school and people are usually tired after a long summer of outdoor activities.
The only real drawback to fall in the mountains is increasing danger of winter weather. In the Northern Rockies or the White Mountains of New Hampshire, to name two infamous examples, snowstorms can come in August, and by October it's basically winter already. Nevertheless, early fall in the mountains is definitely one of the great undiscovered pleasures of the outdoors.
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