So, I'm planning on taking a nice long trip west of the Mississippi for the first time this summer. I don't have any specifics on where I'm headed yet, but I know there will be some mountains involved, and I was curious about needing to pack differant gear than I do here in the Southeast for a summer trip. Can I get a little help?

My typical gear list for summer in KY:

poncho tarp or TT Contrail, depending on forecasts or trip length
20* bag, which winds up staying unzipped and half-off me all night (first down quilt is coming soon!)
rain gear is almost always a poncho only
aqua mira unless I'm hitting an area with a clay bed, then I go with a filter and clean it regularly
my typical clothing stuff sack only has a lightweight baselayer and a vest or zip-up for insulation

My concerns deal mainly with bringing enough to stay safe, temp-wise. Any input is appreciated!

Personally I would say that you are fine to hike almost anywhere through 3-seasons with your current gear list:

- Poncho tarp or TT Contrail: I use my Contrail on all of my 3-season adventures in Colorado.

-20* bag: 15-25 degree bags are ideal for 3-seasons. Some even say that 30 is fine too, but here in Colorado it is not uncommon to wake up to frost on the ground, even in July!

- Rain gear is almost always a poncho only: Personal preference, I take a rain jacket and pants, but many people only take a poncho.

- Aqua mira and filter: Once you determine where you are going, then you can may be able to drop one. I generally just take the Aqua.

-My typical clothing stuff sack only has a lightweight baselayer and a vest or zip-up for insulation: The only things that I would add are a fleece hat and gloves, certainly if you will be at altitude.

For the Sierra if headed here, clothes wise, I have a couple shirts, down vest, Polartec 100 top, wind pants, raingear. I don't have much in the way of layering clothes, so I may be too light. I am warm when moving and I can always get in my bag at camp. May want a watch cap and gloves. Expect freezing temps in the Sierra year round at elevation. I also only use a 35 degree bag with a silk liner. Running shoes are great, did a XC bp trip a few years ago and had no issues with my 50+ old bones on a week long trip.

I'd use the Contrail; the Rockies and the Northwest do have mosquitoes.

Your clothing depends on whether you want to sit on a rock to watch the sunset or whether you hike late and crawl into your sleeping bag as soon as you pitch camp. If, like me, you do the former, you might need bit more clothing. I take a baselayer (which I wear for sleeping as well as when it's cold), shirt and pants for hiking, puffy insulated (or fleece) jacket, rain jacket and pants, fleece hat, gloves, rain mitts. I also take a sun hat. The sun at high altitudes can really be fierce! If you use trekking poles, you might want sunshields to keep the backs of your hands from burning. During mountain thunderstorms, there's lots of wind, so a poncho might not be so good. If you do use the poncho, take a wind shirt.

The 20* sleeping bag should be fine. Be sure you've got a good warm pad under it! If you use a NeoAir, use a supplemental CCF pad, at least 3/8" thick, for below freezing nights.

It can and does snow at high altitude at any time during the summer. Last summer in Wyoming's Wind Rivers it snowed a foot at Titcomb Basin (11,000 ft.) on Aug. 8 and again on Aug. 15. Be prepared!

It makes a huge difference which mountains. A poncho is ok some places, rain gear (pants and jacket) is advisable in the PNW, rain gear can keep you warmer so it does dual purpose. I'd have 200 weight fleece pants and at least a 200 fleece jacket to sleep in as temps up high can drop. You need to err on the side of warm dry clothes in the mountains.
Jim

Definitely rain pants in the PNW, where, when if it's not raining, you'll get soaked to the waist from dew or rain on brush, which we have a lot of! Most trails are brushed out only every 3-5 years (or longer) because of greatly decreased trail maintenance budgets. The brush grows a lot faster than that!

You may find the same problem in the Rockies, especially at lower altitudes where the grass and brush are taller.

One more item for the Rockies--plan to do most of your hiking up high (but allow time to acclimatize on the way), because at lower altitudes where lodgepole pines predominate, most of the trees are dead from the bark beetle. Especially watch where you're camping in these dead tree areas--you are pretty much forced to camp out in meadows, like it or not, for your own safety. In those areas I try to pitch the tent at bedtime and take it down before breakfast in hopes of reducing the effect on the vegetation under and around the tent. There's still plenty of outstanding scenery, though, especially up in the alpine areas! I notice and mourn the dead trees going in, forget about them up in the alpine and don't really notice them on the way out (probably because I'm remembering the beauty of the alpine areas).