Sierra in Late-September

I'd go for the warmer bag. I've experienced many ~15 degree nights at high elevations in late september and likely no ~40 degree ones.

Hey Nate, I just read trip report(about the JMT)and he said every night was below freezing but warmed up in the morning sun to 70's.
Me,Ken and Josh did a early October hike in Emigrant Wilderness last year and it snowed on us in the afternoon..I would be prepared for the worst case scenario..It was very cold and think a tent is a must..I froze in my tarp—-The shoulder season.

Nate, I agree with Jay. Better carrying a warmer bag. Since I have done this trip and know the elevation's of that hike, you are going to be at 8-12,000 feet. Not a time to mess with trying to stretch it. Also, please have a back up plan for bailing if possible. The weather in late Sept. is sublime with cooler days, the changing of seasons, and such. But that can change on a dime. Have some plans so that if you do run into bad weather you can make a judgement call and move along. It is beautiful up there. The view from Dusy Basin towards Mt. Langille is awesome, as well as the Black Divide. Muir Pass is very cool. Just like on Casey's post of his trip, we took off cross country towards Lamarck Col and did not go through Evolution Basin or Piute Pass. Have fun, it is incredible there. And please be safe!

The warmer bag it is.

I'm heading north because the bail out options are so straight forward. Plan to take each pass and plod along if weather holds or scurry out at MTR, VVR, or Red's Meadow if things go badly. I would very much like to see some snow, just not too much of it.

Okay,

So here's the gear list for this trip. It's looking to come out to a baseweight of about 11 lbs. (I didn't add in phone, earbuds, and a few other non-essential items. Also debating on what to do about filtration since I lost my SteriPen in the Emigrant wilderness….probably go with drops but might just get another one.) Just wanted to run this by you guys to make sure it's weather appropriate.

Packing
Osprey Stratos 40 37
Packliner 1

Sleeping
Kelty Light Year + 15 43
Pillow 1.2
Neo Air 9
GG Thinlight 1
PolyCro Ground Cloth 1.5

Shelter
Ti Goat Bivy 7.8
GoLite Hut 1 16.7
8 TiStakes 1.6

Clothes
REI MTS Baselayer Bottoms 4
Propore Rain Suit 10
North Face 100 weight Fleece 8
Mountain Hardwear Fleece Baclava 2.8
Outdoor Research Wool Gloves 2.5
REI Mountaineering Wool Socks 2.2 (Cut off the tops)
Under Armour BaseLayer Top 4.1

Cooking/Hydration
REI TiCup 1.8
RedBull Stove 0.8
Nalgene 32 oz Canteen 2

Lighting/Miscellany
Petzl Tikka Plus 2.5
Cuben Stuff Sack / Bear Bag 2
First Aid 2

Total ~10.5 lbs.

It looks like you're just bringing a baselayer and a 100 weight fleece for your top insulation. That's not nearly enough in my opinion. I'd keep those pieces and add a down jacket, something warmer than a Montbell Ultralight.

Leaving on the 23 rd to do the JMT Southbound so I've been having the same debate myself. I'm hoping to see a bit of snow but not too much.

I'm taking a WM ultralite with a Mont Bell thermawrap parka inside a bivy bag and ID silshelter. Will be sleeping on the same insualtion as you.
My alternative is a 2pd 10F Versalite and leaving the bivy sack behind. Not sure what makes more sense.

I'm taking an MSR windpro canister stove however, don't trust alcohol stoves below freezing.

ARe bear canisters not required where you are going in the sierras?

you can avoid Bear Cannister requirements. Go to http://www.sierrawildbear.gov and look at the interactive map.

For my trip, I'll camp near Little Pete Meadow Night 1, then near MTR night 2, then near VVR night three (resupply there) then near Duck Creek night 4 (just before Ansel Adams restrictions start). After that, I'll have to camp at designated camping spots with bear boxes (Red's Meadow, Agnew Meadows, Vogelsang, Tuolomne). Plenty in that area so I'll have some options. I might do a side trip up the "Crossing" (Minaret Lake over to Lake Ediza), but in any case, yah, with planning you can avoid bear cannisters on the North Side of the JMT.

(Supposedly, it can be done in the SEKI area, but might require a special permit and more planning.)

Jack,

I don't intend on hanging around camp, and since I'm lugging the heavy bag, I figured if it's cold I can just crawl in that. Under Armour, plus Long sleeve nylon-polyester blend shirt plus fleece plus rain jacket should be enough to keep me warm when I'm on the move. When I'm stopped for the evening, I'll be in the 15 bag–cooking and what not from inside it if it's frigid. This is a common technique used by UL'ers, right?

Otherwise…I have a very heavy-duty Sierra Designs down jacket that is WARM (Chicago Winter Warm) and I could bring that. But it's heavy and I'd probably go with the lighter bag and sleep in it to make it worthwhile?

Or just take both….

I haven't been cold on a trip in so long (years) that maybe I'm not as worried about it as I should be. I dunno.

I can't really speak to whether using your sleeping bag as primary evening insulation is common technique or not. I do know that it's common theory, just not sure how many people actually suck it up and do it! I've mostly thought about it as an option if it needs to be done, vs my only plan. I'd say that very few hikers go out with in shoulder season without insulation enough to keep them warm after about 5pm while stationary. It could easily be too cold for your clothing setup after about that time.

Even if I hike a long day, there is usually an hour or so where I'm in camp and I don't yet want to be in my sleeping bag. And if I'm not dead tired from the hiking, I like to be out wandering around my camp checking things out until a little after dark.

If you are willing to sacrifice a LOT of comfort for the goal of a light pack, you may be able to get by with your current setup. All gear choices are compromises.

I'm generally not an advocate of buying a lot of stuff, but you could consider how a warm puffy layer complements an ultralight setup. For this type of trip, and other hiking in CA, I'd consider the Montbell Alpine Light as an option. As for your current down jacket, that's probably not something I'd want to carry outside of winter. Could you scrounge another thick fleece from somewhere? A down vest?

I'm heading out next week and I'll be bringing essentially what you are, plus a Montbell UL Down Inner jacket. I think I'll be cold for the minimal out of bag camp time that I'll have at ~12,000ft.

This could all be bad advice, it could be warmish at night (mid 30s) or it could be low teens.

I've been doing some work planning my JMT trip which starts next week. Here are two maps and temperature profiles along the trail for September and October using average minimum temperatures from 1970-2000.Note the color ramps are slightly different between the two maps. Hope this is useful to someone other than me.

There's a lot of good water in that area, so you might not even need a filter. I've used chlorine dioxide tablets, without waiting the prescribed time, and never had any problems.