Calling all Yosemite veterans

In August, I prepare for freezing temperatures or a few degrees worse. By late September, I prepare for another ten degrees worse than that.

–B.G.–

Unlikely to get below 25F and 30F is more likely. Canyons are worse than surrounding higher country.

Check weather forecast for snow also. I've been snowed out in early Oct., and have seen snow storms through out the Sierra about that time in Sept too. If it passes, it turns nice again. Funny to see a flyer on your car windshield saying Tioga Pass is closed after being out 6 nights, with last four days of very nice weather. :)
Duane

I've seen low 20's in the 3rd week of September around Tuolumne Meadows. Even had it snow one time on the 19th where the temperature was about 17F.

Planning for a low of 20F should be safe.

30's on a good night, most likely 20's Bring a extra layer!

Many thanks to everyone. This is great news…sounds like perfect hiking weather to me.

I usually am prepared for being comfortable at 20F any time I go out into the mountains, simply because it can be that cold any given night, depending on the weather the mountains decide to have that day. It's already been snowing (though not in the sticking, wintery way) and raining up there in the high elevations. We've seen snow every month of the year. So as usual I'll say be prepared for 20F.

I concur – be prepared for 20's.
Also, give yourself time to acclimate to the elevation. Be sure to drink lots -more than you think. Your stomach may be upset at elevation also. Consider bringing extra fuel & hot drinks for chilly afternoons. Enjoy! It's beautiful up there!!

Mid sept can be cold at night and the always unexpected storm can roll through. I have a 15 degree bag that served me well on the SHR two years ago. I can always vent it if I'm too warm. We had planned on a 2nd part of the SHR last year and couldn't make it and the Sierras received a nice amount of snow that week. So I'd plan for the worst and have plans in case you do get caught in one of those freak storms and have to hunker down for a day to wait out the melt.

I'm heading out to do the North Lake – South Lake with side trips into Ioninan Basin mid Sept. My go to source for weather is mammothweather.com

"Canyons are worse than surrounding higher country."

Quite true, even on a small scale. A rise with 20-30 feet more of elevation can be 5-10 degrees warmer than being low in a valley or stream shed where you can get katabatic winds from higher elevations (colder, denser air flow down slope, following the same path water would). In the Sierra, they are typically 5-10 mph (plus being colder), whereas downtown Juneau, Alaska can have them to 90 mph (sidewalks have handrails for assistance) and 300 kph off the Greenland ice sheet. They arise when a cold lake or glacier above you cools the air and/or if radiant cooling on a clear night cools the ground surface above you.

A different weather effect, on clear, windless nights: being under trees, near bushes, or along a rock face will result in less net radiant cooling, less condensation on your tent/tarp, and a little warmer night.