I'll be doing this in early August. Do I need to be concerned at all with rain gear? Does it rain much at all during that time?

Dozens of days in Sierra's in August. One rain/hail storm that lasted about an hour and one snow storm. I take a wind shirt as my only "rain gear." Others will have different experience. One thing about rain gear is it also has a second purpose. If you plan on sitting around at all it could be helpful to keep the blood suckers at bay.

Usually no rain. Sometimes chance of afternoon thunder showers. Can rain cats and dogs occasionally. Check weather forecast prior to trip (usually you can get a 10 day forecast), if you trust meteorologists.

"Do I need to be concerned at all with rain gear? Does it rain much at all during that time?"

If you want to be prepared for the freak storm, pick up an O2 Rainshield or Dri Ducks WPB jacket for minimal cost/weight and relax. You won't even know it's in your pack and it'll do the job if it does get nasty. Also, as Greg said, it'll protect you from the little vampires while you're sitting around in camp.

You bring rain protection. Depending on what the weather is in the San Juaquin you might not get any rainfall on a trip or you will get some nice thunderstorms...I have seen the weather change on a dime and have seen the temp. drop rapidly. You can get hail, rain, snow and high winds. It is not worth it getting caught in a rainstorm. Get some Dri Ducks and you will be good.

appreciate your input, everybody. Any more general tips about the JMT in early August?

water, weather, etc.

There will be plenty of water. Last year in late August water was plentiful. This year with all the snow there will be even more. I wouldn't plan on carrying more than 1L at any time. What else?--Enjoy yourself, its a fantastic trail.

+1 Nick's general description.

I'd say that rain at that time of year is an unlikely possibility that you should nevertheless have a plan for handling. I've been backpacking in the summer in the Sierra since the 1970's, and have never been rained on hard enough that I really needed rain gear during the day or a tarp at night. A couple of times there's been enough of a late-afternoon shower that I decided to put up a shelter for the night, but in both cases the rain was light enough that if I'd just been out in the rain in my sleeping bag, I would have been fine.

On the JMT this year, as a lighter-weight alternative to the driducks, I'm bringing a garbage bag which I can cut into a rain poncho in an emergency. I guess the trade-off is that there's some chance that I could be forced to sit under my tarp for three days while waiting out a big storm, or that I might have to wait for weather to clear before feeling safe going over a high pass.

On the northern half of the JMT, you're generally close enough to resupply points that if worst comes to worst and you're really feeling soaked and miserable, you can pretty easily get off the trail in one day's hike. It's the southern half that's a little more worrisome to me.

Hi I will be in the Dusy Basin/South Lakes area in Mid August hiking the JMT to Kearsarge pass. What are typical nighttime and mid day temperatures on this part of the JMT? I know Bishop averages 90 degree days but the Jmt averages 10,000 ft plus elevation for the majority of this trail segment. Thanks,
Ryan.

"What are typical nighttime and mid day temperatures on this part of the JMT?"
I was in that area in the summer of 2007. Daytime was very comfortable shirtsleeve temps, not too hot or too cold. We slept one night near Bishop pass (12,000 ft), and were slightly chilly in summer bags. We would have been more comfy if we'd had a tarp to keep the wind off.

I lived in T Meadows last season and spent plenty of time at alltitude above that. Most nights figure avg lows of around 35 (as low as 25 on some chilly nights late august) and highs between 60 and 70.

As others already said, the weather can pop up and suprise you but that time of year they are nearly thunderstorms that give off hail or a freak couple inches of snow.

Afternoon thundershowers are pretty normal in the summer along the JMT. Most of the time they're no big deal and clear out in time for you to have a dry camp.

However, one time when I was coming over Pinchot pass, the storm lasted for hours and was quite severe. We still had a dry camp, but we also had several hours of sopping wet misery.

Also, on Labor Day weekend (yes, that's not August but pretty darn close), we had two nights of rain/snow in the Piute Pass/Evolution Valley area. Pretty miserable quite frankly. All we had was a plastic sheeting tube tent. Light but not able to handle heavy rain.

Chances are, something light will do just fine, but there are times where it will rain at night in August. I think in the last 35 years, I've had a seriously rainy night maybe about four times.

HJ

I've had it rain at night more times than not, but I haven't been that much.

Both times it did I was super glad I had a nice shelter instead of my uberlight, dry weather, emergency set up.

My thoughts, based on my limited number of trips to the area, is that an umbrella ought to do well. I can keep pretty dry with an umbrella since the trail isn't overgrown with brush. I will also have rain chaps. They're light enough I always bring them. They work for a warm layer, too. Afternoon thunderstorms can easily be handled under my big 8x10 tarp. And I'll avoid going over passes late in the day so as not to get caught in thunderstorms on my way over. I am hoping this will be enough. (My umbrella is not metal.)