In response to your request for more info about the Sierra storms I've experienced. I thought about sending via PM instead, but maybe somebody else is interested.
My suggestion is that you build a gear list that lets you cope with a 24-48 hour rain, rather than assuming your worst weather will be afternoon showers. If you plan to set up your shelter before you get wet/chilled, and wait out any rain, even if it takes a day or two, then your strategy should be fine. It's not what I would do, but it will work. Alternately, if you're a very strong hiker, you could set off in the rain and hike quickly to the next backcountry ranger station or out via the next available trailhead, relying on burning calories to keep warm. With the hike-fast strategy you'll only get into trouble if you suffer an injury.
The rain gear I would take for a July high Sierra trip would be a home made silnylon rain coat and rainpants. Many people prefer silnylon poncho instead of coat, either way. You can buy an 8 oz commercial raincoat (like the Marmot Essence), and that will serve as a warm layer in camp too. My silnylon coat weighs 4.5 oz and packs compactly.
About the storms I mentioned.
The 68 hour storm was a friend ~30 years ago. It was in August, and it was a summer monsoon storm that didn't stay down in Arizona where it belongs. He spent 3 nights and 2 days in the tent. He was delayed 2 days in his exit, which caused his wife a lot of worry, but he was safe and sound. Using a SPOT tracker would have allowed him to send an OK message to his wife so she wouldn't freak out.
One of our big storms was in the early 90's, last week of August. We were at the end of a descent of Goddard Creek, into the Middle Fork of the Kings River at Simpson Meadow at 6000'. It started raining late afternoon, before we reached the Kings. It rained/hailed/snowed all night, and all through the next day. Rather than wait it out and let the snow melt (which it definitely would have done given the date), we put on our warm clothes and rain gear and continued -- up the Middle Fork Kings and over Muir Pass. The hike through LeConte was perhaps the most beautiful day I've ever hiked. It rained/snowed intermittently the whole day, and there was a foot of snow on the ground in LeConte Canyon. The intermittent views of the canyon walls in the storm clouds were fantastic. If we hadn't had rain gear we would have stayed in the tent all day, where we would have been FINE and SAFE, just delayed for 1-2 days. In a storm like this, your strategy to wait under your tarp should work. If it happens to you, you'll do your family a favor by having a SPOT device so you can notify them that you are OK even if you don't exit on schedule. Our experience was ~20 years ago, and we had no way to notify our family of a deliberate delay, which was part of the reason we chose to hike through the storm.
The other storm we experienced was a late season storm, mid October. At that time of year the snow will not reliably quickly melt, so you need to get yourself out before the snow accumulates too much. In July, when you are going, all snow will melt within a few days.
The deaths and the high-profile SAR efforts in this storm are described here:
There were at least 4 or 5 SAR efforts for hikers, but they are not described on the SAR site. Here's one press story
My recollection of two of the hiker rescues:
One was a small group that was not skilled enough to be in the mountains in October. They did not have equipment to stay warm, and they did not know how to find their way out in low visibility stormy weather after the trail tread became hidden by snow. SAR team took 3 or 4 days to reach them, and they were very happy to see SAR. They were classic examples of the article I read (people die because they prepare for what they have experienced in the past) -- after the rescue they said that for 20+ years they have taken weekend trips in the Sierra in Oct because there are no crowds and the weather is great. Except for 2004, when the weather was not great.
The second SAR was a small skilled group of hikers. They had appropriate clothing and storm worthy shelter, and they were not worried about navigation. They chose to stay in their tents for the duration of the storm, which lasted 3 or 4 days. SAR was initiated when they didn't exit on schedule because SAR didn't know that they were not seeking a rescue. Also, in this case, a SPOT tracker would have allowed them to send an OK message which would have saved SAR and their families a lot of effort and angst.
In this October 2004 storm, we were off trail on the far side of a boulder/scree pass. We started our exit as soon as it was light, in order to get over the pass and descend to a lower elevation as soon as possible, as snow was rapidly accumulating. With the snow masking the placement of scree and rocks, it was very slow going. After a few hours we crossed the pass and reached the trail. We then had a 10 or maybe 15 mile hike down to the car. As we descended the snow turned to sleet and then rain. It continued heavy precipitation all day. We hiked ~12 hours that day to get back to the car. Without raingear we would have been very cold, although I think we would have been OK even in that case (assuming no injury) because we were strong enough to hike 12 hours without resting.
The interesting thing about the Oct 2004 storm was that it hit us by surprise. In Oct trips in the Sierra, we go out for 4 days max so we can get a reasonably sound weather forecast. And the 4th day was to be on trail, so if it did storm on day 4 it wouldn't have been a challenge. As of evening Oct 15 the 4-day weather forecast was good. We ate dinner on our second evening, Oct 17, and it was perfectly clear and calm. By midnight it was howling. By morning there was 8" snow on the ground, and piles of snow blown onto the tent.
I'm a conservative person when it comes to these sorts of things, and I prefer to be prepared for unexpected problems. On the JMT, getting lost is not a problem unless you are seriously unskilled. But a combo of injury and bad weather can quickly complicate things, and I like to know I'm prepared to cope without assistance.
Again, everybody chooses their strategy and level of risk tolerance. My suggestion is only this -- it is possible to have a 2 or 3 day storm in July or August. Not likely, but possible, so have a plan for what you'll do if it happens to you, and have the gear to support that plan.
Sorry for the long past. But I've now bared my soul and I'll retreat :)
Have a great trip in July, and don't let the mosquitoes suck you dry!