August 7th - 1:39am start, 12:25pm summit, 6:25pm complete.
Acclimatization plan - stay at Mammoth Mountain Inn for 2 nights, taking the gondola up to 11k feet and spending time hiking around and hanging out at that altitude. Plus, Diamox 62.5mg twice per day starting the day I headed up to Mammoth.
Day hike - started 1:39am. Took the Old Trail to start, and otherwise followed the trail. Water crossings were all easy, but there were stretches between Mirror Lake and Trail Camp where water flowed down the trail ("waterfall stairs") so I was happy to have GTX half-rise boots.
The cables and the traverse to Trail Crest are clear of snow (snow is there, but not impacting the trail).
I was moving slow. It took 5hrs to reach Trail Camp. 8hrs to reach Trail Crest. For reference, on my last hike up Mt. Baldy, I did the Backbone route at just over 2mph average on the way up. Here I am averaging just over 1mph.
At 11am, with the Smithsonian house in sight to the north, I look back and the clouds are changing. It is right out of one of the warning guides you'll find on the internet. First, blue sky. Then, a couple puffs. Then, happy puffy clouds. Then, they get tall, and kind of dark at the bottom. Then there are more of them.
At 12noon I hit the one snow crossing you have to deal with. People are saying it is "doable" without spikes. I was glad I had mine. I suppose with poles and no spikes, it would just require some extra care. No poles or spikes would be trickier. It's only about 50 yards, anyway.
At this point I was just rushing up because the weather was clearly turning. And kind of annoyed at myself for ignoring what was obviously happening.
12:25 summit. I took a few pictures, congratulated the people who finished around the same time, signed the book, and started moving back down as fast as I could. Not exactly the summit experience I was hoping for (1st ascent for me) but appropriate given the weather.
1:15 the first "BOOM" of thunder. I was close to Trail Crest. It started hailing.
1:30 racing down the switchbacks. Cracks of thunder and lightning on the other side, where I had just been. Aside - it did not appear when I reached the summit that anyone up there was concerned about weather. People were just kind of chilling and hanging out. So at least 30 people were on the side of the ridge where the lightning appeared to be hitting.
2pm intermittent thunder/lightning and continued hail. The trail was getting slippery and I stumbled a few times, but having double poles kept me from falling.
~3pm the bad weather started to let up. I noted that it took me over 5 hours to get up the from the base of the switchbacks and only 2 hours to get down. Adrenaline/fear is a great motivator!
I am very, very appreciative to the posters on this site. I got some really good advice. I learned about the Old Trail, and where to "dip and drink." Because of that knowledge, I was able to minimize carried weight (I started with only 1L of water). I also learned a lot about T-storm risk here and chose to ignore it, at my own risk. Not sure how much risk I really took but it certainly was not fun to see lightning striking.
Zero AMS. So I definitely recommend MMI+62.5 diamox 2x per day. The prescription was for more, but I had read that less is just as effective without side effects, so I went with less.