For all those heading SOBO on the JMT starting at the JMT northern terminus at Happy Isles in June you'll start crossing paths with some of the PCT NOBOers in the month of June, probably around the second or third week of June(it differs from yr to yr though), so trail could/should be broken even if there's snow on the ground(and there MOST LIKELY will be at least some snow in patches, maybe more extensively so, RELAX!). What I'm saying is, it's not likely you are going to be out there all alone breaking trail on the southern half of the JMT even if there is snow on the ground during the third-fourth wk of June. There should be others who have gone before you, NOBO on the PCT, at least in the southern half of the JMT, you could inquire from about trail conditions you will about to experience JMT SOBO. You might/PROBABLY WILL find that some JMT SOBOers are ahead of you or have completed their JMT SOBO hikes ahead of you too. Still too early to tell what it will be like on the ground in mid to late June though. Just keep an eye on it and adjust accordingly to what you feel comfortable with. have a great hike. IMO, some snow makes for a prettier JMT thru-hike anyhow.