Originally Posted by
map man
Snow and ice on Katahdin is enough of a concern in the fall that there are usually some days in early October (and sometimes days in September) when park officials prohibit hiking to the top of Katahdin. It's unusual to be able to summit Katahdin after Oct. 15, though it does happen sometimes. Until this year overnight camping in Baxter State Park ended Oct. 15, but they are now experimenting for a couple years with keeping a few camping spots open for a few days after this date -- that still doesn't mean that you will be able to summit Katahdin. It depends on the weather.
So a NOBO start in the first week of June means you have about 130 days or less to complete your hike. Only around 20 to 25% of NOBO thru-hike attempters finish, and only about 10% of THOSE finishers do it in around 130 days or less. Long odds, but as some members here will tell you, it's not impossible. If you start the last week of May that gives you a few more days. If you get to northern Virginia and it's obvious you are not on pace to complete by early October you can travel to Katahdin and hike south (a "flip-flop") until you get to the point where you left off -- some hikers do this every year. In any event, starting NOBO in late May/early June means lots of heat down south.
If you choose to go SOBO in most years it would be wise to put off your start until at least mid-June. The problems with a typical late May or early June start is plentiful biting black flies and mosquitoes and having to ford swollen streams in Maine from snow melt and rains. This varies from year to year, but to cut down on these problems you could time your start to be one month after officials in Baxter State Park first open the Hunt Trail to the top of Katahdin -- this varies from as early as May 15 to as late as the first week of June (the "typical" opening date would be sometime in the last dozen days of May).
Down south winter weather in the high elevations like the Smoky Mountains is "usually" not bad in October (with exceptions) but getting more pronounced (episodes of snow or freezing rain) as November and December go along. If you knew you could complete your hike in five months (which is the median completion time for SOBOs who do succeed in finishing), a mid to late June start and mid to late November finish is often a good way to thread the needle when it comes to avoiding weather related trouble at either end.
Good luck whatever you decide.