The CPC outlook for May-June-July (MJJ) is indicating a warmer and drier than normal 3-month average for the entire state. The temperature outlook is calling for warmer than normal temperatures for the state, with slightly higher chances of above normal temperatures in southwestern WA. For precipitation, there are higher than normal chances of below normal precipitation statewide for May-July. (Office of the Washington State Climatologist)
Snow levels above 5000 feet were robust this year, often accumulating when lower elevations were experiencing rain and warmer temperatures. With this in mind I have made forays into Mt. Rainier National Park to scope out conditions these past two weeks and will offer an opinion on Wonderland Trail snow conditions for 2018. First hand observations; once passing the 5000' level, there's lots of snow still in them thar hills.
On May 24th I biked the West Side Road and scoped out Klapatche Park, a backcountry camp that resides at 5500'. I encountered some snow at Round Pass (4000'), which required a short bike push, but this is assuredly melted by now. What I did find, hiking the St. Andrews Creek trail, was the predicted snow line right at 5100'. Clear 'til then, then pretty much continuous snow all the way to the camp. Neither the logs nor the sign was visible next to Aurora Lake, and snow levels in the camp varied from less than a foot to more than 6'. Clad in trail shoes, I encountered no serious post holing, although I did sink consistently above ankle height (no gaiters).
On June 6th I biked the Ipsut Creek trail and proceeded on the Wonderland to Moraine Park at 5700'. Maintenance is just starting, with the 3 person crew hard at work for this part of the mountain. Luckily, they had established a log to cross the Carbon River so I was able to continue up the trail past the Carbon Glacier terminus. No surprise, I encountered continuous snow just above 5100'. Levels varied but in some cases the depth far exceeded 6'. Once I hit the open meadow below Mystic Pass, the snow thinned considerably and I even had lunch on an exposed knoll and rock. On this trek the snow was consolidated enough to stride mostly on top, with only about 4 post holes on the descent later in the afternoon; again, trail runners with no gaiters. I did have to go feet wet on the return leg at Dick Creek due to the creek wandering around the end of the bridge, but the maintenance crews will probably deal with this before thru hikers start doin' their thing.
2015 was a low snow year and I hiked the west 57 miles on June 9th and 10th, providing the comparison photos above for contrast.
Lastly, on June 10th I visited Summerland on a "Junuary" weekend, meaning rain, snow, occasional sun breaks and temps in the 40's. Snow started above 4800' and was continuous above the bridge at Fryingpan creek. Although we attempted to follow the summer trail up the switchbacks, steep runouts with lots of snow coverage dictated a descent via what I call the winter route, essentially end running the switchbacks by heading towards the creek right out of Summerland, then paralleling the creek until regaining the bridge. The weather was such that continuing on to Panhandle Gap, all on snow and climbing into cloud, seemed fruitless....needless to say there's still considerable snow coverage to the Gap and beyond.
So early season hikers, meaning June and well into July, can expect varying levels of snow in the higher elevations of the trail. Consolidation is well under way, so forget the snow shoes (some may opt to pack some manner of microspikes and even an ice axe); your regular footwear should suffice. However, continued warm weather, as the CPC outlook indicates, will help the melt off considerably, especially if we don't encounter "Junuary." IOW, pretty much a normal year. I'm planning a fastpack of the Wonderland in late August or early September, so please say hi! Happy Hiking!
DISCLAIMER: Always check with the NPS and/or visit the website for current conditions. What works for me may not work for you, so always pack for your comfort level and experience. Lastly, don't hesitate to contact me if you have specific questions about the Wonderland Trail if you think my input may help you.