The Wonderland Trail is generally accessible to backpackers from late July through the end of September. Depending on the snowfall levels from the previous winter and the arrival of the autumn storms, this window can shift from year to year. It is best to evaluate the snow levels in early March before you apply for your reservation. A good resource is the latest USDA Natural Resources Conservation Service Mountain Snowpack Map.
Annual Snowfall at Mount Rainier National Park
The Wonderland Trail is inaccessible much of the year. Mount Rainier lies front and center in the path of the jet stream which carries incredible amounts of precipitation in from the Pacific Ocean. Above a few thousand feet, this precipitation comes down as snow – so much so that many trees are completely buried each year. Over fifty feet of snow falls on average annually at the Paradise ranger station. Paradise claims to have the highest annual snowfall for any measuring-station in the world, the record being over ninety feet – suffice to say, much of the Wonderland Trail is under snow until late July and sometimes even August.
El Nino Seasonal Oscillation
The snows typically return in October, but depending on the El Nino/La Nina Seasonal Oscillation, this won’t always be the case. During a La Nina year, rainfall is statistically higher in the summer. Rainfall at sea level does not always mean rain at the higher elevations. Stories abound of late-summer snowstorms. In the 1980′s my father once had to wait out a snowstorm for two days on the Wonderland towards the end of summer. He always packed extra tea, fuel, food, and reading material for those occasions.
It is most-practical to hike the entire route in August or early September. I recommend September for a number of reasons: easier river crossings, fewer insects, lower temperatures, better trail conditions, and fewer hikers in general after Labor Day. September averages a few more rainy days each year than August, but nearly twice the rainfall. Regardless of the time of year, be prepared for rain. In mid September of 2009, I met a number of downtrodden hikers who had spent almost their entire trip hiking in the rain.
Firstly, when snowmelt is at its fastest in early summer, last year’s log bridges are prone to being swept downstream as streams overflow their banks. Every few years trail crews have to go in and install new log bridges on a number of big streams. These installations usually aren’t made until the water levels drop to manageable levels, so if you plan to hike in July, you’re likely to find yourself making some hairy fords. Problem crossings can include the Nisqually River, Kautz and Pyramid Creeks, the south and north forks of the Mowich River, and the west fork of White River. Towards the end of summer, these will probably all be spanned by log bridges with handrails, and the water levels are such that they can be safely forded.
Wildflowers and mosquitoes
Mosquitoes and other insects hatch in the moist soil next the receding snow line. That means that in late July and early August you have an opportunity to test your sanity against an army of bugs whose lives depend on a blood meal. In some places, it can be nearly unbearable. I once hiked the trail in the last week of July and was subjected to a few inconveniences, the largest one being the insects with a taste for stinky hiker. I have never been in worse mosquito conditions than on the west side of the mountain, right along the melting snowline.
Fortunately in late August and early September, the mosquito populations die down as the soil dries out and the nights become cooler. The later you can push your trip the less you will have to deal with mosquitoes. Unfortunately, if you want to see the wildflower meadows in full bloom, you have to come in August when it’s mosquito Mardi Gras. Pushing all wilderness fashion aside, you should consider bringing a mosquito hat, long sleeves and pants if you’re traveling early in the season.
Route Finding above the snowline
Another problem with hiking too early in the season is route finding. In my above-mentioned trip, Panhandle Gap at 6,750 feet was still under snow in late July. Even with great visibility, cross country travel with map and compass can be nerve-wracking if you’re unfamiliar with the territory. In addition, crossing snowfields on steep hillsides can be slow going – especially if you don’t have trekking poles.
In mid to late September the conditions are most-likely to be at their very best. The insects are largely gone, the daytime temperatures are typically mild, and the trail will be groomed, bare, and dry. Just be sure to bring a warm down jacket and gloves for chilly mornings, and be prepared for a few days of rain.