The backpacking season for the Wind River High Route is short: three months, July through September, in an average year.
But the most optimal window is even shorter: August through mid-September, or mid-July through mid-September after a dry winter. By pushing it earlier or later, you are more likely to encounter heavy bug pressure, high-water hazards, and/or excessive snow; or you are at an increasing risk of being shut down by an early winter storm.
1. Winter snowpack.
Between about November and April, the Winds are pummeled by Pacific storms that drop hundreds of inches of snow, especially along the crest.
The snowpack normally begins to melt out in early-April, starting with the lowest elevations. Peak melt occurs in June. And most passes are snow-free in or by August. After a drier-than-normal or wetter-than-normal winter, this timeline can be accelerated or delayed by about a month. Some snow never melts, which contributes to the many glaciers and permanent snowfields in the Winds.
Lingering snowfields can be a blessing, especially if it covers nasty talus or loose scree. But extensive snow coverage makes for more challenging conditions. Expect chronically wet feet, afternoon slogs on mushy snow, and some additional risks, including cornices that can block passage and icy morning crusts that increase the chance of a fall.
While the High Route can be shut down for a few days by a freak summer snowstorm, it will most definitely melt out due to the relatively warm air and ground temperatures, and the intense solar radiation. In a typical year, snow begins to permanently stick in October, especially on north-facing aspects and at higher elevations.
2. Summer monsoon.
The prevailing summertime weather system in the Winds is the North American Monsoon, which normally picks up in early July and fades in September. This pattern also affects Colorado, Utah, Nevada, California, and especially Arizona and New Mexico.
Intense solar heating of the southwest US allows moisture from the Gulf of California, the eastern Pacific, and the Gulf of Mexico to flow into the region. This moisture first manifests in puffy, cottonball-like cumulus clouds that build throughout the day, peaking in size and quantity in late-afternoon before dissipating in the evening hours. Cloud development is often greatest over high land masses, which forces the moisture upwards into colder air.
On days with early build-up of cumulus clouds, afternoon thunderstorms are more likely, and sometimes inevitable. Lookout for a dropping cloud ceiling and dark, vertical cumulonimbus clouds with an anvil cap.
Monsoon thunderstorms are often violent and dangerous. A lightning strike is the risk with the highest consequence, but the odds are relatively low. The more assured risk is exposure, which can lead to hypothermia. The thunderstorms drop torrential rain, and sometimes hail or sleet. And the ambient temperature drops suddenly and significantly, since the dry Wyoming air has little thermal mass to counterbalance strong downdrafts of cold air from thousands of feet above.
In addition to lightning and cold-and-wet conditions, thunderstorms can also cause a loss of visability as low clouds engulf the mountains.
The Wind River High Route is often in high and exposed terrain, the last place you want to be during a thunderstorm. Additionally, off-trail navigation is very challenging and possibly dangerous when visability is limited, even if you have a GPS: it may tell you exactly where you are and where you need to go, but it cannot identify an optimal or safe route to get there.
In July and August, expect afternoon thunderstorms at a minimum, and less predictable thunderstorms during very active monsoon patterns. Summer weather is much more similar to Colorado and Utah than to California. Utilize fully the morning weather window, and be aware of lower alternate routes if you are forced down.
The mosquitoes and biting flies normally reach their peak in July, once the snowpack melts out, the vegetation greens up, and the temperatures warm. The most intense pockets are found in soggy areas when there is little wind. Bug pressure fades through August and ceases completely in September.
The bugs in the Winds are not on par with those in Alaska, but they are as bad as anywhere in the continental US. Bug-resistant clothing (e.g. long-sleeve shirt and full-length pants, treated with permethrin and/or made of tightly woven fabrics) and a headnet is highly recommended. A bugproof shelter is useful, too, although the bugs usually go away at night so long as temperatures are below about 50 degrees.
4. River fords
As a crest route, the Wind River High Route crosses most creeks high in their watersheds, before they can accumulate to dangerous volume and velocity. The section-hikes generally have higher water hazards, so beware especially during the peak melt in June.
There are four potentially problematic river crossings on the High Route: the Middle Fork of Bull Lake Creek, the two upper forks of the North Fork of Bull Lake Creek, and Dinwoody Creek. Fortunately, in all cases there are safe fording locations nearby, where the creek slows down and/or widens out.