To a wilderness backpacking guide, it’s important to understand weather and climate. Not just from the standpoint of scheduling treks at optimal times during the ever-changing seasons, but also because understanding local weather patterns allows us to sometimes adjust the trip plan as the trek unfolds. For example, a mild July morning with mixed cirrus and alto-cumulus clouds, though still sunny, might cause me to re-assess the wisdom of a prolonged walk above tree line that afternoon. That’s because such conditions in mid summer often portend severe afternoon thunderstorms.
Although I took just one meteorology course in college, I’ve spent a lifetime studying this fascinating subject. To me, weather talk is anything but idle talk! Rocky Mountain weather is never boring, but at no time of year is Northern Rocky Mountain weather more fickle than in the spring. I’ve seen it break 90 degrees in May and then snow in June (numerous times), though June snow is more likely than May heat. Generally speaking, springtime in Big Wild country is prolonged, with hot summer weather rare before Independence Day. And again, even in most of the valleys it could snow any time before the summer solstice.
Throughout the Northern Rockies of the U.S. springtime usually begets plenty of precipitation. Not so in the Southern Rockies of Colorado and New Mexico, where spring is much drier. So if you’re backpacking in Yellowstone, for example, make sure you’re prepared for any kind of weather. Generally speaking in the Northern Rockies, winters become milder and wetter as you go north and west. In northern Idaho and far western Montana winter brings dense wet snowfalls and even occasional rainstorms due to low elevations and proximity to the Pacific Ocean. But summers become drier and therefore forest fires become more prevalent west of the Continental Divide. East of the Divide, however, the more open landscape gets clipped by the summer monsoon that sometimes really drenches the American Southwest. So with more rain-producing thunderstorms, summers are often less arid than they are on the west side. And east-side winters are drier and colder, with less Pacific influence and more cold air dropping south from the Arctic.
One other thing to keep in mind. Local weather and climate vary radically over short distances in mountainous country due primarily to two factors. The first is geography. Low elevation valleys on the leeward side of a mountain range are drier and warmer than their sister valleys facing the moisture-laden westerly winds. The laws of physics apply here. Moisture falls on the windward slopes and then the air warms and dries and accelerates as it descends the leeward side, where some locations are extremely windy. The second factor is altitude. As a general rule, air temperature decreases about 3 to 5 degrees F for every 1,000 foot elevation gain. Precipitation also increases with elevation, so in the mountains it’s cooler and wetter than in the valleys. Most folks know this. But more specifically, as spring unfolds, for each 100 foot increase in elevation, spring lags by about one day. So if you gain 1,000 feet of elevation in the spring, it’s not only likely to be 3 to 5 degrees cooler at any given time, but it’s like turning the springtime clock back about 10 days for things such as plant growth, flowering times, and nesting.
Which brings me to our 3 spring Big Wild hiking trips. Our Montana Canoe Trip is out in the Missouri Breaks in the Great Plains at lower elevations than any of our mountain backpacking trips. So it’s usually warm enough for a great canoe trip in mid May (but be prepared for anything!). And our two June mountain trips – the first backpacking trek is in Northern Yellowstone and the other is in Gros Ventre Wilderness – are both in local rain-shadow areas, leeward of high mountains and plateaus, where winter snow is shallower and it melts off relatively early. So these areas are great for spring backpacking. It is this knowledge of local topography and micro-climates that allows us to schedule some great treks in the Rocky Mountain springtime when many folks think the water is still too high and there’s still to much snow in the mountains.