In partnership with:
The Friends of the Utah Avalanche Center Home page is: http://www.utahavalanchecenter.com
Monday, May 02, 2005 9:00 am
Spring, as you know, is the time of flip-flopping—when the weather can’t make up its mind whether it’s winter or summer. We usually get a cold, dry snow storm followed by hot summer-like temperatures and strong sun. During the storms, you need to operate in winter, dry-avalanche mode where you have to carefully check the new snow to see how well it’s bonding to the underlying snow and watch for the ever-present wind slabs, especially up along the wind exposed ridges. As always, you should jump on small test slopes and do slope cuts to test the snow. Always be suspicious of steep slopes with recent deposits of wind drifted snow. Click HERE for a cornice-kicking lesson.
Then, after the storm, temperatures often warm dramatically and the super-strong spring sun quickly turns the cold, dry new snow into damp or wet snow. The mantra of avalanche forecasting is that snow does not like rapid changes, and cold, dry snow quickly turning into wet, soggy snow is one of the most rapid changes possible. The snow gets cranky and shows its emotions by producing roller balls and wet point-release sluffs on steep slopes, especially under rocks that heat up in the sun. Then, if the percolating melt water encounters buried weak-layers or ice crusts in the snowpack, it can create much more dangerous wet slab avalanches because the percolating water dissolves the bonds between the grains of buried weak layers. Click HERE for some photos. If that’s not enough trouble, melt water can also pool up above the ground, which makes what we call “glide” avalanches, in which the entire snow cover moves slowly downhill like a glacier until it finally decides to let loose in a large, catastrophic avalanche. Although wet slab avalanches tend to occur in the heat of the afternoon, glide avalanches can come down any time of day, even in the coldest time of the morning. So in spring you should always avoid crossing underneath glide cracks, which you can find in abundance in places like Stairs Gulch and Broad’s Fork.
Luckily, this kind of monkey business doesn’t last forever. After a week or two of percolating melt water, the snowpack layers disappear, the snow becomes much denser and the whole mess turns into a very strong, consolidated, summer-like snowpack. In other words, when the warm up finally comes, the south facing slopes usually get dangerous first, while the north facing slopes are still cold and dry. After a week or two, the south facing slopes have settled and stabilized and then the east and west facing slopes start producing wet slabs, after a few more weeks, they also stabilize and finally late in spring, the upper elevation north facing slopes produce wet avalanches before they finally stabilize for the summer. In other words, the wet avalanche cycles kind of work their way around the mountain by aspect and elevation.
The take-home point here is to get out early and get home early. The coldest part of the day is usually just before dawn and wet avalanches usually occur in the afternoon. If you’re sinking into wet snow past your ankles, you should get off the slope.
Most people love spring because of the corn snow, or what avalanche folks call melt-freeze snow. The game with corn snow is to get onto it just after the sun has soften up the hopefully frozen and supportable crust, and to get off of the snow before it gets so warm that you punch through the crust into the wet snow below. Start with east facing slopes in the morning, and change to south facing slopes by mid morning and onto west facing slopes by late morning and go home by noon. You can usually tell if the snow surface has refrozen overnight by checking the automated weather stations on the Internet. Luckily, Alta will leave Collins study plot operating through the summer and Snowbird will keep their instruments running through most of May. Remember that snow can loose a tremendous amount of heat just by radiating its heat into a clear sky, so even with air temperatures in the mid to upper 30’s, the snow surface can freeze as long as the sky is clear. On cloudy nights, you will definitely need below freezing temperatures.
For weather forecasts, visit the excellent National Weather Service web site at: http://www.wrh.noaa.gov/slc/.
For weather computer models, I like
Finally, remember that with the exception of Snowbird, all the ski resorts are closed for the season and they are not doing any avalanche control. So you need to treat them like the backcountry and follow the usual safe-travel ritual, like one-at-a-time, don’t travel above other people and get out of the way at the bottom.
We would still like to know about any avalanche activity especially if people are involved so please continue to leave us messages at 524-5304 or 1-800-662-4140, or e-mail us at [email protected]. Fax is 524-6301.
Thanks for your support this season and we will talk to you again in November.