Eastern Sierra avalanche advisory
Posted March 19, 2010 by Sue Burak
Today the avalanche danger is estimated to be mostly LOW. Recent dry weather has resulted in a relatively stable snowpack. Generally safe avalanche conditions exist, but continue to watch for isolated shallow wind slabs along upper elevation ridgelines and in alpine terrain. Be on the lookout for large, growing, pinwheels of snow rolling downhill, or sinking into wet glop past your boot tops. These are signs that the surface snow is getting wet enough to slide.
With the sun getting higher in the sky sending more solar energy into the upper layers of the snowpack, the Sierra snowpack is in transition from winter into spring. Solid overnight freezes and warm sunny days have strengthened the late winter snowpack. Spring snow and even corn snow conditions can be found on sun exposed slopes facing southeast to west at mid elevations. Wet creamy powder can be found on some slopes above 10,500 ft.
By early afternoon on sun exposed slopes, the pack offers no support for skiers and people are breaking through to the rounded, melting facets at the bottom of the snowpack. North facing alpine terrain is a mix of wet powder on the surface, some dry winter snow in sheltered gullies and bowls and breakable wind crusts.
Today at the 10,000 ft elevation on a southeast aspect in the Rock Creek basin, soft spring snow surfaces became thick melt freeze crusts as the cold front and north winds moved in by mid day. We looked at snow temperatures and found the snowpack has a ways to go before becoming the same temperature from top to bottom. The surface layers are wet by mid morning and point release wet snow avalanches are a concern over the next few days. Avoid traveling under rock outcrops, ridgetops and cornices by mid day.
In the Mammoth Lakes area, Josh found a spring snowpack on shallow east and east southeast slopes around 9,600 ft- snow temperatures were 0 degrees C or 32 F from top to bottom. Moving around to northern aspects, the snow surface was 0C but colder in the middle of the pack. Good winter skiing was found on north slopes at the east end of the Sherwin Ridge.
An observer from the June Lake area reported transition snow conditions with mid elevation glades containing both cold and wet snow. Numerous rollerballs came down on southeast to southwest aspects but no natural avalanches were reported. The buried layer from the January 12 storm and the old rounded depth hoar at the bottom of the snowpack is worth watching closely if nights warm up to above freezing in the next week. Deep wet slab avalanches will then become a concern and a real headache for the forecasters.
Yesterday’s cold front brought strong north winds last night and early morning. Strong north winds gusting to 70 mph will continue over the higher terrain this morning and finally ease off by the early afternoon. Wind gusts exceeding 40 mph were recorded last night around Ellery Lake and the Tioga Pass area but the top of Mammoth Mountain won the award for the highest wind speeds- with gusts over 80 mph.
Please note that the avalanche danger rating in this advisory expires in 24 hours. This advisory is our best interpretation of snow pack conditions and NWS forecasts issued today. Backcountry travelers should be aware that elevation and geographic distinctions are approximate and that a transition zone exists between upper and lower elevations. Avalanches do not happen by accident and most human involvement is a matter of choice not chance. Most avalanche accidents are caused by slab avalanches that are triggered by the victim of member of the victim's party. Even small slides can be dangerous. Always practice route finding skills and carry avalanche rescue gear. Remember that avalanche danger ratings are only general guidelines. Distinctions between geographic areas, elevations, slope aspects and slope angles should be made.
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