Eastern Sierra - Wed Mar 24 2010
Eastern Sierra avalanche advisory |
| The avalanche danger rating this morning for Bishop Creek to June Lake is LOW for all aspects and elevations. The main avalanche concern today will become isolated wet snow instabilities as a result of solar warming of the snow pack. This will only be an issue on East to South to West aspects. . |
Light winds and sunny skies are expected over much of the forecast area today. Temperatures in the 40s at mid elavations and mid 30s at upper elevations (11,000-12,000') are expected. Breezy conditions over the last several days have kept mid to high elevation snow packscooler, but today calmer winds are expected. As a result expect to see moistening of the snow packreaching higher elevations and slightly greater instabilty as aresult on solar(East to South to West) aspects. The main avalanche concern today will continue to bethese wet snow instabilies.
Observations made 2 days ago in the Convict arearevealed that west aspects were showing more signs of recent wet snow instability than east aspects. Also on one particular slope we could see that a slide wastriggered from a cornice collapse. Click here for a photo. There aresome large cornices out there, bewareof being underneith themor stepping ontop of them, especially later in the day. .
.
People are getting out and skiing and riding bigger terrain. The snow remains winter like on northern slopes above 10,000 ft despite increased solar radiation. Overnight temperatures in the teens ensure the snow on north aspects will take time to develop wet snow instability.
Numerous observations showing an absence of widespread wet slide activity suggest the gradual warming trend has allowed the upper snow layers on mid elevation to some higher elevation slopes facing east to south to west, to adjust to the melt freeze cycle. However, as warming continues, melt water will penetrate deeper into the snowpack. If a fine grained layer of snow deep in the pack gets wet, it loses strength until melt water establishes pathways through the layer.
Wet snow avalanches are occurring but their distribution is patchy rather than widespread. That being said, the large wet snow avalanche that probably occurred on Sunday afternoon above the Whitney Portal Road, is a good example of what can happen when the avalanche danger rating is LOW during the spring.
High thin clouds over the weekend allowed incoming solar radiation to reach the snow but inhibited outgoing longwave radiation. In this case the surface snow gains heat energy rapidly and may weaken in a short period of time. One thing to remember is the depth hoar at the bottom of the snowpack has no strength when it gets wet. Sometimes, a steep drop in temperature over a 24 hour period after a period of warming, followed by a warm day has been associated with wet slab avalanche release in the Rockies and occasionally observed here in the Sierra. The main point to remember is to pay attention to how cold it gets at night, note that high thin clouds add more energy to the pack than a clear day and get off slopes before sinking in over your boot tops.
Today will be mostly sunny with light winds at mid elevation locations. West and southwest winds pick up today over higher terrain with gusts to 45 mph. At mid elevations, winds will be light but pick up by this evening in advance of two shortwave systems to affect our area through Friday morning.
After a mild day, temperatures will fall 5-10 degrees on Thursday. Expect highs in the upper 30’s to low 40’s today at elevations above 9,500 ft. then feel the difference on Thursday when temperatures fall 10 degrees. Mid elevations from 8,000 to 9,000 feet will reach 45-50F today, then fall to around 40F on Thursday.
By Friday, it will feel like winter with highs around 27-29F at the 10,000 ft elevation. Gusty west and southwest winds will plague the higher terrain until Saturday. Wind gusts up to 50 mph are expected today and tomorrow.
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.
ESAC is operating on a limited budget and is supported by its members and the community. Please help support the Center by becoming a member today. Visit the Support area for more information.
