The Eastern Sierra Avalanche Center (ESAC) produces avalanche forecasts for an area stretching from Bishop Creek to Virginia Lakes in the Eastern Sierra. The avalanche forecast is issued daily by 7 AM for the bulk of the season and 3-4 times a week during the shoulder seasons. The Forecast is designed to effectively communicate the avalanche hazard to backcountry recreationists, industry professionals, and emergency responders. Avalanche forecasters analyze professional and public snow observations from across the area, along with daily snowpack and weather forecasts to create the avalanche advisory. Please note, the forecast is not a substitute for professional avalanche education and will be easier to understand after taking an avalanche awareness or Level 1 class.
This page explains the basic concepts of the avalanche advisory and how to read it.
Avalanche Danger Rating
The North American Avalanche Danger Scale is used to communicate the hazard posed by avalanches to backcountry recreationists on any given day. The danger rating is a combination of the expected likelihood, size, and distribution of avalanches. Danger Ratings are assigned to the forecast when ESAC is issuing daily avalanche forecasts. During early and late season advisories are often issued for multiple-day periods and danger ratings are not assigned. This follows guidelines set by the National Avalanche Center, as it is difficult to assign a danger rating for a multiple-day period when conditions can change drastically from one day to the next.
Did you know that most avalanche accidents occur during times of Moderate and Considerable danger? Click HERE for more information on the Avalanche Danger Scale.
The Bottom Line and Mountain Graphic
The Bottom Line highlights the key points of the avalanche advisory and is accompanied by the mountain graphic which provides a general overview of the avalanche danger. While we recommend reviewing the rest of the advisory for more detail, The Bottom Line does provide the salient points clearly and succinctly.
A set of 4 factors (avalanche character, location, likelihood, destructive size) are combined and used to describe the avalanche hazard for the day.
• Avalanche Character or Type – One of 9 potential avalanche descriptions
• Location – Where the avalanche is most likely to exist in the terrain, shown with an Aspect/Elevation diagram
• Likelihood – The chance of triggering an avalanche
• Size – The destructive potential of the expected avalanche
There are many different kinds of avalanches and each has its own unique characteristics that influence terrain and travel choices. In North America we use 9 distinct characters to describe avalanche types. For more information on avalanche character, click HERE.
The Aspect-Elevation Rose
The aspect-elevation rose is a powerful way to understand the general avalanche pattern in a glance by aspect (the direction a slope faces) and elevation (Below, Near, and Above treeline).
This grey-and-white rose is what we call a "locator rose". This diagram is a representation of how the pattern of avalanche conditions may have developed. The grey shaded areas show you the general aspect and elevation where you will MOST LIKELY find the specific avalanche type, not all the places where it might exist within the terrain.
Likelihood describes the probability of a single person triggering that type of avalanche in the terrain specified. The likelihood of triggering an avalanche can change from day to day and sometimes from hour to hour. The Likelihood of triggering an avalanche is described using terms such as Unlikely, Likely, and Certain. The term applied reflects the highest likelihood expected on that day.
The destructive size of an avalanche is based off of the D scale and is described in our advisory using terms such as Small, Large, Very Large, and Historic. These terms reflect the destructive potential of avalanches triggered on that day. Small avalanches (D1) are not large enough to bury a person but are dangerous and potentially deadly if they are encountered in consequential terrain. Large avalanches (D2) are large enough to bury injur or kill a person. Very large avalanches (D3), can bury and destroy a car, damage a truck, destroy a small building or break a few trees. Historic avalanches (D4 and D5) can destroy a railway car, large truck, several buildings, a small village, or a forest area up to 40 hectares.
The Advisory Discussion is used by forecasters to describe in more detail, recent snowpack or weather trends, non-avalanche related hazards, and stability concerns more generally. This section is often used to expand upon specific avalanche problems, to offer detailed travel advice, or to describe forecaster confidence. While the most crucial information on stability is contained in the sections above, the advisory discussion often contains more nuanced descriptions and can provide valuable information to users as they plan their day.
This section contains links to and descriptions of recent snowpack and avalanche observations. These field observations are submitted by forecasters, professional mountain guides, ski patrollers, as well as members of the public and provide point specific information on recent weather or avalanche activity. Field observations are incredibly valuable to the forecaster staff as they create the advisory and are availible for the public to read on our website. Follow this link to submit your own observations, or to view a more extensive list of recent observations please visit the observation page by clicking HERE.
Current Weather and Snow Conditions
This section describes pertinent weather forecast information. A weather discussion describes current conditions and patterns that may be developing. This is followed by a Two-Day Mountain Weather Forecast, listed in a simple table format.
Finally, it is important to remember the avalanche advisory is designed to generally describe avalanche conditions over a large area where local variations always occur. Avalanche conditions can change drastically over a short period of time and from one location to another. The Advisory paints with a broad brush and applies only to backcountry areas located within our forecast area and outside of established ski area boundaries.