The North American Public Avalanche Danger Scale: Its Evolution and Use on Mt. Washington


Since the 1950’s the Snow Rangers on Mount Washington have used a number of forecasting and rating systems to predict the likelihood of avalanches. Regardless of the particular system used, hazard ratings have done two things–helped Snow Rangers explain the expected dangers for the day and given visitors information to make calculated and informed decisions. Looking back over the past 20 years we have gone through four such systems on Mount Washington. In essence they have all been similar but each new version addressed certain new nuances to make improvements on those of the past.

Systems, scales and signal words

20 years ago we worked with a 4-scale system utilizing the familiar ratings of Low, Moderate, High, and Extreme. Its weakness came when moving between Moderate and High. The professional avalanche community felt that too large of a gap existed between both the words as commonly used in the American lexicon and their associated definitions in the danger rating system. As a result the avalanche community in the United States moved to a 5-scale system slipping the term “Mod/High” in the middle during the mid 90’s. The definitions that were crafted to accompany each of the ratings for the 5-scale system were used through the end of the 2009/2010 season but the same cannot be said for the five signal words that were the meat of the system. “Mod/High” was in fact a bridge between its two neighbors on either side but its use was short-lived. After much discussion it was replaced a few years after its birth with “Considerable,” the term that anchors the middle of our current system.

No one word has created as much angst and deliberation within the avalanche community as the 5-scale rating signal word “Considerable.” After a dozen years of discussion, argument, tears, and bloodletting, no other word in the English language has been determined a better fit. Even with the opportunity to change it during the development of the new North American 5-scale system it was determined that we don’t love it, but it’s part of our family now. We’ve utilized it for almost 15 years and avalanche terrain users have assimilated it into their vernacular and dictionary of knowledge. In the end the signal words are certainly important, but it is clear that they are only a link to the (1) travel advice,(2) likelihood of avalanche activity, and (3) size and distribution of avalanches. These three areas are where the subtle yet important changes have occurred with our new system. The five key signal words that we’ve grown to know and love (well sort of!) are still with us will be for the foreseeable future.

Mount Washington does not stand alone

Even if you’ve never been to the jagged peaks of the Sierra or the glacier-laden Canadian Rockies you know that avalanches occur in other mountains. In order to offer consistency and standards that do not change as users move across North America it is imperative that all locales use one unified system. Therein lies the challenge. Can one system be perfect across all regions with different snowpacks, climates, scales of forecast area, sizes of avalanche paths, etc? The simple answer is no but then again avalanche prediction is far from simple. On the other hand, the basic mechanics, the effects on people and infrastructure, and the techniques that we employ to stay safe are applicable to avalanches around the globe. There are enough similarities within the avalanche phenomenon that a standardized forecasting system can be used over vast distances as long as it allows for regional problems to be addressed as necessary.

The forecast area served by the Mount Washington Avalanche Center (MWAC) has its own unique characteristics and the Snow Rangers were highly involved in the process to redesign the system. A rating system crafted for our forecast area alone would undoubtedly look different than the new North American Danger Scale. During the process we realized that to better serve the entire community we would need to be flexible as would the sixteen other U.S. Forest Service avalanche centers, the Colorado Avalanche Information Center, and all the forecasting operations administered by the Canadian Avalanche Center. By utilizing the same language, we feel that we’re helping you as you read the avalanche forecasts in advance of your escape to the Jackson Hole backcountry or the steep chutes of the Cascades.

Why change the old system?

One of the drivers that instigated a new look at the old system has its genesis in the regions that develop persistent weak layers, predominately buried surface hoar. Surface hoar has a proven ability to withstand initial storm loads and retain its original structure even when it’s buried under a meter of snow. This weak layer can lurk below the surface for extended periods waiting for the right load before stress outweighs strength and failure occurs. Buried surface hoar crystals can be a problem for weeks and weeks and in some cases, an intact layer can provide the main concern for the majority of the season. The challenge that forecasters had with the old system was to pick an appropriate rating that didn’t overstate the problem (risking a loss of credibility with users) or understate the problem (sending people into the backcountry in droves). In general, the main issue was that the probability of avalanche activity was on the lower end of the spectrum but if it did occur, the size of the slides would be big and the impacts catastrophic. Nowhere in the standard system was there any reference to these types of avalanches even though they had the potential to catch people off guard with devastating consequences.

It was generally agreed that when the snowpack was either stable or very unstable the system handled these scenarios pretty well, namely under the Low, High, or Extreme ratings. The challenge was the language under the Moderate and Considerable ratings. Under a Moderate rating in our previous system natural avalanches were unlikely and human triggered avalanches were possible. The definition for the Considerable rating stated that natural avalanches were possible while human triggers were probable. Which one should a forecaster choose when the buried layer of surface hoar had been in place for six weeks without failure? If they chose Moderate then the official travel advice associated with the rating was to “use caution in steeper terrain on certain aspects.” If they opted for Considerable the associated travel advice was to “be increasingly cautious in steeper terrain.” Many avalanche professionals felt the travel advice was far too nebulous to be truly helpful for people. This was compounded by the acknowledgment that conditions under the Moderate and Considerable ratings were some of the most challenging for making travel decisions and spatial variability was often elevated.

Avalanche probability and distribution were important aspects of the old system to retain but a better system needed to incorporate size or consequence in some fashion. It would also need to provide travel advice that would actually be helpful to skiers, riders, climbers and mountaineers. There needed to be a way to capture the fact that in some instances there were gigantic ticking time bombs that would be triggered when a mere inch of snow fell or the tenth person down the slope skied or rode over the sensitive spot.

So what exactly changed?

At first glance the new 5-scale system looks nearly identical to the old system. In reality, the two are very similar but the devil is in the details. The four major changes to the system are as follows. The new system provides more detailed and, in our opinion, more valuable travel advice. This is especially true in the Moderate and Considerable levels where the new language emphasizes identifying areas of concern and using route-finding skills to avoid these areas.

The new system now includes avalanche size and distribution as a component of each rating. We may discuss size in our advisories but due to the smaller and more focused size of our forecast areas, you will rarely see us utilize the distribution terms associated with the current ratings. It is hard to justify using the phrase “small avalanches in many areas” when we forecast Considerable avalanche danger for the Little Headwall! More likely you will see us emphasizing the Low definition’s “small avalanches in isolated areas” when pockets of instability exist but do not dominate a forecast area. A classic example of this is a wind-loaded pillow of unstable snow that covers most of the start zone in Left Gully. 90% of the gully may have the stability to justify a Low rating but an avalanche in that one isolated area could still have significant consequences!

There are also a few changes in the words that are used to describe the likelihood of avalanches under a certain rating. There is no longer a distinction between natural and human-triggered avalanches under the Low rating and both are now described as “unlikely.” The Considerable definition used to state that human triggered avalanches were “probable” but they are now described as “likely.” How much of a difference lies between these two words? It all depends who you ask. In the end it was felt that the two were basically equivalent but “probable” was used much less frequently in the modern lexicon and as a result should be retired. Lastly, the likelihood of avalanches under the High rating was slightly tweaked. Instead of both natural and human-triggered avalanches being described as “likely” as was the case in the old system, the two types of triggers were split so that natural avalanches remained “likely” while

human-triggered avalanches are now described as being “very likely.” Some may feel like this change is splitting hairs but several avalanche professionals felt like there needed to be more emphasis on the fact that humans are a very effective trigger.

The last change that was incorporated into the new 5-scale system is the use of icons to quickly convey a particular danger rating. The standard color scheme (green for Low, yellow for Moderate, etc.) continues to be utilized but some forecasting operations needed a tool to convey the danger to a wide audience from backcountry users to suburban residents living in avalanche country. Other areas serve as international destinations and language barriers were hampering avalanche awareness. The icons that were developed may be used on the front page of a newspaper next to the partially sunny icon or they may be utilized on a road sign as you approach an avalanche-prone mountain pass. We do not currently have plans to utilize the icons on a regular basis on Mount Washington but instead will stick with the tried-and-true slatboard system that visitors have grown to understand and expect.

The Snow Rangers at the Mount Washington Avalanche Center believe that the new system is a benefit to the entire avalanche community. That includes you and all the others who come to visit the steep flanks of the Northeast’s biggest mountain. We encourage you to familiarize yourself with the definitions that accompany each signal word as well as the travel advice that goes along with them. It is important to remember that though there are five discrete ratings they are part of a continuum where avalanche hazard changes from one rating to the next, sometimes over the course of an hour or less. We will use the new system to continue relaying the hazards of the mountain but you control your own level of risk by using the information to make your own well-informed decisions.