Figure 4-4 Colorado Avalanche Information Center Forecasts Zones

Source: Colorado Avalanche Information Center

There are few areas of the County where slopes are 30% higher. The majority of vulnerable area in the County lies west of the C-470 corridor, with isolated areas along North and South Table Mountains, the hogback formations and Green Mountain. Most of the areas east of the foothills have strict development restrictions, which minimizes the exposure of the population. In the mountainous areas, the greatest areas of potential occurrence which may impact developments lie along Highway 6, Bear Creek Canyon, Coal Creek Canyon, Ralston Creek Road, and Clear Creek Canyon. Not unexpectedly, these areas are also the areas with greatest potential for rock falls, landslides, or unstable slope events. However, while these areas demonstrate a slope with a known vulnerability to avalanches, the occurrence and tracking records indicate that the areas lack some other element that contributes to avalanche events, such as consistent snowpack.

Based on this information, the geographic extent rating for avalanches in Jefferson County is negligible

or, at most, limited.

Previous Occurrences

The Colorado Avalanche Information Center (CAIC) database recorded 298 occurrences in the State of Colorado between late 1996 and January 2021. However, the database only captures accidents with unusual circumstances, fatalities, and injuries, and therefore represents only a fraction of occurrences.

According to the 2018 State Hazard Mitigation Plan, Jefferson County has had 4 avalanches that caused damage between 1960 and 2008, causing $8,333 in damage. The HMPC could not find any additional details on these, likely due to the small amount of damage. There have been many more occurrences in neighboring Clear Creek County, which have indirect impacts on Jefferson County. Clear Creek County falls almost entirely in the Front Range forecast zone, with the western-most area falling into the Vail- Summit forecast zone. These zones are explained in the Geographic Extent section below. Impacts from avalanches as far away as Summit County can also impact Jefferson County. Avalanches along the I-70 corridor and US Highway 6 threaten transportation routes into Jefferson County from the Western Slope, and may threaten water supplies for downstream residents by jamming creeks, damaging dams, or destroying infrastructure. Several previous occurrences which indirectly impacted the planning area are recounted below, but none of them were within Jefferson County. These occurrences help establish the threat of secondary impacts of avalanches on Front Range counties.

March 23, 2003. The CAIC database recounts a very large avalanche just west of Silver Plume. The avalanche extended all the way down the mountain into Clear Creek and across I-70, spilling into the eastern lanes of the highway and damming the creek, which in turn threatened down-stream water supplies. The event was considered unusual because of its long run out in an area that normally is not avalanche prone.

December 30, 2007. The Channel 7 website reported that avalanche dangers and high winds closed all six lanes of I-70 stranding almost 2,000 travelers along the highway from Floyd Hill to Vail. Interviews with stranded travelers indicate a range of destinations, including the Denver International Airport, sporting events, and New Year’s Eve celebration destinations, which underscores the economic impact of the danger on the entire state.

January 7, 2008. The Channel 7 website records avalanche mitigation efforts along I-70 halfway between the Eisenhower Tunnel and Silverthorne covered all six lanes of the highway and ranged from 6 to 10 feet deep. Other efforts closed down I-70 over Vail Pass and various other Colorado and U.S. highways across the western slope, heightening the dangers that avalanche conditions pose to travelers.

March 3 and 7, 2019. Media broadcasts reported avalanches on March 3rd and March 7th which swept across Interstate-70 in the Ten Mile Canyon between Frisco and Copper Mountain and trapped vehicles in several inches of avalanche debris. Fortunately, no injuries or property damages were reported but a large stretch of I-70 required closing down for several hours due to avalanche remediation work.

Probability of Future Occurrences

Jefferson County has only experienced four recorded avalanches in the past 60 years. This corresponds to a probability of future occurrences rating of unlikely.

Magnitude and Severity

According to the CAIC, there have been no reported deaths in Jefferson County due to avalanches between 1950 and 2014. Indirect impacts of avalanches on the planning area, such as economic losses due to road closures, are a matter of speculation rather than quantifiable data. With no reported damage amounts and no impact to the operation and delivery of critical services and functions it is difficult to consider the hazard very severe.

Information from the event of record is used to calculate a magnitude and severity rating for comparison with other hazards, and to assist in assessing the overall impact of the hazard on the planning area. In some cases, the event of record represents an anticipated worst-case scenario, and in others, it is a reflection of common occurrence. There is no record of damages for Jefferson County; therefore the magnitude and severity ratings for avalanches must remain negligible until additional information becomes available.

Climate Change Considerations

Climate change is likely to continue to alter the frequency and severity of avalanches in the future. In the last decade many experts in western states have pointed out increased avalanche risk associated with a changing snow, precipitation, accumulation, and overall warmer winter patterns. Snow may fall early in the winter and is then followed by a long period without snow. This creates a thin snowpack that becomes structurally weaker as winter goes on. New layers of snow may not bond well to the weak base layer, creating prime conditions for avalanches. Periods of sporadic snowfall in early and mid-spring in Colorado also contribute to this process of creating structurally weaker snowpack, which can lead to avalanche activity as snow accumulation has already begun to thaw with the warmer season. As Colorado experiences winters with higher average temperatures and lower average precipitation, these conditions that increase avalanche risk become more common. More intense and continuous storms over multiple days can also increase the potential for major avalanche cycles, as was experienced in March 2019.

Vulnerability Assessment

Due to limited available data, few recorded impacts, and the low significance rating, a detailed vulnerability assessment was not conducted for avalanche.

Overall Hazard Significance

Avalanches in Jefferson County do not have a significant impact on the planning area. In general, the impacts of avalanches for Jefferson County will be secondary. Avalanches in counties with a higher risk or vulnerability, such as Clear Creek County, may close roads and access points into Jefferson County or those counties may request mutual aid assistance to deal with the event occurrence. The geographic extent of the hazard is considered negligible. The probability of future occurrences is considered unlikely and the magnitude/severity for the event of record is negligible. In addition, the HMPC considers the hazard to have a low impact on the County. This equates to an overall impact rating of low.