Hazard Assessment & Forecsting Heuristics

There are several applications of hazard assessment and forecasting heuristics; these can include contributory factors or threshold values and red or yellow flags.

Contributory factors and threshold values provide shortcuts for operations to apply weather and snowpack parameters in their operational decision making and risk management practice (heuristics). These values are often developed or adapted from decades of observation within an area or operation. Forecasters typically build logistics, operational needs and staffing needs around these factor values.

Cautionary Note: Heuristics can form operational biases. If habit overrides critical judgement, then a flawed forecast outcome is the result. The prudent forecaster will evaluate the threshold values presented here against local observations to determine relevance to their local snow climate..

Thresholds are values or ranges for various contributory factors that have been arrived at through observation and analysis for a given location. Such as avalanches never reach highway abc unless there is 254 cm or more on the HS stake at xyz weather plot, or the lowest amount of storm snow water content when avalanches have run to the road is 38 mm.

Red flags are warnings, the phrase is typically used when describing conditions that have met values or thresholds indicating increasing hazard.

Yellow flags are a specific type of decision aid where observed snow profile structural values are incorporated in a checklist sum. Critical values or ranges (indices) relative to increased hazard for particular snow climates are determined statistically. Observations are contrasted with the critical values, the resulting answer is converted to a discrete value which is then typically summed for the assessment and compared (above or below?) to specific score thresholds.

Contributory Factors for Avalanches in General

Old snow depth

Considerations: ground irregularities need to be covered and adequate size of area connected.

Threshold values: Need 30cm minimum over smooth ground. 60cm or more over average ground.

Old snow surface

Considerations: thickness, stiffness, widespread uniformity and connectedness. Hand hardness, foot penetration, grain type (faceted, crust)

Settlement of storm snow

Considerations: Settlement is measurable with penetration, snow cones, snowboard comparison. Compare settlement in HST and HS. Settlement in HS is considered "good" settlement, settlement in HST is considered slab formation.

Rule of thumb/threshold values: Slow rates of settlement, 15% or less in 24 hours, are associated with slow increase in strength and persistence of instabilities. Greater than 30% settlement for 24 hrs is considered a “very high” rate of settlement and associated with avalanching, particularly when forming over a weak layer. High settlement rate is usually associated with high snowfall intensity and warm temps and thus avalanching.

Settlement calculation rates using study plot HIN and HST Boards for a three-day event:

((Sum of new snow depths) - Storm snow depth)/((Sum of new snow depths) X 100%

Hazard Assessment & Forecasting Heuristics Organized by Avalanche Problem Type.

Contributory factors for Loose Dry

Considerations: Foot penetration provides an indication of the existence and thickness of an unconsolidated layer. Unrimed new snow is more likely to form loose dry problems than rimed snow.

Contributory factors for Storm Slab

Precipitation accumulation

Considerations: Type and intensity (rate +duration) are critical factors. Old axiom: a warming storm is more unstable than storm cooling. Fragmented rimed snow particles bond more quickly than well-formed dendrites and a slab forms over colder, less dense layers. The idea behind threshold values is to identify both patterns of total accumulation and the rate of accumulation.

Threshold Values:

New Snow: 12 inches or 30.5 cm plus some wind is the often quoted requirement for “widespread avalanching” which comes from decades of observations at Alta, Utah, (Source: the Avalanche Handbook). For contrast, the same study determined 4 in (10 cm) with wind was the requirement for the Loveland - Berthoud area in Colorado. This factor depends on the wind speed/direction, the temps and the underlying snowpack characteristics.
Snowfall Intensity: 2-3cm/hr for 10-12 hrs or more, or 1”/hr for 10-12 hrs or more above wind transport threshold (again lower values for Colorado).
Old axiom: 1"/hr for 12 hrs and you have avalanches in steep terrain where stable snow otherwise existed. Greater than 25mm/12 hrs is considered enough of a rapid load to create avalanches on stable snow.
Snowpack loading: adding 20-30% of the total SWE (water equivalent of the total snowpack) This is from Gerald Seligman’s Snow Structures and Ski Fields from the 1930’s, and it still has some practical use. Note the March cycle in CO in 2019 saw these values play out. Experience suggests this has particular value in the continental snow climate areas. Less so in coastal snowpacks in spring.
Precipitation Intensity: this is the amount of SWE and its rate of accumulation. The maximum rate of 0.33 to 0.5 in per hour (8 to 13 mm per hour represents a higher probability of increasing hazard.

Contributory factors for Wind Slab

Snow loading by wind

Considerations: Considered the "most active contributing factor to avalanching after new snow" (Schweizer). Primary factors are snow density, wind speed, temperature. This has been extensively studied. Numerical models have been built around values (Gauer, Doorshot et al, Lehning et al and at least 5 others). Loading is highly variable, often there are observations of localized loading. Variation in wind speed and deposition rates create a layered snowpack. Fragmented grains bond quickly, especially at warm temps.
Threshold values: Wind transport typically can result in deposits of 3-5x (up to 10x) snow accumulation rates in the study plot (from Jamieson, Schweizer and others). Example: 3cm/hr for 3 hours may be considered a rapid load but if there wasn't a buried weak layer may just create good skiing. Add 30kph winds at ridgetop and there could be up to a 45cm slab formed below ridgetop.

Foot or Ski Penetration

Considerations: Foot/Ski pen measures settlement. Old axiom: if your skis or feet penetrate to ‘well below’ the weak layer the snow is too unconsolidated for slab formation. If your foot stops above the weak layer you may have a slab.
If walking or riding around in the fetch, penetration can also indicate snow available for transport. Dry cold snow favors drifting. High humidity and warm temps, while slowing drifting, favors slab formation.

Contributory factors for Loose Wet, Wet Slab, Cornice

Radiation

Considerations: Radiation is more significant than heat in the way it affects the snowpack. Near-surface cooling forms facets and surface hoar. Near- surface warming forms MFcr. Radiation warms black objects (rocks, vegetation, exposed ground) and intense radiation can trigger loose wet, wet slabs, glide slabs, cornices etc. Radiation is hard to measure! There are examples where highways forecasters compare solar panel input readouts that charge the telemetry station batteries (most show the solar panel inputs) and when compared to radiation-initiated avalanches the highways crews have developed parameters they use for forecasting. In another example, two radiation sensors are compared, one mounted in the standard, flat manner, a second tilted to 38 degrees facing south. Factors include snow surface density, surface particulates, slope angle etc but it’s a guideline. Sunburn, warming black gloves...are more observable than measurable.

Air Temperature

Considerations: Warming promotes rapid creep of less dense surface layers. In the short term, warming can destabilize; slower warming over time can stabilize. Cooling usually slows down metamorphism (and thus creep) and thus reduces the strain on the weak layers. Prolonged cooling can facet and weaken (cornice anchors near ridgetop, crevasse bridges, wind slabs etc).
Threshold Values: Temperatures near zero C promote settlement. Old axiom: beware of short term changes (<24 hrs).

Contributory factors for Persistent Slab

Structural Indices for skier triggered slab avalanches

Considerations: Soft slabs (1F or less) are the majority of skier triggered avalanches, especially with thin stiff layers right above the persistent weak layer. (from Jamieson Skier Triggering of Slab Avalanches, Feb. 2000; Characteristics of human triggered avalanches; Schweizer 2001),
Threshold values: Slab is usually less than 1m deep. and overlays a soft coarse weak layer with finer grained stiffer layers. Note the stiffer layers can be above or below. 45cm is the critical depth for ski triggering, 2mm weak layer grain size (equal to or greater.
Weak layer and interface characteristics follow the Snow Profile Checklist indicators 75% of the time in an intermountain snow climate. 5 or 6 flags at the layer/layer interface indicates likely weak layer (from Jamieson, https://vimeo.com/150455199) . Weak layers illustrate propensity for propagation as identified by ECT, RB or PST. May be associated with sudden fractures in compression tests.

Slope Use

Considerations: Compaction: Heliski and snowmobile operations report disruption (i.e. skiing it out early before getting buried) as an effective tool to manage surface hoar problems. Essentially the tracks have to overlap and penetrate through the weak layer to be effective. Many ski areas use bootpacking early season to manage basal facets and depth hoar problems. The compaction has to be done with people side by side and compacting to the ground.

Photo: Margaret Wheeler

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