Avalanche Forecast
Issued: Feb 19th, 2021 1:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet, Persistent Slabs, Wind Slabs and Storm Slabs.
Bill Phipps,
VIAC
Numerous small wind slab avalanches were triggered with ski cuts during avalanche control throughout the day Thursday at Mt Washington up to size 1 on the lee side (NW) of terrain features along the treeline ridges.
Summary
Past Weather
Light to moderate amounts of new snow has fallen since Tuesday night (approx 5-10 cm). Strong SE winds resulted in significant snow transport to NW aspects during the day Thursday Feb 18th. Temps climbed to near 0 Wednesday afternoon at 1500 m but returned to cold Thursday (-2 to -6).
Weather Forecast
Wind wind wind a significant warm up Sunday, all with moderate to heavy amounts of new snow. Winter is not done with us yet!Note the forecast for new snow over the next three days shows significantly high numbers for the west coast and north island vs the east side so factor this in to your decision making process. Friday - 2-5 cm of new snow for the east side, 5-10 cm for the north, 10- 20 cm for the west. Winds moderate to strong SW. Temps for 1500 m -4 to -6. Freezing levels 400 to 800 m. Saturday - 1-3 cm for the east, 5-10 for the north and west. Winds Strong to moderate SW. Temps for 1500 m -4 to -8. Freezing levels 300 to 800 m. Sunday - 10-15 cm for the east side, 30-55 cm for the north and west side of the island. Winds Strong to Extreme SW. Temps WARM -1 to +1. Freezing levels 1000 to 1800 m.
Terrain Advice
Avoid wind loaded zones on NE to NW asp near/below ridgelines Keep track of rising temps Sunday as this will certainly make the snowpack more sensitive to triggering. Avoid ALL avalanche terrain when the danger rating is HIGH, stick to low angled slopes and or well forested terrain.
Snowpack Summary
A slight warm up Wednesday resulted in moist snow surfaces in the PM on solar aspects at lower elevations. This moist snow then froze overnight creating a thin crust. Note non solar aspects escaped the warming. Strong SE winds Thursday resulted in significant snow being transported to lees stripping the 5-10 cm of new snow as well as the old storm snow from last weekend (15-20 cm of low density snow). The persistent weak layers that still linger in the mid snowpack now sits approx 70-110 cm down and have been unreactive to snow travelers.
Snowpack Details
- Surface: Heavily wind affected with widespread wind slabs on lees N-W, some windward zones stripped to a old crust.
- Upper: 5-10 cm of new snow (over a thin crust on solar aspects) over the 15-20 cm of light snow from the weekend
- Mid: A old crust sits on well settled snow above the persistent weak layers (facets and at lower elevations some hoar)
- Lower: Well settled
Confidence
High - Good field data collected, weather models in agreement. Few reports from the public over the past two days.
Problems
Loose Wet
The warm up Sunday may result in some loose wet avalanche activity at low to very low elevations. Be extra cautious when these powerful small devils are combined with terrain traps like cliffs, creeks, holes and bands of trees.
Location: below treeline on all aspects
Likelihood of triggering: likely to human traffic and possible via natural triggers.
Size: 1 to 2.
Likelihood
Expected Size
Persistent Slabs
Our good old persistent weak layers are still in our snowpack but now sit down deeper and have less chance of being triggered. None the less they are still there and with the forecast new snow adding a significant amount of weight and stress to them, we can't write them off yet.
Location: all aspects and elevations
Likelihood of triggering: Human triggering possible, natural occurring slides unlikely. May result as a step down event when an other avalanche over runs lower terrain where these layers sit (ie the first avalanche significantly adds weight to them)
Size: 2-3. If they go they will go big.
Likelihood
Expected Size
Wind Slabs
Significant wind slab development Thursday will only be added to over the next three days. Winds ranging for moderate to strong and even extreme SW are forecast for the weekend. These winds will certainly create touchy wind slabs in specific terrain.
Location: All elevations and found on NW to NE aspects especially at or below ridgetops.
Likelihood of triggering: These slabs will very likely trigger with human activities and we can expect a high likelihood of naturally occurring avalanches as well.
Size: 1 and even up to size 3 (in certain isolated aggressive large bits of terrain)
Likelihood
Expected Size
Storm Slabs
Significant amounts of new snow will fall over the next three days (esp large accumulations are forecast for Sunday). This new snow falls on colder old snow and will result in a upside down upper snowpack (firm snow over softer snow), with a warm up coming this sets our snowpack up to be prime for storm slab avalanches.
Location: All aspects and elevations
Likelihood of triggering: Likely to react to human triggers and possible to natural activity
Size: 1 to 3 ie certainly a big concern and not to be messed with.
Likelihood
Expected Size
Valid until: Feb 20th, 2021 1:00AM
