Friday-Sunday Forecast

Issued: Fri, Mar 02, 2018 at 8AM

Expires: Sun, Mar 04, 2018

Caution advised in glacier areas: this year’s alpine snowpack is thinner than normal and our glaciers are more likely to have thin snow bridges over crevasses.

Above 2,500ft Considerable

1,500 to 2,500ft Considerable

Below 1,500ft Moderate

Degrees of Avalanche Danger ?

1. Low
2. Moderate
3. Considerable
4. High
5. Extreme

Avalanche Danger Rose ?

Avalanche Problems ?

Problem Details

Bottom line: We’ve had a few days for recent snow to settle, but dangerous conditions will persist this weekend due to cold weather, recent wind loading, and persistent weak layers. Wind loaded slopes steeper than 30 degrees that have not avalanched in the last week will be loaded guns, waiting for a trigger.

Strong north – northwest winds blasted our zones Wednesday-Friday, whipping around the fresh snow that fell in the last week (about 20″ in the Lutak zone, ranging to about 6″ in the Pass). The new snow sits over a bed of weak facets about 30-60cm deep. Beneath that, we have a few different mid-pack ice crusts, with faceted layers in between, and re-frozen depth hoar at the ground in thin areas. See snow profile below.

Problem #1: Wind Slab

Location: W-S-E aspects and cross-loaded gullies, at all elevations. With the strong northerly outflow winds over the last few days, expect to find thick recent deposits on lee aspects, and expect cross-loaded gullies to be harboring dangerous wind slabs. These slabs will likely be hard and hollow, with weak storm snow and buried surface facets underneath. Due to the nature of the weak layers and the continued cold weather, bonding will be poor. These slabs could be anywhere from 15-60cm thick, and will be sensitive to human triggering on slopes steeper than 30 degrees. 

Resist the temptation to try out big terrain this weekend. We have not had a stabilizing event to reduce these surface instabilities, and we have not had enough time or warm weather to bond the weak layers.

Problem #2: Persistent Slab

Trend: steady. Location: all aspects above 1500ft. Likelihood of triggering: possible. Confidence: moderate. Avalanche Size: moderate-large. Below the upper rain crust sits a thick layer of weak facets, another crust, then more weak facets. We continue to get reports of whumping within the facet layer below the upper ice crust (about 60cm deep now), which shows that this ice crust is decomposing and overloaded. With the added stress from new snow on top, we now have a “loaded gun” type of scenario.

Remote triggering and wide propagation will be possible, leading to large avalanches. Surface avalanches may step down to these facet layers 45-60+cm deep.

Recent Avalanche Activity

In the last week, lots of whumphing has been reported (failing on facets beneath the upper ice crust), as well as shooting cracks within the recent storm snow, which is sliding on a buried facet layer above the upper ice crust.

There has been sporadic natural wind slab activity to size 3 in the Lutak and Transitional zones, mostly on wind loaded south aspects over the last few days (see photo 1). In a few heavily-cross loaded areas around 4000ft, natural slides broke below the ice crust on persistent facets (photo 2).

Recent Weather

The last few days’ strong north winds will be easing up today, and becoming calm for Saturday and Sunday. Alpine temperatures will be in the low teens, and skies mostly clear.

 Snow Depth [in] Last 24-hr Snow/SWE [in] Last 3-days Snow/SWE [in]  Today’s Freezing Level [ft]  Today’s Winds Next 24-hr Snow/SWE
Mount Ripinsky @ treeline
50″ 0″ / 0.00 8″ / 0.70 0  mod, N 0″/ 0.00 *
Flower Mountain @ treeline
 38″ 0″ / 0.00 4″ / 0.30 0 mod, NW 0″/ 0.00 *
Chilkat Pass @ 3,500ft
 18″* 0″ / 0.00 * 2″ / 0.15 * 0 light, NW 0″/ 0.00 *

( *star means meteorological estimate )

Additional Info & Media

If you get out on the snow, send in your observations!

Posted in Transitional Zone Forecasts.
Erik Stevens

Forecaster: Erik Stevens