Avalanche Forecast for Wednesday, January 23, 2019
This information was published 01/23/2019 at 7:10 AM.
The Bottom Line
Hard, wind slabs built during the extreme wind that followed the MLK weekend winter storm remain a concern today. Settlement and bonding that usually occurs after a snowfall were put on hold as temperatures hovered well below zero. Use normal precautions if you go looking for smooth snow for skiing today. Crampons and an ice axe may be useful on the more wind exposed steep areas near ridgetops. The combination of hard wind slabs, some smaller but softer and possibly more reactive wind slabs, and warming temperatures keep our range-wide rating elevated at MODERATE today. It remains possible for a person to trigger an avalanche on specific terrain features today. Evaluate snow and terrain carefully. Rain on snow conditions tomorrow will likely keep avalanche danger elevated with challenging travel conditions.
A frigid air mass moved in after the recent winter storm deposited 13” of increasingly dense snow on the range. That cold air was ushered in on extreme NW wind that raged over 100 mph for close to 12 hours in the early hours of Tuesday morning. After reaching a low of -31F just prior to the wind event, the mercury is continuing its upward journey from Arctic to temperate through the day and into tomorrow. This warm up will bring sleet and freezing rain late this afternoon, with rain later tonight. Some snow showers will fall today but shouldn’t amount to much more than an inch. Temperatures on the summit will climb from the current 10F to 29F by dark to close to 40F tomorrow. Rain tomorrow seems likely to fall all the way to the summit.
Primary Avalanche Problem – Wind Slab
Winds over 100 mph pounded snow into our primarily east facing terrain. Extreme winds often scour ridgetops while depositing snow and building stubborn slabs lower down the slope, especially beneath steep features and along gully walls. Softer slabs may have formed as wind was slowing so look for more reactive slabs further around the terrain rose. Lower elevation areas weren’t as wind affected but most of the snow was capped by a temperature crust which is likely unreactive.
What is a Windslab Avalanche?
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Numerous crown lines were observed during the brief period of clear conditions yesterday afternoon with most avalanche activity likely having occurred Sunday night. Refilled and eroded crown lines indicate that easterly aspects avalanched more than once. Significant scouring occurred in the higher elevation, but small fetch, Huntington Ravine with similar effects likely in other high elevation areas. The upside down nature of the storm made for touchy conditions on Sunday but this wetter layer has refrozen since then and is likely lacking the energy to propagate a crack anywhere. As stated earlier, the biggest concern is the low probability, high consequence hazard of finding and triggering a thin, weak spot in a large wind slab. The human-triggered avalanche in the Lower Snowfields avalanche in 2013 followed similar weather and wind conditions. The MLK weekend storm and more specifically the wind that followed, filled in and developed many of the more ephemeral east and south east facing avalanche paths like the ‘69 slide path on the Summer Lion Head trail.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch. Several large trees were down yesterday on the Tucks and Sherburne ski trails with wind damage likely elsewhere around the range.
Safe travel in avalanche terrain requires training and experience. This forecast is just one tool to help you make your own decisions in avalanche terrain. You control your own risk by choosing where, when, and how you travel.
Avalanche danger may change when actual weather differs from the higher summits forecast.
For more information contact the US Forest Service Snow Rangers, AMC visitor services staff at the Pinkham Notch Visitor Center, or the caretakers at Hermit Lake Shelters or seasonally at the Harvard Cabin (generally December 1 through March 31). The Mount Washington Ski Patrol is also available on spring weekends.
Posted 01/23/2019 at 7:10 AM.
USDA Forest Service
White Mountain National Forest