Avalanche Forecast for Tuesday, November 27, 2018

This forecast was published 11/27/2018 at 6:50 AM.
A new forecast will be issued tomorrow.

NOT THE CURRENT FORECAST

This is an archived avalanche forecast and expired on 11/27/2018 at midnight.


The Bottom Line

New snow and wind will continue today, making natural avalanches possible. Dangerous avalanche conditions exist on and below steep slopes.  Avalanche danger due to these wind slabs will remain elevated today and tomorrow as snow continues to fall and is redistributed by the wind. The new snow fell on a slick bed surface of refrozen snow created by recent warm temperatures and rain. This surface is likely to make wind and storm slabs even more sensitive to human triggering. All forecast areas have CONSIDERABLE avalanche danger.  Today seems like a good day to visit your local ski area, glade or ski trail and give the storm a chance to clear out and the new snow to settle and bond.

Forecast Area

Mountain Weather

The Presidential Range is receiving yet another early winter storm. Overnight temperatures in the teens plus orographic effect at higher elevations has made more lower density snow on the summits than in the adjacent valleys. As of midnight, the summit of Mount Washington has recorded .65″  of water for somewhere around 8″ of snow. Hermit Lake reported 31cm of 10% snow at 6:30am. 8-14″ more snow is in the forecast for today on the summit. Lower elevation locations also received a sizable amount of snow. In addition to significant amounts of new snow, wind in the 50-60 mph range blew from the east and east-southeast through the night but will diminish slowly through the day. Lower elevations may see some rain on the new snow which could make wind and storm slabs weaker and more prone to failure though areas above 1500′ should see all snow.

Primary Avalanche Problem – Wind Slab

Wind Slab

Aspect/Elevation

Likelihood

Size

Large wind slabs developed on west aspects and behind terrain features as strong easterly winds blew last night. Be on the lookout for cross-loaded slopes as well as softer and more reactive wind slabs as winds diminish today.

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.

Secondary Avalanche Problem – Storm Slab

Storm Slab

Aspect/Elevation

Chance

Size

Given the strong winds recorded last night, it seems unlikely that you’ll find much snow that wasn’t affected by wind. If you do, remember the refrozen crust that you may find below will make for an effective sliding surface for an avalanche. Even a short ride could carry you into rocks or trees or into a terrain trap.

What is a Storm Slab Avalanche?

  Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

Snowpack and Avalanche Discussion

Low visibility and continued snowfall will hamper safe observations today but it is highly likely that west facing avalanche paths grew in size and length overnight. While we still have an early season snowpack, there has certainly been enough snow and avalanche activity to create bed surfaces that are almost full length. That said, trees and bushes remaining in paths will make any ride more dangerous. High east facing terrain likely received a good hammering by the wind and may even have been scoured down to old surface in a few places. Look for smaller wind slabs scattered around the usual slopes and gullies rather than the thick wind slabs that are normally built by our more typical west wind. Continued snowfall and wind will combine with an unusual wind pattern will create a tricky avalanche problem today. Keeping it low angle until visibility improves and wind slabs heal is probably a good idea.

Snow Plot Information

DateHN24HN24W
(SWE)
Density (%)HSTTotalAir TT MaxT MinSkyPrecipComments
02/18/19
05:22
Trace Trace NC201 CM-14.5 C-9.0 C-16.5 COvercastSnow
02/17/19
05:24
Trace Trace Trace204 CM-16.5 C-9.5 C-16.5 CClearNo precipitation
02/16/19
05:22
13 CM 10.7 MM 12%13CM205 CM-10.0 C-2.0 C-10.0 COvercastNo precipitation
02/15/19
05:22
Trace 0.1 MM30CM195 CM-7.0 C-5.0 C-15.0 COvercastNo precipitation
02/14/19
05:24
11 CM 6.5 MM 10%NC196 CM-14.0 C-6.0 C-15.0 COvercastSnow

Avalanche Log and Summit Weather

Daily Observations


Thank you Mount Washington Observatory for providing daily weather data from the summit of Mount Washington.

DateMax TempMin TempTotal (SWE)24H Snow & IceWind AvgWind Fastest MileFastest Mile DirAvalanche Activity
02/17/1914 F-3 F 0.00 in 0 in37 MPH74 MPH

310 (NW)

View
02/16/198 F-5 F .05 in .8 in52.4 MPH84 MPH

310 (NW)

02/15/1925 F7 F .37 in 2.7 in45.3 MPH84 MPH

240 (WSW)

02/14/1919 F-4 F .13 in 1.1 in56.2 MPH90 MPH

280 (W)

02/13/1916 F0 F .87 in 5.1 in46.3 MPH92 MPH

280 (W)

View
02/12/1912 F2 F .54 in 3.3 in37.5 MPH76 MPH

130 (SE)

02/11/196 F-9 F 0 in 0 in59.0 MPH86 MPH

330 (NNW)

02/10/19-4 F-13 F 0 in 0 in65.3 MPH101 MPH

290 (WNW)

02/09/19-7 F-14 F .01 in .2 in87.5 MPH148 MPH

280 (W)

02/08/1937 F-8 F .20 in .2 in71 MPH124 MPH

230 (SW)

02/07/1934 F22 F .04 in .2 in32.2 MPH56 MPH

250 (WSW)

02/06/1932 F17 F .23 in .4 in30.9 MPH68 MPH

330 (NNW)

Please Remember:

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 11/27/2018 at 6:50 AM.

Frank Carus, Lead Snow Ranger
USDA Forest Service
White Mountain National Forest