Avalanche Forecast for Thursday, December 20, 2018

This forecast was published 12/20/2018 at 7:12 AM.
A new forecast will be issued tomorrow.

NOT THE CURRENT FORECAST

This is an archived avalanche forecast and expired on 12/20/2018 at midnight.


The Bottom Line

Wind slabs are widely distributed throughout the terrain with the largest slabs on eastern aspects, particularly in Tuckerman Ravine and the Gulf of Slides. Due to shifting and the long duration of the wind, you’ll find wind deposited snow on other aspects, as well as at lower elevations. Give these slabs some space or travel one at a time when crossing them. Larger areas of these slabs in steep terrain should be respected due to the possibility of a person finding a thin spot and triggering a slab. Remember that the ice crust and nearby weak layer of sugary facets are still in play. For these reasons, plus the potential for some warming and weakening of these slabs today, wind loaded areas earn a MODERATE rating. Evaluate snow and terrain carefully if you are out today. More heavily scoured areas, such as the higher elevation Huntington Ravine, are rated LOW due to the scattered distribution and smaller size of these wind slabs. Look for these wind slabs on isolated terrain features.

2018-12-20 Printable PDF

Forecast Area

Mountain Weather

Strong winds will finally relax today following two days of havoc wrought by northwesterlies. Warm air moving in will allow the mercury to climb near near 40F on the summit with apparent temperature in sunny, calm lower elevations even warmer. Mid-elevations (4,000’) outside of drainages are already 40F at 6am. Cloud cover will increase later today as tropical air flips the mercury and precipitation in the wrong direction later tonight and tomorrow. The NWS has issued a Flood Watch for Friday and Friday night with 2-3” rain expected after a brief period of freezing rain. Snow returns for the weekend.

Primary Avalanche Problem – Wind Slab

Wind Slab

Aspect/Elevation

Likelihood

Size

Widely distributed wind slabs sitting on softer snow or an ice crust with weak faceted snow beneath should remain on your radar today. These wind slabs will be hard to trigger but should be viewed suspiciously due to their recent formation and variable and not particularly strong snowpack structure beneath.

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.

Snowpack and Avalanche Discussion

Northwesterly wind transported a lot of snow around the mountains in the past two days with one small natural avalanche observed from low in the Center Bowl area of Tucks that ran on the Dec. 3 rain crust. Plumes of snow continued to swirl around ridges and pour down steep slopes even yesterday afternoon, scouring deep sluff channels in the process. Deep drifts collected in a variety of locations with medium to firm smooth wind slabs (mostly 1F to P hardness) throughout Tuckerman Ravine. One report of a collapse, assumed to be in the new snow beneath the wind slab, was reported at the mouth of Lobster Claw. D1 avalanche debris was also reported in Shoestring Gully at 2,200’ on a WSW aspect on Webster with unreactive 1F slab over a thick layer of faceted snow.

Additional Concerns

The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.

Snow Plot Information

DateHN24HN24W
(SWE)
Density (%)HSTTotalAir TT MaxT MinSkyPrecipComments
01/21/19
05:15
19 CM 26.6 MM 16% 32 CM  175 CM-20.5 C-7.0 C-24.0 COvercastNo precipitationView
01/20/19
05:20
13 CM 12.5 MM 11% NC CM  158 CM-15.5 C-12.0 C-17.5 COvercastSnowView
01/19/19
05:18
2 CM 0.7 MM 8% 2 CM  145 CM-16.0 C-10.0 C-16.0 CClearNo precipitation
01/18/19
05:18
0 CM 0 MM 0 CM  145 CM-14.5 C-13.0 C-20.0 CBrokenSnow
01/17/19
05:18
1 CM 1.4 MM 1 CM  147 CM-20.0 C-9.0 C-22.0 CClearNo precipitationView

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 12/20/2018 at 7:12 AM.

Frank Carus
USDA Forest Service
White Mountain National Forest