Avalanche Forecast for Friday, December 28, 2018

This forecast was published 12/28/2018 at 7:18 AM.
A new forecast will be issued tomorrow.

NOT THE CURRENT FORECAST

This is an archived avalanche forecast and expired on 12/28/2018 at midnight.


The Bottom Line

Expect freshly forming wind slabs to be touchy to human triggers today. As new snow and wind drifts accumulate through the morning, natural avalanches will become likely, and may become more likely as snow turns to sleet and rain this afternoon. Avalanche danger may rise to CONSIDERABLE today in all mid and upper elevation forecast areas not facing directly southwest.

You can avoid these avalanches by staying on low angle or lower elevation terrain that does not have deep new snow or wind drifted snow. Travelling in longer avalanche paths will expose you to the risk of natural avalanches from above. Gusty and erratic wind that loads slopes with more snow could trigger these avalanches unexpectedly.

 

Forecast Area

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Mountain Weather

Light snow began falling around 2am this morning and has picked up in intensity with sleet mixing in already, despite the ground level air temperature of 7F. Snow will continue through the morning on westerly then southwesterly wind around 60 mph. So far this morning, wind has been gusty and a bit erratic. Sleet will likely continue to mix in this morning, adding weight to the forming wind slabs, before a change over to freezing rain and rain later in the day. Precipitation will switch back to snow tonight with upslope snow showers delivering another couple of inches tomorrow.

Primary Avalanche Problem – Wind Slab

Wind Slab

Aspect/Elevation

Likelihood

Size

Small to medium sized wind slab avalanches will be able gather enough snow to be dangerous, particularly considering the icy bed surface beneath. These wind slabs will grow increasingly unstable through the day as new mixed precipitation types falling on the snow adds stress to the weak snow near the icy bed surface.

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

Air and upper snowpack temperature recordings this morning are not favorable for new snow to bond to the existing, icy snow surface. This hard rain crust is widespread through our terrain and was the result of the 3” rain event on December 21-22. Despite losing almost 25cm of snow during that event, our avalanche paths are still well developed but with rocks, boulders, and ice cliffs now showing again. Given the glazed over snowpack and cold temperatures associated with this morning’s new snow, it would be wise to view the exposed boulders, bushes and cliffs in our avalanche paths as potential obstacles and not as anchors. Today’s avalanche problem is more of an angry badger backed into a corner than a burly grizzly bear.

Additional Concerns

The Sherburne and Gulf of Slides ski trails have patches or water ice and some recently exposed rocks mixed with the refrozen rain crust. New snow will obscure these obstacles.

Snow Plot Information

DateHN24HN24W
(SWE)
Density (%)HSTTotalAir TT MaxT MinSkyPrecipComments
05/31/19
05:20
0 CMTrace 0CM0 CM8.0 C11.0 C0.5 CBrokenNo precipitation
05/30/19
05:25
0 CM 0.0 MM0CM0 CM7.5 C7.5 C1.0 CScatteredNo precipitation
05/29/19
05:25
0 CM 22.3 MM0CM0 CM1.5 C4.0 C0.0 COvercastNo precipitation
05/28/19
05:25
0 CM 0.0 MM0CM0 CM0.5 C11.0 C0.5 CClearNo precipitation
05/27/19
05:15
0 CM 0.0 MM0CM10 CM8.0 C15.5 C8.0 CClearNo precipitation

Avalanche Log and Summit Weather

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
05/30/1946 F36 F 0 in 0 in27.9 MPH55 MPH

290 (WNW)

05/29/1947 F33 F 0 in 0 in20 MPH48 MPH

290 (WNW)

05/28/1934 F28 F .71 in 3.7 in20 MPH48 MPH

290 (WNW)

05/27/1940 F27 FTrace 0 in38.9 MPH68 MPH

300 (WNW)

05/26/1948 F39 F .77 in 0 in48.7 MPH75 MPH

290 (WNW)

05/25/1947 F31 F .42 in 0 in17.7 MPH63 MPH

240 (WSW)

05/24/1942 F32 F .66 in 0 in44.8 MPH105 MPH

05/23/1944 F30 F .16 in 0 in26.8 MPH71 MPH

270 (W)

05/22/1934 F21 F 0 in 0 in36.2 MPH115 MPH

330 (NNW)

05/21/1934 F23 F .57 in 1.9 in73 MPH135 MPH

330 (NNW)

05/20/1951 F33 F 0.57 in 0.0 in48 MPH82 MPH

250 (WSW)

05/19/1951 F34 F .6 in 0 in34.2 MPH66 MPH

250 (WSW)

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 12/28/2018 at 7:18 AM.

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