Avalanche Forecast for Tuesday, April 21, 2020

This information was published 04/21/2020 at 6:50 AM.
A new forecast will be issued tomorrow.

NOT THE CURRENT FORECAST

This is an archived avalanche forecast and expired on 04/21/2020 at midnight.


The Bottom Line

Wind drifting snow will push avalanche danger to Considerable this afternoon. Natural avalanches will be possible in steep lee terrain where drifted snow accumulates. Human triggered avalanches are possible. East facing ravines are targets for this avalanche problem.

Watch for the wind to shift from the SW to the W later today; if snow is falling, wind slabs are building.


Mountain Weather

Yesterday the summit of Mount Washington received 0.8” of snow in the early morning hours on WNW wind at 70-90 mph. Skies cleared by 8am and wind remained light under 30 mph for the rest of the day. The summit reached a high temperature of 23F and Harvard Cabin snow plot reached 51F.

Today, clear skies early give way to clouds and snow in the afternoon with 1 to 3” possible. The highest rate of snowfall is forecast to be late afternoon, and will accompany SW wind shifting W then NW increasing from 25-40mph to 50-70mph. An additional trace to 2” is possible tonight from snow showers. Temperatures on the summit will reach the lower 20s F. Precipitation today is forecast to be all snow at ravine levels and up, with over 0.5” of SWE possible.

Tomorrow. Snow showers continue with a trace to 2” possible with a high temperature of 10F on the summit. NW wind increases to 60-80mph with gusts over 100mph.

Primary Avalanche Problem – Wind Slab

Wind Slab

Aspect/Elevation

Likelihood

Size

Afternoon snowfall with increasing intensity coincides with wind shifting from SW with limited fetch to the W and NW with abundant fetch, creating conditions to rapidly build wind slabs in eastern lee terrain. The timing and size of natural avalanches will depend on the rate of snowfall and when the wind shifts, but weather forecasts suggest peak instability will occur late in the afternoon & evening. If we see the upper end of the forecast snow totals, natural wind slab avalanches capable of burying a person will be possible in eastern ravines with Headwall area of Tuckerman Ravine and the main gullies of the Gulf of Slides the most likely targets.

  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.

Forecast Discussion

The likelihood of today’s wind slab avalanche problem is dependent on the amount of snow the higher summits receive. The MwOBS has 1-3” and the NWS over 2” forecast for today. At the lower end of this range for total snowfall, we may not reach Considerable, though with over 0.5” of SWE available and cold temperatures it seems likely that we’ll see the upper end, or possibly even exceed the 3” forecast. 

Additional Information

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

The Lion Head Winter Route remains the easiest route to the summit from Pinkham Notch but requires an ice axe, crampons (not just micro-spikes) and possibly a rope. This is a mountaineering route and requires solid skills for a safe, timely ascent.

Details on daily snowfall totals, precipitation type, total depth of snow and other information can be found on our page devoted to snow study plot data. Click here to check it out.

Snow Plot Information

DateHN24HN24W
(SWE)
Density (%)HSTTotalAir TT MaxT MinSkyPrecipComments
04/13/20
06:15
0 CM 0.1 MM0CM206 CM-3.5 C7.5 C-7.0 COvercastRain
04/12/20
06:20
Trace 1.8 MMTrace216 CM-7.0 C-4.0 C-7.0 CFewNo precipitation
04/11/20
06:20
14 CM 23.1 MM 14%62CM222 CM-8.0 C-6.0 C-8.0 COvercastNo precipitation
04/10/20
06:30
53 CM 49.5 MM 12%NC200 CM-6.0 C-2.0 C-6.5 COvercastSnowView
04/09/20
06:25
0 CM 0.0 MM0CM158 CM-1.0 C8.0 C-4.5 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
05/25/2054 F46 F 0.0 in 0.0 in18.7 MPH44 MPH

260 (W)

05/24/2054 F40 F 8.3 in 21 in8.3 MPH21 MPH

280 (W)

05/23/2055 F43 F 0.0 in 0.0 in15 MPH45 MPH

300 (WNW)

05/22/2055 F40 F 0.0 in 0.0 in37.4 MPH67 MPH

290 (WNW)

05/21/2053 F38 F 0.0 in 0.0 in29.4 MPH59 MPH

280 (W)

05/20/2050 F34 F 0.0 in 0.0 in29.4 MPH59 MPH

280 (W)

05/19/2041 F27 F 0.0 in 0.0 in15.3 MPH36 MPH

100 (E)

05/19/2050 F34 F 0.0 in 0.0 in11.4 MPH25 MPH

30 (NNE)

05/18/2045 F30 F 0.0 in 0.0 in14.7 MPH36 MPH

70 (ENE)

05/17/2040 F30 F 0.0 in 0.0 in17 MPH35 MPH

270 (W)

05/16/2037 F28 F .16 in 0.0 in38.8 MPH101 MPH

310 (NW)

05/15/2042 F32 F .95 inTrace 31.5 MPH95 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 04/21/2020 at 6:50 AM.

Jeff Fongemie
USDA Forest Service
White Mountain National Forest