Avalanche Forecast for Monday, December 17, 2018

This forecast was published 12/17/2018 at 7:04 AM.
A new forecast will be issued tomorrow.

NOT THE CURRENT FORECAST

This is an archived avalanche forecast and expired on 12/17/2018 at midnight.


The Bottom Line

Moderate amounts of snowfall occurring today will result in increasing avalanche danger. We expect avalanche danger to peak after dark tonight. Additionally, the relatively small amount of snow which has accumulated on light wind in much of our terrain will mask the varied snow surface which preceded the current storm. Today’s avalanche danger ratings are based on a fairly uncertain weather forecast, and actual avalanche danger may be greater or lesser based on how the weather plays out. As always but particularly today, please consider our forecast a starting point for your snowpack observations and terrain decisions. All avalanche terrain may reach CONSIDERABLE before this forecast expires at midnight.

2018-12-17 Printable PDF

Forecast Area

Mountain Weather

It’s currently snowing with just under an inch of snow recorded overnight at our Hermit Lake snow plot. Expect snowfall to initially taper off through midday before upslope snow showers build later today and tonight. All told we could see up to an 8 inch storm snowfall total. There is a fair bit of uncertainty in that number due to potential variability of upslope snow totals which account for most of this forecast storm total. Wind is more certain to be a player in our stability equation, with the current light to moderate SE wind forecast to shift through E to NW today and begin to increase towards sustained summit winds around 80 mph tomorrow. The current temperatures which are just below freezing in the alpine will steadily drop overnight by nearly 30 degrees on the higher summits, to just below 0F by tomorrow morning.

Primary Avalanche Problem – Wind Slab

Wind Slab

Aspect/Elevation

Likelihood

Size

New wind slab developing on today’s snowfall and increasing NW wind will be sensitive to a human trigger. Size and distribution will depend on timing and amount of new snow and wind. There is potential for new wind slabs to be large and likely to human trigger by the time this forecast expires at midnight tonight. Older wind slabs, ranging greatly in hardness and sensitivity, may be possible to human trigger and are likely masked by this morning’s new snow. Ultimately an avalanche in new snow today could step down to these older layers, though significant spatial variability makes this challenging to predict. Wind from the NW this afternoon will combine with previous scouring to generally minimize these avalanche problems on the western half of the compass rose.

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

New wind slab will build today, tonight, and continue to be affected by sustained extreme wind tomorrow. Size and sensitivity of new slabs in the alpine will be directly determined by precipitation amount and timing of the increase in NW wind later today. Peak instability will likely occur after dark today, and if wind forecast numbers hold true we should see slabs become increasingly hard and stubborn tomorrow. These new wind slabs will form on a varied surface, particularly on easterly aspects. This surface includes soft to firm wind slab of varying sensitivity, areas previously scoured to the Dec. 3 crust, and some areas of sun crust. Facets have been observed around the Dec. 3 crust, particularly just below the crust, but likely won’t be a factor in much of our terrain due to the crust remaining relatively robust. Limited snow accumulation at elevations below approximately 3500 feet will result in minimal if any development of a new avalanche problem today.

Additional Concerns

Ski trails in the Pinkham Notch area remain snow covered.

Snow Plot Information

DateHN24HN24W
(SWE)
Density (%)HSTTotalAir TT MaxT MinSkyPrecipComments
03/21/19
05:23
0 CM 0 MM0CM213 CM-1.5 C0.0 C-11.5 COvercastNo precipitation
03/20/19
05:28
0 CM 0 MM0CM214 CM-11.0 C-4.5 C-14.0 CClearNo precipitation
03/19/19
05:25
0 CM 0.0 MMNC215 CM-13.0 C-9.5 C-15.0 CFewNo precipitation
03/18/19
05:20
Trace 0.1 MMNC216 CM-15.5 C-13.0 C-15.5 CBrokenSnowView
03/17/19
05:26
4 CM 3.7 MM 9%4CM218 CM-14.5 C-2.5 C-15.0 CBrokenNo precipitation

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
03/20/1917 F1 F 0.0 in 0.0 in32 MPH58 MPH

270 (W)

03/19/194 F-4 FTrace Trace 43.4 MPH63 MPH

300 (WNW)

03/18/193 F-6 F 0.03 in 0 in52.1 MPH82 MPH

280 (W)

03/17/192 F-6 F 0.07 in .5 in61.5 MPH110 MPH

270 (W)

03/16/1925 F2 F 0.24 in 2.3 in67.5 MPH97 MPH

280 (W)

03/15/1941 F25 F 0.04 inTrace 54 MPH105 MPH

250 (WSW)

03/14/1940 F17 F Trace in Trace in27.7 MPH75 MPH

200 (SSW)

View
03/13/1924 F11 F .07 in .8 in29 MPH52 MPH

290 (WNW)

03/12/1911 F-1 F 0.14 in 1.9 in65 MPH104 MPH

280 (W)

03/11/1917 F6 F 0.35 in 4.4 in77 MPH114 MPH

280 (W)

03/10/1925 F4 F .35 in 3.7 in47.2 MPH94 MPH

150 (SSE)

View
03/09/1919 F0 F 0 in 0 in36.1 MPH70 MPH

290 (WNW)

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 12/17/2018 at 7:04 AM.

Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest