Monday February 26, 2018

Wednesday February 28, 2018
Tuesday February 27, 2018
Monday February 26, 2018
Confidence: High - ample field data and confidence in weather forecast models

Main Concerns

Cornice Fall, Wind Slab - view Avalanche Problems for detailed information.

Travel & Terrain Advice

Identify lee and cross-loaded features and approach with caution.

Be cautious as you transition into areas of wind affected terrain.

Avoid steep slopes below cornices.

Avalanche Summary

Avalanche control team reports from Mount Washington indicate some activity with control teams ski cutting small size one avalanches on steep isolated north aspects. These avalanches where wind slab on cross loaded features in open terrain at tree line. Failing on the exposed melt freeze crust, easily trigger and sensitive to human triggering.

Avalanche Problems

Cornice Fall

Expect already large cornices to continue to grow on Wednesday becoming fragile and very touchy to human triggering. Cornices are located on predominantly north aspects at both the treeline and the alpine and if triggered these could produce a large avalanche big enough to bury injure or kill a traveler , size two. Take the time to identify these features and ensure a wide berth when travelling either above or below them.

Wind Slab

New heavy snow fall Wednesday with sustained and strong southwesterly wind will produce new wind slabs on features lee to the wind. Current wind slab and cross loading exist and wind slab will be wide spread on all aspects and found in the alpine, at tree line and below tree line. Wind slab avalanches when first formed will be touchy to human triggering and when trigger will produce large avalanches , size two.

Snowpack Summary

Variable surface conditions exist dependent on exposure to the wind. On open and exposed terrain, the majority is scoured to melt freeze crust. Areas protected from the wind have up to 50 cm of low density unconsolidated snow sitting on the melt freeze crust. Past moderate to strong wind from all directions with the last big wind event coming from the south west have transported a large amount of available low density snow. Numerous snow pack test have produced easy results failing on or just above the melt freeze crust initially buried on February 14. Observers have reported easy sudden planar results on compression tests and while travelling in avalanche terrain have experienced easily heard and felt snow pack settlements. The valentines day crust can be found down between 20 and 60 cm and the crust can be up to 80 cm thick. Below the crust well settled and strengthening snow

Snowpack Details

Surfacenew low density snow
Uppermelt freeze crust from 30 to 90 cm thick
Midwell settled
Lowerwell settled

Past Weather

Cool conditions persisted with overnight lows below minus 10 and up to 20 cm of new snowfall. Winds continued from the north west with variable velocity ranging from light to strong. Overnight on Saturday strong south west wind transported available new and low density snow to north aspects.

Sunday afternoon gave way to a variety of conditions including clear sky with sun, convective snow showers, graupel and or hail. On solar aspect and at lower elevation surface snow became moist with even the slightest touch of sun.


Weather Forecast

A strengthening low approaches the island region late Wednesday into Thursday bringing to the forecast area strong south east wind and moderate to heavy snowfall.

Monday 1 to 5 cm new snowfall. Temperatures - 10 overnight rising to 0 mid day, winds light to moderate south west. Freezing level 0 m rising to 800 m mid day.

Tuesday 1 to 5 cm new snowfall. Temperatures -3 overnight rising to near 0 by mid day, winds moderate west. Freezing level 500 m rising to 800 m mid day.

Wednesday 25 to 40 cm new snowfall. temperatures -5 rising to -1 mid day, winds Strong South East . Freezing level 250 m rising to 500 m mid day.

Posted on Monday February 26, 2018 by Jesse Percival

Past Bulletins