Avalanche Advisory: Thursday - Apr 19, 2018 | Eastern Sierra Avalanche Center

THIS AVALANCHE ADVISORY EXPIRED ON April 21, 2018 @ 6:59 am
Avalanche Advisory published on April 19, 2018 @ 6:59 am

Issued by Doug Lewis - Eastern Sierra Avalanche Center

Bottom Line

Thursday – Cloud skies and cool temperatures today will limit Loose Wet activity to primarily Friday when skies clear and temperatures quickly rebound into the 40’s and 50’s. Recently formed Wind Slabs (4/16) have generally settled and strengthen and are primarily limited to small isolated pockets in steep and complex terrain. Northerly winds are forecasted to increase overnight into Friday but the lack of abundant transportable snow will limit new Wind Slabs to isolated areas where there is a large upwind fetch. Natural avalanches are unlikely, small isolated triggered releases possible in steep or complex terrain.

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Above Treeline

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Near Treeline

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Below Treeline

Avalanche Problem 1: Wind Slab

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A quick moving spring storm moved through the region 4/16 with moderate to strong SW winds and 2” to 9” (~5 to ~20cm) of new snow, which formed Wind Slabs on NW-NE-SE aspects near and above treeline (~10,000’ and above). As the storm exited the region, winds switched to the NW-NE, forming a new round of Wind Slabs on NW-SW-SE aspects. Generally, enough time has passed for the Wind Slabs to begin bonding to the underlying snow with the recent new snow settling and strengthening and capped by a melt/freeze crust on many aspects, limiting available snow for transport, and constrain the distribution to localized channeled areas where there is still snow available for transport. North winds are forecasted to increase Thursday night into Friday above ~10000’, which may form additional small isolated Wind Slabs in the Alpine terrain (primarily above 10000’). Any new Wind Slabs should be small, relatively shallow, and isolated. Thursday – Natural avalanche are unlikely, small stubborn isolated triggered releases possible today (Thursday) and becoming somewhat increasingly possible Friday in steep or complex terrain from approximately 10000 and above, especially areas that promote drifting (rock outcrops, gullies, leeward side of ridges, etc.) Use additional caution in and around terrain features that promote drifting, under cornices, tops of chutes, and gully sidewalls.

Avalanche Problem 2: Loose Wet

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Thursday- forecasted mostly cloudy skies and cool temps should limit Loose Wetactivity today to primarily solar aspects (E-S-W) from treeline and below (~10500”). Small triggered Loose Wet releases are possible Thursday.

Friday, temperatures are forecasted to climb into the mid 40’s and upper 50’s, which will increase the potential for Loose Wet avalanches during the day from ~10500’ (possible higher in sheltered rocky terrain) and below, primarily on E-S-W-NW aspects but may creep around the compass to include N-NE aspects as well, especially if there is a weak freeze overnight. Signs of wet snow instabilities include: pinwheels, rollerballs, deep ski or boot penetration, and point releases. Loose Wet avalanches can trigger larger deeper releases, especially in the lower elevations (~10500’ and below). Small natural avalanches are possible, triggered releases likely, especially in sheltered, steep, or rocky terrain. Signs of wet instabilities: rollerballs, small sluffs, and deep ski or boot penetration. If it’s becoming ncreasingly hard to turn, it’s likely past time to move onto to shadier, less steep slopes slopes. Use extra caution in the middle and lower elevations near rocky or vegetated areas.

advisory discussion

Changeable spring weather continues to change the avalanche problems as quick moving spring storms pass through the region with cold temps, new snow, and moderate to strong winds followed by rapid warming and a quick shift to spring conditions. The latest storm to pass through the region (4/16) left 2’ to 9” in its wake and was accompanied by strong SW winds which veered to the North as it departed, which formed Wind Slabs throughout much of the Alpine terrain (~10000’ and above) on top of a firm slick melt/freeze crust. This combination slowed the normal bonding process and the new snow can still easily peeled off on steep slopes. Recent warm temps have resulted in good snow settlement and the formation of a melt/freeze crust on most aspects below ~10500’, which will limit the snow available for transport as Northerly winds increase tonight into Friday limiting Wind Slab formation through the forecast period to isolated channeled terrain where there is a large fetch up wind. Low and mid (creeping above 10500') elevations are transitioning toward spring corn quickly on solar aspects. Watch overnight temperatures for sustained freezes to guage how quickly the snowpack may thaw during the day.

recent observations

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Gaylor Peak: New Snow Stability (4/18)

Gaylor Peak: Isolated Wind Slabs & Small Loose Wet Avalanches (4/18)

Sherwins: Stubborn Wind Slabs, Warming (4/17)

Black Mountain: Avalanche Ob (4/17)

South Peak: New Snow Stability, Wind Slab (4/17)

Mammoth Rock Trees: New Snow Stability (4/16)

TJ Bowl, East Trees (4/16)

Bishop Creek: Rain Effect (4/15)

Temperatures @ 0400

Loc New” Highs/Lows

Virginia Ridge, 9409’: ~1”     38/24 deg F., 9 hrs below freezing,
Tioga Pass, 9972’: ~T       33/13 deg F., 13 hrs below freezing.
Agnew Pass, 9355’: ~1”     40/18 deg F., 10 hrs below freezing
June Mt., 9148’: ~T       36/21 deg F., 10 hrs below freezing

Mammoth Pass, 9500’: ~T       38/18 deg F., 10 hrs below freezing
Sesame Plot, 9014’: ~T       34/19 deg F., 12 hrs below freezing
Ch. 22-MMSA, 10,067’: 27/15 deg F., 24 hrs below freezing
Summit-MMSA, 11,053’: 22/13 deg F., 24 hrs below freezing
Rock Creek, 9600’: ~4”     36/18 deg F., 10.5 hrs below freezing
South Lake, 9580’: N/A     39/21 deg F., 10 hrs below freezing

Sawmill, 10,200’: ~T 35/18 deg F., 12 hrs below freezing

Weather and CURRENT CONDITIONS

weather summary

Thurs thru Friday- An upper low will move through the region today bringing isolated to scattered showers through the day. Some light snow accumulations are possible for higher elevations of Mono County as the upper flow switches to the north, which favors banking of moisture along the Sierra Front increasing precipitation chances along the eastern Sierra and Sierra Front with cloudy and scattered showers through the day. Temps will remain cool today due to brisk northerly flow across the region with gusts of 30’s mph with highs in the mid 20s to 30’s. The upper low circulation exits into Utah overnight tonight with high pressure building across the region by Friday. Winds will drop off as the pressure gradient weakens. Temperatures will begin to rebound into the 40s to low 50’s.

Saturday thru Wednesday- After a dry and mild Saturday, the models solutions start to become a bit more aggressive with a short wave trough Sunday. Earlier runs showed this mainly in the ECMWF but with limited convection. The GFS is now taking up the mantle as well and driving a short wave east along the Oregon border followed by weak short wave ridging for early Monday. The ECMWF carves an upper level low into the base of a short wave trough and drops it south well off the coast for the Monday through Wednesday time frame. For Wednesday, the POPs spread south along the Sierra. High temperatures generally stay above normal with enough low- to mid-level instability Tuesday and Wednesday for isolated thunderstorms.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS

For 8,000 ft. to 10,000 ft.

	Thursday	Thursday Night	Friday
Weather:	Mostly cloudy. Scattered snow showers.	Mostly cloudy. Scattered snow showers.	Sunny.
Temperatures:	33 to 39 deg. F.	21 to 26 deg. F.	49 to 55 deg. F.
Mid Slope Winds:	Light winds becoming northwest	Northwest	Light winds.
Expected snowfall:	up to 2 inches in.	up to 1 in.	0 in.

Over 10,000 ft.

	Thursday	Thursday Night	Friday
Weather:	Cloudy. Scattered snow showers.	Mostly cloudy then becoming partly cloudy. Scattered snow showers.	Sunny.
Temperatures:	26 to 31 deg. F.	15 to 20 deg. F.	42 to 47 deg. F.
Ridge Top Winds:	Northwest	North	North
Expected snowfall:	up to 2 in.	up to 1 in.	0 in.

Disclaimer

This Avalanche Advisory is designed to generally describe avalanche conditions where local variations always occur. This product only applies to backcountry areas located outside established ski area boundaries. The information in this Snowpack Summary is provided by the Eastern Sierra Avalanche Center, who is solely responsible for its content.