Tahoe Backcountry Snow

Backcountry obs.:

Colder overnight lows have finally returned offering solid overnight refreezes to the snowpack and a slowing of last week’s dramatic melt. Last weekend’s disturbance brought in a cooler, more seasonable air mass and spritzes of new snow across the backcountry.

A tour through the Rose backcountry between 8650′-9700′ at dawn on Mon (3/2) revealed a solidly refrozen and supportable surface, comprised of inches-thick rain/melt freeze crust. Runnels from the recent rains still exist but have mellowed, making for smoother skiing. The Sun (3/1) system that brought rain to lower elevations, dropped a trace to .5″ of snow across the Rose backcountry. Treed lee areas retained the most new snow, while ridges and open bowls had been scoured down to bare crusts. Even the deepest pockets of dust offered little more than a dampening of the fast, scratchy chatter made with each turn. Ski crampons and whippets recommended.

Now that the Sierra seems to have definitively shifted out of powder season, the corn chase is on. Enough cold air has finally returned to refreeze the surface each night, while sunny skies and above freezing temps each afternoon are surely softening the surface into buttery corn.

Worth noting is that below 7500′, the snowpack is quickly retreating, especially on solar aspects. There’s not much ski season left.

Weather and forecast thoughts:

Spring-like weather is here as noted by the arrival of warm sunny days interspersed with quick-hitting cool disturbances. Today (3/3) will be sunny and warm with high temps reaching the low-mid 40s in the backcountry with lows cooling back to the upper 20s. Tomorrow (3/4) a relatively weak and moisture-starved shortwave trough drops in from the PacNW. Winds and clouds will increase, along with the chances for snow showers, especially over higher terrain. Late week and into the weekend, dry conditions return with near to above normal temps, continuing on into the second week of March.

As noted above, tomorrow’s quick hitting system splits off from a trough of low pressure in the PNW, then drops down across the Sierra and Western Great Basin on Weds and early Thurs (3/5 – 3/6). Due to its trajectory in from the NW, the system will be cooler with snow levels around lake level. However, despite middling lifting dynamics and a cooler source region, the system is unlikely to create much impactful precip with total snow amounts between 1-4″ with the highest amounts along the Sierra Crest.

Atmospheric lifting dynamics, as noted by vorticity and 500mb heights through early Fri AM (3/6), do show sufficient lift and instability to create modest rain and snow, especially with the added influence of springtime surface warming on Weds afternoon.

But despite the dynamics, models for the same period consistently show the system starved for moisture, as noted by meager precipitable water anomalies:

By late Thurs night into Fri (3/5-3/6) total liquid precip. amounts look very light, with only up to .5″ falling near the Sierra Crest.

Behind the system, cool N and NE flow will keep temps down near seasonal levels Thurs – Sat (3/5 – 3/7) with highs above 7000′ in the 30s and 40s. Moderate N and NE winds will also chill the peaks through Sat AM before slackening over the weekend.

Sun (3/8) into next week, warmth will slowly rebuild as a long wave ridge of high pressure rebuilds over the East Pacific, also blocking any potential storms from rescuing the quickly receding snowpack.

Apart from this week, it’s time to shift the backcountry mindset from pow to corn.

Backcountry obs.:

The backcountry snow pack has taken a big hit over the past week. Warm air more reminiscent of May, replaced the seasonably cold conditions earlier in the week, bringing rain as high as 11,000′ on Tues and Weds (2/24-2/25). Gone is the powder and refreshed snow pack from mid Feb replaced by soggy, saturated mush at the lowest elevations, and poorly refrozen rain/melt freeze crust above 8000′.

I toured in the Mt. Rose backcountry between 8650-9900′ at dawn on 2/27 and found a snow pack still greater in volume than pre-storm, but not by much. Rain and above freezing temps melted and compressed the snow pack over the past week. Luckily, efficient overnight cooling had refrozen the snow surface in most areas above 9000′, creating a supportable, 1″+ thick melt-freeze crust, making skinning and skiing quick and efficient. Runnels, beautifully smooth and curved, riddled the entire snow surface, even up to 9900′, evidence of liquid water percolating down through the snow, compressing and pulling its weight onto the topography below.

Skiing on the smooth, runneled surface crust was still fun and fast, scrapey and loud. East Coast-trained skiers will excel on these surfaces. In steeper, more consequential terrain, whippets are necessary to prevent an uncontrollable slide in the event of a blown edge. By mid-morning – early afternoon, soft, buttery corn is likely on offer courtesy of solar surface softening, but only in open areas receiving sufficient overnight refreezes. Forested and lower elevation zones not benefiting from overnight refreezes are soft, rotten, and unsupportable, especially after mid-morning.

And just like that, it’s feels like late spring in the backcountry.

Weather and forecast thoughts:

Anomalously warm, moist air will continue to be a feature of the backcountry over the weekend and early work week (2/28-3/3). The ridge of high pressure left in the wake of last week’s AR responsible for continued warmth, will flatten over the course of the weekend as a cut off low approaches CA from the Eastern Pacific. The low will bring more clouds, muggy air, and the chance of afternoon rain/snow showers–and even the chance of a few rumbles of thunder–to the backcountry.

Highs this weekend will be in the low 50s on Sat (2/28) above 7000′ under a mix of clouds and sun, cooling to the 40s on Sun (3/1) with mainly overcast skies. Lows each night will cool into the mid to low 30s. Refreezes will be weak at best, but improving each day especially over higher terrain. Chances for scattered showers increase each afternoon through Mon (3/2), but rain/snow accumulations will likely be generally light and localized.

Snow levels this weekend will start near and above 9000′, before falling to around 6500′ by Mon morning as the low begins to depart the region and chances for precip fall.

There are modest lifting dynamics with the weekend system which will likely ignite a few showers, especially in the afternoon with the added energy of late winter solar heating.

Despite the dynamics and afternoon instability, total available moisture looks to wane through the weekend, just as the best dynamics arrive late on Sun.

The result will be light and spotty precip with most areas receiving less than .10″ rain or a few inches of snow, at most.

Once this weekend’s system moves out, high pressure returns bringing sunny skies and mild temps through midweek.

There’s some indication of another weaker frontal passage around mid week that would further cool temps and bring back chances of rain and snow above 7000-8000’. Total amounts look light (a few inches at best). Models still disagree significantly, keeping forecaster confidence low.

Longer term models show signs of another modest system reaching the Sierra by next weekend, followed by cooler temps closer to seasonal averages.

Despite the unsettled pattern, what doesn’t appear into the first days of March are signs of prolonged cold and storminess: a full blown return to winter. It might not be official on the calendar, but springtime weather is here way ahead of schedule.

Backcountry obs.:

I started the week yesterday (2/23) at dawn with a quick north facing tour in the Mt. Rose backcountry between 8400′-9200′. Despite the warmth late last weekend, cold, soft powder had held on in shady, north facing protected areas. Leaving the shadows, snow quickly turned transitional with every variety of breakable crust imaginable on offer: thin and zippy to thick and deadly to stuffed crust! One lap on the bipolar snowpack was enough to start the week off right.

As of this writing (2/24), any remaining cold powder from yesterday has likely been soaked with warm rain at all elevations, even above 10,000′. Much like the endangered pika, there’s no more mountain to climb to escape the warmth of this system.

Weather and forecast thoughts:

Fresh on the heels of the coldest cycle of the season, the warmest cycle of the season is now impacting the Tahoe backcountry with rain for Tues (2/24), tapering off tomorrow (2/25). A subtropical moisture feed has already pushed snow levels with this system to around 9000′ and these will continue rising throughout the day as rain intensity increases. At the storm’s peak this evening, snow levels will likely be above 10,000′! Rain tapers off tomorrow (2/25) and in its wake, warm temps and sunny skies for the rest of the work week will make memories of cold powder feel like a fever dream.

Taking a look at the big picture, the pattern responsible for the glorious, cold snow from last week has flipped completely, replaced by a ridge of high pressure over the west and a fetch of warm, moist air pumping in from the subtropics. 500mb height anomalies tell the story with the black lines pointing out into the Pacific SE of Hawaii.

NE of Hawaii is an area of low pressure, spinning counter clockwise and funneling moisture into the west in a weak-moderate AR (atmospheric river), as noted in the normalized precipitable water (PWAT) anomaly analysis. This shows how much more precipitable moisture is in the total atmosphere than the climatological norm.

The result of this warm, wet connection is rain for all elevations of the Tahoe backcountry. Models are in good agreement for freezing levels topping out above the terrain between 11,000′ – 12,000′ later this afternoon, before falling to between 10,000′ – 11,000′ this evening and into tomorrow (2/25).

The one saving grace is that the rain will likely not be too heavy in the Sierra, with only a few inches of liquid (1-3″) falling, with the highest amounts N and W along the crest.

Despite the robust moisture, the location of the low pressure way out in the Pacific tempers the dynamic forcing of the system, which in turn limits the strength and amount of precip that the Sierra will receive. If the low were closer to the coast, rainfall would be heavier, potentially creating flooding concerns with more rain on snow melt. Note the lack of strong vorticity (atmospheric spin) which favors dynamic lifting in the atmosphere.

In this case, we just get to contend with a significantly degraded backcountry snow pack.

Behind this week’s system, warm and dry weather returns going into next weekend (2/28-3/1). The low responsible for spinning the subtropical moisture into Tahoe today may eventually trek into the Sierra late this weekend, bringing somewhat cooler temps and chances for rain and high elevation snow Sun into Mon (3/1-3/2). But at this point, backcountry impacts look to be modest.

Looking way out into the beginning of March, models agree on a return of warm and generally dry conditions with no big, cold systems yet in sight.