Tahoe Backcountry Snow

Backcountry obs.:

The recent cool and showery pattern of recent days continues to foster varied spring conditions across the Tahoe backcountry on both trails and the remaining snowpack. As of dawn on Sun (4/26) and Tues (4/28), usable snow had become interrupted with large dirt patches at TH elevation in the Mt. Rose area, requiring some route finding to enter and exit with skis. Above 9000′, coverage is largely consistent on N and E aspects, yet on a deflating snowpack: rocks and stumps are emerging, along with the occasional section of summer trail. Recent cold temps have been great for the melt freeze cycle, creating firm, supportable and delightfully-textured snow. Morning ascents tend to be quick and grippy underfoot, while descents are firm and fast, yet confidently edgeable.

High elevation N facing bowls near 10k ft, still hold pockets and panels of soft, wind-deposited fluff, composed of layers of light accumulations from recent afternoon snow and pellet showers. If they haven’t already, these areas will transition quickly to melt freeze crust with incoming warm and sunny days for the rest of the week.

Weather and forecast thoughts:

Today (4/29) marks the first day truly sunny day since the past weekend with mostly sunny skies and no afternoon showers across Tahoe. The rest of the work week and first part of the coming weekend look to bring moderating temperatures and a mix of clouds and sun through Sat (5/2) as a transient ridge of high pressure sets up over the Pacific NW, largely shielding the Sierra from unsettled weather.

Above 7000′, highs will range from the upper 40s to mid 50s on Thurs (4/30), climbing a few degrees higher Fri-Sun (5/1-5/3), before cooling off back into the 40s for the beginning of next week. Lows for the period will cool into the mid to upper 30s most nights with valleys and depressions cooling to around freezing in the calm, stable airmass. Surface refreezes on the remaining snowpack are a good bet for the backcountry through the weekend.

The reprieve from unsettled weather is thanks to a building ridge of high pressure over the Pacific NW, which model ensembles show remaining in command until late Sat and into Sun (5/2-5/3). However, around midday Sat, a cut off low looks to pinch off the Pac NW ridge, then drop down the coast, deepening and strengthening along the way.

As the low approaches CA, it looks to strengthen, bringing good dynamic forcing along with it, as noted in 500mb height, wind, and vorticity (atmospheric spin) analysis below. The low and its favorable, lifting dynamics approach late Sat, then meander and spin their way slowly into Central CA through Tues (5/5), before ejecting out to the east.

Moisture with the system is not terribly organized, so a widespread, heavy rain/snow event currently looks unlikely. However, when coupled with the incoming dynamics of the approaching low, there will be sufficient precipitable moisture to help fire afternoon showers and thunderstorms, especially Sun – Tues (5/3-5/5).

This far out, forecast confidence for snow level predictions is pretty low, but based off the NW origin of the low, temps will fall with its arrival, and so too will snow levels. Both the ECMWF and GFS have quite a bit of spread in their freezing level predictions, but do show agreement on a marked drop in snow levels–and an increase in precip chances– throughout the day Sun into Mon (3/3-3-4) and persisting into the middle of the first week in May.

This is a tricky pattern. Although the incoming system looks similar those throughout much of the past month, it too will have its own personality. Expect changes to the forecast timing and character of the system as its arrival draws nearer.

For now, expect warmer and drier weather through Sat, with mainly sunny skies. Clouds will increase on Sat with the approaching system, along with SW winds. Cool temps and periods of showers and afternoon thunderstorms are likely Sun lasting through the middle of next week.

Looking beyond to end of next week, models hint at the return of warmth and sunshine once this weekend’s system finally kicks out, perhaps lasting longer than just a few days.

Get out and enjoy the spring sun and remaining snow while they last. More change is in the offing.

Backcountry obs.:

Spring is in full effect across the Tahoe backcountry with fresh snow, sunshine, and variable conditions. This week a cold storm Tues-Weds (4/21-22) dropped anywhere from 4″ to nearly a foot of cold powder across the area, with highest amounts on the NW side of the lake. In the Mt. Rose backcountry, 4-6″ of new dust on crust delivered a surface reset by dawn Weds (4/22), filling in tracks since the last system. The new snow skied cold, light and fast.

Since the midweek storm, sunshine, warm highs and below freezing lows have restarted the corn cycle process just in time for the weekend.

Weather and forecast thoughts:

While most high elevation snow might be in prime spring skiing shape this weekend, an unsettled pattern of afternoon showers and storms, will keep conditions highly variable into early next week. No significant accumulations are expected, however a coating to a few inches of snow and graupel could fall out of any group of showers/storms that get hung up on terrain around the area, especially south of Tahoe on Sat.

Expect highs in the backcountry above 8000′ in the low-mid 40s on Sat (4/25), before falling into the mid-upper 30s on Sun (4/26). On Mon temps will start a steady rebound back through the 40s, edging a few degrees higher each day through the end of the week.

Lows this weekend through the beginning of the work week look to fall below freezing in most areas, promoting surface refreezes each night. This could further aid the corn cycle in areas that don’t receive new snow, or preserve whatever coating falls in afternoon showers.

The culprits behind this unsettled and complicated pattern of late day showers and storms are two areas of troughing in the upper atmosphere and their associated pulses of low pressure: one to the NE and another low coming ashore on the central CA coast that eventually merge late Sat into Sun (4/25-4/26) before meandering off to the east.

These troughs have the upper level dynamics in place to promote rising motions in the atmosphere, creating clouds and precip., as noted in the 500mb vorticity analysis through the weekend.

In addition to the forcing provided by these low pressure systems, daytime heating on the mountains–especially any sunny mornings and early afternoons–will add a convective forcing element, lifting air skyward in growing cumulus clouds. Look for cumulus clouds bubbling up each late morning and afternoon this weekend as precursors to showers and thunderstorms.

In addition to the dynamics, there’s also plenty of moisture across the Sierra to condense out into precip. as noted in the Euro model for mid level relative humidity levels from this weekend through Mon (4/27). Notice the slug of moisture south of Tahoe late Sat into early Sun.

Most of the precip that falls across the backcountry above 8000′ will be snow this weekend, especially late Sat and Sun. Model forecasts for freezing levels snow colder air arriving late Sat which will drive snow levels down towards lake level by Sun morning, before lifting to around 6500′-7000′ by later in the afternoon.

All of the elements are coming together this weekend to create a mixed bag of weather: sun, clouds, snow showers, and even some thunder. Just as the backcountry now has multi-sport conditions, so too will the weather be “multi-impact”.

Looking ahead into next week, the pattern looks to settle down a bit towards the middle to end of next week with sunnier weather and near normal temps reaching the upper 40s-50s by midweek.

Backcountry obs.:

Cool nights, sunny days and occasional winds helped promote excellent spring skiing across the highest Tahoe backcountry zones this past weekend (4/17-19).  Heading out at dawn on 4/19, I found mostly supportable melt-freeze conditions across the majority of aspects above 8600′ in the Mt. Rose backcountry. Transitional, yet frozen winter snow, adorned with breakable crusts, topped the snowpack in high north facing bowls above 9000′. Well-frozen spring corn at various degrees of development blanketed the rest of the backcountry on E-S-W aspects. High clouds and a 10-20 knot SW wind slowed surface melt until mid-late morning on Sun (4/19) when clouds dissipated, and the sun finally got to work softening surface snow. 

Popular zones are heavily tracked with multiple skin tracks and deep ruts left behind from late day skiers sinking deep in water-logged snow, while untracked, clean surfaces are more common than not further afield. A refresh would be nice.

Wx. and forecast thoughts: 

A deep, closed low inching its way into northern CA today will deliver the desired refresh to keep backcountry season going.  Today (4/21) rain and snow showers will increase in coverage and intensity during the day and into tonight, potentially dropping up to a foot+ of new snow along the Sierra Crest and highest terrain by dawn Weds (4/22). Snow levels will start around 7000′ at daybreak today, then fall to lake level and below by this afternoon. East of the crest and below 7000′, expect a nuisance few inches for most Tahoe communities. Behind today’s system, cool temps and sunshine return to the area for Weds into Thurs (4/22-4/23), before temps warm to near average for Fri and the weekend with renewed chances for isolated showers across higher terrain.

IR mid-level water vapor satellite imagery this morning shows the storm spinning away and slowly advancing towards the Sierra. (Blues and whites denote greater moisture whereas yellow signifies drier air.)

And underneath those clouds and moisture, a north-south band of concentrated precip is nudging its way into the Sierra.

Worth noting is the N-S flow in both images this morning. This wind flow isn’t conducive to efficient spillover east of the Sierra Crest because it’s hitting the Sierra at an oblique angle, rather than closer to 90°. Expect rain and snow to “hang up” on the crest before finally moving in later this morning.

Throughout the day, the storm will edge eastward into Tahoe, bringing rounds of rain and snow into the mountains with snow levels falling throughout the day. Precip rates will likely increase later today as afternoon warmth and peak dynamics of the low combine, embedding convective showers and thunderstorms into the mix, especially near the Sierra Crest.  Afternoon instability will also promote thunderstorms to fire across the Central Valley (some potentially severe), then push eastward into the mountains, bringing locally heavy bursts of snow and pellet showers.

Tonight snow showers will continue before tapering off around daybreak tomorrow (4/22) as the bulk of the moisture and dynamics moves eastward. Lingering instability will likely create a buildup of clouds and occasional snow/pellet showers throughout the day on Weds, before clearing out Weds night. Temps on Weds. will range from the upper 20s-mid 30s above 8000′, before cooling off into the low 20s on Weds night.

High resolution models indicate highly variable total precip amounts due to the convective nature of this system, in addition to the S-N flow favoring areas along and west of the Sierra Crest. Notice the steep drop off in precip in this morning’s HRRR model for total liquid precip through late Weds afternoon.

Snow conditions in the backcountry will rapidly change throughout the storm and immediately thereafter in the dynamic spring environment. Cold powder on Weds AM will quickly warm and transition to mashed potatoes and “hot pow” on all but the shadiest north aspects throughout the day. Once the sun sets on Weds, any sun-softened snow will quickly refreeze into breakable crusts by Thurs AM, initiating the melt-freeze cycle and journey back to corn snow. 

Thurs (4/23) looks to be cool and largely sunny with highs above 8000′ ranging between the upper 30s to mid 40s depending on elevation. Lows Thurs night will drop well below freezing under mainly clear skies. Inversions may develop, but the return of dry air and clearer skies will promote overnight refreezes and potentially, another weekend corn cycle. 

For the coming weekend, model ensembles suggest the potential for more unsettled spring weather starting on Fri and lasting through Sun (4/24-4/26) as a trough of low pressure develops over the west.

While confidence in any given outcome is low, chances for showers over the mountains and near to below normal temperatures in the 30s and 40s above 8000′ looks like a good bet the upcoming weekend.

Despite a lackluster winter and low tide conditions this spring, the corn cycle is on and about to get another refresh. Backcountry season chugs on.