Tahoe Backcountry Snow

Backcountry Obs.:

There’s fresh snow out there. Daily thunderstorms from the most recent storm dropped a few inches of snow at the highest elevations, mainly above 9000′. The storm system responsible for the unsettled weather arrived on Sat (5/2), triggering a daily barrage of showers and storms through today (5/5). Snow levels during the event started high, then dropped to nearly 7000′ by the evening of 5/4, and by midnight, a few inches of snow had fallen across the Mt. Rose area above 7500′. Despite the fresh snow, it mainly fell on dirt except at the highest elevations and in N facing zones where access now requires hiking in, before reaching a continuous, skiable snowpack.

Trail conditions below 8000′, however, are currently excellent–and fragrant too with the multitude of flora bursting forth for the season. Daily rainfall since Sat (5/2) has been generally light to moderate with .10″-.50″ falling each afternoon, tamping down dust and creating a soft, springy surface on which to run, hike, or carve by bike. Below 8000′, trails are generally open with the occasional snow patch on shady N facing terrain. Above 8000′, and along the west side of the lake above 7000′, trails are still snow covered, but quickly burning out.

The recent rains and retreating snowpack make for an abundance of water flowing in most creeks and rivers, yet at moderate, past peak flows, facilitating simple, stress free creek crossings. Because peak melt already passed in late March and early April in response to the multi-week March heatwave, stream flows will steadily decline with spring’s progression.

Weather and forecast thoughts:

The storm system responsible for firing off daily storms since Sat (5/4) will finally move out of the area this evening (5/5), setting the stage for sunny skies and warm temps for the remainder of this week and the coming weekend. Beyond next weekend, models are in good agreement for even more warmth and sunshine as robust ridging builds in across CA into the week of May 11th.

The stubborn, cut off low that has meandered off the CA coast since the weekend created some impressive storms and rainfall across Tahoe. Storms brought a mixed bag of lightning, hail, graupel and even snowfall on Monday evening. I even received a report of a backcountry traveler in Johnson Canyon having to shelter from nearby lightning on Sun afternoon (5/3)!

The heaviest and most consistent rain set up on Mon (5/4/) as a deformation zone formed right over Tahoe and the I-80 corridor, bringing a train of heavy showers and storms that transitioned to a wet snowfall late in the evening above 7000′. By Tues morning, the rain and 2″ of snow melted down to a total of 1.08″ of liquid at a 7650′ remote sensor above Incline Village.

This evening the low pressure responsible for the active weather is finally moving off to the east. In its wake, clearing skies and cool overnight lows are expected tonight with lows in the low 40s in the mountains, with mid-upper 40s around the lake and in Truckee. Dense morning fog is possible for valley floors tomorrow (5/6) as abundant low level moisture and calm, clear conditions provide an ideal setup for nocturnal radiation fog.

Tomorrow (5/6) high pressure begins to build over the area keeping skies clear and boosting afternoon temps. Highs will reach the mid- upper 50s above 7000′ with 60s in mountain communities. As the week wears on, temps will climb a few degrees each day going into the Mother’s Day weekend with highs into the 60s in the upper elevations with low – mid 70s possible around the lake.

Nighttime lows throughout the period look to cool efficiently each night in the dry airmass under mostly clear skies. Where snowpack does persist at the highest elevations (around and above 10k ft), a weak corn cycle may still hold on across open N facing slopes and bowls.

Looking out into next week, model ensembles show good agreement in the ridge of high pressure peaking early before gradually ebbing later in the week.

In response, temps will likely peak during the first half of the work week before cooling off towards the weekend with little chance of any precip during the period.

After nearly a month of warmish weeks and cool, unsettled weekends, the pattern looks to finally flip this coming week heralding spring’s arrival to the mountains.

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.