Weather West: Following Whiplash Transition From Record Warmth to Heavy Mountain Snowfall, yet Another Swing Back Toward Warmth

The below article is reposted with permission by the Weather West Blog.

A remarkable snowy interlude during what has thus far been a record warm winter: Disruptive, and even deadly, in the Sierra Nevada

Last week’s snowfall in the Sierra Nevada–and also at much lower elevations, for the first time this season, into even the lower foothills and Coast Ranges–marked a dramatic shift from a nearly 40-day stretch of dry and unusually warm conditions. Moreover: this sudden flip occurred during what is still the warmest winter on record for most the Western U.S., including many of California’s mountain areas. It brought widespread travel disruptions, especially for those caught unprepared, with all major mountain highways in the Sierra Nevada closing for extended periods.

But the most serious and unexpectedly tragic consequence of this rapid weather pattern shift was the avalanche near Castle Peak (not too far from Donner Summit, and just west of Truckee) that claimed the lives of nine backcountry skiers–making not only the deadliest avalanche in contemporary California history, but also the single deadliest avalanche in the United States since 1981.

This event was facilitated by a combination of well-understood, and well-predicted, antecedent risk factors: the prevalence of a weak and icy snow layer due to warm temperatures and freeze/thaw cycles in the weeks leading up to the event, and the subsequent arrival of an intense, blizzard-like snowstorm that dropped as much as 4-6 feet of snow on the nearby peaks over just a 2-3 day period. The combination of a weak base layer and very intense “snow loading” during the storm itself led to an exceptionally high risk of snow slab failures, and one of these ultimately occurred in the path of travel of the ski group and their guides.

Much has already been said in the wake of the tragedy; I have no desire to speculate beyond what has been publicly reported here. But I will reiterate this important fact: the remarkable and rapid shift to very heavy Sierra Nevada snowfall, and the resultant very high backcountry avalanche risk, was very well predicted and had been communicated in advance by both the National Weather Service and the Sierra Avalanche Center (which had issued early Avalanche Watches and later an Avalanche Warning as the system approached). This recently published piece in SF Gate does a good job breaking down the timeline and details, and also framing some of the bigger questions that will inevitably be pondered in the aftermath.

Snowfall, while not record-breaking as some have claimed, did make into some top-10 lists in a few orographically-favored Sierra western slope locations. The Central Sierra Snow Lab logged its 10th snowiest 24 hour period on record during this event, as well as its 9th snowiest 72-hour period–but the most notable statistic is the 5-day accumulation of 111 inches, which marks the 3rd snowiest such period on record. The CSSL benefited not only from excellent orographic forcing (i.e., lifting of air by topography) during this event but also from near-ideal alignment of the recurrent heavy precipitation along the Interstate 80 corridor as well as convective enhancement (there was even some Sierra thundersnow at times locally–not surprising given that the same storm brought severe thunderstorms with damaging wind gusts to portions of SoCal).

As one might expect, this heavy Sierra snowfall bolstered central and southern Sierra snowpack quite substantially; many high elevation observing sites are now at near to above-average snow water equivalent (SWE) levels for the time of year. Many lower elevation sites, however, as well as those in the Northern Sierra and Shasta Mountains, remain below average even following last week’s accumulations. Overall, though, California’s water situation looks pretty solid at this point given the very wet start to the season in central and southern CA followed by this mid-winter top-up of critical central and southern Sierra snow water.

It’s a very different story, though, nearly everywhere else in the West. Last week’s snow deluge centered pretty heavily on California; while snow also fell in the Cascades and Rockies to the east, amounts were not nearly so impressive. In fact, while overall snowpack conditions did improve slightly across most of the West, the only place where snowpack came even close to average was in California and across a portion of the northern Rockies.

Extremely low snow conditions persist in the Pacific Northwest, Great Basin, and central/southern Rockies–where many locations remain, even after last week’s snowfall, near or at record low SWE. The Colorado River Basin remains in trouble; Upper Basin SWE as of yesterday (again, after all the snow) are still at period-of-record lows (since the early 1980s in the NRCS dataset). And dry/warm conditions are set to return to much of the West in the coming days…as discussed below.

An exceptionally warm and moist atmospheric river will affect NorCal and the PacNW this week–but will bring largely (and thankfully) unexceptional impacts despite some flood risk

There has been a bit of social media buzz surrounding a very warm, and very moist, atmospheric river that will take aim at California and the broader West Coast over the next few days. And, in a literal sense, this is true–a remarkably warm and moist plume of air, with origins in the subtropics, is indeed aimed directly at Northern California. And it will produce widespread rain atop recent heavy snowpack in the mountains, leading to a rain-on-snow event across essentially all of Northern California’s mountainous areas.

All of that might sound pretty alarming, at face value. And there are some plots, including the “scary-looking” time-integrated vapor transport plot above, which might understandably conjure flood fears. Yet while there will be some flood risk across portions of NorCal with this event, it is likely to be somewhat localized and rather modest in magnitude. In fact, right now, it generally appears that most of the upcoming flood risk will be on the “nuisance to minor” end of the scale versus something worse. It’s possible a couple of isolated spots could see more moderate flooding, but it’s pretty clear at this point that widespread or severe flooding is unlikely during this event. So what’s really going on here?

Well, as indicated above in the plot from NOAA’s Situational Awareness tool, the absolute amount of water vapor in the atmosphere will indeed be at near-record levels for the time of year across California and adjacent coastal waters. And it is also true that the airmass will be exceptionally warm–so much so that snow levels will be above 10,000 feet nearly everywhere, and at times over 12,000 feet. So nearly all precipitation, event on many mountain peaks and passes, will fall as rain.