Atmospheric Rivers 101 with Dr. Thomas Painter

INTERVIEW: Donny O’Neill | IMAGES: Peter Morning/Mammoth Mountain

Atmospheric rivers bearing down on California, dropping gargantuan snow totals on the Sierra Nevada are not necessarily new events for any local residents.  California has often been caught between this boom or bust cycle of dry weather with sudden extreme weather events.

Protect Our Winters’ Science Alliance member, Dr. Thomas H. Painter, is a research scientist of hydrology who serves as a principal scientist at the Jet Propulsion Laboratory (JPL)/California Institute of Technology. He’s also the principal investigator of the JPL Airborne Snow Observatory, a system that gives the first-ever comprehensive, near real-time maps of snow water equivalent and snow reflectivity in mountain basins. Saying Dr. Painter loves snow is simply an understatement.

Dr. Painter also lives near Mammoth Mountain and has experienced these atmospheric events firsthand. POW caught up with him to discuss what exactly an atmospheric river is, why it happens frequently in California and how future, more extreme weather events like this could impact local mountain communities in the Sierra Nevada. Before jumping into our interview, Dr. Painter panned his computer camera to show his deck, covered in snow, stating that he’s already had to clear off the deck three times since the beginning of the storm. What an indication of the intensity of these storms.

The following is a transcript from POW’s interview with Dr. Thomas Painter in 2021

Protect Our Winters (POW):

Can you explain exactly what an atmospheric river event is, what’s occurring over California right now?

Dr. Thomas Painter:

An atmospheric river is just what it sounds like; it’s a convergence out over the Pacific Ocean of water vapor that’s being carried toward the continent. Instead of a really broad distribution of that water vapor it gets narrow and barrels down on the continent. When that river of water vapor comes blasting into the mountains and is lifted, then you get very, very concentrated precipitation in a much narrower band. And, that’s why it creates these massive dumps, and it’s also what creates the massive flood potential when the precipitation falls as rain.

POW:

Can you shed some light on what this kind of extreme atmospheric river event means in terms of precipitation amounts?

Dr. Thomas Painter:

Well, over half of California’s annual precipitation will fall, depending on the year. This is especially the case in a year like this, where we’ve been limping along in terms of moisture. Going back to the beginning of December we had a [smaller] atmospheric river in there, then just a nominal northwest flow event right after Christmas and then nothing. And then, wham, [a huge spike] in the last two days. And so it makes a huge difference in these years. 

Dr. Painter shows POW a snow study plot graph, highlighting huge jumps that bear resemblance to an ascending staircase.

Notice the large steps in this graphic that signal atmospheric river events.

Dr. Thomas Painter:

What you see are these big steps, and when you see big steps, you’re seeing atmospheric rivers. So if you went in and you took away each of those steps there’s damn little snow. They represent pulses to the snowpack, which itself is a reservoir, it’s a freshwater reservoir. All of a sudden, you just dump tons of water into a reservoir. And for water managers [the atmospheric rivers] are a godsend up high. But if they’re wet down low, if it rains down low, then they can be huge trouble. Because of that pulse of water, and in a changing climate, that’s going to get harder and harder because the temperature at which those make landfall and approach the mountains is increasing. And therefore, the rain-snow line is going to increase in elevation. And so more and more of that precipitation is going to fall as rain. And that’s going to have a monstrous effect then on reservoir operations, flood mitigation and it will trash ski area snowpacks.

POW:

Not all of these atmospheric rivers come in as cold as the one currently impacting California, many times they’re a lot warmer with higher snow levels. What generally happens, specifically to ski areas, with these warmer events?

Dr. Thomas Painter:

This is a bit of a cold storm, yeah. When you go out into this meter of snow out here on my deck, which my sons are going to do here pretty soon to shovel and then jump off the deck, you can pretty much just wade through it. It’s nice blower powder. But if you had this amount of precipitation falling, and had an 8,000-foot rain-snow line, then a place like Tahoe would be completely trashed. There’s so much to look at and think about with our better understanding of atmospheric rivers in the context of a changing climate. Because these are such big pulses, then the changes in them carry so much impact and so much potential for destruction.

POW:

Is there any kind of projection regarding the impact of future atmospheric river events in the context of a changing climate?

Dr. Thomas Painter:

The models so far seem to suggest that we’re not going to be getting any more atmospheric rivers; the frequency doesn’t seem to be headed towards increasing but the extreme ones appear to be heading to be more extreme. The hydrologic cycle is headed to more and more extremes. And so there will be drought intensification and then precipitation intensification. In terms of the atmospheric rivers, increasingly those are going to be bigger and bigger.

POW:

These kinds of atmospheric events that pop up are going to continue and be more extreme, and as the climate continues to get warmer, they’re going to come in with much more rain than snow. What does that mean for a winter snowpack and ski industry in California?

Dr. Thomas Painter:

We’ve experienced this at Mammoth, Tahoe’s experienced this and even less so in the Rockies. There’s the wholesale thrashing of the snowpack that we’re going to experience out here in California. While these events create a huge snowpack in the present and past climate, heading into the future, they can still produce a bigger snowpack. But the rain-snow line is going to increase in elevation. And so that can completely thrash the snowpack and destroy some of the mid-mountain and lower snowpack on a lot of these ski areas. As that keeps moving higher and higher, you can see rain at 11,000 feet. And if it’s in the course of one of these atmospheric rivers, which we used to call pineapple expresses, then it just ravages the ski area and backcountry snowpack. So from that recreation standpoint, there’s potential for more snowfall over the next couple of decades in the higher elevations, but what also comes with it is more rain. 

From a water resource standpoint, the way that civilization out here in the West has built up is by relying on the mountain snowpack as a reservoir. Humans captured all that water, and then it comes out as the snow melts but generally, it comes out in a predictable way. You start getting a release in April, and then it’s picking up speed in May and then peaks in June. Humans started building their own reservoirs to capture a bunch of that water, so they didn’t lose it all in one year and could hold on to it. 

Back in the ‘70s, California had half the population that it has today and there weren’t federal requirements on flows in the rivers either. The demand for that water resource wasn’t anywhere near what it is now. And we’re going to continue to drive that demand. We’re going to need to get to every drop, we’re going to need to get to every snowflake. With these reservoirs, if more snow starts falling as rain, it’s going into the reservoirs immediately. This reservoir of the mountain snowpack has to be increasingly put into this reservoir. Well, we’ve already built the reservoirs to maximum capacity. And so in essence, this human-built reservoir system is going to have to start accommodating this earlier runoff, and it can’t, and therefore the water supply is going to shrink, plain and simple. Coupled with climate change, the increased evaporation of surface water and the reduced flow, it’s not good, it’s not good for civilization in places like this. 

POW:

It seems like California has been the ultimate boom or bust. Either it’s stuck in catastrophic drought or these huge atmospheric rivers show up and drop massive amounts of precipitation that cripples the infrastructure. What causes these drastic swings?

Dr. Thomas Painter:

The seasonal forecasts are problematic. If you look at data about April through July snowpack runoff, half of the time, one in two years, the forecast errors are 20 percent or more off. But the way that water management is going and the [increased] demand on the water resources, they need to be more along the lines of about 5 percent. And, one in five years, those forecasts are 40 percent or more off. 

And if you look at the natural flow of the river into the watershed, the Tuolumne River, for example, the average almost never happens. The data proves it. There’s a standard deviation (the amount of variance in a set of values) in 1950 of 765,000; the deviation is 1.1 million here in 2017. It will continue because of the drastic variability. And so what’s happening is that it is in fact getting more and more extreme.

And just as the modeling forecasted the hydrologic cycle is going to get more intense. And California is intensely dry and then it’s intensely wet. We had an intense drought from 2013 to 2015, and then we got 2016, which was a roughly average year and then, boom, 2017 was above average, and then 2018 was a below-average year, and then, boom, 2019 was a monster year again. And it’s just this whipsaw for water management and snowpack.

And that is going to be increasingly the case in a changing climate—unless POW is able to get legislators to get our shit together. No pressure.

POW:

How do these unpredictable swings impact these outdoor-centric communities, like Mammoth, where you live?

Dr. Thomas Painter:

The impacts on recreation and on mountain communities are huge. I’m in Mammoth and I’m on the school board. We had an intense drought and we watched as revenues decreased in town, then property values don’t go up, as people aren’t selling houses and therefore the property taxes aren’t there. And that contribution to the school board isn’t there. And, in the school district, we’re having to lay people off, because the cost of benefits continues to go up. It just reaches everywhere in these communities.

If we can have a good handle on the physics of it, the earth science, the societal implications and the financial implications of this, we can actually do something about it. We can actually stem the tide of heading towards where the amplitude keeps getting bigger and bigger and bigger. That’s only going to thrash people more and more. We’re in desperate need of addressing this.

POW:

In communities like Tahoe or around Mammoth, backcountry skiers and riders always have a pulse on the snowpack and are always analyzing avalanche conditions. Do these big swings between drought and large precipitation events make snow study and avalanche analysis even more unpredictable?

Dr. Thomas Painter:

Yes, it’s a huge brain reset. The Sierra usually doesn’t get much depth hoar faceting and the snowpack heals pretty quickly. The very rough rule of thumb is 48 hours after a storm, it’s probably going to be safe. At any rate with a year like this, where it’s been drought riddled, and at times it’s been quite cold, you can have a lot of faceting and a pretty unstable snow setup. Then, you get a monster snowpack on top of that and it’s nerve-racking. In terms of recreation and backcountry skiing, because you’re getting such a huge dump, you have to look at it carefully. And you have to not go out there because you’ve got such a discrepancy from the snowpack that you had leading up to it.

POW:

What about the actual ski resorts? From afar, people see 100 inches of snow and think the skiing must be great, but it can actually be a bit nightmarish, right?

Dr. Thomas Painter:

Yeah, right now there’s no uphill travel to the ski area. And people are stuck down in Bishop, people are stuck on side streets. It does tend to be a pulse of excitement, a pulse of snow and a pulse of travel fiasco. But then, Lordy, it’s going to be awesome on Friday. 

But, the other thing is when I look out during these big atmospheric rivers and I look at every snowflake, every snowflake is extending the spring skiing season. And that makes me really happy. The other thing is every snowflake keeps us on an earth with snow. I study snow, but I study snow because I love snow. I’m obsessed with it. And if I was a tattoo person, I’d have a snowflake tattoo right here on my forehead. For me, every time it snows, and in particular, with these kinds of dumps, it’s a breath of fresh air of still being able to have snow on earth this way and to ski and to have that chance to still save this kind of snow.