New System Built to Better Predict Runoff During Historic Storm Season

The amount of water present in these systems is similar to the average flow of water at the mouth of the Mississippi River

As the region catches up with shoveling massive amounts of snow, the weather forecast hints toward change. Back-to-back atmospheric river systems are moving inland and forecasters are calling for high snow levels and impressive amounts of rain. This is what meteorologists call a rain-on-snow event – and it is not a rare phenomenon.

In 2017, a similar, albeit larger, series of cold storms followed by warmer systems caused flooding across the region. Back in 1997, a white Christmas turned into some of the worst floods Reno and the surrounding regions have seen in decades. This Twitter post provides a sense of how the snow levels are anticipated to change over the next several days.

In anticipation of increased runoff, the City of Reno has set up sandbag stations ahead of the storm and urges people to prepare for the worst. Additionally, they have issued a memo highlighting the anticipated action to take place in the North Valleys, around Swan and Silver Lakes. Both areas have seen flooding before, including the serious flooding of Swan Lake back in 2017 that eventually led to the city paying out $1.1 million to affected homeowners. But forecasts are subject to change as more data comes in. It is best to check with the National Weather Service every six to eight hours for weather updates.

“Our street is often not plowed or a low priority and our house sits at the bottom of a loop- getting out of the neighborhood requires going uphill, regardless of which direction we go,” said Emily Walsh, a resident of south Reno. She has had to contend with steady snowfall and daily shoveling. “Despite the travel complications and manual labor required to maintain access to the street, we’ve loved the snow.”

Walsh added that she is grateful for the ability to work from home. Despite shoveling multiple times a day and treacherous driving conditions, she said these recent storms reminded her of the storms she experienced as a child, like the cold winter of 2005-06.

“It is not a major atmospheric River, this is not a 1997 type of event. This really isn’t even a 2017-type event,” explained Anne Heggli, a graduate research assistant with the Desert Research Institute in Reno, Nev. Rain-on-snow events are variable and nuanced; the rain does not just melt the snowpack as is commonly thought. At the same time, the snowpack does not act like a sponge, absorbing the rain. The interaction between snow and rain is subtle and a challenge to understand.

With the current snowpack being so cold and comprised of light, fluffy snow, there is some room for the water. Known as preferential flow and similar to the way icicles form, rainwater will trickle down through the snowpack, finding the path of least resistance. Some will take up the pore space within the snow while some water will be released as runoff.

“We don’t know exactly what the throttle is or how much it will take on and how much it will release and at what point it will be released in these deep snow packs,” said Heggli. She has been studying weather and hydrology, particularly atmospheric rivers using applied science to develop a real-time runoff forecast system. Her research with DRI could lead to a forecasting system that would advise policymakers in a by-the-hour time frame about when and where water gets released by a snowpack during a rain-on-snow event.

Atmospheric rivers (AR) are long, narrow regions in the atmosphere that transport water vapor out of the tropics. They move similarly to normal weather patterns, but carry a large amount of moisture. The National Weather Service says the amount of water present in these systems is similar to the average flow of water at the mouth of the Mississippi River. When this system hits land, the moisture precipitates in the form of rain or snow.

What this looks like on the ground is wet storm after wet storm after wet storm. There have been a couple of ARs moving through the region this winter, resulting in one of the largest snowpacks in several years.

In a way, scientists feel they are flying blind when it comes to monitoring and forecasting ahead of an atmospheric river. This is something Heggli caught onto during her master’s research and led her to build a runoff forecast system. “This snowpack runoff decision support tool right now works over the SNOTEL, or the Snow Telemetry Network, which is part of the Natural Resource Conservation Service as a part of the US Department of Agriculture,” said Heggli.

This data is available on the SNOTEL portal and is just one point of information being utilized by Heggli for research. Courtesy USDA

The SNOTEL system collects all sorts of data used by scientists to determine the water content of a snowpack, including soil temperature, ambient weather and temperature, and much more. Typically this data is looked at on a daily basis, but Heggli has zeroed into the data on an hourly basis and compares that to what is happening in the sky.

“The soil moisture sensors have a very prominent signal response when the snowpack releases water midwinter, especially from these rain-on-snow events,” said Heggli. This allows her to use the soil moisture data and identify signals or trends in the data to determine when the snowpack is actively releasing water. When combined with other SNOTEL stations in a given river basin, she can better understand if there will be an increase in water levels downstream. “We’re leveraging observed data to help us learn about the patterns that are present and then communicate that in real-time to the decision makers so they can have improved situational awareness.”

The Truckee River flowing through Wingfield Park in downtown Reno.

By consolidating the information of data that is already being collected, this tool can be utilized by water managers, local officials, and media outlets. “Days ahead of the event the water managers and emergency managers can start looking at the probabilities of the worst case and maybe the least impactful scenarios and what they should really be preparing for,” she explained.

Heggli hopes this powerful information can find a home in forecasts, ultimately helping the public understand the nuances of incoming storms and how to adequately prepare. She explained this tool will help water managers forecast runoff timings and amounts, allowing for more informed management of water flow. Proving crucial, this information could provide instrumental knowledge in navigating the future where snowpacks are expected to be far smaller due to anthropogenic climate change. With these tools in hand, reservoirs can be better managed for dry spells and flood risk can be reduced through refined mitigation.

A current look at the snow water equivalent for a station near the Mt. Rose Ski Area. Courtesy USDAThe same data for a station near Truckee, Calif. indicates the probability for water runoff for the year as the snowpack melts throughout the spring and summer. Courtesy USDA

As of this morning, the Truckee River basin’s water content is at 183% of its usual amount at this time. The Carson River basin’s water content is at 235%. The area of most concern for Heggli and others is the low-elevation snowpack that has accumulated over the past several weeks. This snow is shallower and has less insulating space from the heat rising from the earth and input from the atmosphere. Essentially, lower-elevation snowpacks warm and melt more quickly.

“We have seen quite a bit of relief from severe drought,” said Heggli. For residents like Walsh, the snow and rain is a welcome sight despite the frustrations, something she understands is adding to the water resiliency of the region.

Though Heggli said we are still experiencing drought conditions in our region, this winter has helped relieve some of the stress on natural systems. But, she emphasized that we are not out of the weeds just yet. “What we really need is a couple more years like this to really take us out of the drought.”

This story was originally published with the Sierra Nevada Ally.

Copyright Richard Bednarski
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