2021 Season Recap

October 2021 - Meghan Collins, Monica Arienzo, and Keith Jennings

Check out the results from the second season of Mountain Rain or Snow, then known as Tahoe Rain or Snow. Click here to see the original story map version of this report-back.

A person standing in the woods surrounded by snow, holding a cell phone and looking at the sky.

Did you know that snow can exist at temperatures above freezing? It depends, in part, on the humidity level. 

This means (as shown in the image below) that in areas with high humidity such as the west side of the Cascade Range in Washington, snow transitions to rain at 32-34⁰F. In areas with lower humidity, such as the east side of the Sierra Nevada, snow can exist at even warmer temperatures. 

A comparison of two illustrated mountain ranges with clouds above them. One mountain range is entitled "High humidity (such as the west side of the Cascade range)" and it shows snowflakes turning into raindrops at a high elevation which is marked 32 degrees Fahrenheit. The other illustrated mountain range is entitled "Low humidity (such as the rainshadow of the Sierra Nevada)" and it shows snowflakes turning into raindrops at a low elevation, which is marked 39.5 degrees Fahrenheit.

Credit: Lindsey Funseth/DRI. 

This brings us to the goal of Tahoe Rain or Snow

We are working to reduce inaccuracies in determining precipitation type by estimating the temperature of the rain-snow boundary, which is used in weather forecasts and hydrologic models. 

Normally, we use weather station observations of air temperature and precipitation to predict whether it's raining, snowing, or a mix. Then, computer weather and hydrologic models would indicate the type of precipitation. With help from Tahoe Rain or Snow weather spotters, we have been able to record evidence of snow consistently falling at above-freezing temperatures in the Sierra Nevada, as shown in the image below. 

A comparison of two illustrated mountain ranges with snow falling in both. One mountain range is entitled "Previously" and there is an illustrated weather station with the label "Rain at 37 degrees Fahrenheit" and a red X indicating that the prediction is wrong. The other mountain range is entitled "With your help" and there is an illustrated person holding a cell phone, with the label "Snow at 37 degrees Fahrenheit" and a green checkmark indicating that the report is a correct representation of the environment.

Credit: Linsey Funseth/DRI. 

Want to know more about the ways that citizen scientists are helping to solve this problem? Read Dr. Keith Jennings’ helpful article on the topic:  

A person standing in a snowy field holding an umbrella and a cell phone, and looking out at the field.

Credit: Gareth Blakemore

Weather spotters like you are helping to clarify the temperature threshold where snow transitions to rain. This can only be done with accurate observations of the weather on the ground! 

Between 2020 and 2021, citizen scientists submitted nearly 2500 reports of rain, snow, and mixed precipitation. The next figure shows the number of observations (x-axis) per type at different elevations (y-axis) in the Tahoe area. They are clustered around 4800 ft (Reno/Carson City) and 6200 ft (Tahoe/Truckee) because they are the larger population centers in the region. 

A chart showing the amount of observations of rain, snow, and mixed precipitation (separated) compared to elevation. The chart shows clusters of observations around 6200 feet and 5000 feet.

Number of observations of rain, snow, and mixed precipitation in 2020 and 2021. Credit: Dr. Keith Jennings. 

The interactive map below shows the observations submitted from February 8, 2021 to February 24, 2021. You can see how the rain-snow transition shifts through this series of storm events. Click play at the top of the map to view the simulation. 

Ground-based observations submitted by the Tahoe Rain or Snow community in 2021 showed that a much warmer temperature threshold of 39°F (3.9°C) for splitting precipitation into rain and snow may be more accurate for our mountain region.

How did we identify this threshold? Well, it is based on probability.  

Based on data collected by you, our citizen scientists, we calculated a 39°F/3.9°C air temperature threshold for partitioning rain and snow in the Tahoe Sierra and Truckee Meadows region. It's at this air temperature that snowfall probability reaches 50%. When it's warmer than 39°F/3.9°C the majority of precipitation falls in liquid form (i.e., rain or mixed).  

The threshold is important because many hydrologic, land surface, and climate models use 32°F/0°C or slightly warmer values to estimate the split between snow and rain. In very cold regions like the Arctic or high peaks of the Rocky Mountains this might not matter, but in the Sierra Nevada a lot of precipitation falls right near freezing.  

In fact, you submitted 999 reports at air temperatures between 32°F/0°C and 39°F/3.9°C, 569 of which were snow. This means if we were using a simple 32°F/0°C threshold to split rain and snow in the study, we would have incorrectly identified snow as rain 57% of the time in that temperature range.  

This season, we will be growing this program into new regions beyond the Sierra Nevada, including the Cascade Range and the Rocky Mountains! Keep an eye out for alerts about “Mountain Rain or Snow” and our forthcoming web app. 

Thankfully, we had citizen scientists on the ground reporting rain and snow, so we can work to improve modeled air temperature thresholds and remote sensing products of precipitation phase. 

This work wouldn’t be possible without hundreds of volunteers making observations from different areas of the storm. This way of doing community science is unique. We appreciate you very much! 

This map shows all of the data points that were submitted by Tahoe Rain or Snow observers during the 2020-2021 winter season. Click on data points to explore the data.

If you would like to access the complete dataset and analysis, please  click here to continue to GitHub. 

Funding provided by Nevada NASA EPSCoR Grant 20-23, 19-40 to Monica Arienzo, Meghan Collins, and Keith Jennings. 

Mountain Rain or Snow logo with graphics of mountains, a snow crystal, and a rain drop.