Typically, I write the OpenSnow forecast for Alaska.

Today, I want to share a couple of insights on what makes snow level forecasts so challenging and what you can do to help improve them through a crowdsourced science project called Mountain Rain or Snow.

Deciding whether precipitation falls as rain or snow should seem simple. We all know that snow melts above 32°F (0°C), but anyone who’s been watching the thermometer while driving up to the mountains on a warm-storm day knows that snow often falls above freezing.

That’s because snowflakes don’t instantly melt as they fall. The temperature and humidity of the air column matter, as does the time it takes for snow to melt on the way down. Evaporative cooling can also keep snow crystals intact, even when air temperatures are in the mid-30s. Snowmaking crews know this well, as they can still make snow in above-freezing temperatures if the wet-bulb temperature is low enough. In other words, using 32°F as a hard cutoff doesn’t match what happens in the mountains.

This problem gets even harder in mountainous regions. The precipitation type can change rapidly in a short distance or a quick period, as rugged terrain produces sharp changes in the vertical column of temperature and humidity. At the onset of precipitation, valleys often host surprisingly chilly temperatures with cold, dense air having settled in low-lying areas. On the other hand, mountain tops are exposed to turbulent winds that can produce rapid fluctuations in the precipitation type as heavier precipitation temporarily produces cooling. 

Even more, mountain weather stations are sparse and often lack the instruments that directly measure precipitation type. While weather station data can tell us whether precipitation is falling and the current temperature, between about 32°F and 40°F, it’s often unclear whether it’s rain, snow, or a mix. Thankfully, what is difficult to observe from our monitoring networks is easy for us as humans. 

How can you help? Sign up to submit real-time precipitation observations.

Our eyes function as excellent precipitation type identifiers, and the Mountain Rain or Snow project offers the platform to participate. With the support of NASA funding, Mountain Rain or Snow allows people to report whether they are observing rain, snow, or mixed precipitation from their phones. The project aims to increase the coverage of precipitation type observations in mountainous terrain across the United States. Since the launch in 2020, there have been over 100,000 observations across the United States.

Where do your reports go?

These observations can be viewed in two places. Firstly, they appear on a real-time map made available to operational partners, giving decision makers, like National Weather Service meteorologists and Sierra Avalanche Center forecasters, a clearer picture of what’s happening on the ground during a storm. A second map is accessible to the general public at RainorSnowMaps.com with a slight delay in reporting to allow for quality control.

Feeding these crowd-sourced observations back into forecast algorithms helps improve precipitation type forecasts, which means better recommendations to find pow days, safer trips to the mountains, and more accurate estimates of the water stored in snowpack for the dry months. If you’re out recreating in the mountains, your precipitation-type observations will help clarify what’s happening where most of the snowpack exists.

The fraction of precipitation type observations with height in the Sierra Nevada. The greatest number of observers and skiers are located in Tahoe, which is why we see so many observations around 6,000ft. Observations from locations above and below populated ski towns help fill in the gaps.

How might your reports affect your ski day?

While I can’t promise that your precipitation reports will instantly lower the snow level, your reports are useful for forecasters to know where it is raining and snowing, and also, in the long term, will help validate forecast models to help improve snow level forecasts.

More real-time observations add critical context during marginal storms, exactly when forecasts struggle the most. For example, one of our MRoS team members, Dr. Anne Heggli, analyzed how the forecasted snow levels varied across terrain using Mountain Rain or Snow observations and how those forecasts related to observed avalanches in the Tahoe region. The results showed that a greater number of observations adds valuable context to snow level forecasts that avalanche and weather forecasters rely on.

Panel (a) shows the snow level forecast data and Mountain Rain or Snow observations over time for this storm from mid-February, and panel (b) shows a map of where those Mountain Rain or Snow observations were located. 

Sign up to be a Mountain Rain or Snow observer!

With only a couple of clicks on your smartphone, send a quick report anytime you see rain, snow, or a wintry mix. The most useful reports come when the storm starts, when the precipitation type changes, from the passenger seat up when heading over mountain passes, or every 30 minutes if conditions are holding steady. Reports from less-populated mountain areas are especially valuable. Every report helps improve mountain forecasts. If you’re already out in the storm, putting your eyes on the sky is one of the easiest ways to contribute.

Convinced?

From your phone, follow the link here to sign up or scan this QR code to text to sign up and access the web-based app. Tap Here.