Is it going to rain or snow?
Answering that age-old question is vital for weather forecasters and climatologists, hydrologists, farmers and ranchers, municipal and agricultural water supply managers, transportation officials, tourism and recreation professionals, and anyone wondering whether to don ski boots or muck boots.
Now, a new citizen-science initiative aims to increase understanding of local rain-snow transitions, with the goal of better predicting them. This winter, the program seeks volunteer observers from Missoula and across western Montana, where winter is predicted to be unusually snowy. Unless it's rainy.
The program, Mountain Rain or Snow, came about primarily because satellites, a cornerstone of modern weather forecasting and monitoring, can struggle to answer the basic question of whether it's raining or snowing. When forecasters detect a possible rain-snow transition in precipitation, the program will send an alert text encouraging volunteers to use a web app (no download required) to report whether they observe rain, snow or a mix at their location.
According to the United Nations, an estimated 7,500 satellites crowd around planet earth just beyond its atmosphere but still barely within the grasp of earth's gravity. The satellites provide for internet access and telecommunications, high-resolution surveillance and imagery, weapons systems, geographic data collection, and weather forecasting. Despite all they can do, the satellites that monitor precipitation are not particularly good at figuring out when rainfall transitions to snowfall, or snow to rain, when air temperatures are around freezing.
And temperatures are often around freezing in western Montana. According to National Weather Service data from 1991 — 2020, average daily low temperatures around Missoula are in the 30s or colder from October through May — eight months out of the year — and Missoula spends four months of the year with daily highs near or slightly above freezing. Missoula's average daily high in November is 39.9 degrees. The average daily high is 30.9 degrees in December, 31.6 degrees in January, and 37 degrees in February.
"A threshold of 0 degrees Celsius (32 degrees Fahrenheit) for rain-snow transition isn't necessarily accurate all of the time ... but the nature of that transition varies by region," said Meghan Collins, an associate research scientist in science communication at the Desert Research Institute and the engagement leader for Mountain Rain or Snow. "Snow can fall above freezing. It's not a change in the laws of physics — it's often a factor of weather, and we see it a lot in our mountain regions."
The regionally variable nature of rain-snow transition means that rain may shift to snow at a different temperature depending on location, Collins explained during a Pacific Northwest Drought Early Warning System online seminar last week.
Data from the program last winter show that in the mountains of Vermont and New Hampshire, if there's precipitation when the air temperature is exactly freezing — 32 degrees Fahrenheit — there are equal chances of the precipitation being rain or snow. Below freezing, snow is more likely. Above freezing, rain is more likely. But in the Sierra, precipitation that falls when air is 32 degrees comes down as snow about 75% of the time, and snow is more likely than rain even when it's slightly warmer than freezing. In the southern Rockies, snow is more likely than rain up to about 40 degrees, and snow is almost certain when precipitation occurs at 32 degrees or colder.
But satellites are often wrong about when that transition takes places, she said. They errantly see rain as snow, or snow as rain, and they're even wrong in different ways in different places. Satellites may more often see rain as snow at 31 degrees in one region, while more often mistaking snow as rain at 35 degrees in another place. In the Northeast U.S., for example, they often missed freezing rain.
"Right now, they don't do as good of a job as ground-based observers at partitioning rain-snow mix," Collins said in a video call Thursday. "This is a greater challenge in arid, high-elevation regions," where low humidity and other traits of dry mountain climates "can make the snow melt more slowly as it falls."
Those errors have real-world implications.
"Intuitively we know that an inch of snow is different than an inch of rain," Collins said. "They might be forecasting rain at 35, 38 degrees Fahrenheit, but actually we've got snow flurries and there's fender-benders everywhere."
It's not just weather forecasters who are interested. Collins said she's heard "a lot of interest from avalanche communities. The elevation at which that rain-snow transition happens can have big implications."
That's where volunteer "citizen scientists" come in: "The best way to get these observations is real people," Collins said on Monday.
"These patterns aren't consistent across regions," she said Thursday. "To improve our estimates across different areas, we need locally place-based data." And the more data the group collects, the better resolution estimates they can develop over time.
The data and the group's estimates — trying to figure out when, where, at what elevation or in what conditions rain will shift to snow in a particular region — will be available to regional avalanche forecast centers, Collins said. She said that she's already been in communication with the Flathead Avalanche Center and hopes to also work with the West-Central Montana Avalanche Center, based in Missoula.
"We work in numerous mountain ranges from coast to coast, and this year we're interested in growing our observer base in Montana," she said. "That's part of what we'd like to give back to avalanche centers: If they request the data, they can see it."
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