Why It's So Hard to Predict Snowfall Accurately

It's one thing when there's talk of flurries or a light dusting. But when your local meteorologist starts mentioning serious snowfall or the snowflake icon is prominent on your weather app, it can cause havoc.

Before you rush out for bread and milk, here's a quick explainer on what to expect. The predicted snow inches you hear often are much higher or lower than what you actually get. Here's why.

Snow forecasts are more accurate than ever, but they're still challenging for meteorologists, says the National Snow and Ice Data Center (NSIDC).

There are so many conditions to take into consideration: If it will snow, how much it will snow and exactly where it will snow. All those factors are, in turn, impacted by other issues.

A small amount can make a big difference

If there's just a little more or less precipitation than predicted, it can have a big effect on the amount of snowfall.

"A small difference in the precipitation amount will have a big difference in the accumulation in inches of snow," meteorologist Jeff Haby explains. "For example, a 1/10th of an inch of liquid equivalent can produce 1 inch of snow while 4/10ths of an inch of liquid equivalent can produce 4 inches of snow."

Snowfall can vary in close distances

Snow doesn't fall evenly everywhere. You might remember winter storms where one neighborhood was blanketed while another neighborhood just a few miles away barely got a dusting.

During intense snows, sometimes the heaviest snowfalls will happen in very narrow bands, according to NSIDC. And it will occur on such a small scale that forecast tools won't see it.

These bands may be as narrow as 5 to 10 miles wide, reports The Weather Channel. They can produce snowfall rates of more than 1 inch per hour, while an area just a few miles away gets much less, or even no snow.

"At the local scale, variations in snow depth are caused primarily by wind during and after the storm, and by melting after the storm," according to NSIDC. "At the larger scale, say across an entire state, it also depends on the storm track. Places in the middle of the storm track may receive significant snowfall, while locations along the edges of the storm may receive much less."

Temperature matters

How cold it is during a snowfall also affects how much snow — and even the type of snow — that ends up on the ground.

If it's relatively warm as the snow falls, it could melt by the time it hits the ground, turning to slush on the roads and sidewalks and never accumulating. Then, when temperatures drop again overnight, that slush and wetness will turn to ice. If it's cold enough, the snow will keep piling up as it falls.

Haby says the temperature also impacts whether snow is fluffy or wet. And as the video above explains, temperature and other conditions can even affect the shape of the snowflakes, which also affects what it will do on the ground.

"A good typical value is a 10:1 ratio which means 10 inches of snow will occur from each inch of liquid equivalent. Depending on the temperature profile, the snow can be a fluffy 20:1 ratio or a wet snow with a 5:1 ratio. Thus, it is important to forecast the temperature profile in order to determine how fluffy or dense the snow will be."

And for meteorologists, getting the temperature wrong by a hair can make a huge impact in a snowfall forecast.

"The extremely small temperature differences that define the boundary line between rain and snow make large differences in snow forecasts," writes NSIDC. "This is part of the fun and frustration that makes snow forecasting so interesting."

Forecasts change

Meteorologists can't predict snowfall with much accuracy more than a few days ahead of time. So when you see or hear a 10-day forecast, take it with a huge grain of salt.

"Even when we're close enough to begin issuing specific snowfall forecasts, there can be considerable remaining question marks," says the Weather Channel's senior meteorologist Jonathan Erdman.

Normally snow falls to the north and northwest of the track of a low pressure center, Erdman says. If the track changes, so does the chance of snow.

Early forecasts may be based on a system more than 1,000 miles away. As it gets closer, it can change along with the snow it may or may not bring with it.

Add to that changes in moisture and temperature and winds and other elements that can impact wintry precipitation, as well as the limits of technology used to determine forecasts.

"The atmosphere is very random, and there are lots of things that interact — water, the structure of the atmosphere, friction from the land," Eli Jacks, chief of fire and public weather services at the National Weather Service, told Live Science. "To me, it's quite amazing that we can capture it at all."