If you’ve been a weather nerd and checking the National Weather Service’s hourly precipitation tallies, you might have noticed something appears to be off with the measurements of Tuesday’s snowstorm.

Precipitation measurements are in the tenths and hundreds of an inch, though widespread measurements of the blanket of snow falling across the state are in the several inches.

Baton Rouge, for instance, had a reported 6 inches of snow by midday Tuesday, though hourly measurements from Baton Rouge Metropolitan Airport only measured 0.27 inches of precipitation.

What’s the deal?

Vincent “Vinny” Brown, an LSU climatologist, said those National Weather Service precipitation measurements are in liquid amounts, not snow accumulation totals.

The ice crystals that make up snow take up more volume than liquid water, but exactly what that ratio is in any given snowstorm can involve a number of factors.

One old rule of thumb is 10-to-1, or 10 inches of snow for every 1 inch of liquid, but forecasters have developed other ways to predict snow accumulations.

The Kuchera snow ratio, for example, is a widely used statistical method that relies on the warmest temperatures in the air column to try to estimate snowfall. 

But it can be a difficult task. According to the National Weather Service, factors such as the amount of ice in a snow cloud, the types of snowflakes being formed, how warm it is between the snow cloud and the surface of the Earth, and windiness can all increase or decrease the amount of snow hitting the ground.

Deep cold can push the snow-to-water ratio to as much as 20-to-1, the Weather Service says. Some academic papers published by the American Meteorological Society say that ratio can range from 3-to-1 to 100-to-1.

Phil Grigsby, a forecaster with the National Weather Service in Slidell, said published snowfall tallies are measurements of actual accumulations on the ground or other surfaces that the service’s network of observers are reporting.

“Those are actual people going out and measuring them … on patio tables and decks and things like that,” he said. 

He added that this difference between snow volume and water volume means the eventual melting of south Louisiana’s coating of snow is unlikely to have much of an impact on local waterways.

Grigsby added that the snow falling across south Louisiana is on the dry side and that means the snow-to-water ratio is even wider than the typical rule of thumb.

That means even less water will ultimately run off once the snow melts.

“This won’t cause any flooding issues at all,” Grigsby said.