Meteorologists like to categorize and rank things, and snowstorms are no exception. That’s where the Regional Snowfall Index comes in.

NOAA’s National Centers for Environmental Information started officially using the Northeast Snowfall Impact Scale (NESIS) in 2005. NESIS ranks snowstorms not just on weather, but on societal impacts.

It uses population and historical snowfall information from the eastern two-thirds of the U.S. and applies that to Northeast snowstorms. Big snows falling on big cities have big impacts, so NESIS accounts for that by incorporating how much snow fell over an area and the population affected.

All that data leads to a score, which corresponds to a 1-5 scale.

However, the calculation doesn’t work well in other areas. For example, a snowstorm in North Carolina that’s significant by Southern standards might not even rank using the NESIS formula.

That’s where the Regional Snowfall Index comes in. The RSI ranks storms based on the historical particulars of smaller regions. Like the NESIS, it gives storms a 1-5 rating based on population and snowfall.

Like the Enhanced Fujita tornado damage scale, NESIS and RSI rate snowstorms after they’ve happened.

Wondering what the RSI says the top three snowstorms are in the South?

#1: Feb. 18-22, 1921

RSI: 31.885, Category 5

This snowstorm just brushed North Texas, dumping the biggest totals in Oklahoma into Arkansas.

#2: Jan. 5-9, 1988

RSI: 22.641, Category 5

Just like the #1 snowstorm, the heaviest snow fell in a wide swath north of Texas. However, just north and east of Dallas got a few inches of snow and double-digit totals fell in the Panhandle.

#3: Dec. 19-24, 1929

RSI: 21.134, Category 5

This snowstorm brought heavy snow to places that don't usually see much of it. Well over a foot of snow fell south of Dallas, while a few inches occurred from San Antonio to Houston.