November 20

Coming Soon: Category 4 Winter Storms?

Man looking at snow on deck

Digging Out. AerisWeather senior meteorologist Paul Douglas, assessing the aftermath of the Category 4, April 12-17 blizzard in the Twin Cities of Minnesota.

Capital Gang meteorologist Sammy Sleet here, with an update on the crazy Nor’easter impacting the Mid-Atlantic states this morning. Latest models are in, and the good folks at NOAA have upgraded this system to a rare Category 4! A combination of heavy, wet snow, ice and high winds will result in severe coastal flooding and power outages. Travel in many communities west of I-95 may become impossible later today as snowfall rates increase – best to hunker down and stay put until this beast blows over. You heard it right, a Category 4 winter storm is on the way!

A far-fetched scenario? Don’t rule it out, at some point in the not-to-distant future. We rate everything these days, from Uber drivers and customer service calls to meals and hotel rooms. The same rating mentality now extends to severe storms. Americans have become comfortable with a 1-5 rating scale for hurricanes, and a 0-5 rating scale for tornadoes. Why rate severe storms at all? With hurricanes – to set expectations about destructive potential; to provide perspective to consumers trying to prepare for these Texas-size storms. With the tornado EF (Enhanced Fujita) scale, meteorologists assess debris in damage swaths to estimate tornado wind speeds. But with improvements in Doppler and real-time social media posts, some meteorologists now estimate strength and EF-rating while the tornado is still underway. Will this will be a small, garden-variety storm, a meteorological nuisance – or a true knock-down, drag-out weather disaster, with significant risk to life and property?

Meteorologists Paul Kocin and Louis Uccellini developed such a scale to rate Northeast snowstorms in 2003, and a resulting NESIS (Northeast Snowfall Impact Scale) has been in use ever since. According to NOAA, “NESIS scores are a function of the area affected by the snowstorm, the amount of snow, and the number of people living in the path of the storm. NESIS gives an indication of a storm’s societal impacts. This scale was developed because of the impact Northeast snowstorms can have on the rest of the country in terms of transportation and economic impact.”

Using this winter storm ranking system, researchers isolated 2 extreme, Category 5 winter storms (1993 and 1996) and a total of 10 crippling, Category 4 storms impacting the Northeast since 1960. Again, it’s all about setting expectations.

Storm Category Chart

NESIS was beta, but a successful trial. In fact the scale worked so well in the Northeast that NOAA expanded the concept into a regional snowfall-rating scale: RSI, a Regional Snowfall Index, one that uses region-specific parameters and thresholds for its calculations. For example, 2 inches of snow in Buffalo, where winters are consistently severe, is a minor nuisance, a meteorological rounding-error. For Atlanta or Dallas, 2 inches of snow is a potential calamity. The NESIS scale takes regional variations and long-term weather average into account. According to NOAA NCDC: “NCEI has analyzed and assigned RSI values to over 500 storms going as far back as 1900. New storms are added operationally.

As such, RSI puts the regional impacts of snowstorms into a century-scale historical perspective. The index is useful for the media, emergency managers, the public and others who wish to compare regional impacts between different snowstorms.” NOAA has analyzed and assigned RSI storm values to over 500 storms going back roughly 100 years – to provide historical perspective on what constitutes a major, crippling or extreme storm, on a region-by-region basis, for 6 regions of the USA.

Map with weather layers

RSI Map Viewer for April 12-17, 2018 Upper Midwest Blizzard.

NOAA NCDC (National Climate Data Center) provides a free map viewer link that allows users to call up a specific storm to see the rating. The blizzard that stalled over the Upper Midwest in mid-April of 2018 was a long-duration winter system that paralyzed travel. In retrospect, based on metrics unique to the Upper Midwest, NOAA calculates it was a rare Category 4 winter storm. The RSI Scale shows that since 1900 there have been only three extreme, Category 5 winter storms in the Upper Midwest: 2 in 1985, and the infamous “Halloween Blizzard” of 1991.

The Northeast has experienced four separate Category 5 snowstorms since 1900: in 1969, 1978, 1993 and 1996. Snow buffs may be interested to learn that of the 6 of the 15 Category 4+ winter storms in the Northeast have occurred since 1993. As a snow lover I like those odds!

Meanwhile, the Southeast, with a lower threshold for mayhem (and a tendency for more ice than snow) has recorded a total of five extreme, Category 5 winter storms since 1900, most recently from January 6-9, 1996, when over 30 million people were impacted by wind, ice and snow.

It’s important to note that both the NESIS and RSI Snowfall Rating Scales are forensic, backward-looking, a summary of what already happened. But it may not be a stretch of the imagination that, at some point, NOAA may adapt RSI to be forward-looking, helping Americans prepare for a storm forecast to develop – helping to set snowy expectations.

Black Map with town and roads on it

Recent research suggests a rapidly warming Arctic, warmer oceans and higher water vapor levels may be fueling heavier snowfalls, especially near coastal areas. New times require new tools. With the Weather Channel naming winter storms, it’s probably inevitable that a rating scale similar to one currently tapped for hurricanes and tornadoes may find its way into winter predictions and your favorite web site and weather app at some point in the future.

Then again, that’s a forecast, and to quote Yogi Berra, “It’s tough to make predictions, especially about the future.” Stay tuned, and don’t be surprised if we soon go from naming winter storms to chatting about a NOAA-assigned rating, based on a myriad of factors. We still can’t do anything about the weather, but I stand by my prediction: it’s just a matter of time before we transition from “how many inches!” to “what’s the winter storm rating!” Wait for it.


AerisWeather co-founder and senior meteorologist Paul Douglas lives and works in the Twin Cities of Minnesota but has numerous snow-crazed family members living in the DC area.

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