Much of the Northeast experienced the first significant storm ofthe winter from Jan. 3–5, with many locations in and around Bostonseeing more than 12 inches of snow and heavy flooding.

We can take a few inches in stride, but a major Northeastsnowstorm, or worse — aseries of them as we had in 2015 — can have seriouseconomic impacts.

Although most snowstorms don't have the sudden catastrophicimpact of hurricanes or tornadoes, they still can bring a potentcombination of wind, low temperatures and heavy snow. Attemperatures near freezing, snow can have high water content,producing gravity (downward) loads that can stress buildingconnections or even lead to roof collapses.

Severe snowfall can place enormous stresses on civil governmentsand business operations, causing costly travel disruptions anddamage to power and communication networks that can bring everydaylife to a standstill.

Understanding rating scales

As a society, we rely on rating or relative impact scales as apractical means for assessing the severity of various events. Inatmospheric science, the most familiar impact scales are thosedeveloped for tornadoes (Fujita) and hurricanes (Saffir-Simpson).Did you know that there is also a rating scale for Northeastsnowstorms?

Paul Kocin, formerly of The Weather Channel, and Louis Uccellini of theNational WeatherService developed the Northeast Snowfall Impact Scale (NESIS) to quantifythe impact of these storms on populations. NESIS provides arelative measure of Northeast winter storm impact based on totalsnowfall amount, its geographic distribution and populationdensity.

The NationalClimatic Data Center (NCDC) computes NESIS values when asignificant snowstorm hits the 13-state Northeast region —definedas West Virginia, Virginia, and northeastward through New York andthe New England states. To capture the entire storm history, NESISvalues are computed using total snowfall distributed east of theRocky Mountains.

About a week after the event, preliminary values are calculatedusing a geographic information system (GIS) and posted to the NCDCwebsite. These values are often refined after additional qualitychecks or as more observations become available and final NESISestimates are released at the end of the winter season.

Related: What insurance agents need to know about winterflood risks

Wide range of severity

The wide range in the severity of winter storms typicallyresults in NESIS values between 1 and 13, corresponding to one offive categorical assignments:

1. Notable.
2. Significant.
3. Major.
4. Crippling.
5. Extreme.

Based on historical analysis

The NESIS scale was calibrated based on an analysis of 30Northeast snowstorms that occurred from 1956 to 2000, using theaverage area covered by at least 10 inches or more of snowfallaccumulation and the average population (as of the 2000 census)within the affected area. The mean NESIS value for thesecalibration events is 5.0 (Category 3). The 10 most severe stormson record are ranked by NESIS value in Table 2.

Related: 6 keys to successfully weathering thestorm

Societal impact

NESIS is different from most other scales in that it takes intoaccount population density in addition to meteorological data andcan indicate a storm's societal impacts. The overall impact of awinter season can be assessed by summing the NESIS values ofindividual storms.

A very intense winter season can be the result of a singlecrippling event or multiple less intense ones. For example, theGreat Blizzard of 1993 holds the record for maximum NESIS value fora single storm at 13.20. In comparison, the seasonal NESIS valuefrom 1977–78 was 12.31, but this resulted from two Category 3events.

The NESIS scale can provide valuable information regarding thepotential social and economic impacts of an impending snowstorm bycomparing its projected track and intensity to similar storms inthe NESIS historical database.

A more robust and reliable representation of the full spectrumof extreme winter events is provided by a well-constructedcatastrophe model. Its primary benefit is to anticipate thepotential impact of extreme events before they occur. Furthermore,by simulating winter storms using state-of-the-art numericalmodeling techniques, regional correlation in risk is properlyenforced, as is the intra-seasonal “clustering” of snowfall eventsthat has been observed on occasion.

As a final point, it's important to note that NESIS doesnot consider the replacement value of property. Thisbrings into focus the value of a well-constructed industry exposuredatabase in a catastrophe model, which — along with robust hazardsimulation and expert understanding of how structures respond tosnow loads — can provide a fully probabilistic assessment ofpotential damage and loss.

Related: Mitigating the No. 1 natural disaster inthe U.S.

Todd Keller is a risk consultant at AIR-Worldwide, a Veriskbusiness. He can be reached at [email protected].This article was first published on AIR-Worldwide and is republished withpermission.