Weather is democratic. A tornado rated an EF-3 (out of 5) is potentially as destructive in Nashville as it is in Pittsburgh. A Category 2 hurricane threatening Tampa will have impacts similar to a Category 2 hurricane pinwheeling toward Houston.
But winter storms are a different beast altogether. A mere coating of flakes in Washington at precisely the wrong time can result in nightmare and death on the Beltway. Half a foot of snow will probably paralyze Dallas and Atlanta, but six inches of powder falling on Minneapolis or Denver in January? Just another Thursday. The effects, the overall impacts, vary wildly based on a matrix of factors, including latitude, timing, land use, population density and general acclimation to snow and ice.
As meteorologists, how do we do a better job of setting expectations with the public? Despite Doppler and high-resolution weather models, it’s still all but impossible to predict snowfall amounts down to the inch. Yet consumers and weather-sensitive businesses demand reliable forecasts to lower risk to life, property, supply chains and logistics. “How bad will this storm be? Give me the information I need to prepare.”
For several years the National Weather Service has been experimenting with a winter storm rating system. The WSSI, or Winter Storm Severity Index, is a tool that helps to set expectations about winter storm impacts 72 hours into the future. Designed to complement traditional winter storm warnings, watches and advisories, the WSSI is modeled on five categories, ranging from “limited impacts” (think of this as Category 1) with little inconvenience to “extreme impacts” (Category 5), with “extensive and widespread severe property damage where lifesaving actions will be needed.” Extreme winter storms cause extreme disruptions to daily life but are mercifully rare.
The WSSI is a work in progress; the art is in the fine-tuning of the scale over time. Private weather companies are working to improve the formula, localizing it to better set expectations.
You may be aware of Capital Weather Gang’s similar scale, but CWG’s is personalized for the Washington metro area. The WSSI’s descriptions of these five categories range from “limited” to “extreme.” The CWG scale ranges from “nuisance” to “crippling/historic.”
The one big difference between the two scales is that CWG’s categories (below) specify the types of effects you would expect for schools and air travel at different levels, whereas the WSSI descriptions of impacts (above) are more generalized.
Predicting winter storm impacts is an evolving science. The WSSI takes into account expected snowfall amounts, snowfall rates, blowing snow and a snow load index that factors the weight of snow and potential damage to trees and power lines.
CWG’s scale includes additional variables, some not obvious, including: day of the week; time of day; time of year and sun angle; temperatures before, during and after the storm; snowfall rates and visibility; wind and whether snow mixes with ice or rain.
CWG’s scale places a particular emphasis on timing. When it comes to storm impacts, timing really is everything. Heavy snow falling on a Saturday night has a much lower impact than on a Monday morning, when many people are returning to work and school.
The Weather Service has a good informational document that explains the factors that go into the WSSI rating scale and how to interpret the maps now publicly available. At least 116 National Weather Service offices are participating in the WSSI test, adding local experiences to make the scale as relevant, accurate and actionable as possible in a real-world setting.
The experimental WSSI is available from the Weather Service’s Weather Prediction Center on an interactive U.S. map, along with specific regions and individual forecast offices.
The WSSI is a high-resolution spatial product soft-launched around the nation.
Every private meteorologist could potentially suggest ways to improve the scale for their hometown. Ways to make it more useful for the Washington region might include accounting for temperatures leading up to a storm and sun angle, which are important variables for determining how quickly and readily frozen precipitation will accumulate on roads.
It’s only a matter of time before a simple, numerical winter storm rating scale shows up in your local forecast on a routine basis. But it’s naive to believe you can capture the essence of any storm with just one number. Just like hurricanes, winter storms are complex, and the types and severity of hazards can change during the course of a storm. A number offers a convenient overview but is no substitute for detailed briefings from trusted sources on how a storm will evolve, regional differences, uncertainties and how best to prepare.
Any winter storm is a golden opportunity for meteorologists to provide the things an app cannot: context, perspective and analysis.
Every swirl of snow, ice and wind is a new creation — a unique, vexing puzzle for meteorologists to unpack. The challenges for meteorologists are twofold: Accurately predict the weather and then accurately communicate impending weather to the public. Winter storm ratings, combined with supporting information, are another tool for advancing weather communication.
Jason Samenow is chief meteorologist for the Capital Weather Gang.