Get Ready for “Category 5” Winter Storms
“A Category 5 Winter Storm?” NOAA Testing New Winter Rating Scale
Weather is democratic. An EF-3 tornado is just 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. Half a foot of snow will all but paralyze Dallas and Atlanta, but 6 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, 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? In spite of Doppler and high-resolution weather models, it’s still all but impossible to predict snowfall amounts down to the inch. And 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.” It’s a golden opportunity for meteorologists to provide the things an app cannot: context, perspective and analysis.
For several years NOAA 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, WSSI is modeled on 5 different 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 life-saving actions will be needed.” Extreme winter storms are mercifully rare, resulting in extreme disruptions to daily life.
Dan Luna is MIC (Meteorologist in Charge) of the Twin Cities National Weather Service, one of several snowy NWS offices around the nation that fine-tuned the WSSI scale. “It incorporates our criteria for winter storm warnings, which does vary from location to location. It looks at whether a storm event is all-snow, ice or rain — as well as wind gusts and temperature trends” Luna explained during a recent interview. Due to population density and congested urban highways, storm impacts tend to be much greater in cities than outlying rural areas, and this is reflected in the scale. Luna sees advantages to the experimental WSSI scale. “It utilizes GIS datasets, which means we know where the urban areas are compared to rural areas. I saw this recently, where the Twin Cities had major impacts and the surrounding (more rural areas) had moderate impacts, even though the snow amounts were similar.” Because not all winter storm impacts are created equal.
Predicting winter storm impacts is an evolving science. 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 powerlines. Other factors include potential for icing, ground blizzards and a flash-freeze index, when the mercury is expected to fall below freezing during or just after precipitation.
One sobering reality: weather rhymes, but never repeats. A winter storm may be similar to a previous system, but never identical. Every swirl of snow, ice and wind is a new creation — and a new and vexing puzzle for meteorologists to unpack.
Much like hurricanes, which produce multiple threats (storm surge, wind damage, tornadoes embedded in spiral bands and severe inland flooding), winter storms create a myriad of hazards. Consumers often fixate on the total amount of snow expected, when the greatest impacts can come from freezing rain (glaze ice) or high winds capable of power outages. NOAA has a good informational document that explains the factors that go into the WSSI rating scale, and how to interpret the maps now available in the public domain.
The Winter Storm/Severity Index is not meant to replace traditional warnings and watches. It is not designed to be the sole source of information for an upcoming storm. And WSSI is a forward-looking product; it doesn’t factor snow, ice or wind impacts in the recent past. Weather is rarely one-size-fits-all, and NOAA meteorologists stress that winter storm impacts may vary greatly within the same county, based on GIS, population density and land-use considerations.
At least report 116 National Weather Service Offices were participating in the WSSI test, adding their own local experiences to bear to make the scale as relevant, accurate and actionable as possible in a real-world setting.
WSSI Operational Forecasts. The experimental Winter Storm Severity Index is available from NOAA WPC on an interactive map for the USA, specific regions and individual National Weather Service forecast offices.
WSSI is a high-resolution, 2.5km spatial product that is being soft-launched around the nation. “I do see this as a step forward” Luna said. The forecasts for winter storm impacts are designed with continuity and consistency in mind. “WSSI encompasses one picture for the entire country that is seamless and consistent in nature from one area to the next, one meteorologist to the next and one event compared to another. This product can be utilized by the public, decision makers and the media in a fairly simplistic manner” according to Luna.
Growing up, my father always said, “Once you have all the information, the decision is easy.” A persistent treadmill of winter storms produces a steady diet of anxiety and dread among consumers, nationwide. Impacts extend beyond slow commutes and icy driveways. One in three businesses in the USA are directly impacted by weather. Increasingly, companies aren’t content to play defense. They are going on the offensive, pulling in continuous data streams, weather API’s, to fine-tune logistics, lower risk to staff and operations and avoid costly weather delays. WSSI is a work in progress, an intuitive winter rating scale that may be the next step in better setting consumer expectations, and reducing potentially unpleasant meteorological surprises.
- Paul Douglas is co-author of “Caring for Creation: An Evangelical’s Guide to Climate Change and a Healthy Environment”, and co-founder and Chief Meteorologist at AerisWeather, located in Minneapolis Minnesota.
