How Forecasters Use the Winter Storm Severity Index to Warn of Dangerous Weather

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Even though the calendar doesn’t show the lower 48 is in the depths of winter, the National Oceanic and Atmospheric Administration’s Weather Prediction Center is working around the clock to update the winter storm severity index for the agency, to warn of the impacts of approaching storm systems.

The WSSI is a newly developed tool intended to aid in decision making when snow, ice, wind and freezing temperatures could be disruptive.

The approach is similar to the Storm Prediction Center’s categorical forecast for thunderstorms, but instead of warning of hail, wind, and tornadoes, the WPC product warns of winter weather impacts.

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The index takes into account non-meteorological data such as population, land use and vegetation to determine the expected severity of the system.

An area that is home to more people or has vulnerable vegetation would likely rank higher on the WSSI due to the potential for greater impacts.

The scale goes from zero to extreme, and impacts can start to be felt when an area is highlighted in yellow. Limited impacts include small accumulations of snow and ice, but for the most part society can continue with normal routine.

Greater impacts begin to be felt when an area is placed below a moderate or orange zone. This designation equates to an ice or snow event that may take a day or two to recover.

In the rarest of events, forecasters can subject communities to an extreme level of impact. This would be equivalent to a historic snowstorm or blizzard. Recovery in terms of clearing roads and restoring utilities can take several days to more than a week.

The National Weather Service warns that the scale is not meant to be the sole source of information about the effects of a winter storm, but could be used to clarify expected impacts.

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With the experimental product, the impact outlook only extends to 72 hours, but can be broken down into snow amount, snow load, blowing snow, ground blizzard, ice accretion, and flash freeze events.

Each component has a similar purpose as the main global outlook, to show where winter weather could disrupt travel, utilities and other vital functions of daily life.

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