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Should There Be a Ranking System for Thunderstorms?

By By Heather Buchman, Meteorologist
April 13, 2011, 7:29:53 AM EDT

Meteorologists classify tornadoes and hurricanes, but what about thunderstorms?

Thunderstorms can have such a wide range of impacts, with some producing wind damage, others dropping large hail, some causing flooding, and violent ones spawning tornadoes. The severity of these impacts varies, and some thunderstorms produce a combination or all of these things.

Instead of predicting "severe thunderstorms" for an area, while mentioning what the main risks are, would it be helpful if categories were assigned to thunderstorms? Once thunderstorms are under way, would it be beneficial for people to know that a certain category thunderstorm is headed toward their local area?

The ability of people to recognize categories of thunderstorms, as they do hurricanes, may help them have a better perception of the risk at hand and how they may be affected.

AccuWeather.com Senior Meteorologist Henry Margusity has developed a classification system for thunderstorms using what he calls a "TS Scale," similar to the Enhanced Fujita scale used for classifying tornadoes and the Saffir-Simpson scale used for categorizing hurricanes.

Margusity's thunderstorm scale consists of five categories: TS1, TS2, TS3, TS4 and TS5, with TS5 being the most severe in terms of damage and impact.

The scale ranks thunderstorms by a combination of their average rainfall rate, maximum wind gusts, hail size, peak tornado potential, lightning frequency and storm impact.


590x386_01031707_tstormscale

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The following are some characteristics of the different thunderstorm types, according to the scale: TS1: WEAK No tornadoes, no hail, winds less than 25 mph, only a few lightning strikes total, rainfall rates 0.03-0.10 of an inch per hour, no damage.

TS2: MODERATE No tornadoes, no hail, winds 25-40 mph, 1-10 lightning strikes per five-minute interval, rainfall rates 0.10-0.25 of an inch per hour, little damage--mainly limited to breakage of small tree branches and movement of lawn furniture.

TS3: HEAVY EF0 tornado possible, hail 0.25-0.75 of an inch in diameter, winds 41-57 mph, 10-20 lightning strikes per five-minute interval, rainfall rates 0.25-0.55 of an inch per hour, minor damage to small branches and roofs, with street flooding and lightning-sparked house fires possible.

TS4: INTENSE EF1-EF2 tornado possible, hail 1.00-1.50 inches in diameter, winds 58-70 mph, 20-30 lightning strikes per five-minute interval, rainfall rates 0.55-1.25 inches per hour, moderate damage--wind damage to trees and buildings, possible tornado damage, hail dents in cars, damage to crops, power outages, and flooding along streams, creeks and roadways.

TS5: EXTREME EF3-EF5 tornado possible, hail larger than 1.50 inches in diameter, winds greater than 70 mph, more than 30 lightning strikes per five-minute interval, rainfall rates greater than 1.25 inches per hour, severe damage--significant, widespread damage to trees and property, flooding, hail damage to property and crops, EF3-EF5 tornado damage possibly devastating, and widespread power outages.

While a thunderstorm may have characteristics of multiple categories, it would be categorized according to its highest-ranking criterion. For example, if a thunderstorm produces a 57-mph wind gust but no hail, it would be classified as a TS3 rather than a TS2.

The scale can be incorporated into radar data and used as a "nowcast" tool that determines thunderstorm type as storms develop. This would enable people to know more specifically what type of thunderstorm is headed for their local area in the middle of a thunderstorm outbreak.

In addition to providing "nowcast" information, the thunderstorm scale could also be used in advance of the outbreak to forecast what types of thunderstorms will develop and where. Below is an example of what such a forecast map would look like.


400x266_04071648_2011-04-07t-stormfcst


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