By Lauren Casey

PHILADELPHIA (CBS) — One year ago, Tropical Storm Isaias generated the most tornadoes in a single day in the Delaware Valley at seven. Then came last Thursday, when a non-tropical storm system produced an eye-popping nine tornadoes across the region to set a new single-day record dating back to 1950.

The most intense of these, the EF-3 Somerton-Trevose-Bensalem tornado with winds of 140 mph, is amongst the most violent the area has ever experienced.

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Our current system for “rating” tornadoes is based on estimated wind speeds and the resultant damage, categorized by the Enhanced Fujita Scale, or EF Scale, which became operational in February 2007.

The EF Scale is a revised version of the original Fujita, or ‘F Scale’, developed by renowned University of Chicago meteorologist Dr. Tetsuya “Ted” Fujita.

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The original scale was updated to provide more detailed examinations of tornado damage surveys, and the new scale incorporates how modern-day structures are designed.

When tornado-related damage is surveyed by a team of meteorologists, it is compared to a list of Damage Indicators (DIs) and Degrees of Damage (DoD), which estimates the range of wind speeds that the tornado likely produced. From that, a rating (from EF0 to EF5) is designated.

Looking back into the tornadic history of the Delaware Valley, only five other EF-3 or F-3 tornadoes are documented in the storm events database.

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Chester County registered two F-3’s; one in March 1955 and one in July 1994, which traveled into Montgomery County. Monroe and Northampton Counties each have one F-3 on record, in March 1976 and July 1981, respectively. And in July 1983, an F-3 occurred in Ocean County, New Jersey.