Here is the most comprehensive look at Tornado Statistics you will find for North Texas anywhere.
TOTAL NUMBER OF KNOWN TORNADOES 1950-2013
Why have there been more tornadoes in Johnson, Dallas and Tarrant Counties? A big reason is that there are more people in those counties to see brief tornadoes. Many brief tornado touchdowns in rural counties were never counted because no one ever saw them.
AVERAGE NUMBER OF TORNADOES A YEAR IN EACH COUNTY (1950-2013 AVERAGE)
TOTAL NUMBER OF TORNADO WARNINGS FROM 1986-2013
AVERAGE NUMBER OF TORNADO WARNINGS A YEAR (1986-2013 AVERAGE)
TOTAL NUMBER OF SEVERE THUNDERSTORM WARNINGS 1986-2013
AVERAGE NUMBER OF SEVERE THUNDERSTORM WARNINGS A YEAR (1986-2013 AVERAGE)
SOURCE: National Weather Service Office Fort Worth