The graph at the right shows the number of tornadoes reported each year since 1950.  The numbers shown are likely an underestimate of the true tornado count since some events, especially in rural areas, go unreported.  In the early 1990s, a major modernization program within NWS took place, which included the deployment of Doppler radar and a more vigorous spotter training program.  In addition, the 1990s saw an increasing number of storm chasers using video cameras and cell phones to document and report severe weather.  These facts are likely responsible for the dramatic increase in annual tornadoes reported since 1990. Okay, and maybe the movie "Twister"!


Another way to illustrate the increase in reported tornadoes in recent years is to take a look at the number of tornado reports in each decade since 1950.   From the 1950s through the 1980s, the number of tornadoes reported in the Tri-State area was fairly consistent with 65 to 85 events logged in each ten year period.  Since 1990 however, the number of tornado events logged is about three times that of prior decades.
Still another way to look at this trend is to again divide the data into decades, but now calculate the average number of tornadoes each year within that decade.  For example, in the 1950s, 87 tornadoes were reported over the ten years, giving an average of 9 tornadoes each year from 1950-1959.  Again notice that from the 1950s through the 1980s, the average annual tornado count was consistent between 7 and 9 tornadoes.  In the past two decades however, over 200 tornadoes have been logged during each 10-year period, giving a more recent annual average of 23 or 24 tornadoes per year.
The next two charts illustrate the month-to-month variation in tornado reports.  Although tornadoes can occur in any month given the right atmospheric conditions, over the past 59 years, no tornadoes have occurred between November 1 and February 28.  The peak months for tornadoes in the Tri-State area historically are May and June, and to a lesser extent, July.
This chart takes the monthly information above and breaks each month's tornado count down into a percentage of the total.  Taking June as an example, 33 percent of all tornado reports received since 1950 have occurred during that month, while 31 percent have occurred in May.
Dr. Ted Fujita developed the F-Scale in 1971 to provide a method to rate the intensity of tornadoes.  The intent of the scale was to distinguish between weak and strong tornadoes.  Dr. Fujita's scale was quickly accepted by the NWS and came into wide use by the mid-1970s.  Values on the scale range from F0 for tornadoes producing minimal damage, to F5 for tornadoes causing devastating damage.  A rough estimate of the wind speeds responsible for the damage could then be inferred using the F scale.  In February 2006, this scale was modified and is now referred to as the "EF" scale, or Enhanced Fujita Scale.  Sixty four tornadoes in the historical records were not assigned a rating, perhaps due to lack of information about the tornado.  These unrated tornadoes are shown in the first column and labeled EF.

This chart expresses the EF ratings in the chart above as a percentage.  Tornadoes rated EF0 and EF1 are considered weak, tornadoes rated EF2 and EF3 are considered strong, and those with ratings of EF4 and EF5 are considered violent. 

It would appear that most tornadoes in the Tri-State area are weak since 84% are either EF0 or EF1.  This is likely not the case however.  Tornado ratings are based on observed damage.  In rural areas with vast expanses of open fields there are simply less things to damage compared to urban areas.  Thus, a tornado which in reality might be violent, will receive an EF0 rating if it remains over open country producing no damage.


The number of tornadoes reported in each of the 19 counties in the Goodland CWA varies quite a bit over the years.  Sherman county Kansas takes the dubious honor of having the most tornadoes reported since 1950, with Kit Carson and Yuma counties in Colorado coming in 2nd and 3rd.  Norton county Kansas has had the fewest number of tornadoes reported at 19.  Also note there are two Cheyenne counties.  The one at the far left is Cheyenne county Kansas, while the one toward the right is Cheyenne county Colorado.

The graph in the previous slide is somewhat misleading in that the size of the counties is not uniform.  It would seem logical that if county A is twice as large as county B that there would be more tornadoes reported in county A, all other things being equal. 

To remove some of that bias, each county's tornado count in the slide above was divided by the area of the county and normalized to 1000 square miles.  The graph to the right shows a more uniform tornado tally among the counties when this factor is taken into account.  Sherman county Kansas still comes in with a high number of tornado reports.  And no, the presence of the Doppler radar in Goodland is not to blame!


 

 


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