Climatology of Colorado Tornadoes Are Colorado Tornadoes Growing Longer and Stronger in Time?

Climatology of Colorado Tornadoes Are Colorado Tornadoes growing longer and stronger in time? CHRISTOPHER K. SPEARS Graduate Student, Mississippi State University ABSTRACT Over the past half century, the number of tornadoes reported in the United States has doubled from roughly 600 per year in the 1950s, to around 1,200 per year in the 2000s (Verbout et al. 2006). The trend in Colorado has been more aggressive with the number of average annual tornadoes quadrupling during the same time period, from 12 to 48 per year on average. This trend can be attributed to a number of factors, including an improvement in record keeping and better tornado detection through Doppler radar. It is also widely accepted throughout the meteorological community that a growing population and more public awareness are contributing factors. There are numerous studies about U.S. tornado trends, but few that focus specifically on potential trends in Colorado. When posing the question ‘are tornadoes growing longer and stronger with time,’ a long-time resident might be inclined to say yes, simply due to recent back- to-back killer tornadoes in the late 2000s. One of those storms stuck in late March 2007. Typically, Coloradoans are more concerned about a blizzard rather than a killer tornado that early in the spring season. So are we seeing tornadoes grow longer and stronger with time? It is the goal of this paper to answer this question and more through a detailed analysis of all documented Colorado tornadoes between 1950 and 2012. Having a better understanding of this weather hazard will help improve public awareness and preparedness that could potentially save lives. INTRODUCTION. On March 28, 2007, thunderstorms that produce tornadoes are a large and violent EF-3 tornado swept most common in the mid-afternoon and through the small town of Holly, Colorado, early evening hours. In addition to the odd just before 8 p.m. local time. Holly is time, this thunderstorm moved very quickly located in the southeast corner of the state in toward the north-northwest, and was Prowers County, on the western fringe of an surprisingly strong and long-lived for being area in the United States known as tornado in such close proximity to the Front Range, alley. The storm was part of a major where weaker tornadoes are more common outbreak of tornadoes that spanned three (Schumacher et al., 2010). The storm days and impacted eight states. The tornado tracked over 30 miles northwest into the tore a 28-mile path across Prowers County, mountains of southeast Wyoming, causing damaging roughly one-third of the homes in significant damage along the way. It was the small town of Holly, and resulted in the labeled a geographically rare event that first tornado-related deaths in the state since could only occur in a limited number of 1960. Less than an hour later, a second places (Finch and Bikos, 2010). long-track tornado touched down in Kiowa County. This storm tracked 11.6 miles into Kansas, but luckily, remained over open CLIMATOLOGY. A 1953 research country, and did not cause any damage. paper about tornadoes in New Mexico, Colorado and Wyoming showed that Colorado tornadoes were an infrequent occurrence, primarily experienced during the late spring and early summer months, with a peak date between June 10-15 and the peak time between 12-7 p.m., along or east of the 105th meridian (Cook, 1953). For the most part, that assessment is still valid today. An analysis of all Colorado tornadoes documented between 1950-2012 (Fig. 2) shows that June is by far the peak month for tornado activity with May and July tied for second place. Since 1950, there have been Figure 1: Severe weather reports around the United States tornadoes documented in every month of the on March 28, 2007. year except November, December and January. (While not reflected in the modern In the following year, on May 22, 2008, record, there was a rare killer tornado prior another killer EF-3 tornado struck Colorado, to 1950 that occurred during the month of but this time, it was much further north and November. More information on that event west in the town of Windsor, located along can be found later in this section.) the Interstate 25 urban corridor between The most opportune time to experience a Denver and Fort Collins. While this tornado Colorado tornado is between 11am-11pm touched down at a time of the year when with the main peak being between 2pm-7pm residents expect severe weather, there were local time (Fig. 3). While extremely rare, several aspects of the storm that caught most overnight tornadoes (between 12am-6am) off guard. The storm formed during the late have occurred 26 times since 1950. There morning hours, contrary to what one has never been a tornado documented in the typically might expect in Colorado, where modern record during the 4am, 5am or 8am frequencies were then plotted at the hours. midpoint of each 5-day interval (Fig 4). The results show that Colorado’s tornado 900 800 714 season typically begins in April and builds 700 to a first peak by the end of the month. After 600 a slight decline in early May, the probability 500 422 422 400 of a tornado rapidly increases to a peak that 300 192 occurs somewhere between May 29th and 200 90 June 7th. The graph is almost identical to the 100 2 22 45 39 Number of of Number Tornadoes 0 one that was produced by Cook in 1953 (Figure 5). The only difference is Cook’s Jul Jun Oct Apr Feb Aug Mar May Sept peak for Colorado tornadoes was closer to th Figure 2: Colorado tornadoes by month (1950-2012). June 10 , and there was not as much detail in the graph since it was built off a much lower number of annual tornadoes. 350 300 250 30 200 25 150 2012 20 100 - 50 15 0 10 5 Number of of Number Tornadoes 0 2:00 PM 4:00 PM 6:00 PM 8:00 PM 2:00 AM 4:00 AM 6:00 AM 8:00 AM between 1950 12:00 PM 10:00 PM 12:00 AM 10:00 AM Number of Tornadoes of Number Tornadoes 3-Jun 1-Apr 5-Aug 15-Jul 24-Jun 22-Apr 16-Sep 26-Aug Figure 3: Colorado tornadoes by hour (1950-2012). 13-May Figure 4: Smoothed frequency distribution of Colorado tornadoes for 5-day intervals, April through September, To determine the peak date for tornado 1950-2012. activity in Colorado, a smoothing technique was applied to the modern tornado record. It was designed to highlight the period of maximum probability for a tornado. This calculation was modeled after the distribution created by Cook in 1953, where the number of tornadoes were tabulated for 5-day intervals between April 1 and September 30. The frequencies were then smoothed by successively overlapping three consecutive 5-day intervals (with Figure 5: Cooke’s smoothed frequency distribution of tornadoes in Colorado, Wyoming and New Mexico for 5- frequencies a, b and c) and using the day intervals, April through September, 1916-1951. formula to smooth the data. The 푎+2푏+푐 4 Tornadoes in Colorado are most common on area. The zone can often be found extending the eastern plains but can occur anywhere at from Weld County or Morgan County, south any time, even in the mountains. Since 1950, toward El Paso County. The DCVZ all but11 of Colorado’s 64 counties have develops as southeast low-level wind flow documented at least one tornado. The interacts with an east-west ridge known as counties without a recorded tornado include the Palmer Divide (located between Denver Broomfield, Dolores, Garfield, Gilpin, and Colorado Springs), and the north-south Gunnison, Hinsdale, Lake, Ouray, San Juan, axis of the Front Range of the Rocky San Miguel and Summit Counties. (Note: Mountains (Pietrycha et al., 1998). The top Broomfield County was formed in 2001 10 counties for tornadoes in Colorado lie from parts of Boulder, Adams and Weld along and just east of the DCVZ and account Counties) for 59% of the state’s total tornadoes since For the most part, the counties without a 1950. documented tornado contain some of the highest and most rugged terrain in the Top 10 Counties central Rockies, but elevation doesn’t make a location exempt from a tornado. On July Colorado Tornadoes 29, 2012, the second highest elevation 1950-2012 tornado ever recorded in the United States Weld 252 touched down 40 miles southwest of Denver Adams 158 near the summit of Mount Evans, in Clear Washington 133 Creek County, at 11,900 feet above mean Elbert 101 sea level. The tornado tracked 1.75 miles and did not cause any damage or injuries. Kit Carson 94 Lincoln 90 Arapahoe 85 El Paso 85 Yuma 81 Kiowa 78 Figure 6: Mt. Evans tornado on July 29, 2012. Photo courtesy of 9News.com. COLORADO’S VERY OWN TORNADO ALLEY. Colorado’s most tornado-prone counties are located along and east of Interstate 25 in an area that is heavily influenced by the Denver Convergence- Vorticity Zone (DCVZ). The DCVZ is an Figure 7: Colorado's top 10 counties for tornadoes with the area most often impacted by the Denver area of convergent winds, typically 50 to Convergence-Vorticity Zone highlighted in the green 100 km in length, oriented in a north to circle. south fashion just east of the Denver metro The area along and east of the Interstate 25 • May 22, 2008 (Weld County, urban corridor between Fort Collins and Larimer County) – a massive EF-3 Colorado Springs could conceivably be tornado traveled across two counties considered Colorado’s very own “tornado leaving a path of destruction when it alley.” The good news is the vast majority of touched down around 11:30 am.

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