Colorado eastern plains tornado warning, hail

The National Weather Service issued a tornado warning for west-central Logan County at 6:05 p.m. MDT on Thursday, May 21, after radar indicated rotation in a severe thunderstorm near Pawnee Pass. The warning said the storm was moving east at 15 mph and could produce a tornado and quarter-size hail. Earlier forecasts from the Denver/Boulder office had warned that strong to locally severe thunderstorms were likely across the plains east of Interstate 25 through the afternoon and evening. Storm reports later showed larger hail and multiple tornado sightings farther south on the Eastern Plains. A preliminary National Weather Service local storm report logged estimated 4.5-inch hail, roughly apple-sized, at 5:50 p.m. about 7 miles southeast of Last Chance in Washington County. The same report listed tornado sightings near Thurman between 5:35 p.m. and 5:47 p.m., including one storm chaser report that said a tornado lasted four minutes. ### Where was the tornado warning in effect? The 6:05 p.m. warning covered west-central Logan County in northeastern Colorado, including Pawnee Pass and Logan Corners. The National Weather Service said the warning was in effect until 6:45 p.m. The warning text said people in the path should move to an interior room on the lowest floor of a well-built building and stay away from windows. It also said people outdoors, in mobile homes or in vehicles should move to the closest substantial shelter and protect themselves from flying debris. ### How large was the hail that was reported? A National Weather Service local storm report issued at 8:37 p.m. listed the largest hail at 4.5 inches in diameter near Last Chance in Washington County. Other reports in the same summary included 3-inch hail near Thurman, 2.75-inch hail near Thurman, 2.5-inch hail near Lindon and Last Chance, and 1.75-inch golf-ball hail south of Last Chance. The Denver/Boulder forecast office had said earlier Thursday that the strongest storms in Lincoln, Washington and Logan counties could produce hail up to about 2 inches in diameter, along with isolated strong wind gusts or a weak tornado. The observed hail reports exceeded that forecast in parts of Washington County. ### Why were forecasters focused on the Eastern Plains? The Denver/Boulder office said in its 1:03 p.m. area forecast discussion that storms were expected to develop after 3 p.m., mainly east of Interstate 25 and off the Palmer Divide. Forecasters said the more unstable air was expected over the eastern plains, roughly east of a line from Fort Morgan to Agate. The same discussion said 45-60 knot bulk shear could support a few supercells in Lincoln, Washington and Logan counties late Thursday afternoon into early evening. Forecasters said those storms would be capable of large hail, isolated strong wind gusts and a weak tornado. ### What did the tornado warning say about the storm itself? The National Weather Service said the warned storm was located over Pawnee Pass, or 13 miles west of Sterling, when the alert was issued. The agency said the hazard was a tornado and quarter-size hail, with damage possible to roofs, windows and vehicles and likely tree damage. The warning said flying debris would be dangerous to people without shelter and that mobile homes could be damaged or destroyed. The source for the tornado threat was radar-indicated rotation, according to the warning text. ### What comes next for Colorado weather? The National Weather Service said in a Friday morning forecast discussion that the stronger Thursday storms had moved out and that cooler air would hold temperatures in the 60s across northeast Colorado on May 22. The office said scattered showers and thunderstorms were still possible later Friday, mainly south of Interstate 70. The Denver/Boulder office said warmer and drier weather is expected for the weekend, with only isolated showers possible Saturday afternoon. The office also said mild conditions should continue next week, with a return of afternoon shower and thunderstorm chances.