Tornadoes are rare west of the I-25 corridor; however, one notable exception was an F3 tornado that struck 4.4 miles south of downtown on June 15, 1988. On the other hand, the suburbs east of Denver and the city's east-northeastern extension (Denver International Airport) can see a few tornadoes, often weak landspout tornadoes, each spring and summer especially during June with the enhancement of the Denver Convergence Vorticity Zone (DCVZ). The DCVZ, also known as the Denver Cyclone, is a variable vortex of storm-forming air flow usually found north and east of downtown, and which often includes the airport. Heavy weather from the DCVZ can disrupt airport operations. In a study looking at hail events in areas with a population of at least 50,000, Denver was found to be ranked 10th most prone to hail storms in the continental United States. In fact, Denver has received 3 of the top 10 costliest hailstorms in United States history, which occurred on July 11, 1990; July 20, 2009; and May 8, 2017 respectively.