Crane Accidents

Tornado Topples Crane at Mormon Church

08/11/99

Salt Lake City, Utah

A Scientific Report

by

Clayton Brough, Climatologist, KTVX
Dan Brown, HMT/Webmaster, NWS
David James, Geographer, BYU
Dan Pope, Meteorologist, KSL
Steve Summy, OPL, NWS

Tornado in Salt Lake City, UtahAn F2 tornado touched down in the metropolitan area of Salt Lake City. The tornado lasted ten minutes and killed one person, injured more than 80 people, and caused more than $170 million in damages. It was the most destructive tornado in Utah’s history, and awakened the entire state’s population to the fact that the Beehive State does experience tornadoes.

Generally speaking, atmospheric conditions are rarely favorable for the development of tornadoes in Utah due to its dry climate and mountainous terrain. On fact, Utah ranks as having one of the lowest incidences of tornadoes in the nation, averaging only about two tornadoes per year, with only one F2 or stronger tornado once every seven years. From January 1950 to August 11, 2005, 121 tornadoes and 22 waterspouts have been reported in the state.

In the central U.S., tornadoes are commonly one-fourth of a mile wide and often cause considerable destruction and death. However, Utah tornadoes are usually smaller in size–often no more than 60 feet wide (at the base)–with a path length usually less than a mile and a life span of only a few seconds to a few minutes. They normally follow a path from a southwesterly to a northeasterly direction and usually precede the passage of a cold front.

About 73% of all Utah tornadoes have occurred in May, June, July and August, when severe thunderstorms occasionally frequent Utah. Also, 69% of all Utah tornadoes have occurred between the hours of Noon and 5:00 PM (MST), while 55% of all waterspouts have happened in the morning hours.

There have probably been more tornadoes and waterspouts in Utah than the following statistics and accounts indicate. In fact, in recent years an increasing number of these storms have been reported–probably due to Utah’s increasing population and greater public awareness about twisters. However, sometimes people have mistaken whirlwinds (or dustdevils), microburst winds and other natural phenomena as tornadoes. Thus, every report of possible tornadic activity that appears in this publication has been carefully reviewed and analyzed to assure the greatest degree of accuracy possible.

Synoptic Analysis: On the morning of August 11, 1999, an upper level trough of colder air moved into northern Utah from Nevada. In advance, warm breezy southerly winds blew over the Salt Lake Valley. By Noon, there was evidence that either an old frontal boundary existed or a convergence zone had developed across the Salt Lake Valley due to breezes from the Great Salt Lake meeting up with the southerly winds that prevailed through the majority of the valley. The Salt Lake morning sounding indicated some vertical shearing of the winds (differences in wind speeds) along with the jet-stream over northern Utah. As this happened, thunderstorms began to form over the Oquirrhs in the Herriman area and over the south end of the Great Salt Lake/north end of the Oquirrhs in the Magna area. By 12:35 PM, there was a thunderstorm over the north portion of the Salt Lake Valley–with clouds tops extending up to 41,000 feet high–that rapidly intensified and generated a rare F2 tornado.

Event Analysis: On August 11, 1999, an F2 tornado (having winds of 113 to 157 mph) did considerable damage as it tracked northeastward across the metropolitan area of Salt Lake City. One person was killed and over 80 people were injured–with 15-20 serious injuries reported. The tornado produced F0 wind damage at 12:41 PM from about 400-500 South/Navajo (1340 West) to about 300 South/Goshen (1040 West). The tornado reached F2 strength by 12:45 PM.

From 300 South/1040 West the tornado tracked northeast producing widespread damage at the Delta Center–including the destruction of one of the large outdoor tents set up for the Outdoor Retailers Convention. The collapse of the large tent facility killed one man: Allen Crandy. The tornado also damaged the Wyndham Hotel, which had to be closed for several days until the damage could be repaired.

Crane Toppled at Mormon Church, UtahFrom the Wyndham Hotel, the tornado continued its northeast track, knocking down scaffolding and shearing off a crane at the site of the LDS Church’s new Assembly Hall that was under construction. Next, it went up Capitol Hill and along the southeast side of the Capitol, through Memory Grove, and up along the northwest portion of the Avenues–just barely missing the LDS Hospital. It then lifted off the ground at about Edge Hill/Terrace Hill (20th Avenue and P Street). Along its path through the Avenues, houses experienced from minor to major damage, with hundreds of trees either uprooted or damaged. Throughout much of the tornado’s destructive path, vehicles were tossed around and many were damaged or totaled by falling trees.

This F2 tornado was on the ground from 12:45 PM to 12:55 PM (10 minutes). It traveled a distance of about 3-3/4 miles, and had a width of about 100 to 200 yards. From F0 to F2 intensity, the tornado traveled 4-1/4 miles, lasted 14 minutes, and traversed an elevation difference of 1,095 feet (from 4,225 feet to 5,320 feet).

Category: Accident Report

Tags: , , ,

Leave a Reply