When to Chase
by Robert Prentice
Storm Track, November-December 1992 REVISED Jan 97
© Copyright 1992, 1997 Robert A. Prentice
Storm chasers have traditionally been told the best time of the year to chase is the last three weeks of May. This is because tornadic supercell frequency is maximized across favorable terrain (the Southern/Central Great Plains) and the storms move more slowly.
I wanted to verify this for myself, but was unable to due to a lack of a data. Publication of Significant Tornadoes 1880-1989 by Thomas P. Grazulis finally provided the comprehensive data set necessary for such a study.
The two areas sampled in this study (see map) were subjectively chosen on the basis of favorable tornado frequency, visibility and lighting (Miller Type I "loaded-gun" sounding dominates), landforms (few hills), vegetation cover (few trees) and road network.
In his study of Southern Plains tornado outbreaks, Moller (1979) defined a tornado "cluster" as four or more tornadoes which occur within a roughly circular area of about 5,600 square miles, while a tornado "corridor" was defined as an outbreak of three or more tornadoes that advances generally from west to east within a narrow corridor of land. Moller determined that 57% of the "corridor" events take place from March 1 - May 15 with a well defined peak in late April; and that 77% of the "clusters" occur during the late spring from May 16 - June 30 with a well defined peak in early June. Tornado "cluster" events tend to be most concentrated in an area extending from Lubbock, TX to Wakeeney, KS.
Storm chasers should prefer to chase the slow-moving/stationary "cluster" events or at least chase later in the spring when "corridor" events move slower. This is because less time would be spent driving to keep pace with the storm.
Storm Chase season across the preferred regions of the southern and Central Great Plains follows quite closely to calendar Spring (late March through late June). The height of tornado reports extends from April 26 - June 9 with the peak from May 16 - May 20 (MAY18 on the graphs). However, there are a few caveats. First, the graphs do not differentiate tornado "corridor" outbreaks from the more prized "cluster" outbreaks. The period April 26 - April 30 (APR28 on the graphs) seems to correspond to Moller's "corridor" peak, while June 4 - June 9 (JUN7 on the graphs) corresponds to Mollerís "cluster" peak. Also, numerous reseach papers and personal experience suggest that tornadoes are under-reported across the Great Plains particularly across the sparsely populated Hi Plains and prior to the 1970's.
The last three weeks of May certainly is a good time to chase and coincides rather closely with the greatest number of tornadoes and tornado days (days with one or more reported tornadoes). However, the period June 4 - June 9 (JUN7 on the graphs) is a close second to May 16 - May 20 and many spectacular, slow-moving events have occured during this peak. Due to this and previously mentioned caveats, I belive any period between May 11 and June 9 is best.
Obviously these graphs cannot show yearly flucuations of tornado/supercell occurence or when the most visually spectacular/photogenic storms will occur, but hopefully they will provide storm chaser's with objective guidance when they plan their future chase activities.
Note: This is an updated version of an article that originally appeared in StormTrack Magazine in 1992.