Organized severe thunderstorm episodes can occur anywhere in the United States in any month of the year. The synoptic environments in which these storms develop can vary in many ways depending on region of the country and time of the year. Climatologically most of the severe thunderstorm episodes in the United States occur in an area bound by the continental divide on the west side and a line approximately 1000 - 1200 miles east of the continental divide on the east side. The part of the United States east of this high frequency area has a large number of severe thunderstorm episodes but not near the number in the high frequency area. The part of the United States west of the continental divide has an extremely low frequency of severe thunderstorms when compared to the high frequency area and the eastern area.
Experience in using the tools needed to forecast severe thunderstorm episodes is extremely important and essential if a forecaster is to be successful in forecasting these storms. Forecasters located outside the high frequency area mentioned above have limited exposure to severe thunderstorm situations and therefore are limited in being able to increase their skill and confidence in forecasting them. Individual forecasters, at locations outside the high frequence area, may work a severe thunderstorm situation only once or twice a year. At some locations west of the continental divide, some forecasters may only work one or two severe thunderstorm situations in their entire career.
One of the ways to augment a forecaster's experience is to study various synoptic analyses and other tools that describe the environment in which severe thunderstorms develop. The purpose of this web document is to identify organized severe thunderstorm episodes and to organize them chronologically. The events provided will appear as a somewhat standardized package of synoptic analyses and other tools so an interested forecaster can review and compare them to other severe thunderstorm episodes and/or to a current situation. The set of events provided here is not intended to be a set of case studies. The more detailed analysis required for case studies is left to the individual. This set of events will provide forecasters and researchers a quick look at the synoptic environments related to various severe thunderstorm episodes and allow them to decide if further study of a specific event is desirable.
The selection procedure used here is more structured than it was in
the previous technical memorandums containing events from July 1985 through
June 1992. The selection criteria which follow will reveal that the thresholds
used in and west of the Rocky Mountains are somewhat lower than those used
east of the Rocky Mountains and may vary a little from the strict definition
of severe thunderstorm...Definition: A severe thunderstorm is a thunderstorm
which produces hail 3/4 inch, and/or damaging winds or wind 50 knots, and
or a tornado. Since the major focus of this web document is on organized
severe thunderstorm episodes, the criteria for determining which days are
to be used will be those days when there is considerable severe thunderstorm
activity confined to a relative small area (ranging from approximately
the size of Kansas to about four times the size of Kansas) and over a relatively
short time interval (6, 12, and 24 hours). These severe thunderstorm events
are keyed to well-organized severe thunderstorm events most capable of
damage and/or injury. They are not intended to cover every isolated or
marginally severe thunderstorm. Pulse-type thunderstorms, consisting primarily
of solitary brief severe downdrafts are not considered to be organized.
Convection of this type and thunderstorms barely meeting severe thunderstorm
criteria will not be considered when determining severe thunderstorm cases
for this web document except on days when unusually dense and/or large
areas of marginally severe thunderstorms are reported.
Event Day Selection Guidelines for Areas East of the Rocky Mountains
Event Day Selection Guidelines for Areas in and West of the Rocky Mountains