Introduction

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

  • When two or more F2 or 1 or more F3 or greater tornadoes (Fujita and Pearson, 1973) are reported regardless of time period or area coverage.
  • When 30 reports of hail 1 inch and wind events and tornadoes, or 30 reports of hail 1 inch and wind events, or 30 reports of hail 1 inch and tornadoes, or 30 reports of hail or 30 reports of wind events and tornadoes, or 30 reports of wind events, or 10 tornadoes occur within an area of 60,000 square miles and within a 6 hour time interval.
  • When 60 reports of hail 1 inch and wind events and tornadoes, or 60 reports of hail 1 inch and wind events, or 60 reports of hail 1 inch and tornadoes, or 60 reports of hail or 60 reports of wind events and tornadoes, or 60 reports of wind events, or 20 tornadoes occur within an area of 120,000 square miles and within a 12 hour time interval.
  • When 120 reports of hail 1 inch and wind events and tornadoes, or 120 reports of hail 1 inch and wind events, or 120 reports of hail 1 inch and tornadoes, or 120 reports of hail or 120 reports of wind events and tornadoes, or 120 reports of wind events, or 40 tornadoes occur within an area of 240,000 square miles and within a 24 hour time interval.
  • In the period September 30 through March 31, the number of reports will be reduced by one half.
  • Any tornadoes, and/or wind damage, and/or hail that result in 1 or more fatalities, 10 or more injuries, or damage $1,000,000 or more.
  • Seriously consider days with an area of unusually dense but marginally severe reports or a large area with a large number of marginally severe reports even though they did not quite meet the number criteria suggested above. The Storm Prediction Center (SPC) Science and Operations Officer (SOO) and/or the National Severe Storms Laboratory ( NSSL) /Forecast Research and Development Division (FRDD) /Mesoscale Applications Group (MAG) Leader were consulted as needed.
  • All days when Moderate or High Risk are forecast.
  • Day suggested by a SPC Lead Forecaster and/or SPC SOO. Inclusion of such cases was approved by the SPC SOO and NSSL/FRDD/MAG Leader.
  • Day suggested by a NSSL Scientist. Inclusion of such cases was approved by the NSSL/FRDD/MAG Leader and SOO for approval.

 

Event Day Selection Guidelines for Areas in and West of the Rocky Mountains

  • Any tornado report regardless of F rating.
  • When 10 reports of hail 1 inch and wind events, or 10 reports of hail 1 inch, or 10 reports of wind events occur within an area of 120,000 square miles and within a 12 hour time interval
  • When 20 reports of hail 1 inch and wind events, or 20 reports of hail 1 inch, or 20 reports of wind events occur within an area of 240,000 square miles and within a 24 hour time interval.
  • Any tornado, and/or wind damage, and/or hail that resulted in 1 or more fatalities, 3 or more injuries, or damage $500,000 or more.
  • In the period September 30 through March 31, the number of reports will be reduced by one half.
  • Seriously consider days with an area of unusually dense but marginally severe reports or a large area with a large number of marginally severe reports even though they did not quite meet the number criteria suggested above. The SPC SOO and/or NSSL/FRDD/MAG Leader were consulted as needed.
  • All days when Moderate or High Risk are forecast.
  • Day suggested by a SPC Lead Forecaster and/or SPC SOO. Inclusion of such cases was approved by the SPC SOO and NSSL/FRDD/MAG Leader.
  • Day suggested by a NSSL Scientist. Inclusion of such cases was approved by the NSSL/FRDD/MAG Leader and SPC SOO.