1. Quantifying the Significance of the 26-27 April 2011 Severe Weather Outbreak Matthew S. Stalley, Chad M. Gravelle, Charles E. Graves Saint Louis University John P. Gagan NOAA/NWS Springfield, MO 2011 National Weather Association Annual Meeting Birmingham, Alabama 17 October 2011

2. Motivation Question: When severe weather is expected, how likely are major severe weather outbreaks? More specifically: How likely is a significant tornado outbreak when the environment is similar to 27 April 2011? The answer may provide some (more appropriate?) measure of tornado outbreak significance and situational awareness. When the synoptic-scale pattern is similar, does an outbreak occur 1 of 4 times or 1 of 50 times (i.e., probabilistic awareness)?

3. Defining an Outbreak 48-h tornado reports within the domain (having at least 1 severe report) were ranked between 1 January 1979 and 31 December 2010. For each category (i.e., tornado, long-track tornado (≥10km), and significant tornado), outbreaks were defined as being in the top 2%. Outbreak Thresholds: Tornado: 23 Long-track Tornado: 6 Significant Tornado: 6 26-28 April 2011* Tornado: 217 Long-track Tornado: 76 Significant Tornado: 73 * Statistics are preliminary SPC reports within the domain shown above. 48-h Tornado reports ending 1200 UTC 28 April 2011

4. Assessing the Similarity Between Environments - Example 1800 UTC 27 April 2011 1800 UTC 11 April 2008 The similarity was assessed between the April 2011 case and historical data using the North American Reanalysis from 1 January 1979 to 31 December 2010 (46,752 possible historical events). 500-hPa Height Correlation: 0.985 500-hPa Height Mean Absolute Error: 23.322 m 500-hPa Height Field Score: 0.961

5. Tornado Outbreak Likelihood: 500-hPa Heights 1800 UTC 27 April 2011 Top 20 matches ranked using the 500-hPa score. Troughs are located in similar positions. 13 of 20 have closed circulations in the northern Plains.

6. Tornado Outbreak Likelihood: 500-hPa Heights 1800 UTC 27 April 2011 Top 117 (0.25%): Tornado: 14 (12.0%) Long-Track: 15 (12.8%) Significant: 15 (12.8%) Top 20: Tornado: 3 (15.0%) Long-Track: 3 (15.0%) Significant: 2 (10.0%) Tornado Outbreak Sig. Tornado Outbreak Long-Track Tornado Outbreak

7. Tornado Outbreak Likelihood: 500- and 850-hPa Heights 1800 UTC 27 April 2011 Top 20 matches ranked using the sum of the 500-hPa and 850-hPa height scores. 16 of 20 have closed circulations. 850-hPa south- southwest wind maximums are evident over the area of interest.

8. Tornado Outbreak Likelihood: 500- and 850-hPa Heights 1800 UTC 27 April 2011 Top 117 (0.25%): Tornado: 17 (14.5%) Long-Track: 18 (15.4%) Significant: 19 (16.2%) Top 20: Tornado: 5 (25.0%) Long-Track: 3 (15.0%) Significant: 5 (25.0%) Tornado Outbreak Sig. Tornado Outbreak Long-Track Tornado Outbreak

9. Tornado Outbreak Likelihood: 500-, 850-hPa Heights, and PWTR 1800 UTC 27 April 2011 Top 20 matches ranked using the sum of the 500-hPa height, 850-hPa height, and precipitable water scores. High atmospheric moisture content is evident over the area of interest.

10. Tornado Outbreak Likelihood: 500-, 850-hPa Heights, and PWTR 1800 UTC 27 April 2011 Top 117 (0.25%): Tornado: 22 (18.8%) Long-Track: 20 (17.1%) Significant: 19 (16.2%) Top 20: Tornado: 6 (30.0%) Long-Track: 5 (25.0%) Significant: 4 (20.0%) Tornado Outbreak Sig. Tornado Outbreak Long-Track Tornado Outbreak

11. Tornado Outbreak Likelihood: Supplemental Fields Mass and Wind: 300-hPa Heights 500-hPa Heights 500-hPa Isotachs 850-hPa Heights Sea-level Pressure Moisture: 2-m Dewpoint 850-hPa Mstr. Trans. Precipitable Water 600-hPa Dew. Dep. Temperature: 2-m Temperature Instability: Lifted Index It has been shown that adding atmospheric fields in the assessment methodology identifies environments more representative for tornado outbreaks. The atmospheric fields chosen to supplement the previous assessments were based on previous research and the Storm Prediction Center’s Convective Outlooks for the 26-28 April 2011 event.

12. Tornado Outbreak Likelihood: Supplemental Fields 1800 UTC 27 April 2011 Top 20 matches ranked by total score using the sum of the 11 atmospheric fields. Top 117 (0.25%): Tornado: 32 (27.4%) Long-Track: 34 (29.1%) Significant: 29 (24.8%) Top 20: Tornado: 10 (50%) Long-Track: 9 (45%) Significant: 10 (50%)

13. Tornado Outbreak Likelihood: Supplemental Fields Median = 13 Super Tuesday Tornado Outbreak Examining the distribution of tornado reports for each event in the top 117 similar historical environments provides a probabilistic expectation of tornadoes. Similar environments are conducive to multiple tornadoes, however extreme outbreaks are still very rare. Outbreak Criteria (23) 26-28 April 2011 21-22 January 1999

14. Tornado Outbreak Likelihood: Supplemental Fields Median = 2 Super Tuesday Tornado Outbreak Although 28% of similar environments did not have a long-track tornado, 25% had 6 or more long-track tornadoes. Given a severe weather 48-h period (7155), the likelihood of at least 1 long-track tornado is about 6%. However, similar environments have at least 1 long-track tornado 72% of the time. Outbreak Criteria (6) 26-28 April 2011 21-22 January 1999

15. Tornado Outbreak Likelihood: Supplemental Fields Median = 2 Super Tuesday Tornado Outbreak Similar to the long-track tornado distribution, the probability of at least one significant tornado is 73%. Consequently, not only is the April 2011 environment associated with tornadoes, but also with long-track and significant tornadoes. Outbreak Criteria (6) 26-28 April 2011 28 February and 1 March 1997

16. 1974 Super Outbreak But what about the comparison to the 1974 “Super Outbreak”? 148 tornadoes 97 significant tornadoes 80 long-track tornadoes Compared to 26-28 April 2011: 277 tornadoes 79 significant tornadoes 78 long-track tornadoes Fujita 1975

17. 1974 Super Outbreak Using the Global Reanalysis Dataset, the similarity was computed using the 11 atmospheric fields between the April 2011 case and… 1. the 1974 Super Outbreak. 2. the top 20 NARR similar environments. Scores for Top 20 Historical Environments 8.458 6.549 (#1) (#20)

18. 1974 Super Outbreak Scores for Top 20 Historical Environments 8.458 6.549 1974 Super Outbreak 8.176 (#6) (#1) (#20) Using the Global Reanalysis Dataset, the similarity was computed using the 11 atmospheric fields between the April 2011 case and… 1. the 1974 Super Outbreak. 2. the top 20 NARR similar environments.

19. Conclusions With an environment similar to the April 2011 event, tornado, long- track tornado, and significant tornado outbreaks are anticipated. The potential is even higher when analyzing the most similar environments (top 20). However, it is difficult to anticipate the magnitude and extent of the April 28 outbreak because the likelihood, even under tornado outbreak conditions, of this event is extremely rare. When the number of long-track and significant tornadoes are compared to the 1974 Super Outbreak, results are similar. Furthermore, these environments are statistically similar. This leads to speculation that there may be signals in the large-scale data that suggest the potential to anticipate catastrophic tornado outbreaks.

20. Questions and Comments? mstalley@slu.edu Questions