Significant Tornadoes in the Northern Plains: An Environmental and Pattern-Based Climatological Risk Assessment

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1 Significant Tornadoes in the Northern Plains: An Environmental and Pattern-Based Climatological Risk Assessment Chauncy J. Schultz NOAA/NWS Bismarck, ND

2 Significant tornadoes have occurred in all corners of North Dakota (and the region). Northwood EF4 (2007) Fargo F5 (1957) Credit: Nathan Heinert and JP Martin (NOAA/NWS Bismarck, ND)

3 Does relying only on reports quantify the true annual risk of significant tornadoes? Average Days/Year ND 2.0 SD 2.6 MN 2.5 Observed F/EF2+ Tornadoes ( ) Average Events/Year ND 3.0 SD 4.5 MN 5.4

4 Past F/EF2+ tornadoes in the region occurred with moderate MLCAPE and SRH. 00 UTC Soundings within 185 km of F/EF2+ Tornadoes in the Northern Plains were collected ( ; 96 total) MLCAPE ML LCL Height 0-1-km SRH Field Mean Value 1471 J/kg 1204 m AGL 123 m2/s2 Significant Tornado Parameter 1.45 Vorticity Generation Parameter 0.33 What can we learn from the variability in these ingredients? What drives significant tornado potential in the region?

5 An unconditional environment climatology was created using a vast sounding sample. March-November soundings were collected from 1998 to 2017 (20 years; ~16,500 total soundings) KBIS (681) Soundings with MLCAPE > 150 J/kg were used to develop the final climatology. KABR (864) KMPX (778)

6 0-1-km SRH is in the significant-tornadofavorable range only ~10-20% of the time. 92% 22% 16% 29% 25% 10% 28% 26% 25% MLCAPE and SRH thresholds based on regional climatology; BRN thresholds per Thompson et al. (2003)

7 Supercell Composite Parameter (SCP) can also be used to assess mesocyclone risk. 29%

8 ML LCL heights are >1500 m AGL in 39% of favorable SRH and SCP environments.

9 STP values in a significant-tornadofavoring range occur infrequently. 7.6 (24 May 2010) Calendar Day is shown (Not Sounding Date) 16.0 (23 July 2005) 10.9 (22 May 2010) Soundings with STP >1.5 ABR 38 (4.4%) BIS 16 (2.3%) MPX 88 (11.3%) Average Events/Year ABR 1.9 BIS 0.8 MN 4.4 Significant Tornado Parameter Extremes

10 Another way to assess climatological risk is looking at a BIG outbreak - 17 June Only 9 days since 1950 have had more than one violent tornado in the region.

11 The 17 June 2010 outbreak featured a strong low along the Canadian border.

12 An analog-matching system was used to help quantify the rarity of the 2010 event. North American Regional Reanalysis from compared against 00 UTC 18 June 2010 Fields Mass Temperature Moisture 300 mb Height Precipitable Water 500 mb Height mb RH 850 mb Height 850 mb Temperature 850 mb Mixing Ratio MSLP 2 m Temperature 2 m Dewpoint Scoring is done by combination of area-averaged mean absolute difference and spatial correlation. Out of ~14,000 potential analogs, 61 close matches were found to the 2010 outbreak.

13 On average, there were 1.63 average analog matches to 17 June 2010 per year. Little correlation between frequency and ONI or PDO. 5 (2005) 4 (2016) 3 (1991) 3 (1993) 3 (2012) 0 (1984) 0 (1985) 0 (1995) 0 (2003)

14 A majority of the close analog matches produced at least one tornado. Nontornadic Tornadic Of those cases that produced at least one tornado, over 50% of them produced an EF2+ event. Distribution for all 61 Analogs Distribution for Analogs Producing Tornadoes

15 Several regionally-memorable tornado events are among the analogs. The Mountain, ND Outbreak was the 32 nd -best match to 17 June Other significant events with high overall analog scores: 6 June 2004 (ND) 11 July 2008 (MN)

16 The top-scoring analog was actually a null event for ND/SD/MN tornadoes, that is. 17 June July 2015

17 Sub-synoptic subtleties may ultimately dictate tornado potential in this pattern. The other two events in the top-ten analog matches that were non-tornadic in ND, SD, and MN were still significant-severe-producing synoptic waves.

18 Concluding Remarks An environment- and analog- based approach to significant tornado risk assessment provides opportunities for training, simulations, and community preparedness for extreme events. ML LCL height may be of particular importance in fostering significant tornado environments across the region, but what would an in-depth, STP-focused analysis reveal? Acknowledgements: Tara Schultz (scripts), Dr. Chuck Graves (analogs), and Greg Gust (talk suggestion).