Jonathan Garner Storm Prediction Center Norman, OK

1. A Study on the Environments Associated with Significant Tornadoes Occurring Within the Warm Sector versus Those Occurring Along Boundaries Jonathan Garner Storm Prediction Center Norman, OK

2. Objectives • Examine similarities and differences between environments supporting significant tornadoes (F2+/EF2+) in the warm sector versus along boundaries during the period 1999-2010 • Basic climatological aspects associated with each category • Convective/tornado parameter space

3. Tornadoes along Boundaries Maddox et al. (1980) Markowski et al. (1998)

4. Warm Sector Significant Tornadoes Newton 1967 Fujita

5. Methodology • Storm Data, Severe Plot, and 2010 NWSFO storm survey information • Subjective surface analysis and visible satellite imagery • Classification of events (warm sector or boundary) • Archived regional reflectivity • Storm longevity • Appearance of 35 dBZ echo in regional reflectivity • Dissipation of cell or morphology into another storm type • Level II data used to evaluate rotational characteristics • Convective mode

6. Methodology • Rapid Update Cycle (RUC) hourly analysis proximity soundings • 46 soundings from Thompson et al. (2003) • 39 soundings from Thompson et al. (2007) • 6 soundings collected during 2010 • Modified for surface temperature/dewpoint and wind representative of the inflow sector of each tornadic storm • 35 Significantly Tornadic Warm Sector Events • 56 Significantly Tornadic Boundary Events

7. Climatalogical Aspects • Tornado Path Length • Supercell Longevity • Convective Mode • Boundary Interaction • Warm Sector Initiating Boundary

8. Tornado Path Length Warm Sector Sig Mean – 25 mi Boundary Sig Mean – 10 mi (99% Confidence Level)

9. Supercell Longevity Warm Sector Mean – 4.7 hrs Boundary Mean – 3.8 hrs (99% Confidence Level)

10. Convective Mode Discrete – well spaced cells with reflectivity values < 25 dBZ between each cell (Dial et al. 2010)

11. Convective Mode Linear – solid continuous line of 35 dBZ or greater reflectivity values with a length to width ratio of 5 to 1 (Dial et al. 2010)

12. Convective Mode Mixed – discrete cells occurring adjacent (usually downstream) to a linear storm mode

13. Convective Mode 51% 55% 34% 29% 3% 4% Warm Sector Boundary

14. Boundary Interaction 54% 16% 14% 12% 4% Boundary Events

15. Warm Sector Initiating Boundary 46% 37% 9% 6% Warm Sector Events

16. RUC Thermodynamic Parameters Mean Values **Difference in means is statistically insignificant for all thermodynamic parameters except for the MLLCL height.

17. MLLCL Height Warm Sector Sig Mean – 955 m Boundary Sig Mean – 1061 m (95% Confidence Level)

18. MLLCL Height • More humid boundary layer air mass would promote longer-lived supercells and significant tornadogenesis • Warm sectors which are hot, well-mixed, and less humid would make significant tornadoes less probable • However, observational uncertainty suggests that the difference in warm sector and boundary MLLCL heights cannot operationally distinguish between the two environments

19. RUC Wind Parameters Mean Values

20. 850 mb Wind Speed Warm Sector Sig Mean – 41 kt Boundary Sig Mean – 27 kt (99% Confidence Level)

21. 0-1 km Bulk Shear Warm Sector Sig Mean – 29 kt Boundary Sig Mean – 22 kt (99% Confidence Level)

22. 500 mb Wind Speed Warm Sector Sig Mean – 58 kt Boundary Sig Mean – 43 kt (99% Confidence Level)

23. 0-6 km Bulk Shear Warm Sector Sig Mean – 56 kt Boundary Sig Mean – 47 kt (99% Confidence Level)

24. Bunkers (ID Method) Storm SpeedBunkers et al. (2000) Warm Sector Sig Mean – 39 kt Boundary Sig Mean – 25 kt (100% Confidence Level)

25. Summary • Warm sector versus boundary events • Stronger ambient wind fields and vertical wind shear parameters for warm sector environments • Weaker ambient shear would confine significant tornadoes to boundaries…where shear and instability would be augmented • Thermodynamic parameters were similar for both significant tornado categories • Mean and median MLLCL heights were lower for warm sector significant tornado events • Observational uncertainty suggests this result cannot operationally distinguish between the two environments

26. Summary • ID method for predicting supercell speed • Much stronger for warm sector significantly tornadic storms • This likely reflects upon the stronger deep-layer wind fields over the warm sector • Significant tornadoes observed along low-level boundaries are most likely to occur within 10 km on the warm side to 30 km on the cold side of a boundary (Markowski et al. 1998) • A storm which has a slower storm motion would allow it to interact with a boundary for a greater amount of time, increasing the potential for a significant tornado, compared to a storm which quickly passes across the boundary • On the other hand, warm sector tornadoes have fast storm motions, thus the width of the unstable air mass must be great enough to allow time for significant tornadogenesis to occur before the parent storm moves into a more hostile downstream environment

27. Future Work • Examine in greater detail boundaries which do and do not support tornadic storms • What is the pattern of destabilization along pre-existing boundaries which support tornadic thunderstorms? • How is moisture, instability and shear distributed along boundaries? • What is the most favored mode of storm-boundary interaction?

28. Acknowledgments Steve Weiss and Rich Thompson