Merging Storms & Tornadogenesis

1. Merging Storms& Tornadogenesis Jeff Schild, NWS RAP Matt Bunkers, NWS RAP 8th Annual Northern Plains Convective Workshop Sioux Falls, South Dakota 14-16 April 2004

2. Overview • 1st part of presentation will focus on literature review of merging storms and tornadogenesis • 2nd part of presentation will focus on a specific example near the Black Hills

3. Background • Relatively unstudied phenomenon • May disrupt tornadogenesis • May promote tornadogenesis • Review of studies suggests a bias toward “favorable” interactions • Merging storms and tornadoes • Can be merger of two discrete storms • Can be merger of small elements from flanking line, or even outflow from another storm

4. Stout and Hiser (1955) Tornado developed as two echoes merged – formed near the edges of where the echoes first touched.

5. Lemon (1976) Convective cells along the flanking line can lead to the intensification of the adjoining supercell through a merger process. No discussion about tornadoes, but an F2 tornado occurred at 1700 CST.

6. Bluestein and Parker (1993) Three storms merged into one large cell, and a small tornado was produced 20 minutes later.

7. Browning et al. (1997) F3 tornado developed after outflow (from an old storm, A) reached its southern flank. Also see Weaver and Purdom (1995) for similar interactions.

8. Kulie and Lin (1998) • Modeled the 28 November 1988 Raleigh, NC tornadic supercell • Storm’s intensity maintained through frequent mergers of updraft maxima (from flanking line) • Similar to Lemon (1976)

9. Finley et al. (2001) A merger of supercells (S2 and S1) lead to an intensification of the main updraft, and was followed by tornadogenesis. The supercell eventually transitioned to HP and then bow echo. Also see Klimowski et al. (2003, 2004).

10. Dowell and Bluestein (2002) T Tornadogenesis right near time of storm merger along the flanking line.

11. Lindsey and Bunkers (2004) Right-moving supercell produced a tornado both before and after a merger with a left-moving supercell. This storm eventually produced the long-track Pierce City, MO tornado.

12. Other local examples • 9 June 2003 – merger about 45 min prior to tornado at Winner • 24 June 2003 – merger about 45 min prior to tornado at Manchester • 17 March 2004 – several mergers with supercells in eastern OK, but no tornadoes • 26 March 2004 – two “flanking line” mergers with supercell in central Meade County (storm briefly intensified), but no tornadoes (despite favorable environment)

13. Interim Summary • What few studies exist on this subject support a constructive interaction between mergers and tornadogenesis • Merger of discrete cells – may be constructive or destructive • Merger of flanking line echoes with main storm – typically intensifies the storm/updraft • Local experience presents a mix of results • Physical processes attending storm mergers and tornadogenesis poorly understood (mostly empirical) • More study needed – climatologies and modeling

14. Merging Storms & Tornadogenesis 2nd part

15. Objectives • Show example of merging storms. • Discuss velocity data associated with storm before and after merger. • Discuss when storm became tornadic with respect to merger. • Discuss storm traits after merger.

16. Example of Merging Supercells • June 23, 1998 • 2 cells merged. • Became tornadic upon initiation of the merger. Photo taken by NWS employee from the Rapid City NWS office facing north.

17. Loop of Storm

18. Tornado Reports • Three tornadoes affiliated with this storm. • First at 00:02 UTC (during merger) • Second at 00:30 UTC (after merger) • Third at 00:45 UTC (after merger)

19. 22:56 Z Reflectivity -Strong storm moving slowly eastward in western Meade County. -Over 1 hour before first tornado report.

20. 22:56 Z Relative Velocity -Velocity not showing any indication of rotation at this level, but at 10,000 ft AGL rotation is indicated. -Apparent area of 30 to 40 knot inflow at 3,300 ft AGL with areas of over 50 knot inflow at ~1,000 ft AGL level.

21. 23:21 Z Reflectivity -Storm is starting to give indication of a “hook echo”. -On Northwest side of cell, second storm is just starting to form.

22. 23:21 Relative Velocity -Outbound of over 50 kts at 4,000ft AGL on northwest side of apparent hook. -Not apparent on lower elevation scan. -On 2.5° scan appears as over 50 knots inbound. -Not apparent on previous scan or on later scan. -Conclusion: Bad velocity data for that area on this scan.

23. 23:36 Z Reflectivity -2 cells moving parallel to each other to the east at approximately 5 knots. -Cell to south still has the “hook” signature.

24. 23:36 Z Relative Velocity -Southern cell is showing DV >50 kts over 0.7 km.

25. 23:46 Z Reflectivity -Storm to the north is about to make a southerly move and merge into the original storm.

26. 23:46 Z Relative Velocity -One can clearly see couplet at this point. -Gate to gate DV of over 60 kts. -Tornado is not reported for another 20 minutes.

27. 23:56 Z Reflectivity -Merging is occurring at this point while storm to the north comes around and into the back-side. -First report of tornado is at 00:02.

28. 23:56 Z Relative Velocity -Couplet remains very apparent. -Gate to gate DV of over 70 kts.

29. 00:11 Z Reflectivity -At this point storms almost appear completely merged. -Tornado still on ground.

30. 00:11 Z Relative Velocity -Gate to gate DV has weekend to less than 50 kts. -Tornado still on ground -Rotation towards the west is becoming apparent.

31. 00:31 Z Reflectivity -Second tornado report at 00:30 UTC. -2 “hooks” appear, tornado is affiliated with the one towards the west.

32. 00:31 Z Relative Velocity -Though the couplet towards the east appears much stronger at this point, the one towards the west is the one producing the tornado.

33. 00:46 Z Reflectivity -2 “hooks” very apparent. -Forward motion of storm is increasing at this point. -Tornado affiliated with hook to the west.

34. 00:46 Z Relative Velocity -Though shear towards the east appears stronger, storm report affiliates tornado with western hook. Gate to gate DV values for 1.5° scan: >42 kts for western hook >50 kts for eastern hook for 0.5° scan: >52 kts for western hook >72 kts for eastern hook Affected by distance from radar? East: 11nm West 13nm

35. 01:11 Z Reflectivity -Merged storm now moves eastward at 25 knots.

36. One last look

37. Summary • Storm was not tornadic until the apparent onset of the merging process. • Upon merger, relative forward speed of the storm system increased from 5 knots to 25 knots. • After merger when two “hooks” were apparent, the one on the west side produced the tornadoes, though the one towards the east had an apparently stronger couplet.