Merging StormsMerging Storms& &

TornadogenesisTornadogenesisJeff Schild, NWS RAPJeff Schild, NWS RAP

Matt Bunkers, NWS RAPMatt Bunkers, NWS RAP

88thth Annual Northern Plains Convective Annual Northern Plains Convective WorkshopWorkshop

Sioux Falls, South DakotaSioux Falls, South Dakota

14-16 April 200414-16 April 2004

OverviewOverview

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

BackgroundBackground

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

Stout and Hiser (1955)Stout and Hiser (1955)

Tornado developed as two echoes merged – formed near the edges of where the echoes first touched.

Lemon (1976)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.

Bluestein and Parker Bluestein and Parker (1993)(1993)

Three storms merged into one large cell, and a small tornado was produced 20 minutes later.

Browning et al. (1997)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.

Kulie and Lin (1998)Kulie and Lin (1998)

Modeled the 28 November 1988 Raleigh, Modeled the 28 November 1988 Raleigh, NC tornadic supercellNC tornadic supercell

Storm’s intensity maintained through Storm’s intensity maintained through frequent mergers of updraft maxima (from frequent mergers of updraft maxima (from flanking line)flanking line) Similar to Lemon (1976)Similar to Lemon (1976)

Finley et al. (2001)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).

Dowell and Bluestein Dowell and Bluestein (2002)(2002)

Tornadogenesis right near time of storm merger along the flanking line.

T

Lindsey and Bunkers Lindsey and Bunkers (2004)(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.

Other local examplesOther local examples 9 June 2003 – merger about 45 min prior to tornado at 9 June 2003 – merger about 45 min prior to tornado at

WinnerWinner

24 June 2003 – merger about 45 min prior to tornado 24 June 2003 – merger about 45 min prior to tornado at Manchesterat Manchester

17 March 2004 – several mergers with supercells in 17 March 2004 – several mergers with supercells in eastern OK, but no tornadoeseastern OK, but no tornadoes

26 March 2004 – two “flanking line” mergers with 26 March 2004 – two “flanking line” mergers with supercell in central Meade County (storm briefly supercell in central Meade County (storm briefly intensified), but no tornadoes (despite favorable intensified), but no tornadoes (despite favorable environment)environment)

Interim SummaryInterim Summary What few studies exist on this subject support a What few studies exist on this subject support a

constructive interaction between mergers and constructive interaction between mergers and tornadogenesistornadogenesis Merger of discrete cells – may be constructive or Merger of discrete cells – may be constructive or

destructivedestructive Merger of flanking line echoes with main storm – Merger of flanking line echoes with main storm –

typically intensifies the storm/updrafttypically intensifies the storm/updraft Local experience presents a mix of resultsLocal experience presents a mix of results

Physical processes attending storm mergers and Physical processes attending storm mergers and tornadogenesis poorly understood (mostly tornadogenesis poorly understood (mostly empirical)empirical) More study needed – climatologies and modelingMore study needed – climatologies and modeling

Merging Storms & Merging Storms & TornadogenesisTornadogenesis

22ndnd part part

ObjectivesObjectives

Show example of merging storms.Show example of merging storms. Discuss velocity data associated with Discuss velocity data associated with

storm before and after merger.storm before and after merger. Discuss when storm became Discuss when storm became

tornadic with respect to merger.tornadic with respect to merger. Discuss storm traits after merger.Discuss storm traits after merger.

Example of Merging Example of Merging SupercellsSupercells

June 23, 1998June 23, 1998 2 cells merged.2 cells merged. Became tornadic upon Became tornadic upon

initiation of the merger.initiation of the merger.

Photo taken by NWSemployee from the Rapid City NWS officefacing north.

Loop of StormLoop of Storm

Tornado ReportsTornado Reports

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

22:56 Z Reflectivity22:56 Z Reflectivity

-Strong storm moving slowlyeastward in western MeadeCounty.

-Over 1 hour before firsttornado report.

22:56 Z Relative Velocity22:56 Z Relative Velocity

-Velocity not showing anyindication 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 ftAGL with areas of over50 knot inflow at ~1,000 ft AGL level.

23:21 Z Reflectivity23:21 Z Reflectivity-Storm is starting to give indication of a “hook echo”.

-On Northwest side of cell,second storm is just starting to form.

23:21 Relative Velocity23:21 Relative Velocity-Outbound of over 50 ktsat 4,000ft AGL on northwestside of apparent hook.

-Not apparent on lower elevationscan.

-On 2.5° scan appears as over50 knots inbound.

-Not apparent on previous scanor on later scan.

-Conclusion: Bad velocity data for that area on this scan.

23:36 Z Reflectivity23:36 Z Reflectivity-2 cells moving parallel to each otherto the east at approximately 5 knots.

-Cell to south still has the “hook” signature.

23:36 Z Relative Velocity23:36 Z Relative Velocity-Southern cell is showingV >50 kts over 0.7 km.

23:46 Z Reflectivity23:46 Z Reflectivity-Storm to the north is aboutto make a southerly move and merge into the original storm.

23:46 Z Relative Velocity23:46 Z Relative Velocity

-One can clearly see coupletat this point.

-Gate to gate V of over 60 kts.

-Tornado is not reported for another 20 minutes.

23:56 Z Reflectivity23:56 Z Reflectivity-Merging is occurring at this pointwhile storm to the north comes around and into the back-side.

-First report of tornado is at 00:02.

23:56 Z Relative Velocity23:56 Z Relative Velocity

-Couplet remains very apparent.

-Gate to gate Vof over 70 kts.

00:11 Z Reflectivity00:11 Z Reflectivity-At this point stormsalmost appear completelymerged.

-Tornado still on ground.

00:11 Z Relative Velocity00:11 Z Relative Velocity-Gate to gate V has weekend to less than 50 kts.

-Tornado still on ground

-Rotation towards the westis becoming apparent.

00:31 Z Reflectivity00:31 Z Reflectivity-Second tornado report at00:30 UTC.

-2 “hooks” appear, tornadois affiliated with the one towardsthe west.

00:31 Z Relative Velocity00:31 Z Relative Velocity-Though the couplet towardsthe east appears much strongerat this point, the one towards thewest is the one producing the tornado.

00:46 Z Reflectivity00:46 Z Reflectivity-2 “hooks” very apparent.

-Forward motion of storm is increasing at this point.

-Tornado affiliated with hook to the west.

00:46 Z Relative Velocity00:46 Z Relative Velocity-Though shear towards the east appears stronger, stormreport affiliates tornado withwestern hook.

Gate to gate V 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 fromradar?East: 11nmWest 13nm

01:11 Z Reflectivity01:11 Z Reflectivity-Merged storm now moveseastward at 25 knots.

One last lookOne last look

SummarySummary

Storm was not tornadic until the Storm was not tornadic until the apparent onset of the merging process.apparent onset of the merging process.

Upon merger, relative forward speed Upon merger, relative forward speed of the storm system increased from 5 of the storm system increased from 5 knots to 25 knots. knots to 25 knots.

After merger when two “hooks” were After merger when two “hooks” were apparent, the one on the west side apparent, the one on the west side produced the tornadoes, though the produced the tornadoes, though the one towards the east had an one towards the east had an apparently stronger couplet.apparently stronger couplet.