An Update on TBSSs and Relatively Moist Environments

Matthew J. Bunkers2006 Spring Training

Three-Body Scatter Spike (TBSS)

TBSS Review

• Radar artifact typically associated with hail ≥ 1”(high POD and low FAR)

• Observed down-radial of ~62+ dBZ core(use 4-panel and/or all-tilts displays)

• May also indicate potential for damaging winds?

• More at P:\Training\8thGreatDivide\TBSS

Recent Trends

• At least two events in 2005 with TBSSs but only limited, or no, severe hail

• Another case around the same time with TBSSs and very large hail

• What’s going on here?

#1 – 10 August 2005, 2130z

Max Ref = 68.5 dBZ

Hgt50dBZ = 20,345 ft AGL

TBSS length = 8 nm

POSH = 50 %

MEHS = 1 ¼ “ (250%)

MLT = 11,180 ft AGL

WBZ = 9,849 ft AGL

RH700-300mb = 33 %

Max Obs. Hail = ½ “*** Next five sets of images are all the at same scale

#2 – 11 August 2005, 0214z

Max Ref = 70 dBZ

Hgt50dBZ = 31,590 ft AGL

TBSS length = 12 nm

POSH = 90 %

MEHS = 2 ½ “ (143%)

MLT = 11,180 ft AGL

WBZ = 9,849 ft AGL

RH700-300mb = 33 %

Max Obs. Hail = 1 ¾ “

#3 – 25 July 2005, 0438z

Max Ref = 65.5 dBZ

Hgt50dBZ = 33,689 ft AGL

TBSS length = 8 nm

POSH = 80 %

MEHS = 2 ¼ “ (225%)

MLT = 13,322 ft AGL

WBZ = 9,885 ft AGL

RH700-300mb = 19 %

Max Obs. Hail = 1 “

#4 – 25 July 2005, 0453z

Max Ref = 65.5 dBZ

Hgt50dBZ = 33,931 ft AGL

TBSS length = 10 nm

POSH = 70 %

MEHS = 2 “ (400%)

MLT = 13,322 ft AGL

WBZ = 9,885 ft AGL

RH700-300mb = 19 %

Max Obs. Hail = ½ “

#5 – 20 July 2005, 2224z

Max Ref = 66.5 dBZ

Hgt50dBZ ≥ 31,500 ft AGL

TBSS length = 9 nm

POSH = 70 %

MEHS = 2 “ (73%)

MLT = 12,133 ft AGL

WBZ = 10,911 ft AGL

RH700-300mb = 23 %

Max Obs. Hail = 2 ¾ “

What does this show?

• Not much difference up to this point…– Similar max reflectivity, TBSS lengths, and

MLT/WBZ levels

• Generally small-to-moderate storm cores

• Is there a pattern in the moisture field that can help explain this?

#1 – 10 August 2005, 2131z (wv sat)

#2 – 11 August 2005, 0215z (wv sat)

#3,4 – 25 July 2005, 0445z (wv sat)

#5 – 20 July 2005, 2224z (wv sat)

From AWOC…

• Recall that a moist atmosphere can accelerate the hail melting process

• Flash presentation… http://www.wdtb.noaa.gov/courses/awoc/ICsvr1/ICSvr1-lesson3/ic1-lesson3_color/

• Let’s look at one final case from 2004

#6 – 3 July 2004, 2128z

Max Ref = 74.5 dBZ

Hgt50dBZ = 33,348 ft AGL

TBSS length = 22 nm

POSH = 100 %

MEHS > 4 “ (400%)

MLT = 9,153 ft AGL

WBZ = 8,017 ft AGL

*RH700-300mb = 50 %

Max Obs. Hail = 1 “*** Note this is at a smaller scale than the previous radar images

Produced flash flooding and copious small hail

03 July 2004, 2115z (wv sat)

Summary

• Phone logs indicate these storms with TBSSs and sub-severe to marginally severe hail produced very heavy rainfall and copious small hail

• Some of these storms were supercells, but still struggled to produce very large hail

• The best discriminator for these events may be the water vapor imagery

Summary

• The length of the TBSSs were below average, based on J. Johnson’s local study(however the last event was 22 nm)

0

2

4

6

8

10

12

<5 5-8 9-12 13-16 17-20 21-24 25-28 29-32 33-36 >37

Max TBSS Length (nm)

# o

f T

BS

S's

<5

5-8

9-12

13-16

17-20

21-24

25-28

29-32

33-36

>37

Summary

• The SOO proposes an additional CTA for severe thunderstorm warnings to account for these kinds of events/environments…

“This storm is capable of producing a lot of small to moderate size hail and very heavy rainfall…possibly causing low lying roads to become covered with ice.”

(or something like this)