Interactive Analysis of 28 August 2002 Supercell Event to Determine Warning Type

1
Interactive Analysis of 28 August 2002 Supercell
Event to Determine Warning Type

• Dave Kellenbenz
• National Weather Service Eastern North Dakota
  Grand Forks
• Image courtesy NWS/Rapid City web page (8/8/02)

2
Area of interest where supercell developed
3
Objectives

• Synoptic Overview
• Surface analysis and brief sub-synoptic/mesoscale
  overview
• Stability and shear indices overview
• Evolution of supercell and warning decision
  scenarios along the way
• What weather was produced by supercell
• Hypothesis and Research into event

4
250 Hpa 8/29/02 00 UTC

• Departing Right entrance region of 30kt jet and
  upper divergence near area of interest

5
500 Hpa 8/29/02 00 UTC

• Weak shortwave moves through

6
850 Hpa 8/29/02 00 UTC

• 20-25kt Jet during supercell lifecycle

7
Surface Map at 20 UTC

• Wahpeton, ND Wind Dir _at_ 20Z 169 degrees at 12
  mph

8
Surface Map at 23 UTC

• Wahpeton, ND Wind Dir was 100 degrees _at_ 23Z at 21
  mph, as winds back and increased with enhanced
  inflow into supercell.

9
Model RUC sounding near Wahpeton at 20 UTC

• Fairly weak flow with high instability (CAPE 3870
  J/Kg) and only 3 J/Kg CIN, freezing level 13,200
  ft

10
Hodograph at 20 UTC

• 0-3km SRH between 90-110 M2/S2
• EHI 2-2.5

11
Warning scenario 1 (2113-2128 UTC).5
Reflectivity Loop (KMVX)

• Supercell developing rapidly in 20 minutes

12
Radar Cross Section and 0.5 Reflectivity-KMVX
2113 UTC
2114 UTC

• Supercell begins to develop with elevated core

13
10-15 minutes later
2128 UTC
2124 UTC

• Supercell has developed rapidly with 50-55dbz
  core up to 40k ft

14
Mid level reflectivity 2114 UTC and 2120 UTC
VIL 31 (Kg/m2)
VIL 57 (Kg/m2)

• Reflectivity core grows quickly in 6 minutes, as
  VIL inc by 26 Kg/m2

15
Mid level reflectivity at 2125 UTC and SRM at
2129 UTC
VIL 62 (Kg/m2)

• Some weak cyclonic and anti-cyclonic rotation

16
Warning decision time, what to do?

• No reports of severe weather as of 2129 UTC or
  429 pm.
• A) Tornado warning Richland/Wilkin County?
• B) Severe Thunderstorm warning Richland/Wilkin
  County?
• C) Flash Flood warning Richland/Wilkin County?
• No warning needed, go get some coffee ?
• Quarter size hail reported in Wahpeton at 2133
  UTC, along with numerous other dime-quarter hail
  reports until 2145 UTC

17
Warning scenario 2 (2130-2200 UTC).5
Reflectivity Loop(KMVX)
18
2.4 Reflectivity/SRM 2139 UTC (KABR)
Weak mid level circulation
19
0.5 Reflectivity/SRM 2142 UTC (KABR)

• Weak circulation now showing up in lowest slice

20
Base Reflectivity at 2143 UTC 2149 UTC (KMVX)

• TVS indicated over Wahpeton with inflow notch
  developing

21
Reflectivity Cross Section at 2154 UTC

• Good elevated core with some tilt and slight BWER

22
0.5 Reflectivity/SRM 2153 2158 UTC (KMVX)
2153 UTC
2158 UTC

• Hook echo trying to form
• Spotter report from Wahpeton wall cloud
  beginning to form with scud lifting rapidly into
  updraft _at_ 2155 UTC

23
Warning Decision time, what to do at 2200 UTC?

• A) Keep Severe Thunderstorm Warning going
• B) Upgrade to Tornado Warning
• C) Issue Flash Flood Warning
• D) Drop all warnings and send everyone home
• E) Time for more coffee ?!!!
• Stay Tuned !!!

24
Warning scenario 3 (2158-2245 UTC).5
Reflectivity Loop(KMVX)
25
0.5 Reflectivity 2203 2208 UTC (KMVX)
2203 UTC
2208 UTC

• Hook trying to develop with strong updraft noted
  by donut hole at 2208 UTC

26
0.5 Reflectivity 2213 2218 UTC (KMVX)

• Spotter reports of rapidly rotating wall cloud

27
Base Ref 2233,2238 and 2243 UTC (KMVX)

• Signs of RFD developing and inflow beginning to
  fill in with precipitation, still well defined
  wall cloud/funnels over Wahpeton

28
0.5 Reflectivity/2.4 SRM 2247 UTC (KABR)
Weak Mesocyclone still present one hour after
supercell develops
29
Warning(s) are expiring, what to do at 2245 UTC?

• Gettting reports of wallcloud/funnel clouds from
  2155-2245 UTC near Wahpeton
• VIL ranging from 60 to 65 Kg/m2 during time frame
• A) Flash Flood warning
• B) Severe thunderstorm warning
• C) Tornado warning
• D) Make some coffee because its gonna be a long
  night ?
• Stay Tuned !!!

30
0.5 SRM 2254 UTC and 2259 UTC(KMVX)
gate to gate shear in lowest elevation angle for
consecutive scans
31
Spectrum Width at 2254 UTC
SW indicating turbulence at same time TVS
indicated over Wahpeton
32
Base Ref. 2238-2319 UTC (KMVX)

• Hook quickly fills as precipitation chokes
  supercell updraft

33
Ref _at_ 2254 UTC and SRM _at_ 2259 UTC
TVS triggered 2254 UTC with pronounced hook echo
at 2259 UTC
34
What do you think the supercell produced ?

• Tornado/Hail/Flash Flooding
• Just some Hail
• Hail and Flash Flooding
• Answer C - No Tornadoes were reported, with
  many hail reports up to golf ball size along with
  Flash Flooding
• Wall clouds/funnel clouds and strong
  rotation were reported from 2155Z until 2300Z in
  Wahpeton ND
• Spotters reported that they thought a
  tornado would form at any minute
• Continuous training and re-development of
  cell(s) produced 3 to 6 inches of rain and caused
  flash flooding in Wahpeton, ND and surrounding
  areas

35
Warning thought process

• Numerous reports of wallclouds/funnel clouds in
  combination with radar signatures through
  lifecycle of supercell influenced warning
  decisions
• Radar signatures also indicative of tornadic
  potential although not classic
• Severe thunderstorm warning issued at 2129 UTC
  for Richland/Wilkin County
• Upgraded to tornado warnings at 2200 UTC and
  continued until 2330 UTC
• Flash Flood warnings issued at 2307 UTC with many
  flood reports from 2330 UTC until 0130 UTC

36
Favorable LCL Height
Edwards and Thompson 2000

• LCL height of 831m (2,227ft) fell almost into
  center of non significant tornadoes F0-F1 and
  into upper range for Significant tornadoes
  (F2-F5), lower range for non-tornadoes

37
Favorable LFC Height
Davies 2002

• LFC Height of 1,120m(3,676ft) fell between
  non-significant and significant tornadoes

38
CAPE and 0-3km SRH comparison
Edwards and Thompson 2000

• 4000 CAPE and around 100 SRH in range with both
  tornadic and non tornadic supercells

39
Images of Supercell taken by storm chaser Mike
Hollingshead

• Classic Wall cloud and funnel clouds

40
Favorable factors for tornadogenesis

• Ideal LFC(1,120m or 3676ft) and LCL(831m or
  2727ft) heights from Davies (2002) and
  Edwards/Thompson (2000) guaranteeing a more
  surface based supercell and lower CIN
• Favorable radar signatures for up to one hour
• Approaching low pressure which helped back low
  level flow thus increasing 0-1km SRH and focus
  inflow
• High instability

41
Negative factors for tornadogenesis

• Only little tilt of supercell with precipitation
  core over updraft
• Deep layered shear was rather weak
• Numerous cells eventually developing in same area
  cutting off inflow to supercell
• Little movement of supercell enabled updraft to
  ingest rain cooled air

42
Possible Cold RFD at work
Markowski 2003

• cold downdraft air is not easily ingested by the
  updraft

43
Images of Supercell taken by storm chaser Mike
Hollingshead

• RFD undercutting updraft and wrapping rain cooled
  air into it

44
Schematic and picture of warm RFD
Markowski 2003

• warm downdraft air is readily ingested by the
  updraft

45
Hypothesis and future research

• Deep layered shear may have been a bit too
  weakbut shear was high ?
• Best rotation remained under core of storm, and
  supercell remained nearly stationary which likely
  lead to cold type RFD
• Needs to be more research on tornadogenesis
  failure, as these are the toughest cases
• Possible real time analysis of RFD type
  (cool/warm) using LFC/LCL and proximity
  soundings/sfc reports would help reduce tornado
  FAR
• Local study with UND to compare LCL/LFC height
  with past tornado/non-tornado cases to determine
  in effort to classify RFD type for northern
  plains area with downdraft model written by Mark
  Askelson

46
Questions, Comments ???

• Dave Kellenbenz
• National Weather Service Eastern North Dakota
• Grand Forks
• David.Kellenbenz_at_noaa.gov