# Presentation: 935181

## 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)

## Area of interest where supercell developed

## 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

## 250 Hpa 8/29/02 – 00 UTC

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

## 500 Hpa 8/29/02 – 00 UTC

- Weak shortwave moves through

## 850 Hpa 8/29/02 – 00 UTC

- 20-25kt Jet during supercell lifecycle

## Surface Map at 20 UTC

-    Wahpeton, ND Wind Dir @ 20Z 169 degrees at 12 mph

## Surface Map at 23 UTC

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

## 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

## Hodograph at 20 UTC

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

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

- Supercell developing rapidly in 20 minutes

## Radar Cross Section and 0.5 Reflectivity-KMVX

- Supercell begins to develop with elevated core

- 2114 UTC

- 2113 UTC

## 10-15 minutes later

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

- 2128 UTC

- 2124 UTC

## Mid level reflectivity 2114 UTC and 2120 UTC 

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

- VIL = 31 (Kg/m2)

- VIL = 57 (Kg/m2)

## Mid level reflectivity at 2125 UTC and SRM at 2129 UTC

- Some weak cyclonic and anti-cyclonic rotation

- VIL = 62 (Kg/m2)

## 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

## Warning scenario #2 (2130-2200 UTC).5 Reflectivity Loop(KMVX)

## 2.4 Reflectivity/SRM 2139 UTC (KABR)

- Weak mid level circulation

## 0.5 Reflectivity/SRM 2142 UTC (KABR)

- Weak circulation now showing up in lowest slice

## Base Reflectivity at 2143 UTC & 2149 UTC (KMVX)  

- TVS indicated over Wahpeton with inflow notch developing

## Reflectivity Cross Section at 2154 UTC

- Good elevated core with some tilt and slight BWER

## 0.5 Reflectivity/SRM 2153 & 2158 UTC (KMVX)

- Hook echo trying to form
- Spotter report from Wahpeton – “wall cloud beginning to form with scud lifting rapidly into updraft @ 2155 UTC”

- 2153 UTC

- 2158 UTC

## 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  !!!

## Warning scenario #3 (2158-2245 UTC).5 Reflectivity Loop(KMVX)

## 0.5 Reflectivity 2203 & 2208 UTC (KMVX)

- Hook trying to develop with strong updraft noted by “donut hole” at 2208 UTC

- 2203 UTC

- 2208 UTC

## 0.5 Reflectivity 2213 & 2218 UTC (KMVX)

- Spotter reports of rapidly rotating wall cloud

## 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

## 0.5 Reflectivity/2.4 SRM 2247 UTC (KABR) 

- Weak Mesocyclone still present one hour after supercell develops

## 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 it’s gonna be a long night 
- Stay Tuned !!!

## 0.5 SRM 2254 UTC and 2259 UTC(KMVX)

- gate to gate shear in lowest elevation angle for consecutive scans

## Spectrum Width at 2254 UTC

- SW indicating turbulence at same time TVS indicated over Wahpeton

## Base Ref. 2238-2319 UTC (KMVX)

- Hook quickly fills as precipitation chokes supercell updraft

## Ref @ 2254 UTC and SRM @ 2259 UTC 

- TVS triggered 2254 UTC with pronounced hook echo at 2259 UTC

## 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

## 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

## Favorable LCL Height

- 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

- Edwards and Thompson 2000

## Favorable LFC Height

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

- Davies 2002

## CAPE and 0-3km SRH comparison

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

- Edwards and Thompson 2000

## Images of Supercell taken by storm chaser Mike Hollingshead 

- Classic Wall cloud and funnel clouds

## 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

## 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

## Possible Cold RFD at work

- cold downdraft air is not easily ingested by the updraft

- Markowski 2003

## Images of Supercell taken by storm chaser Mike Hollingshead

- RFD undercutting updraft and wrapping rain cooled air into it

## Schematic and picture of warm RFD

- warm downdraft air is readily ingested by the updraft

- Markowski 2003

## Hypothesis and future research

- Deep layered shear may have been a bit too weak...but 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

## Questions, Comments ???

- Dave Kellenbenz
-  National Weather Service Eastern North Dakota
-  Grand Forks
*David.Kellenbenz@noaa.gov*