# Presentation: 369420

## Radar and Warning Strategies for Tropical Cyclone Tornadoes

- 2008 NOAA in the Carolinas Hurricane Conference

***Presentation by WFO Raleigh, NC***

## Why TC tornadoes are especially difficult

- “Needle in a haystack”: small, shallow feature within a much larger storm
- Often lack signatures seen with classic supercell tornadoes
- Hooks
- Appendages
- Long-lived updrafts
- Factors serving to increase instability are subtle:
- Small areas of thinner clouds or sunshine
- Mid level dry slot can be narrow or difficult to see on water vapor imagery
- Lapse rates often moist adiabatic
- Storm-relative rotation can be difficult to detect within the fast mean layer winds within a TC
- Shallow and narrow supercells can be tough to detect if they’re too far from the radar
- Low level boundaries (favored zone for TC tornadogenesis) can be difficult to discern within a TC environment

## Recent TC Tornado Research

- Schneider and Sharp (WAF, April 2007) reviewed all TC tornado events from 2004 to find the best predicting signatures:

- Gaston (Aug. 2004)

- Frances (Sep. 2004)

- Ivan (Sep. 2004)

- Jeanne (Sep. 2004)

- Tornado in
- Hoke County

- Numerous tornadoes
- across southern NC

- Tornado in
- N Guilford County
- (Stokesdale) and
- 79 mph gust at RDU

- Tornadoes in
- Moore, Richmond,
- and Wake counties

## Recent TC Tornado Research

- Schneider and Sharp (WAF, April 2007) findings:
- All tornadoes had **gate-to-gate velocity couplet** at some point before touchdown.
- All tornadoes had at least 15 kts rotational velocity (Vr); most had a **Vr of 20 kts or greater**.
**Vr of 25 kts or greater had a tornado every time**.
- Nearly every **hook signature** produced a tornado, but not every tornado had a hook signature.
**Enhanced velocities at 1.5****° slice (~7-14 kft)** was often a precursor to a tornado (but not every tornado had this signature).

## TC Tornado Characteristics

- Most of the 2004 TC tornadoes were bottom-up type development – no descending mesocyclone.
- Rotation was typically confined below 15,000 ft and only detectable in the lowest 2 scans
- Rotation signatures may last for only 2 or 3 volume scans – requires a quick warning decision.
- 		So, how can you get good lead time?...

## Situational Awareness

- Know the environment and how it is changing:
- Monitor the SPC mesoanalysis page (i.e. LCL, low level helicity, subtle boundaries, stability gradients)
- Wind profiles, via 88D VWP products, profilers, & land and aircraft (AMDAR) soundings
- Look for boundaries in radar, satellite, and surface data (areas where helicity is potentially enhanced and surface dewpoint spread is narrower)
- North and northeast quadrants are favored areas

## Situational Awareness

- Monitor water vapor imagery for dry slot:
- Can lead to reduced sky cover and more rapid destabilization
- Curtis (WAF, 2004) examined 13 tornado outbreaks associated with tropical cyclones; 11 of 13 identified with mid level dry air intrusion
- Both Ivan and Jeanne had significant dry air intrusion (and both events saw most severe occurrence of wind damage)
- Watch visible imagery for areas of sunshine or thinning clouds

## Jeanne

**Jeanne**
**September 27-28, 2004**
**12z**

## Jeanne

**Jeanne**
**September 27-28, 2004**
**18z**

## Typical TC tornado environment

| Parameter | Low Threat | High Threat |
| --- | --- | --- |
| Location with respect to storm center | 120 - 359 deg | 1 - 120 deg |
| Lifted Index (sfc) | ≥ - 1 | ≤ -2 |
| CAPE (sfc) | < 500 j/kg | ≥ 500 j/kg |
| 0 - 3 km shear | < 20 m/s | ≥ 20 m/s |
| SRH | < 100 m2/s2 | ≥ 100 m2/s2 |
| BRN | < 10 or > 50 | 10 to 50 |
| 850 mb wind speed | < 30 kt | ≥ 30 kt |
| Storm motion | < 10 mph or > 30 mph | 10 to 30 mph |
| Axis of low level convergence over area | NO | YES |

- From McCaul (MWR, 1991)

## Recommended Warning Criteria (used at WFO Raleigh)

_**Gate-to-gate Rotational Velocity of 15 kts**_** = Getting interesting; pay close attention. Warn if there is a hook or velocity enhancement aloft with it or if it is far from the radar. (Good lead time possible, but high FAR)**
**Watch for a tightening circulation to < 3 mi diameter**
_**GTG Rotational Velocity of 20 kts**_** = Go for it (can get 15-20 minutes lead time).**
_**GTG Rotational Velocity of 20 kts + hook or velocity enhancement aloft**_** = WARN NOW! (can get 10-15 minutes lead time)**
**Use lower thresholds when far from the radar, and take advantage**
**of other radars and TDWRs.**
**Reflectivity signatures will not be seen far from the radar.**

## A few of our radar techniques:

- At least one person only focuses on the tornado threat – nothing else
- Sectorization based on the spiral bands
- Use reflectivity to track strongest cells
- We use VCP 121 to reduce range folding
- Lower slices scanned at different PRFs (pulse repetition frequencies) to reduce second trip echoes
- Watch cells with a history more closely...
- Look at other offices’ radars when needed...

## Gaston

**Gaston**
**August 29, 2004**
**Harnett County**
**History of a tornado, plus a hook!**

- Max Vr was 19 kts with this cell...
- but obviously range folding was a problem

## Using other radars

-  Anson County, from KRAX

## A new finding: Velocity Enhancement Signature (VES)

- Typically found at 1.5 - 2.4 ° (~7-14 kft)
- Located above low-level inflow region of storm
- Preceded all but one of the studied tornadoes

## Example:  Hurricane Jeanne 

- Rain bands associated with Jeanne produced several tornadoes over the Sandhills and Piedmont of North Carolina. Tornado touchdowns were reported in Moore, Richmond and Wake Counties.

## Jeanne  18Z Mon Sep 27, 2004

**Jeanne  18Z Mon Sep 27, 2004**

- Boundary in place

- Upper divergence to enhance lift

- Very high SRH

- Instability gradient = boundary

## Jeanne  0.5 Base Ref 2118Z Mon Sep 27, 2004

## Jeanne  4 Panel SRM 2118Z Mon Sep 27, 2004

## Jeanne  0.5 Base Ref 2124Z Mon Sep 27, 2004

## Jeanne  4 Panel SRM 2124Z Mon Sep 27, 2004

## Jeanne  4 Panel Base Ref 2145Z Mon Sep 27, 2004

**Jeanne  4 Panel Base Ref 2145Z Mon Sep 27, 2004**

## Jeanne  4 Panel SRM 2145Z Mon Sep 27, 2004

**Jeanne  4 Panel SRM 2145Z Mon Sep 27, 2004**

## Initial touchdown ~ 541 pm

**Initial touchdown ~ 541 pm**

**534 pm**

- Vr = 32 kts

## Thank you for your attention!

- 2008 NOAA in the Carolinas Hurricane Conference