# Presentation: 756539

## Verification of NHC Forecasts of Extratropical Transition 

**Verification of NHC Forecasts of Extratropical Transition **

- TROPICAL PREDICTION CENTER  NATIONAL HURRICANE CENTER

- JACK BEVEN

**WHERE  AMERICA’S  CLIMATE  AND  WEATHER  SERVICES  BEGIN**

## What is extratropical transition?

**My conceptual model**

**Hart and Evans Cyclone Phase Space**

**ET is the process by which a tropical cyclone loses tropical characteristics in a baroclinic environment.  This can end in a variety of cyclone structures.**

## Why are analyzing and forecasting extratropical transition important?

**Tropical cyclones undergo significant structural changes during ET, which can include (re-)intensification, expansion of the wind field, and modified precipitation distribution (all seen in Noel of 2007)**
**Forecast/warning procedures differ greatly for tropical and non-tropical cyclones**

## Hurricane Lili (1996)

**“****Obviously we have little skill with Lili in forecasting extratropical transition” – Lixion Avila**

| Fcst Time | Mean Error (hr) |
| --- | --- |
| 12 hr | 39.43 |
| 24 hr | 99.00 |
| 36 hr | 27.00 |
| 48 hr | 42.00 |
| 72 hr | 24.00 |

## Verification for ET Forecasts

**Based on yes-no 2-D contingency diagram based on whether ET occurred/was forecast **_**at any time**_** in the forecast period **
**Two versions: One includes forecasts for all TCs, while the other  includes only TC where ET was forecast and/or did occur – both versions done for 5-day and 3-day forecasts**
**Includes verification of the timing of the ET forecast – how wrong temporally was the forecast of ET when ET occurred**

## Verification Table for 2005

| 
663 forecasts | 
Yes | 
No |
| --- | --- | --- |
| 
Yes | 155
(successfully forecast ET) | 57
(false alarms) |
| 
No | 49
(missed transitions) | 402
(successfully forecast non-ET) |

**ET Observed**

**ET Forecast**

## Verification does not include...

**Forecast track, intensity, or wind radii of the cyclone during the extratropical phase**

## Verification Skill Measures

| Skill Measure | Range | Good Values |
| --- | --- | --- |
| Critical Success Index | 0.0 to 1.0 | Close to 1.0 |
| Probability of Detection | 0.0 to 1.0 | Close to 1.0 |
| False Alarm Ratio | 0.0 to 1.0 | Close to 0.0 |
| Percentage Correct | 0.0 to 1.0 | Close to 1.0 |
| Event Bias | 0.0 to ? | Close to 1.0 |
| Gilbert Skill Score | -0.33 to 1.0 | Close to 1.0 |
| Heidke Skill Score | -1.0 to 1.0 | Close to 1.0 |
| True Skill Statistic | -1.0 to 1.0 | Close to 1.0 |

**Verification Skill Measures**

## 5-Day ET Timing Errors/Biases

## 3-Day ET Timing Errors/Biases

## 2005 Errors/Biases by Storm

## 3-Day Individual Time Errors

**Mean ET Forecast Error (hr)**

## 3-Day Individual Time Biases

**Mean ET Forecast Bias (hr)**

## 3-Day Skill Scores

## 3-Day Skill Scores

## Methodology Issues

**TPC discrete forecast intervals and timing**
**Possible best track inconsistencies in determining if transition occurred and transition time, especially before the development of the cyclone phase space**
**Forecasts of system becoming ET, then no actual forecast positions as an ET cyclone**

## Issues with Storms

**Systems that dissipated upon merging with fronts (Cindy 1999, Florence 2000, Cristobal 2002, Kyle 2002, TD14 2002, Irene 2005)**
**Track forecast problems (Katrina 1999, Gamma 2005, Gordon 2006)**
**Intensity forecast problems (Mindy 2003, Bonnie 2004, Debby 2006)**
**Aborted transitions (Alberto 2000)**
**Slow or interrupted transitions (Lili 1996, Gabrielle 2001, Maria 2005, Helene 2006)**
**Unclimatological transitions (Michelle 2001)**
**Unclimatological non-transitions (Humberto 2001, Juan 2003)**
**Transitions or lack thereof over mid-America (Lili 200, Ivan 2004, Arlene 2005, Katrina 2005)**
**Environmental mis-reads? (Henri 2003, Hermine 2004, Matthew 2004, Philippe 2005, Epsilon 2005)**

## Conclusions

**NHC forecasts of extratropical transition for the most part have skill according to the measures of the 2-D contingency table verification**
**ET forecasts have shown a general increase in quality since 1996.  This is likely due to better NWP models, as well as the introduction of the Hart and Evans Cyclone Phase Space diagrams**