Title: Verification of NHC Forecasts of Extratropical Transition
1Verification of NHC Forecasts of Extratropical
Transition
JACK BEVEN
TROPICAL PREDICTION CENTER NATIONAL HURRICANE
CENTER
WHERE AMERICAS CLIMATE AND WEATHER SERVICES
BEGIN
2What is extratropical transition?
My conceptual model
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.
Hart and Evans Cyclone Phase Space
3Why 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
4Hurricane Lili (1996)
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
Obviously we have little skill with Lili in
forecasting extratropical transition Lixion
Avila
5Verification 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
6Verification Table for 2005
ET Observed
663 forecasts Yes No
Yes 155 (successfully forecast ET) 57 (false alarms)
No 49 (missed transitions) 402 (successfully forecast non-ET)
ET Forecast
7Verification does not include
- Forecast track, intensity, or wind radii of the
cyclone during the extratropical phase
8Verification 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
95-Day ET Timing Errors/Biases
103-Day ET Timing Errors/Biases
112005 Errors/Biases by Storm
123-Day Individual Time Errors
Mean ET Forecast Error (hr)
133-Day Individual Time Biases
Mean ET Forecast Bias (hr)
143-Day Skill Scores
153-Day Skill Scores
16Methodology 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
17Issues 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)
18Conclusions
- 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