# Presentation: 250452

## Ensemble, Corrected Consensus, and Weighted 

**Consensus TC Track Forecasts**
**James S. Goerss**
**NRL  Monterey **
**March 4, 2009**

## How to Beat Consensus*

**How to Beat Consensus***
***at least for the Atlantic basin in 2008**

## Track Forecast Guidance

***Interpolated Model Guidance***
**AVNI – Global Forecast System (GFS) run at NCEP**
**GFDI – GFDL model run at NCEP**
**HWFI – Hurricane WRF run at NCEP**
**NGPI – Navy global model (NOGAPS) run at FNMOC**
**GFNI – GFDL model run at FNMOC**
**EGRI ****–** **UK Met Office global model**
**EMXI – ECMWF global model**
**TVCN – Consensus of above models (at least two)**
**AEMI – NCEP GFS ensemble mean**

## Corrected Consensus (TVCC)

** ****	****Possible predictors are consensus model spread; ****	****initial and ****	****forecast TC intensity; initial TC location and forecast ****	****displacement of TC location (latitude and ****	****longitude); TC ****	****speed of motion; and number of members available.**
**	****Regression models to predict TVCN east-west and north-****	****south forecast error for all forecast lengths were derived ****	****using the aforementioned pool of predictors for the 2002-2007 ****	****seasons **
** ****	****The means of the TVCN east-west and north-south forecast ****	****errors for all forecast lengths were also found for the 2002-****	****2007 seasons to be used as bias correctors.**
** ****	****Optimal combinations of the statistical and bias correctors ****	****are used to produce corrected consensus (TVCC) forecasts ****	****for the Atlantic and eastern North Pacific basins.**

## 2008 Atlantic

**2008 Atlantic**
**Non-Homogeneous TC Forecast Error (nm)**

**314**

**251**

**204**

**Number of Forecasts**

**160**

**131**

*****

*****

*****

*****

*****

*** ****Best Non-Consensus Guidance**

**Notes:**
This comparison is homogeneous with respect to OFCL.
For the 24h-72h forecasts, GFDI and EMXI have been the best performers of the individual models.  For the 96h-120h forecasts,
EMXI and AEMI have been the best performers.  The performance of NGPI, EGRI, and GFNI have been roughly comparable.  For
the 96h-120h forecasts, the performance of AVNI, HWFI, and GFDI have been roughly comparable.  For all forecast lengths except
48h, EMXI and TVCN forecast errors were roughly the same.  For the 48h forecasts, EMXI had a 77 nm error vice 86 nm for TVCN.

## 2008 Atlantic

**2008 Atlantic**
**Forecast Availability Percentage**

**Notes:**
Forecast availability is defined as the percent of time a verifying forecast was available from the guidance when a verifying official
forecast was made.  The forecast availability for GFDI, HWFI, and TVCN for the most part approaches 100 percent.  For the 24h-48h
forecasts, the availability of AVNI and AEMI is close to 100 percent, that for NGPI and EGRI is around 90 percent, and that for GFNI
and EMXI is around 80 percent.  For the 72h-120h forecasts, the availability of AVNI is above 90 percent, that for NGPI and EGRI is
around 85 percent, that for EMXI is around 80 percent, that for GFNI is between 70-75 percent, while that for AEMI drops quickly
from just over 80 percent at 72h to about 60 percent at 96h to just over 50 percent at 120h.

## 2008 Atlantic TC Forecast Error

**2008 Atlantic TC Forecast Error**
**Percent Improvement over GFDI**

**Notes:**
This comparison is homogeneous with respect to OFCL.
For the 24h-72h forecasts, GFDI and EMXI have been the best performers of the individual models.  For the 96h-120h forecasts,
EMXI and AEMI have been the best performers.  The performance of NGPI, EGRI, and GFNI have been roughly comparable.  For
the 96h-120h forecasts, the performance of AVNI, HWFI, and GFDI have been roughly comparable.  For all forecast lengths except
48h, EMXI and TVCN forecast errors were roughly the same.  For the 48h forecasts, EMXI had a 77 nm error vice 86 nm for TVCN.

## Weighted Consensus Guidance

**Based on the forecast performance for EMXI and AEMI in the Atlantic for 2008, the following weighted consensus forecast aids were computed:**
**CNW6 – Determined just like TVCN except, that instead of ****	****        equal weight for all models, 6 times the weight ****	****  ****	****        was applied to EMXI (roughly the same as taking ****	****        the average of TVCN and EMXI)**
**CON3 – Determined by taking the average of the TVCN, ****	**** ****	****        EMXI, and AEMI forecasts (at least TVCN).**

## 2008 Atlantic

**2008 Atlantic**
**Homogeneous TC Forecast Error (nm)**

**306**

**249**

**202**

**Number of Forecasts**

**158**

**131**

**Notes:**
Based on the EMXI and AEMI performance, I put together a couple of new aids.  CON3 is the simple mean of TVCN, EMXI, and AEMI and
is simply TVCN if both EMXI and AEMI are missing.  CNW6 is a weighted version of TVCN where I gave 6 times the weight to EMXI
than the other models.  There is very little difference between CNW6 and simply averaging the TVCN and EMXI forecasts.   For the 24h-72h
forecasts, CNW6 was the best performer.  For the 96h-120h forecasts, CON3 was the best performer but CNW6 was still better than
TVCN and TVCC.  TVCC was a bit better than TVCN at 120h.

## 2008 Atlantic TC Forecast Error

**2008 Atlantic TC Forecast Error**
**Percent Improvement over TVCN**

**Notes:**
This comparison is homogeneous with respect to OFCL.
For the 24h-72h forecasts, GFDI and EMXI have been the best performers of the individual models.  For the 96h-120h forecasts,
EMXI and AEMI have been the best performers.  The performance of NGPI, EGRI, and GFNI have been roughly comparable.  For
the 96h-120h forecasts, the performance of AVNI, HWFI, and GFDI have been roughly comparable.  For all forecast lengths except
48h, EMXI and TVCN forecast errors were roughly the same.  For the 48h forecasts, EMXI had a 77 nm error vice 86 nm for TVCN.

## 2008 Eastern North Pacific

**2008 Eastern North Pacific**
**Non-Homogeneous TC Forecast Error (nm)**

**238**

**178**

**130**

**Number of Forecasts**

**90**

**57**

*** ****Best Non-Consensus Guidance**

*****

*****

*****

*****

*****

**Notes:**
The model performance in the eastern North Pacific was quite a bit different from than in the Atlantic.  For the 24h-48h forecasts,
GFDI and EMXI were the best performers.  At 72h, EGRI joined those two models as the best performers.  For 96h-120h,
EGRI was the best performer although its availability was worse than that for the other models.  GFNI was the worst performer at all forecast
lengths except 72h and 96h where AVNI was the worst.  Unlike the Atlantic, EMXI and AEMI did not excel for the 96h-120h forecasts.  
For 72h-120h, the TVCN forecast errors were much smaller than the model errors and for 96h-120h the OFCL errors were smaller 
than the TVCN errors.

## 2008 Eastern North Pacific

**2008 Eastern North Pacific**
**Forecast Availability Percentage**

**Notes:**
Again, in terms of forecast availability, GFDI, HWFI, and TVCN are the clear winners (93-100 percent availability).
The availability of AVNI ranges from above 90 percent (24h – 72h) to just over 70 percent at 120h.  That for
NGPI is above 80 percent out to 96h but drops to 70 percent at 120h while that for EGRI is above 80 percent
(24h and 48h) but drops to just over 40 percent at 120h.  The availability of EMXI ranges from 65-74 percent 
while that for AEMI is over 90 percent out to 72h but drops to 72 percent at 96h and under 60 percent at 120h.
The availability of GFNI was the worst ranging from just under 70 percent at 24h and 48h to just under 40 percent
at 120h.

## 2008 Eastern North Pacific TC Forecast Error

**2008 Eastern North Pacific TC Forecast Error**
**Percent Improvement over Best Model**

**Notes:**
This comparison is homogeneous with respect to OFCL.
For the 24h-72h forecasts, GFDI and EMXI have been the best performers of the individual models.  For the 96h-120h forecasts,
EMXI and AEMI have been the best performers.  The performance of NGPI, EGRI, and GFNI have been roughly comparable.  For
the 96h-120h forecasts, the performance of AVNI, HWFI, and GFDI have been roughly comparable.  For all forecast lengths except
48h, EMXI and TVCN forecast errors were roughly the same.  For the 48h forecasts, EMXI had a 77 nm error vice 86 nm for TVCN.

## 2008 Eastern North Pacific

**2008 Eastern North Pacific**
**Homogeneous TC Forecast Error (nm)**

**236**

**178**

**129**

**Number of Forecasts**

**89**

**56**

**Notes:**
Unlike the Atlantic, the CON3 performance is the worst at all forecast lengths.  For this basin, the addition of AEMI and/or the 
extra emphasis on EMXI does not result in the improvement we saw in the Atlantic.  Except at 72h and 96h, TVCC performed a bit worse
than TVCN.  For the 96h and 120h forecasts, OFCL clearly beats the consensus guidance (judicious application of BAMS perhaps?).

## 2008 Eastern North Pacific TC Forecast Error

**2008 Eastern North Pacific TC Forecast Error**
**Percent Improvement over TVCN**

**Notes:**
This comparison is homogeneous with respect to OFCL.
For the 24h-72h forecasts, GFDI and EMXI have been the best performers of the individual models.  For the 96h-120h forecasts,
EMXI and AEMI have been the best performers.  The performance of NGPI, EGRI, and GFNI have been roughly comparable.  For
the 96h-120h forecasts, the performance of AVNI, HWFI, and GFDI have been roughly comparable.  For all forecast lengths except
48h, EMXI and TVCN forecast errors were roughly the same.  For the 48h forecasts, EMXI had a 77 nm error vice 86 nm for TVCN.

## Summary

**	****For both the Atlantic and eastern North Pacific basins in 2008, the ****	****corrected consensus (TVCC) forecast errors were not much better or ****	****worse than those for TVCN.  **
** ****	****Due to the superior performance of EMXI and AEMI (at 96 h and 120 h) in ****	****the Atlantic, the forecast errors for CNW6 were 6-9 percent better than ****	****those for TVCN at all forecast lengths while those for CON3 were 14-16 ****	****percent better at 96 h and 120 h.  **
**	****Due to the average performance of EMXI and inferior performance of AEMI ****	****in the eastern North Pacific, the forecast errors for CNW6 were 5-12 ****	****percent worse than those for TVCN for forecast lengths greater than 72 h ****	****while those for CON3 were 14-26 percent worse for forecast lengths ****	****greater than 24 h.**
** ****	****The Atlantic results indicate that weighted consensus TC track forecast ****	****aids are capable of producing improved track forecasts if one or more of ****	****the models perform considerably better than the others.  However, whether ****	****such improved model performance is consistent from season to season ****	****has yet to be determined.  The eastern North Pacific results suggest that ****	****such performance and the utility of weighted consensus track forecasts ****	****are likely to be basin dependent.**

## Questions?