Verification of Forecasts of Tropical Cyclone Activity 
over the Western North Pacific in 2001

 Issued on 15 Jan 2002

Our forecasts for the tropical cyclone (TC) activities for 2001 turned out to be another success other than the number of TCs making landfall over the South China coast (see Tables 1, 2 and 3).

Over the entire western North Pacific, the number of TCs was correctly predicted by the CSL-6 scheme (the one issued in June, see Chan et al. 2001) and also by the CSL-4 scheme (the one issued in April, see Chan et al. 1998 and Chan et al. 2001) within the margin of error.  As in the year 2000, two systems (28W and 29W) were considered by the Joint Typhoon Warning Center (JTWC) as having reached tropical storm intensity but were only treated as tropical depressions by the Regional Specialized Meteorological Center (RSMC) in Tokyo.  Since we developed our forecasts based on the JTWC database, our verifications should also follow their numbers, which means that the CSL-6 scheme was again perfect and the CSL-4 scheme near perfect.  Even if we use the RSMC-Tokyo data, our forecasts were still correct.  For the number of typhoons, JTWC designated Vamei (32W) as a typhoon but only for one 6-hourly period.  Again, regardless of the database, our forecasts were again correct.  As stated in the forecasts issued in April 2001, and again, in June 2001, we predicted a near to above-normal activity, which was exactly what happened.

Over the South China Sea (SCS), the number of TCs was again correctly predicted although almost all of them reached at least tropical storm intensity (except 29W, which was not considered a tropical storm by RSMC-Tokyo).  In any case, both the CSL-4 and CSL-6 schemes made the correct predictions that the TC activity over the SCS was below normal.  For the number of TCs with at least tropical storm intensity, our predictions were still correct if Vamei was excluded.  This TC formed at ~1oN, which was extremely unusual and no statistical model would be able to predict the occurrence of such a system.  Thus, we would still maintain that our predictions were within the margin of error.

The other "failure" in the forecasts for 2001 is in the number of TCs making landfall along the South China coast.  The TC that ruined the statistic was Nari that traversed the Island of Taiwan before turning westward to make landfall along the northern coast of south China.  This was a very unusual storm.  It made several loops to the northeast of Taiwan, then moved south along the Central Mountain Range without much weakening.  Statistical models would not be able to predict such systems.  It is therefore fair to say that we did correctly predict the number of landfalling TCs to be below normal.  In 10 out of the 15 years in which the number of landfalling TCs is above normal, they tend to occur in later months (August and beyond).  In 2001, most of the TCs that made landfall occurred in July.

To summarize, we consider our forecasts to be a near-complete success for 2001.  We also made correct predictions for 2000.  Therefore, it appears that our schemes may be working well.  However, some of our recent research on ENSO suggests that we might have overlooked some factors that determine whether a cold event would transition to a warm one.  In fact, we might be issuing our ENSO outlook within the next few weeks.

The forecasts for TC activity in 2002 will be issued in mid April 2002.

References

Chan, J. C. L., J. E. Shi and C. M. Lam, 1998:  Seasonal forecasting of tropical cyclone activity over the western North Pacific and the South China Sea. Wea. Forecasting, 13, 997-1004. Abstract

Chan, J. C. L., J. E. Shi and K. S. Liu, 2001: Improvements in the seasonal forecasting of tropical cyclone activity over the western North Pacific. Wea. Forecasting, 16, 491-498. Abstract

Table 1.  Forecasts of TC activity in 2001 using the CSL-4 and CSL-6 schemes.

2001

 Forecast Observed Normal
  CSL-4 CSL-6    

Western North Pacific 
No. of TCs 31 3 32 3 32 31
No. of TCs with at least tropical storm intensity 27 3 28 3 28 (26) 27
No. of typhoons 18 2 18 2 20 (19) 17
    

South China Sea

No. of TCs 10 2 11 2 12 13
No. of TCs with at least tropical storm intensity 9 2 9 2 12 (11) 10
No. of TCs making landfall along the South China coast 3 1 3 1 5 5

Note: Numbers in parentheses are the observed TC numbers if the RSMC-Tokyo data are used.

Table 2. 2001 summary of tropical cyclones over the western North Pacific. 

Western North Pacific (including South China Sea)

2001

 Tropical cyclones Tropical cyclones with at least tropical storm intensity Tropical cyclones with typhoon intensity
   01. 01W (no name)
02. 02W (no name)
03. Cimaron
04. Chebi
05. Durian
06. Utor
07. Trami
08. 08W (no name)
09. Kong-rey
10. Yutu
11. Toraji
12. Man-yi
13. Usagi
14. Pabuk
15. 15W (no name)
16. Wutip
17. Sepat
18. Fitow
19. Danas
20. Nari
21. Vipa
22. Francisco
23. Lekima
24. Krosa
25. Haiyan
26. Podul
27. Lingling
28. 28W (no name)*
29. 29W (no name)*
30. Kajiki
31. Faxai
32. Vamei		 01. Cimaron
02. Chebi
03. Durian
04. Utor
05. Trami
06. Kong-rey
07. Yutu
08. Toraji
09. Man-yi
10. Usagi
11. Pabuk
12. Wutip
13. Sepat
14. Fitow
15. Danas
16. Nari
17. Vipa
18. Francisco
19. Lekima
20. Krosa
21. Haiyan
22. Podul
23. Lingling
24. 28W (no name)*
25. 29W (no name)*
26. Kajiki
27. Faxai
28. Vamei

 

 

01. Chebi
02. Durian
03. Utor
04. Kong-rey
05. Yutu
06. Toraji
07. Man-yi
08. Pabuk
09. Wutip
10. Danas
11. Nari
12. Vipa
13. Francisco
14. Lekima
15. Krosa
16. Haiyan
17. Podul
18. Lingling
19. Faxai
20. Vamei #

 

 

 

 

 

 
  * The TD was considered by JTWC as having reached tropical storm intensity
# Vamei was considered by JTWC as having reached typhoon intensity
Total number 32 28 (26) 20 (19)
Predicted number
(issued in April)		 31 3 27 3  18 2 
Predicted number
(issued in June)		 32 3 28 3  18 2 

Note: Numbers in parentheses are the observed TC numbers if the RSMC-Tokyo data are used.

Table 3. 2001 summary of tropical cyclones over the South China Sea and making landfall along the South China coast.

South China Sea

  Tropical cyclones Tropical cyclones with at least tropical storm intensity Tropical cyclones making landfall along the South China coast
  01. Cimaron
02. Chebi
03. Durian
04. Utor
05. Yutu
06. Usagi
07. Fitow
08. Nari
09. Lingling
10. 29W
11. Kajiki
12. Vamei		 01. Cimaron
02. Chebi
03. Durian
04. Utor
05. Yutu
06. Usagi
07. Fitow
08. Nari
09. Lingling
10. 29W
11. Kajiki
12. Vamei		 01. Durian
02. Utor
03. Yutu
04. Fitow
05. Nari

 

 

 
Total number 12 12 (11) 5
Predicted number
(issued in April)		 10 2 9 2 3 1
Predicted number
(issued in June)		 11 2 9 2 3 1

Note: Numbers in parentheses are the observed TC numbers if the RSMC-Tokyo data are used.