FORECAST OF ATLANTIC HURRICANE ACTIVITY FOR SEPTEMBER AND OCTOBER 2006 AND SEASONAL UPDATE THROUGH AUGUST

 

Following below-average activity in August, we are significantly reducing our seasonal forecast. We expect slightly above-average activity in September and that October will likely have below-average activity.  We now predict that total seasonal activity will be slightly below the long-term average.

 

(as of 1 September 2006)

 

 

 

By Philip J. Klotzbach[1] and William M. Gray[2]

 

with special assistance from William Thorson[3]

 

 

 

This forecast as well as past forecasts and verifications are available via the World Wide Web at http://hurricane.atmos.colostate.edu/Forecasts


Emily Wilmsen, Colorado State University Media Representative, (970-491-6432) is available to answer various questions about this forecast

 

 

 

Department of Atmospheric Science

Colorado State University

Fort Collins, CO 80523

Email: amie@atmos.colostate.edu

 

ATLANTIC BASIN SEASONAL HURRICANE FORECAST FOR 2006

 

Forecast Parameter and 1950-2000

Climatology (in parentheses)

Full Season Adjusted

 3 August 2006 Fcst.

Observed Activity Through August

Updated Sept.-only Fcst.

Updated Oct.-only Fcst.

Forecast Activity After 1 Sept.

Full Season Adjusted 1 Sep. 2006 Forecast1

Named Storms (NS) (9.6)

15

5

5

2

8

13

Named Storm Days (NSD) (49.1)

75

17.50

20

10

32

50

Hurricanes (H) (5.9)

7

1

3

1

4

5

Hurricane Days (HD) (24.5)

35

0.25

10

3

13

13

Intense Hurricanes (IH) (2.3)

3

0

2

0

2

2

Intense Hurricane Days (IHD) (5.0)

8

0

4

0

4

4

Net Tropical Cyclone Activity (NTC) (100%)

140

18

59

12

73

90

 

 

1 We assume one weak named storm in November that accrues two named storm days.

 

 


Notice of Author Changes

 

By William Gray


The order of the authorship of these forecasts has been reversed from Gray and Klotzbach to Klotzbach and Gray.  After 22 years (since 1984) of making these forecasts, it is appropriate that I step back and have Phil Klotzbach assume the primary responsibility for our project’s seasonal, monthly and landfall probability forecasts.  Phil has been a member of my research project for the last five years and has been second author on these forecasts for the last four years.  I have greatly profited and enjoyed our close personal and working relationships.

 

Phil is now devoting more time to the improvement of these forecasts than I am.  I am now giving more of my efforts to the global warming issue and in synthesizing my projects’ many years of hurricane and typhoon studies.

 

Phil Klotzbach is an outstanding young scientist with a superb academic record.  I have been amazed at how far he has come in his knowledge of hurricane prediction since joining my project five years ago.  I foresee an outstanding future for him in the hurricane field.  I expect he will make many new forecast innovations and skill improvements in the coming years.  I plan to continue to be closely involved in the issuing of these forecasts for the next few years. 



ABSTRACT

 

Information obtained through 31 August 2006 shows that we have so far experienced only 18 percent of the average full season Net Tropical Cyclone (NTC) activity.  We significantly over-estimated August activity.  In an average year, 33 percent of the seasonal average NTC of 100 occurs before the end of August.

Our September-only forecast calls for 5 named storms, 3 hurricanes, 2 major hurricanes and NTC activity of 59 which is slightly above the September-only average value of 48. Our October-only forecast calls for 2 named storms, 1 hurricane, 0 major hurricanes and NTC activity of 12 which is below the October-only average value of 18.

We now anticipate that the 2006 Atlantic basin tropical cyclone (TC) season will be considerably less active than the seasonal activity we anticipated in our earlier forecasts and in our updated 3 August forecast. We now expect that the 2006 hurricane season will have slightly less hurricane activity than the long-term average.  This is due to an unexpected increase in tropical Atlantic mid-level dryness (with large amounts of African dust) and a continued trend towards El Niño-like conditions in the eastern and central Pacific.   


AUGUST FORECAST VERIFICATION

 

Eric Blake spent from 1998-2001 as a graduate student at Colorado State University. His research efforts went into the development of an Atlantic basin August-only hurricane forecast scheme which was used for our 3 August 2006 forecast. See Blake (2002) or Blake and Gray (2004) for background information. Our August 2006 forecast called for well above-average activity, but this forecast did not verify.  Below-average activity occurred during the month.  A more in-depth analysis of why we think August 2006 was an inactive month follows in our discussion (Section 4).

 

 

 

CSU forecast and verification of August-only hurricane activity made in early August.

 

Tropical Cyclone Parameters and 1950-2000 August Average (in parentheses)

August 2006

Statistical Forecast

Adjusted August 2006 Forecast

August 2006 Verification

Named Storms (NS) (2.8)

3.3

4

3

Named Storm Days (NSD) (11.8)

21.1

22

12

Hurricanes (H) (1.6)

2.9

3

1

Hurricane Days (HD) (5.7)

8.1

11

0.25

Intense Hurricanes (IH) (0.6)

0.7

1

0

Intense Hurricane Days (IHD) (1.2)

2.0

3

0

Net Tropical Cyclone Activity (NTC) (26.4)

53.6

50

12

 

 

Our August 2006 forecast was a bust and not typical of our previous six August-only forecasts for 2000-2005 or our hindcasts of August-only activity as contained in our original developmental datasets over the period 1949-1999.  Our developmental data sets showed considerable skill.  Table A shows the skill of our prior six August-only forecasts for Net Tropical Cyclone (NTC) activity over the 2000-2005 period.  Note that we have correctly predicted above- or below-average in five out of the prior six years. 

 

Table A: Predicted, observed, August-only 2006 forecast (bottom line) and previous five-year mean and climatological NTC for our six August-only forecasts of 2000-2005.   Evaluation of skill with respect to average error and mean square error are also shown. 

 

 

Year        

Observed NTC

Predicted NTC

Previous Five-Year Mean NTC

Climatological NTC

2000

42

33

48

26

2001

9

22

42

26

2002

7

18

31

26

2003

26

22

33

26

2004

89

35

28

26

2005

41

50

35

26

 

 

 

 

 

Average Error (2000-2005)

 

16.7

23.0

21.7

Mean Square Error (MSE) (2000-2005)

 

569

923

851

Skill of Prediction (relative to MSE)

 

 

0.38

0.33

2006

12

50

34

26

 



1        Introduction

 

Our Colorado State University research project has shown that a sizable portion of the year-to-year variability of Atlantic tropical cyclone (TC) activity can be hindcast with skill significantly exceeding climatology. These forecasts are based on a statistical methodology derived from 55 years of past global reanalysis data and a separate study of prior analog years which have had similar global atmosphere and ocean precursor circulation features. Qualitative adjustments are added to accommodate additional processes which may not be explicitly represented by our statistical analyses. We believe that seasonal forecasts must be based on methods showing significant hindcast skill in application to long periods of prior seasonal and monthly data.

 

 

2        Predictions of Individual Monthly Atlantic TC Activity

A new aspect of our climate research is the development of TC activity predictions for individual months. There are often monthly periods within active and inactive Atlantic basin hurricane seasons which do not conform to the overall season. For example, 1961 was an active hurricane season (NTC of 222), but there was no TC activity during August; 1995 had 19 named storms, but only one named storm developed during a 30-day period during the peak of the hurricane season between 29 August and 27 September. By contrast, the inactive season of 1941 had only six named storms (average 9.3), but four of them developed during September. During the inactive 1968 hurricane season, three of the eight named storms formed in June (June average is 0.5).

We have conducted new research to see how well various sub-season or individual monthly trends of TC activity can be forecast. This effort has recently been documented in papers by Blake and Gray (2004) for August and Klotzbach and Gray (2003) for September. These reports show that it is possible to develop skillful prediction schemes for August-only and September-only Atlantic basin tropical cyclone activity. We have also developed a separate October forecast scheme. On average, August, September, and October have about 26%, 48%, and 17% or 91% of the Atlantic basin's NTC activity. Initial August-only forecasts have now been made by Blake for the last seven years (2000-2006), and the verification of these forecasts is promising, despite this year’s significant over-forecast. The verification of the September-only and October-only forecasts also appears to show skill.

2.1 Seasonal Update Through August 2006

Through August, the 2006 hurricane season has had 18 percent of the NTC activity of the average hurricane season. June-July 2006 had approximately average activity while August has had below-average activity, experiencing only one-half of average August tropical cyclone activity. As of 1 September, 5 named storms, 1 hurricane and no major (Cat. 3-4-5) hurricanes have developed.  Through August, the climatological (1950-2000) average number of named storms, hurricanes and major hurricanes is 4.2, 2.4, and 0.7, respectively. Through August of 2006, the Atlantic basin has witnessed 120, 42, and 0 percent of average named storm, hurricane, and major hurricane activity, respectively.

 

2.2 September Forecast

      Figure 1 and Table 1 list the predictors used in the September-only hindcast (Klotzbach and Gray 2003) for each of the seven different forecast parameters. The table also shows hindcast skill for the 51-year period 1950-2000, as well as the independent jackknife hindcast skill over this period. Table 2 gives the predictor values for September 2006. Table 3 gives our independent statistical prediction for September 2006.  Predictor values for September 2006 are mixed, so our final forecast is calling for slightly above-average activity for the month.

 

Figure 1:  Predictors selected for the end of August forecast of September tropical cyclone activity.  The numbers in each area are keyed to the description given in Table 1.