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

 

We have experienced average hurricane activity through September.  August was inactive, but September had above-average activity.  We expect October to have below-average activity largely due to developing El Niño conditions in the central and eastern Pacific.  November activity in El Niño years is very rare.

 

(as of 3 October 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


[1] Research Associate

[2] Professor Emeritus of Atmospheric Science

[3] Research Associate

 

ATLANTIC BASIN SEASONAL HURRICANE FORECAST FOR 2006

 

 

Full Season Tropical Cyclone Parameters and their 1950-2000 Climatology (in parentheses)

 

Full Season Adjusted 3 Aug. 2006 Forecast

Full Season Adjusted 1 Sep. 2006 Forecast

 

Observed Activity Through September

 

Updated Oct.-Nov. Forecast

 

Full Season Adjusted 3 Oct. 2006 Forecast

Named Storms (NS) (9.6)

15

13

9

2

11

Named Storm Days (NSD) (49.1)

75

50

48

10

58

Hurricanes (H) (5.9)

7

5

5

1

6

Hurricane Days (HD) (24.5)

35

13

18.50

4

23

Intense Hurricanes (IH) (2.3)

3

2

2

0

2

Intense Hurricane Days (IHD) (5.0)

8

4

3

0

3

Net Tropical Cyclone Activity (NTC) (100%)

140

90

83

12

95

 

 

 

 

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 six years and has been second author on these forecasts for the last five 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 six 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 30 September 2006 shows that we have so far experienced an average Atlantic basin hurricane season.  August had substantially below-average activity (only 45% of average) while September had above-average activity (about 140% of average).  US landfall has been well below average.  No hurricanes have made landfall along the US coastline this year.  Eighty-three percent of the average full season Net Tropical Cyclone (NTC) activity has occurred so far this year.  In an average year, 80 percent of the seasonal average NTC of 100 occurs by the end of September.

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 forecast no tropical cyclone activity in November. Our below-average prediction for October-November activity is largely due to the rapid emergence of an El Niño event during the latter part of this summer.

Atlantic basin tropical cyclone (TC) activity for the 2006 season will be considerably less than the seasonal activity we anticipated in our earlier forecasts issued in early December, early April, late May and early August.  We judge this reduced seasonal activity to be due to mid-level dryness in the tropical Atlantic (with large amounts of African dust) which greatly reduced August activity and to the rapid late summer development of an El Niño event which we and nearly all ENSO forecasts did not anticipate.   


VERIFICATION OF SEPTEMBER MONTHLY FORECAST

 

CSU forecast and verification of September-only hurricane activity issued on 2 September.

 

 

 

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

 

September 2006

Statistical Forecast

 

Adjusted September 2006 Forecast

 

September 2006 Verification

Named Storms (NS) (3.4)

3.4

5

4

Named Storm Days (NSD) (21.7)

17.2

20

30.50

Hurricanes (H) (2.4)

3.2

3

4

Hurricane Days (HD) (12.3)

5.5

10

18.25

Intense Hurricanes (IH) (1.3)

1.7

2

2

Intense Hurricane Days (IHD) (3.0)

2.5

4

3

Net Tropical Cyclone Activity (NTC) (48.0)

45.4

59

66

 

Our September 2006 forecast verified quite well.  Even though conditions in August were not favorable for Atlantic basin tropical cyclone activity, we predicted that they would likely become more favorable for hurricane development in September, and this prediction verified quite well.  Dry air continued to predominate across the tropical Atlantic in September; however, vertical wind shear was below average for most of the month.  Four named storms (Florence, Gordon, Helene and Isaac) formed during September, and all four of these storms became hurricanes.  Gordon and Helene became major hurricanes.  A more in-depth discussion of our September-only forecast is in section 5.



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.

 

2        2006 Atlantic Basin Activity through September

 

As of the end of September, the 2006 hurricane season has had 83 percent of the NTC activity of the average hurricane season. June-July 2006 had approximately average activity while August had greatly reduced (approximately 45%) TC activity compared with the typical August.  September activity was somewhat above the long-term average with 4 named storms, 4 hurricanes and 2 major hurricanes forming during the month.  As of 1 October, a total of 9 named storms, 5 hurricanes and 2 major (Cat. 3-4-5) hurricanes have developed this season.  Through September, the climatological (1950-2000) average number of named storms, hurricanes and major hurricanes is 7.5, 4.5, and 2.1, respectively. Through September 2006, the Atlantic basin has thus witnessed 120, 111, and 95 percent of average named storm, hurricane, and major hurricane activity, respectively.  Table 1 shows observed Atlantic basin tropical cyclone activity by storm, and Figure 1 displays Atlantic basin tracks for the 2006 season. 

 

Table 1: Observed 2006 Atlantic basin tropical cyclone activity through September.

 

 

Highest

Category

 

 

Name

 

 

Dates

 

Peak Sustained Winds (kts)/lowest SLP (mb)

 

 

NSD

 

 

HD

 

 

IHD

 

 

NTC

TS

Alberto

June 11-14

60 kt/995 mb

2.75

 

 

2.7

TS

Beryl

July 19-21

50 kt/1001 mb

2.75

 

 

2.7

TS

Chris

Aug. 1-4

55 kt/1001 mb

3.25

 

 

2.8

TS

Debby

Aug. 23-26

45 kt/1000 mb

3.25

 

 

2.8

H-1

Ernesto

Aug. 25–Sep. 1

65 kt/988 mb

6.00

0.25

 

6.8

H-1

Florence

Sep. 5-12

80 kt/972 mb

7.50

2.75

 

9.0

IH-3

Gordon

Sep. 8-20

105 kt/955 mb

9.25

7.50

1.25

24.2

IH-3

Helene

Sep. 14-24

110 kt/954 mb

10.75

7.50

1.75

26.4

H-1

Isaac

Sep. 28-Present

70 kt/989 mb

2.50

0.50

 

5.7

Totals

9

 

 

48.00

18.50

3.00

83.1

 

 

 

Figure 1:  2006 Atlantic basin hurricane tracks through September.  Figure courtesy of Weather Underground (http://www.weatherunderground.com/).

 

 

3        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.

3.1       Independent October-only Forecast

 

We had previously developed a statistical forecast of October-only activity; however, we did not have an explicit ENSO predictor in this scheme, and therefore we have discontinued its use for this year.  Our research over the past couple of years indicates that ENSO is of critical importance in determining October and November activity. We intend to develop a new statistical forecast by next summer for October activity that takes into account El Niño conditions.  This forecast will likely be developed using a similar methodology to what was used for our new August seasonal statistical forecast that uses predictors obtained from surface data.  Using only surface data, we can develop the scheme on data extending back to 1900.  Our new October forecast scheme will be developed on 1949-2005 and then tested on 1900-1948.  A forecast scheme that shows skill over 106 years is likely to be quite robust. 

 It is expected that the currently rapidly developing El Niño in the central and eastern Pacific will likely bring an early end to the Atlantic basin hurricane season.  El Niño conditions typically cause a greater suppression of the latter part of the hurricane season than they do in the earlier portion.  Table 2 displays average (1950-2000) October TC activity compared with years when the August-September Nino 1+2 and Nino 3 indices average greater than 0.5ºC above average.  For our October forecast calculation, this will be our definition of an El Niño.  Based on correlation data from 1949-2005, October Atlantic basin hurricane activity is more affected by SSTs in the eastern Pacific than SSTs in the central Pacific.

Table 2 shows that there is typically a marked reduction in October Atlantic hurricane activity in El Niño years.  Table 3 gives probabilities of percentage reductions in October TC activity in El Niño years.  The probabilities for one or more intense hurricanes, two or more hurricanes and an October NTC value of 20 or greater drops considerably when SSTs in the east-central Pacific (Nino 3 temperatures) are greater than 0.5ºC above normal.  These probabilities drop even more when SSTs are above normal in the eastern Pacific (Nino 1+2 temperatures greater than 0.5ºC above normal).  We estimate that the Nino 1+2 and Nino 3 indices for August-September will average around 0.7ºC above average.  Note the large reduction in October activity, especially for intense hurricane and intense hurricane days in El Niño years.  No intense (or major) hurricanes have been observed to form after 1 October in El Niño years since 1950.

 

Table 2: Average October Atlantic basin tropical cyclone activity compared with October Atlantic basin tropical cyclone activity in El Niño years (defined as the August-September Nino 1+2 index and the August-September Nino 3 index averaging greater than 0.5ºC above normal).

 

TC

Parameter

Average October (1950-2000)

El Niño

October

 

Ratio (El Niño/Average)

NS

1.6

1.3

81%

NSD

8.9

6.5

73%

H

1.1

0.7