SUMMARY OF 2006 ATLANTIC TROPICAL CYCLONE ACTIVITY AND VERIFICATION OF AUTHOR'S SEASONAL AND MONTHLY FORECASTS

SUMMARY OF 2006 ATLANTIC TROPICAL CYCLONE ACTIVITY AND VERIFICATION OF AUTHOR’S SEASONAL AND MONTHLY FORECASTS

The 2006 Atlantic basin hurricane season had activity at slightly less than average (1950-2000) levels. This activity was much less than predicted in our seasonal forecasts.

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 verification.

Department of Atmospheric Science

Colorado State University

Fort Collins, CO 80523

Email: amie@atmos.colostate.edu

17 November 2006

[1] Research Associate

[2] Professor of Atmospheric Science

[3] Research Associate

Acknowledgment

We are grateful to the National Science Foundation (NSF) and Lexington Insurance Company (a member of the American International Group (AIG)) for providing partial support for the research necessary to make these forecasts. We also thank the GeoGraphics Laboratory at Bridgewater State College (MA) for their assistance in developing the Landfalling Hurricane Probability Webpage (available online at http://www.e-transit.org/hurricane).

The second author gratefully acknowledges valuable input to his CSU research project over many years by former graduate students and now colleagues Chris Landsea, John Knaff and Eric Blake. We also thank Professors Paul Mielke and Ken Berry of Colorado State University for much statistical analysis and advice over many years.

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.

DEFINITIONS

ATLANTIC BASIN SEASONAL HURRICANE FORECASTS FOR 2006

Forecast Parameter and 1950-2000 Climatology (in parentheses)	6 Dec 2005	Update 4 April 2006	Update 31 May 2006	Update 3 Aug 2006	Update 1 Sept 2006	Update 3 Oct 2005	Observed 2006 Total
Named Storms (NS) (9.6)	17	17	17	15	13	11	9
Named Storm Days (NSD) (49.1)	85	85	85	75	50	58	50
Hurricanes (H) (5.9)	9	9	9	7	5	6	5
Hurricane Days (HD) (24.5)	45	45	45	35	13	23	20
Intense Hurricanes (IH) (2.3)	5	5	5	3	2	2	2
Intense Hurricane Days (IHD) (5.0)	13	13	13	8	4	3	3
Net Tropical Cyclone Activity (NTC)* (100%)	195	195	195	140	90	95	85

*NTC is a combined measure of the yearly mean of six indices (NS, NSD, H, HD, IH, IHD) of hurricane activity as a percent deviation from the 1950-2000 annual average.

Figure courtesy of Weather Underground (http://www.weatherunderground.com)

ABSTRACT

This report summarizes tropical cyclone (TC) activity which occurred in the Atlantic basin during 2006 and verifies the authors’ seasonal and monthly forecasts of this activity. A forecast was initially issued for the 2006 season on 6 December 2005 with updates on 4 April, 31 May, 3 August, 1 September and 3 October of this year. These forecasts also contained estimates of the probability of U.S. hurricane landfall during 2006. The 3 August forecast included forecasts of August-only, September-only and October-only tropical cyclone activity for 2006. Our 1 September forecast gave a seasonal summary to that date and included individual monthly predictions of September-only and October-only activity. Our 3 October forecast gave a seasonal summary to that date and included an October-November forecast. Our 2006 seasonal hurricane forecast was not successful. We anticipated a well above-average season, and the season had activity at slightly below-average levels. We did catch this downward trend beginning with our early August update We attribute a large portion of this forecast over-prediction to a late-developing El Niño and increased mid-level dryness in the tropical Atlantic.

Our August-only forecast was a bust. Our September-only forecast was quite successful, especially when evaluated against the Net Tropical Cyclone (NTC) activity metric. The October-only forecast also successfully called for activity at well below-average levels, and no tropical cyclone activity occurred after October 2. Our first forecast for the 2007 season will be issued on Friday, 8 December 2006.

1 Introduction

A variety of atmosphere-ocean conditions interact with each other to cause year-to-year and month-to-month hurricane variability. The interactive physical linkages between these many physical parameters and hurricane variability are complicated and cannot be well elucidated to the satisfaction of the typical forecaster making short range (1-5 days) predictions where changes in the momentum fields are the crucial factors. Seasonal and monthly forecasts, unfortunately, must deal with the much more complicated interaction of the energy-moisture fields with the momentum fields.

We find that there is a rather high (50-60 percent) degree of year-to-year hurricane forecast potential if one combines 4-5 semi-independent atmospheric-oceanic parameters together. The best predictors (out of a group of 4-5) do not necessarily have the best individual correlations with hurricane activity. The best forecast parameters are those that explain the portion of the variance of seasonal hurricane activity that is not associated with the other variables. It is possible for an important hurricane forecast parameter to show little direct relationship to a predictand by itself but to have an important influence when included with a set of 4-5 other predictors.

In a five-predictor empirical forecast model, the contribution of each predictor to the net forecast skill can only be determined by the separate elimination of each parameter from the full five predictor model while noting the hindcast skill degradation. When taken from the full set of predictors, one parameter may degrade the forecast skill by 25-30 percent, while another degrades the forecast skill by only 10-15 percent. An individual parameter that, through elimination from the forecast, degrades a forecast by as much as 25-30 percent may, in fact, by itself, show little direct correlation with the predictand. A direct correlation of a forecast parameter may not be the best measure of the importance of this predictor to the skill of a 4-5 parameter forecast model. This is the nature of the seasonal or climate forecast problem where one is dealing with a very complicated atmospheric-oceanic system that is highly non-linear. There is a maze of changing physical linkages between the many variables. These linkages can undergo unknown changes from weekly to decadal time scales. It is impossible to understand how all these processes interact with each other. It follows that any seasonal or climate forecast scheme showing significant hindcast skill must be empirically derived.

2 Tropical Cyclone Activity for 2006

Figure 1 and Table 1 summarize the Atlantic basin tropical cyclone activity which occurred in 2006. All the seasonal forecast parameters of NS, NSD, H, HD, IH, IHD and NTC were near their long-period averages. See page 4 for acronym definitions.

3 Individual 2006 Tropical Cyclone Characteristics

The following is a brief summary of each of the named tropical cyclones in the Atlantic basin for the 2006 season. See Fig. 1 for the tracks of these tropical cyclones, and see Table 1 for statistics of each of these tropical cyclones. Online entries from Wikipedia (http://www.wikipedia.org) were very helpful in putting together this tropical cyclone summary.

Figure 1: Tracks of 2006 Atlantic Basin tropical cyclones. Figure courtesy of Weather Underground (http://www.weatherunderground.com).

Table 1: Observed 2006 Atlantic basin tropical cyclone activity.

Highest