SUMMARY OF 2005 ATLANTIC TROPICAL CYCLONE ACTIVITY AND VERIFICATION OF
AUTHOR’S SEASONAL AND MONTHLY FORECASTS
The 2005 Atlantic basin hurricane season was the most active and destructive season on record.
By William M. Gray[1] and Philip J. Klotzbach[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
Brad Bohlander and Emily Wilmsen, Colorado State University Media
Representatives, (970-491-6432) are available to answer various questions about
this verification.
Department of Atmospheric Science
Email: amie@atmos.colostate.edu
18 November 2005
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
first 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
Notice of Author Changes
By William Gray
Beginning with the issuing of our first seasonal forecast for 2006 Atlantic basin hurricane activity (on Tuesday, 6 December 2005), the order of the authorship of these forecasts will be 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.
|
Forecast Parameter and
1950-2000 Climatology (in parentheses) |
3 Dec 2004 |
Update 1 April 2005 |
Update 31 May 2005 |
Update 5 Aug 2005 |
Update 2 Sept 2005 |
Update 3 Oct 2005 |
Observed 2005 Total |
|
Named Storms (NS) (9.6) |
11 |
13 |
15 |
20 |
20 |
20 |
23 |
|
Named Storm Days (NSD)
(49.1) |
55 |
65 |
75 |
95 |
95 |
100 |
103.25 |
|
Hurricanes (H) (5.9) |
6 |
7 |
8 |
10 |
10 |
11 |
13 |
|
Hurricane Days (HD)
(24.5) |
25 |
35 |
45 |
55 |
45 |
40 |
45.25 |
|
Intense Hurricanes (IH)
(2.3) |
3 |
3 |
4 |
6 |
6 |
6 |
7 |
|
Intense Hurricane Days
(IHD) (5.0) |
6 |
7 |
11 |
18 |
15 |
13 |
16.75 |
|
Net Tropical Cyclone Activity (NTC)* (100%) |
115 |
135 |
170 |
235 |
220 |
215 |
249 |
*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.
ABSTRACT
This report summarizes tropical
cyclone (TC) activity which occurred in the Atlantic basin during 2005 and
verifies the authors’ seasonal and monthly forecasts of this activity. A forecast was initially issued for the 2005
season on 3 December 2004 with updates on 1 April, 31 May, 5 August, 2
September and 3 October of this year. These
forecasts also contained estimates of the probability of
Our monthly forecasts for August-only and September-only activity were quite successful, especially when evaluated against the Net Tropical Cyclone (NTC) activity metric. The October-only forecast also successfully called for a very active month; however, we did not anticipate that this would be one of the most active Octobers on record. Overall, we consider our seasonal and monthly forecasts for the 2005 hurricane season to be one of the most skillful that we have issued. Our first forecast for the 2006 season will be issued on Tuesday, 6 December 2005.
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 much less 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. No one can completely understand the full complexity of the atmosphere-ocean system or develop a reliable scheme for forecasting the myriad non-linear interactions in the full-ocean atmosphere system.
2
Tropical Cyclone Activity for 2005
Figure 1 and Table 1 summarize the Atlantic basin tropical cyclone activity which occurred in 2005. All of the seasonal forecast parameters of NS, NSD, H, HD, IH, IHD and NTC were well above their long-period averages as was predicted in our seasonal forecasts.
3
Individual 2005 Tropical Cyclone Characteristics
The following is a brief summary of
each of the named tropical cyclones in the Atlantic basin for the 2005
season. See Fig. 1 for the tracks of these
tropical cyclones, and see Table 1 for the statistics of these tropical
cyclones. Table 2 displays the minimum
pressures recorded for the five tropical cyclones in the
Figure 1: Tracks of 2005
Table 1: Observed 2005 Atlantic
basin tropical cyclone activity.
|
Highest Category |
Name |
Dates |
Peak Sustained Winds
(kts)/lowest SLP (mb) |
NSD |
HD |
IHD |
NTC |
|
TS |
Arlene |
June 9-11 |
60 kt/989 mb |
2.50 |
|
|
2.6 |
|
TS |
Bret |
June 29-29 |
35 kt/1002 mb |
0.50 |
|
|
1.9 |
|
TS |
Cindy |
July 5-6 |
60 kt/992 mb |
1.25 |
|
|
2.2 |
|
IH-4 |
Dennis |
July 5-11 |
130 kt/930 mb |
5.75 |
4.00 |
2.00 |
23.1 |
|
IH-4 |
Emily |
July 12-21 |
135 kt/929 mb |
9.25 |
6.75 |
3.75 |
32.0 |
|
TS |
|
July 22-29 |
60 kt/997 mb |
8.00 |
|
|
4.5 |
|
TS |
Gert |
July 24-25 |
40 kt/1005 mb |
1.00 |