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 to this year. 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

       An aspect of our climate research besides our seasonal forecasts is the development of TC activity predictions for individual months. On average, August, September and October have about 26%, 48%, and 18% or 92% of the total Atlantic basin NTC activity. Initial August-only forecasts have now been made for the last six seasons, September-only forecasts have been made for the last four seasons, and October-only forecasts have been made for the last three seasons.

There are often monthly periods within active and inactive hurricane seasons which do not conform to the overall season. To this end, we have recently developed new schemes to forecast August-only, September-only and October-only Atlantic basin hurricane activity by the beginning of each respective month. These efforts have been recently documented in papers by Blake and Gray (2004) for our August-only forecast and by Klotzbach and Gray (2003) for our September-only forecast - see the references at the end.

Quite skillful August-only, September-only and October-only prediction schemes have been developed based on 51 years (1950-2000) of hindcast testing using a statistically independent jackknife approach. Predictors are derived from prior month, usually June and July NCEP global reanalysis data for all three (August-only, September-only and October-only) individual monthly forecasts and include August's data for the early September update of the September-only and October-only forecasts. Table 1 gives an outline and timetable of the different forecasts and the verifications we issue after the end of each month.

Through August, the 2005 hurricane season has had 110 percent of the NTC activity of the average hurricane season. June-July 2005 had the largest values of Net Tropical Cyclone activity for any June-July period on record. August was also very active, experiencing about 150 percent of average August tropical cyclone activity. As of 1 September, twelve named storms, four hurricanes and three major (Cat. 3-4-5) hurricanes have developed. This 2005 activity is to be compared with the average number of named storms based upon the 1950-2000 climatology. Through August, the climatological average number of named storms, hurricanes and major hurricanes is 4.2, 2.4, and 0.7, respectively. Through August of 2005, the Atlantic basin has witnessed 262, 167, and 429 percent of average named storm, hurricane, and major hurricane activity, respectively.

2.1  September Forecast

      Table 2 and Fig. 1 list the nine September-only predictors with their location and sign for higher TC activity. Table 3 shows which of the nine predictors are used for each forecast parameter of NS, NSD, H, etc.

Table 4 lists the value of each September-only predictor and whether its 2005 value indicates above or below average September-only TC activity. Six out of nine predictors indicate an above-average month. We are calling for a very active September, due in part to the greatly enhancing Atlantic SST pattern presently in place.

Our early September update of September TC activity keeps our forecast for September the same (see Table 5). We have not seen any conditions which would give us pause in forecasting another very active month. Sea surface temperatures are very warm, and vertical wind shear remains quite favorable for development in the tropical Atlantic and Caribbean. Also, when July and August are active in the deep tropics (as they have been during 2005), September activity tends to also be enhanced.

2.2  Independent October-only Statistical Forecast

       Through examination of the NCEP/NCAR reanalysis, we have discovered four predictors that in combination explain about 60 percent of the October cross-validated variance in Net Tropical Cyclone activity (at 1 September) for the hindcast period of 1950-2001. We are currently unable to find combinations of predictors that explain large amounts of variance for the individual tropical cyclone parameters (i.e., named storms, hurricane days, etc.). Therefore, our October forecast consists of predicting NTC and consequently increasing or decreasing October's values for the other parameters accordingly. For example, if October NTC was 150 percent of normal, and a typical October had two named storms, we would forecast three named storms for October.

Table 6 and Fig. 2 list the four October-only predictors with their location and sign for enhanced TC activity.

Table 7 lists the value of each October-only predictor and whether its 2005 value indicates above or below average October-only TC activity. A majority of the October predictors call for below-average activity.

Three out of the four October predictors are negative. However, our early August forecast for October indicated above-average activity, and we believe that the current favorable conditions are likely to last for the remainder of the 2005 hurricane season. We are reducing our October forecast slightly, though, due to the fact that three of the four predictors for October are negative for storm activity. During the 2004 hurricane season, activity was at record levels for August and September but died off in October due to a developing El Niño event which increased vertical wind shear. We do not expect an El Niño event to develop this fall, and therefore, we expect an active end to the hurricane season. We continue to call for well above-average October activity with an NTC of about 175 percent of the typical October value (Table 8). In round numbers, we are forecasting 3 named storms, 2 hurricanes, 1 intense hurricane and an NTC of 30 for October.

Table 9 displays a summary of this year's hurricane activity through August and our projection for the rest of the season. We expect September and October activity to be above their climatological averages. We assume no activity in November.

3  U.S. Landfall Probability Forecast for September and October

       We have recently developed a methodology for calculating the probability of hurricane landfall along the entire U.S. coastline for the months of September and October. Based on a 1950-2001 dataset, the probability of various intensity classes of tropical cyclones making landfall in September and October is based on September and October NTC values. For our forecast of a September 2005 NTC of 80, our United States landfall probabilities are given in Table 10. Table 11 displays the landfall probabilities for October. Landfall probabilities for the U.S. are above average for this September and October based on an above-average forecast of Atlantic basin NTC for both September and October.

4  United States Major Hurricane Landfall Since 1995

       The global climate signals since 1995 have been similar to the global climate signals of the mid-1920s to the mid-1960s when many more major hurricanes struck the US East Coast and Florida. Between 1995-2003, only 3 of 32 major Atlantic basin hurricanes (Cat. 3-4-5) made United States landfall (Opal (1995), Fran (1996), Bret (1999)). Since August of last year, we have had five major hurricanes make landfall (Charley (2004), Ivan (2004), Jeanne (2004), Dennis (2005), and Katrina (2005)). The long-term average is about one in three. Even though the US has experienced five major hurricane strikes since August 2004, in terms of the number of U.S. major hurricane strikes during the last 11 years (8 of 41 versus one of three for the century-long average) we have been, from this perspective, rather fortunate.

5  Difference in 2005 versus 2004 June-July TC Activity

       Although both the 2004 and 2005 seasons will likely have nearly comparable seasonal NTC activity, the June-July period of each season was very different. June-July 2004 had no TC activity while June-July 2005 had the most early season tropical cyclone activity on record (7 named storms, 2 hurricanes, 2 major hurricanes and an NTC of 68).

These June-July activity differences are well explained by the large June-July increases in 2005 over 2004 for several enhancing Atlantic tropical cyclone features including:

1. Higher Atlantic sea surface temperatures (SST)

2. Lower Atlantic sea level pressures (SLP)

3. Weaker low-latitude trade winds

4. Stronger low-latitude upper-tropospheric easterly winds; the combination of 3 and 4 results in reduced vertical wind shear

5. Moister lower and middle atmosphere

6  United States Landfalling Hurricane Webpage Application

       Over the past four years, we have been compiling and synthesizing our landfalling hurricane data and have developed a webpage application with extensive landfall probabilities for the Gulf and East Coasts of the United States. In partnership with the GeoGraphics Laboratory at Bridgewater State College, a web application has been created that displays landfall probabilities for eleven regions, 55 subregions and all 205 U.S. coastal and near-coastal counties from Brownsville, Texas to Eastport, Maine. Individual probabilities of sustained winds of tropical storm force (40-75 mph), hurricane force ( ³ 75 mph) and intense or major hurricane force ( ³ 115 mph) are also given. These probabilities are based on the current forecast of NTC activity and on current values of SSTA*. Probabilities of winds in the vicinity of a subregion and county as well as 50-year probabilities for winds of tropical storm force, hurricane force, and intense or major hurricane force are also provided. These probabilities have recently been updated with data from the latter part of the 19th century with the release of the first part of the HURDAT reanalysis (Landsea et al. 2005). Table 12 summarizes the data currently available on the webpage.

Figures 3 and 4 display example screens of data that is available on this website. The user can select tracks of all intense hurricanes that have made landfall in a given area over the last 100 years. This webpage is currently available at http://www.e-transit.org/hurricane. One can also reach this webpage from a link off the CSU Tropical Meteorology Project homepage: http://hurricane.atmos.colostate.edu.

7  The 1995-2005 Upswing in Atlantic Hurricanes and Global Warming

       Many individuals have queried whether the unprecedented landfall of four destructive hurricanes in a seven-week period during August-September 2004 and the landfall of two more major hurricanes in the early part of the 2005 season is related in any way to human-induced climate changes. There is no evidence that this is the case. If global warming were the cause of the increase in United States hurricane landfalls in 2004 and 2005 and the overall increase in Atlantic basin major hurricane activity of the past eleven years (1995-2005), one would expect to see an increase in tropical cyclone activity in the other storm basins as well (ie., West Pacific, East Pacific, Indian Ocean, etc.). This has not occurred. When tropical cyclones worldwide are summed, there has actually been a slight decrease since 1995. In addition, it has been well-documented that the measured global warming during the 25-year period of 1970-1994 was accompanied by a downturn in Atlantic basin major hurricane activity over what was experienced during the 1930s through the 1960s.

We attribute the heightened Atlantic major hurricane activity between 1995-2005 to be a consequence of the multidecadal fluctuations in the Atlantic Ocean thermohaline circulation (THC) as we have been discussing in our Atlantic basin seasonal hurricane forecasts for several years. Major hurricane activity in the Atlantic has been shown to undergo marked multidecadal fluctuations that are directly related to North Atlantic sea surface temperature anomalies. When the Atlantic Ocean thermohaline circulation is running strong, the central Atlantic equatorial trough (ITCZ) becomes stronger. The stronger the Atlantic equatorial trough becomes, the more favorable are conditions for the development of major hurricanes in the central Atlantic. Since 1995, the THC has been flowing more strongly, and there has been a concomitant increase in major hurricanes in the tropical Atlantic.

8  Forecast Theory and Cautionary Note

      Our forecasts are based on the premise that those global oceanic and atmospheric conditions which precede comparatively active or inactive hurricane seasons in the past provide meaningful information about similar trends in future seasons. It is important that the reader appreciate that these seasonal forecasts are based on statistical schemes which, owing to their intrinsically probabilistic nature, will fail in some years. Moreover, these forecasts do not specifically predict where within the Atlantic basin these storms will strike. The probability of landfall for any one location along the coast is very low and reflects the fact that, in any one season, most US coastal areas will not feel the effects of a hurricane no matter how active the individual season is. However, it must also be emphasized that a low landfall probability does not insure that hurricanes will not come ashore.

9  Forthcoming Update Forecasts of 2005 Hurricane Activity

      We will be issuing a final seasonal update of our 2005 Atlantic basin hurricane activity forecast on Monday 3 October 2005. This 3 October forecast will include a separate forecasts of October-only Atlantic basin tropical cyclone activity. A verification and discussion of all 2005 forecasts will be issued in late November 2005. Our first seasonal hurricane forecast for the 2006 hurricane season will be issued in early December 2005. All these forecasts will be available at our web address given on the front cover: http://hurricane.atmos.colostate.edu/Forecasts .

10  Acknowledgments

       Besides the individuals named on page 2, there have been a number of other meteorologists that have furnished us with data and given valuable assessments of the current state of global atmospheric and oceanic conditions. This includes Arthur Douglas, Richard Larsen, Todd Kimberlain, Ray Zehr and Mark DeMaria. In addition, Barbara Brumit and Amie Hedstrom have provided excellent manuscript, graphical, and data analysis assistance over a number of years. We have profited over the years from many indepth discussions with most of the current and past NHC hurricane forecasters. The first author would further like to acknowledge the encouragement he has received for this type of forecasting research application from Neil Frank, Robert Sheets, Robert Burpee, Jerry Jarrell, former directors of the National Hurricane Center (NHC), and from the current director, Max Mayfield and their forecast staffs. Uma Shama and Larry Harman of Bridgewater State College, MA have provided assistance and technical support in the development of our Landfalling Hurricane Probability Webpage. We also thank Bill Bailey of the Insurance Information Institute for his sage advice and encouragement.

The financial backing for the issuing and verification of these forecasts has in part been supported by the National Science Foundation and by the Research Foundation of Lexington Insurance Company (a member of the American International Group). We also thank the GeoGraphics Laboratory at Bridgewater State College for their assistance in developing the Landfalling Hurricane Probability Webpage.

11  Citations and Additional Reading