Information obtained through July 2005 indicates that the 2005 Atlantic hurricane season will be an extremely active one. We estimate that 2005 will have about 20 named storms (average is 9.6), 10 hurricanes (average is 5.9), 95 named storm days (average is 49.1), 55 hurricane days (average is 24.5), 6 intense (category 3-4-5) hurricanes (average is 2.3) and 18 intense hurricane days (average is 5.0). We expect Atlantic basin Net Tropical Cyclone (NTC) activity in 2005 to be about 235 percent of the long-term average. The probability of U.S. major hurricane landfall is estimated to be well above the long-period average. This year is expected to continue the past-decade trend of above-average hurricane seasons.

This early August forecast is based on our newly devised monthly forecasts for August, September and October which utilize 55 years of past global reanalysis data. Analog predictors are also utilized. We have increased our forecast from our 31 May prediction due to the seven named storms and the two major hurricanes that have already formed at low latitudes and the increase in favorability of several seasonal predictors over the past two months. This is the highest seasonal forecasts of hurricane activity we have ever made.

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 data. It is only through hindcast skill that one can demonstrate that seasonal forecast skill is possible. This is a valid methodology provided the atmosphere continues to behave in the future as it has in the past. We have no reason for thinking that it will not.

Reanalysis data sets are available from the late 1940s and offer exciting and unique opportunities for the development of new and more skillful extended range empirical climate forecasts. Through extensive analyses of the recently available NOAA/NCEP reanalysis products, we have developed a new post-1 August seasonal forecast based on the sum of our individual August, September, and October forecasts.

2  Predictions of Individual Monthly Atlantic TC Activity for August, September, and October

       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 five years (2000-2004), and the verification of these forecasts looks promising. The verification of the September-only and October-only forecasts also appears to show skill.

2.1  Independent August-Only Statistical Forecast

       Figure 1 and Table 1 list the predictors used in the August-only hindcast (Blake and Gray 2004) 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 August 2005. Table 3 gives our independent statistical prediction for August 2005. These predictors indicate above-average activity for August 2005. The most skillful August predictors, in general, call for a very active month, so we are calling for considerable activity during the month.

2.2  Independent September-Only Statistical Forecast

       Figure 2 and Table 4 portray and list our 1 August predictors for September-only activity for this year. Table 5 gives the predictor values for September 2005. Table 6 gives our independent September statistical forecast and our adjusted final forecast.

Statistical data available through the end of July indicates that September 2005 will have about average activity. Our three most skillful predictors (Predictors 2, 3, and 8) all call for well above-average activity, and therefore, we believe that September will be quite active. An updated September-only statistical forecast will be issued on 3 September. This early September forecast will have the advantage of August data.

2.3  Independent October-only Statistical Forecast

       Through examination of the NCEP/NCAR reanalysis, we have discovered four predictors that in combination explain about 50 percent of the October cross-validated variance in Net Tropical Cyclone activity 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. The predictors utilized in our initial October prediction are displayed graphically in Figure 3, and their 2005 values are displayed in Table 7. Three of the four predictors are above-average for storms. Therefore, we are calling for a very active October with an NTC of about 200 percent of the climatological average. In round numbers, we are forecasting 3 named storms, 2 hurricanes, 1 intense hurricane and an NTC of 35 for October. Table 8 displays our initial statistical forecast and our adjusted forecast for October tropical cyclone activity. Additional updates for the October-only forecast will be issued in early September and in early October.

2.4  Monthly Prediction Summary

       Table 9 summarizes our individual monthly predictions and our monthly adjustments to these predictions. Based on jackknifed hindcast data from 1950-2000, the sum of the August, September, and October forecasts explains 79% of the variance in seasonal TC activity.

3  Seasonal Analogs for 2005

       Table 10 lists our best seasonal analogs for 2005. We selected years with a warm tropical Atlantic, neutral ENSO conditions, and, in general, active early seasons. Our seasonal analogs continue to point toward a very active season.

4  Forecast Adjustments

       Table 11 provides a comparison of all of our forecast techniques along with the final full season adjusted forecast. Given the current (July) global conditions and other information we have, we anticipate comparable activity to what is indicated by the sum of our three adjusted individual monthly predictions.

5  Comparison of Forecast Techniques

       Table 11 provides a comparison of our statistical and analog forecast techniques along with the final adjusted forecast and climatology. Column 1 gives activity prior to 1 August. Column 2 gives the 3-month sum of our monthly forecasts. Column 3 is our adjusted final after 1 August forecast, Column 4 is our analog scheme, column 5 is the total season adjusted forecast and column 6 is the 1950-2000 climatology.

The reader will note that we have raised our forecast from our earlier forecasts. Atlantic SST conditions are close to being the highest on record. Also, we have already witnessed two major hurricanes forming in the deep tropics which is very favorable for an extremely active season.

We believe that the current active period is quite similar to the 1930s, where we had many active hurricane seasons, even though other features typically associated with active seasons in the 1950s and 1960s were not present. The 1930s were also a period of strong global warming similar to the global warming of the last decade. From the limited data available during the 1930s and 1940s, we deduce that the Atlantic was quite warm, similar to conditions that we are presently experiencing. However, other features, such as strong easterly anomalies at upper levels in the tropical Atlantic which were present in the 1950s and 1960s, do not appear to have been present in the earlier period of the 1930s. We have seen a slight increase in tropical Atlantic easterly anomalies since 1995 but have yet to see the easterlies that were present in the earlier decades of the 1950s and 1960s. In addition, the westerlies in the Southern Hemisphere have not yet weakened, even though, a weaker midlatitude circulation in the Southern Hemisphere is typically associated with active Atlantic hurricane seasons.

6  Post-1 August Landfall Probabilities for 2005

       A significant focus of our recent research involves efforts to develop forecasts of the probability of hurricane landfall along the U.S. coastline. Whereas individual hurricane landfall events cannot be accurately forecast months in advance, the total seasonal probability of landfall can be forecast with statistical skill. With the observation that, statistically, landfall is a function of varying climate conditions, a probability specification has been developed through statistical analyses of all U.S. hurricane and named storm landfall events during the last century (1900-1999). Specific landfall probabilities can be given for all cyclone intensity classes for a set of distinct U.S. coastal regions.

Figure 4 provides a flow diagram showing how these forecasts are made. Net landfall probability is shown linked to the overall Atlantic basin NTC (see Table 12) and to climate trends linked to multi-decadal variations of the Atlantic Ocean thermohaline circulation as inferred from recent past years of North Atlantic SSTA*.

Higher values of SSTA* (see prior forecast for definition) generally indicate greater Atlantic hurricane activity, especially for intense or major hurricanes. Atlantic basin NTC can be skillfully hindcast, and the strength of the Atlantic Ocean thermohaline circulation can be inferred as SSTA* from North Atlantic SST anomalies in the current and prior years. These relationships are then utilized to make probability estimates for U.S. landfall. The current (July 2005) value of SSTA* is 62. Hence, in combination with a post-1 August prediction of NTC of 167 for 2005, a combination of NTC + SSTA* of (167 + 62) yields a value of 229.

As shown in Table 12, NTC is a combined measure of the year-to-year mean of six indices of hurricane activity, each expressed as a percentage difference from the long-term average. Although many active Atlantic hurricane seasons feature no landfalling hurricanes, and some inactive years experience one or more landfalling hurricanes, it is found that, on average, the more active the overall Atlantic basin hurricane season is, the greater the probability of U.S. hurricane landfall. For example, landfall observations during the last 100 years show that a greater number of intense (Saffir-Simpson category 3-4-5) hurricanes strike Florida and the U.S. East Coast during years of (1) increased NTC and (2) above-average North Atlantic SSTA* conditions.

Table 13 lists strike probabilities for different TC categories for the entire U.S. coastline, the Gulf Coast, and Florida and the East Coast for 2005. The mean annual probability of one or more landfalling systems is given in parentheses. Note that post-1 August Atlantic basin NTC activity in 2005 is expected to be greater than the long-term average (167 versus 100), and North Atlantic SSTA* values are measured to be above average (62 units). U.S. hurricane landfall probability is thus expected to be well above average owing to both a higher NTC and above-average North Atlantic SSTAs. During periods of positive North Atlantic SSTA*, a higher percentage of Atlantic basin major hurricanes cross the Florida and eastern U.S. coastline for a given level of NTC.

7  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 (> 75 mph) and intense or major hurricane force (> 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 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 14 summarizes the data currently available on the webpage.

Figures 5 and 6 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.

8  The 1995-2004 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 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 the overall increase in Atlantic basin major hurricane activity of the past ten years (1995-2004), 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 hurricane activity over what was experienced during the 1930s through the 1960s.

We attribute the heightened Atlantic major hurricane activity between 1995-2004 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.

9  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. Regardless of how active the 2005 hurricane season is, a finite probability always exists that one or more hurricanes may strike along the US coastline or the Caribbean Basin and do much damage.

10  Forthcoming Update Forecasts of 2005 Hurricane Activity

      We will be issuing seasonal updates of our 2005 Atlantic basin hurricane activity forecast on Friday 2 September and Monday 3 October 2005. These 2 September and 3 October forecasts will include separate forecasts of September-only and 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 on our web site: http://hurricane.atmos.colostate.edu/Forecasts.

11  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.

12  Citations and Additional Reading