The recent upturn in Atlantic basin major hurricane activity which began in 1995 is expected to continue in 2004. We anticipate an above average probability for Atlantic basin major hurricanes and U.S. major hurricane landfall.
(as of 5 December 2003)
This forecast is based on new research by the authors,
along with current meteorological information through November 2003
William M. Gray1 and Philip J. Klotzbach2
with assistance from William Thorson3 and Jason Connor4
[This forecast as well as past forecasts and verifications are available via the World Wide
Web: http://tropical.atmos.colostate.edu/forecasts/index.html] - also,
Brad Bohlander and Thomas Milligan, Colorado State University Media Representatives (970-491-6432) are available to answer various questions about this forecast.
Department of Atmospheric Science
Colorado State University
Fort Collins, CO 80523
|Forecast Parameter and 1950-2000||5 December 2003|
|Climatology (in parentheses)||Forecast for 2004|
|Named Storms (NS) (9.6)||13|
|Named Storm Days (NSD) (49.1)||55|
|Hurricane Days (HD)(24.5)||30|
|Intense Hurricanes (IH) (2.3)||3|
|Intense Hurricane Days (IHD)(5.0)||6|
|Hurricane Destruction Potential (HDP) (72.7)||85|
|Net Tropical Cyclone Activity (NTC)(100%)||125|
1) Entire U.S. coastline - 68% (average for last century is 52%)
2) U.S. East Coast Including Peninsula Florida - 48% (average for last century is 31%)
3) Gulf Coast from the Florida Panhandle westward to Brownsville - 38% (average for last century is 30%)
4) Above-average major hurricane landfall risk in the Caribbean
Seasonal hurricane forecasts have now been issued for 21 years by the tropical meteorology research group of Prof. William Gray of the Department of Atmospheric Science, Colorado State University (CSU). The forecasts, which are issued in December of the prior year, and in April, June, and August of the current year, have steadily improved through continuing research. These forecasts now include predictions of Atlantic basin hurricane activity and U.S. hurricane landfall probabilities for seasonal as well as individual monthly periods.
The National Oceanic and Atmospheric Administration (NOAA) has also recently begun to issue Atlantic basin seasonal hurricane forecasts. The NOAA forecasts are independent of our CSU forecasts although they utilize prior CSU research augmented by their own insights. The NOAA and the CSU forecasts will typically differ in some aspects and details. Chris Landsea and Eric Blake, former CSU project members presently employed by NOAA, have made important contributions to both forecasts.
We are grateful to AIG - Lexington Insurance Company (a member of the American International Group) for providing partial support for the research necessary to make these forecasts. The National Science Foundation has also contributed to the background research necessary to make these forecasts.
Information obtained through November 2003 indicates that the 2004 Atlantic hurricane season will be an active one. We estimate that 2004 will have about 7 hurricanes (average is 5.9), 13 named storms (average is 9.6), 55 named storm days (average is 49), 30 hurricane days (average is 24.5), 3 intense (category 3-4-5) hurricanes (average is 2.3), 6 intense hurricane days (average is 5.0) and a Hurricane Destruction Potential (HDP) of 85 (average is 71). The probability of U.S. major hurricane landfall is estimated to be 30 percent above the long-period average. We expect Atlantic basin Net Tropical Cyclone (NTC) activity in 2004 to be about 125 percent of the long-term average. This forecast is based on our recently developed 6-11 month extended range statistical forecast procedure which utilizes 52 years of past data. Both statistical and analog predictors have been utilized. These include five selective measures of September-November North Atlantic and Pacific surface pressure and 500 mb height fields and a measure of the stratospheric QBO. The influence of El Niño conditions are implicit in these six predictor fields, and therefore we do not utilize a specific ENSO forecast as a predictor. Our predictors indicate that we will likely have weak warm ENSO conditions by next summer.
This is the 21st year in which the first author has made forecasts of the upcoming season's Atlantic basin hurricane activity. 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. The forecasts are based on a statistical methodology derived from 52 years of past data and a separate study of analog years which have similar precursor circulation features to this year. Qualitative adjustments are added to accommodate additional processes which may not be explicitly represented by our statistical analyses. These evolving forecast techniques are based on a variety of climate-related global and regional predictors previously shown to be related to the forthcoming seasonal Atlantic tropical cyclone activity and landfall probability.
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. Our initial 6-11 month early December seasonal hurricane forecast scheme (Gray et al. 1992) demonstrated hindcast skill for the period of 1950-1990. Our new, recently developed forecast scheme uses more hindcast years (1950-2001) and shows improved hindcast skill and better physical insights into why such precursor relationships have an extended period memory.
Through extensive analyses of NOAA-NCEP reanalysis products, Phil Klotzbach of our forecast team has developed a new set of 6-11 month extended range predictors which shows superior hindcast prediction skill over our previous 1 December forecast scheme. The location of each of these new predictors is shown in Fig. 1. The pool of six predictors for this new extended range forecast is given in Table 1. Strong statistical relationships can be extracted via combinations of these predictors (which are available by 1 December) and the Atlantic basin hurricane activity occurring the following year.
|2003 Values for 2004 Forecast|
|(1) - November 500 mb geopotential height (67.5-85°N, 10E-50°W) (+)||+0.8SD|
|(2) - October-November SLP (45-65°N, 120-160°W) (-)||+0.9SD|
|(3) - September 500 mb geopotential height (35-55°N, 100-120°W) (+)||+0.4SD|
|(4) - July 50 mb U (5°S-5°N, 0-360) (-)||-0.4SD|
|(5) - September-November SLP (15-35°N, 75-95°W) (-)||-0.7SD|
|(6) - November SLP (7.5-22.5°N, 125-175°W) (+)||-2.6SD|
The locations and brief descriptions of our 6-11 month predictors are as follows:
Predictor 1. November 500 MB Geopotential Height in the far North Atlantic (+)
Positive values of this predictor correlate very strongly (r = -0.7) with negative values of the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO). Negative AO and NAO values imply more ridging in the central Atlantic and a likely warm north Atlantic Ocean (50-60°N, 10-50°W). Also, on decadal timescales, weaker zonal winds in the subpolar areas are indicative of a relatively strong thermohaline circulation which is favorable for hurricane activity. Positive values of this November index are negatively correlated with both 200 mb zonal winds and trade wind strength the following September in the tropical Atlantic. The associated reduced tropospheric vertical wind shear enhances conditions for TC development. Other features that are directly correlated with this predictor are low sea level pressure in the Caribbean and a warm North and Tropical Atlantic. Both of the latter are also hurricane-enhancing factors.
Predictor 2. October-November SLP in the Gulf of Alaska (-)
Negative values of this predictor are strongly associated with a positive Älaskan pattern" (Renwick and Wallace 1996) as well as a positive "Pacific North American Pattern" (PNA) which implies reduced blocking over the central Pacific with increased heights over the western United States. The negative mode of this predictor is typically associated with current warm eastern Pacific equatorial SST conditions which usually lead to cool ENSO conditions the following year. Low sea level pressure is observed to occur in the Gulf of Alaska with a decaying El Niño event, and anomalously high pressure is observed with a weakening La Niña event (Larkin and Harrison 2002). Negative values of this predictor indicate that La Niña conditions are likely the following year which tends to enhance Atlantic hurricane activity.
Predictor 3. September 500 MB Geopotential Height in Western North America (+)
Positive values of this predictor correlate very strongly (r = 0.8) with positive values of the PNA. PNA values are usually positive in the final year of an El Niño event (Horel and Wallace 1981). Therefore, cooler ENSO conditions are likely during the following year. Significant lag correlations exist between this predictor and enhanced geopotential height anomalies in the subtropics during the following summer. High heights in the subtropics reduce the height gradient between the tropics and subtropics resulting in easterly anomalies at 200 mb throughout the tropics which favor hurricane development.
Predictor 4. July 50 MB Equatorial U (-)
Easterly anomalies of the QBO during the previous July indicate that the QBO will likely be in the west phase during the following year's hurricane season. The west phase of the QBO has been shown to provide favorable conditions for development of tropical cyclones in the deep tropics according to Gray et al. (1992, 1993, 1994) and Shapiro (1989). Hypothetical mechanisms for how the QBO affects hurricanes are as follows: a) Atlantic TC activity is inhibited during easterly phases of the QBO due to enhanced lower stratospheric wind ventilation and increased upper-troposphere-lower-stratosphere wind shear, and b) for slow moving systems, the west phase of the QBO has a slower relative wind (advective wind relative to the moving system) than does the east phase. This allows for greater coupling between the lower stratosphere and the troposphere.
Predictor 5. September-November SLP in the Gulf - SE USA (-)
This feature is strongly related to the following year's August-September sea level pressure in the tropical and subtropical Atlantic. August-September SLP in the tropical Atlantic is one of the most important predictors for seasonal activity, that is, lower-than-normal sea level pressure is favorable for more TC activity. Low pressure in this area during September-November correlates quite strongly with the positive phase of the PNA. In addition, easterlies at 200 mb throughout the tropical Atlantic are typical during the following year's August-September period with low values of this predictor.
Predictor 6. November SLP in the Subtropical NE Pacific (+)
This feature is also strongly related to the following year's August-September sea level pressure in the tropical and subtropical Atlantic. High pressure in this area correlates with low sea level pressure in the tropical Atlantic and easterly anomalies at 200 mb during the following August through September period. According to Larkin and Harrison (2002), high pressure in this area appears during most winters preceding the development of a La Niña event. High pressure in this region forces stronger trade winds in the east Pacific which increases upwelling and helps initiate La Niña conditions which eventually enhance Atlantic hurricane activity during the following summer. In addition, this predictor correlates with low geopotential heights at 500 mb throughout the tropics the following year which is also favorable for more hurricane activity in the Atlantic.
Table 2 shows the degree of hindcast variance explained by our new 1 December forecast scheme based on a 52-year developmental dataset.
To reduce overfitting, the 1 December forecast picks the best combination of five predictors from a pool of six predictors or until the jackknife variance explained no longer increases.
|Variables Selected||Variance (r2) Explained||Jackknife (r2)|
|NS - 1,2,3||0.40||0.29|
|NSD - 1, 3, 4, 5, 6||0.45||0.28|
|H - 1, 2, 3, 4, 5||0.53||0.38|
|HD - 1, 2, 3, 4, 5||0.53||0.35|
|IH - 1, 2, 3, 4, 5||0.69||0.57|
|IHD - 1, 3, 4, 5, 6||0.51||0.41|
|HDP - 1, 3, 4, 5, 6||0.57||0.37|
|NTC - 1, 3, 4, 5, 6||0.62||0.46|
Certain years in the historical record have global oceanic and atmospheric trends which are substantially similar to 2003/2004. These years also provide useful clues as to likely trends in activity that the forthcoming 2004 hurricane season may bring. For this (1 December) extended range forecast, we project atmospheric and oceanic conditions for August through October 2004 and determine which of the prior years in our database have distinct trends in key environmental conditions which are similar to current October-November 2003 conditions. Table 3 lists our analog selections.
Analog Years. We have found four prior hurricane seasons since 1949 which appear to be similar to current November 2003 conditions and projected 2004 August-October conditions. Specifically, we expect the North Atlantic (50-60°N, 10-50°W) warm SST anomalies to remain warm for the 2004 hurricane season due to current negative AO and NAO values and hence that the strong Atlantic thermohaline circulation will persist through the next hurricane season. Also, it is assumed that the conditions of the Northern Hemisphere NAO, PNA, PDO, and AO of seven of the last nine years will persist through 2004. The latter assumptions carry the implication that the recent global atmosphere and ocean circulation regimes which have been present in all but two of the last nine years will continue to be present in 2004.
There were four hurricane seasons since 1949 with characteristics similar to what we observe in November 2003 and what we anticipate for the summer/fall 2004 period. These best analog years are 1953, 1960, 1967, and 1969 (Table 3). We anticipate that 2004 seasonal hurricane activity will be somewhat above the average values for these four analog years due to an anticipated active thermohaline circulation. Thus, based on this analysis, we expect 2004 to be an active hurricane season and in line with the average of seven of the last nine years (1995-1996; 1998-2001; 2003). We anticipate 2004 to be considerably more active than the average season during the inactive 1970-1994 period.
Table 4 shows our final adjusted 1 December forecast for the 2004 season which is a combination of our derived full 52-year statistical forecast and our four analog year forecast. We foresee an active 2004 hurricane season.
|Climatology (in parentheses)||Scheme||Scheme||Forecast|
|Named Storms (9.6)||10.4||11.5||13|
|Named Storm Days (49.1)||44.3||59.0||55|
|Hurricane Days (24.5)||30.5||28.0||30|
|Intense Hurricanes (2.3)||2.7||2.3||3|
|Intense Hurricane Days (5.1)||4.8||5.3||6|
|Hurricane Destruction Potential (72.7)||78.0||84.8||85|
|Net Tropical Cyclone Activity (100%)||107.4||112.8||125|
Our analysis of the current and projected global atmospheric and oceanic circulation patterns indicates that Atlantic tropical cyclone activity should be above average during the 2004 Atlantic basin hurricane season. We anticipate that ENSO conditions will remain neutral to slightly warm and that the warm sea surface temperatures in the North and tropical Atlantic that have been prevalent in most years since 1995 will continue.
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 100 years (1900-1999). Specific landfall probabilities can be given for all cyclone intensity classes for a set of distinct U.S. coastal regions.
Figure 2 provides a flow diagram showing how these forecasts are made. Net landfall probability is shown linked to the overall Atlantic basin Net Tropical Cyclone activity (NTC; see Table 5) 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* 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 (November 2003) value of SSTA* is 44. Hence, in combination with a prediction of NTC of 125 for 2004, a combination of NTC + SSTA* of (125 + 44) yields a value of 169.
As shown in Table 5, 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. Whereas many active Atlantic hurricane seasons feature no landfalling hurricanes, some inactive years have experienced one or more landfalling hurricanes. Long-term statistics show 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 the Florida and U.S. East Coast during years of (1) increased NTC and (2) above-average North Atlantic SSTA* conditions.
|1)||Named Storms (NS)||9.6|
|2)||Named Storm Days (NSD)||49.1|
|4)||Hurricane Days (HD)||24.5|
|5)||Intense Hurricanes (IH)||2.3|
|6)||Intense Hurricane Days (IHD)||5.0|
Table 6 lists strike probabilities for different TC categories for the entire U.S. coastline, the Gulf Coast and Florida, and the East Coast for 2004. The mean annual probability of one or more landfalling systems is given in parentheses. Note that Atlantic basin NTC activity in 2004 is expected to be greater than the long-term average of 100, and North Atlantic SSTA* values are measured to be above average (44 units). U.S. hurricane landfall probability is thus expected to be 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.
|Coastal||Category 1-2||Category 3-4-5||All||Named|
|Entire U.S. (Regions 1-11)||85% (80)||79% (68)||68% (52)||93% (84)||99% (97)|
|Gulf Coast (Regions 1-4)||66% (59)||51% (42)||38% (30)||70% (61)||90% (83)|
|Florida plus East Coast (5-11)||56% (51)||56% (45)||48% (31)||77% (62)||90% (81)|
A major reconfiguration of the distribution of Atlantic SST anomalies began in mid-1995 and has largely persisted through the present. North Atlantic SSTs have become about 0.4 to 0.6°C warmer than normal since 1995, and tropical Atlantic August-October upper tropospheric 200 mb winds have increased from the east, bringing about a significant decrease in tropospheric vertical wind shear. We hypothesize that these strong broadscale SST changes are associated with basic changes in the strength of the Atlantic Ocean thermohaline circulation (ATC). This interpretation is consistent with changes in a long list of global atmospheric circulation features during the last nine years which conform to a prominent shift into hurricane-enhancing Atlantic circulation patterns, particularly the enhancement of major hurricane activity. Historical and geographic evidence going back thousands of years indicates that shifts in the Atlantic multi-decadal thermohaline circulation tend to occur on periods of about 25-50 years. If the recent nine-year shift follows prior occurrences, it is likely that enhanced intense Atlantic basin hurricane activity will persist through the early decades of the 21st century in contrast with the diminished activity that persisted from 1970-1994. We expect that the hurricane season of 2004 will follow this recent upswing in hurricane activity. Our recent verification paper has more discussion on this issue and is available on our website:
Various groups and individuals have suggested that the recent large upswing in Atlantic hurricane activity (since 1995) may be in some way related to the effects of increased man-made greenhouse gases such as carbon dioxide (CO2). There is no reasonable scientific way that such an interpretation of this recent upward shift in Atlantic hurricane activity can be made. Please see our recent 21 November 2003 verification report for more discussion on this subject.
Our forecasts are based on the premise that those global oceanic and atmospheric conditions which preceded 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 2004 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.
We will be issuing seasonal updates of our 2004 Atlantic basin hurricane activity forecast on Friday 2 April, Friday 28 May (to coincide with the official start of the 2004 hurricane season on 1 June), Friday 6 August, Friday 3 September and Friday 1 October 2004. The 6 August, 3 September and 1 October forecasts will include separate forecasts of August-only, September-only and October-only Atlantic basin tropical cyclone activity. A verification and discussion of all 2004 forecasts will be issued in late 2004. All these forecasts will be available at our web address given on the front cover
The first author gratefully acknowledges valuable input to our project research by former graduate students and colleagues Chris Landsea, John Knaff and Eric Blake. A number of other meteorologists 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. 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, but this NSF support is insufficient. We appreciate the financial support of the Research Foundation of AIG - Lexington Insurance Company (a member of the American International Group) for the last two years. We are also grateful to the Research Foundations of the United Services Automobile Association (USAA) and to State Farm Insurance for prior support.
|1999||5 Dec 1998||7 April||4 June||6 August||Obs.|
|No. of Hurricanes||9||9||9||9||8|
|No. of Named Storms||14||14||14||14||12|
|No. of Hurricane Days||40||40||40||40||43|
|No. of Named Storm Days||65||65||75||75||77|
|Hurr. Destruction Potential(HDP)||130||130||130||130||145|
|Major Hurricanes (Cat. 3-4-5)||4||4||4||4||5|
|Major Hurr. Days||10||10||10||10||15|
|Net Trop. Cyclone (NTC) Activity||160||160||160||160||193|
|2000||8 Dec 1999||7 April||7 June||4 August||Obs.|
|No. of Hurricanes||7||7||8||7||8|
|No. of Named Storms||11||11||12||11||14|
|No. of Hurricane Days||25||25||35||30||32|
|No. of Named Storm Days||55||55||65||55||66|
|Hurr. Destruction Potential(HDP)||85||85||100||90||85|
|Major Hurricanes (Cat. 3-4-5)||3||3||4||3||3|
|Major Hurr. Days||6||6||8||6||5.25|
|Net Trop. Cyclone (NTC) Activity||125||125||160||130||134|
|2001||7 Dec 2000||6 April||7 June||7 August||Obs.|
|No. of Hurricanes||5||6||7||7||9|
|No. of Named Storms||9||10||12||12||15|
|No. of Hurricane Days||20||25||30||30||27|
|No. of Named Storm Days||45||50||60||60||63|
|Hurr. Destruction Potential(HDP)||65||65||75||75||71|
|Major Hurricanes (Cat. 3-4-5)||2||2||3||3||4|
|Major Hurr. Days||4||4||5||5||5|
|Net Trop. Cyclone (NTC) Activity||90||100||120||120||142|
|2002||7 Dec 2001||5 April||31 May||7 August||2 Sept||Obs.|
|No. of Hurricanes||8||7||6||4||3||4|
|No. of Named Storms||13||12||11||9||8||12|
|No. of Hurricane Days||35||30||25||12||10||11|
|No. of Named Storm Days||70||65||55||35||25||54|
|Hurr. Destruction Potential(HDP)||90||85||75||35||25||31|
|Major Hurricanes (Cat. 3-4-5)||4||3||2||1||1||2|
|Major Hurr. Days||7||6||5||2||2||2.5|
|Net Trop. Cyclone (NTC) Activity||140||125||100||60||45||80|
|2003||6 Dec 2002||4 April||30 May||6 August||3 Sept||2 Oct||Obs.|
|No. of Hurricanes||8||8||8||8||7||8||7|
|No. of Named Storms||12||12||14||14||14||14||14|
|No. of Hurricane Days||35||35||35||25||25||35||32|
|No. of Named Storm Days||65||65||70||60||55||70||71|
|Hurr. Destruction Potential(HDP)||100||100||100||80||80||125||129|
|Major Hurricanes (Cat. 3-4-5)||3||3||3||3||3||2||3|
|Major Hurr. Days||8||8||8||5||9||15||17|
|Net Trop. Cyclone (NTC) Activity||140||140||145||120||130||155||168|
1Professor of Atmospheric Science