EXTENDED RANGE FORECAST OF ATLANTIC SEASONAL HURRICANE ACTIVITY FOR 2005 EXTENDED RANGE FORECAST OF ATLANTIC SEASONAL HURRICANE ACTIVITY AND US LANDFALL STRIKE PROBABILITY FOR 2005


We foresee a well above-average hurricane season for the Atlantic basin in 2005. Also, an above-average probability of U.S. major hurricane landfall is anticipated. We have adjusted our forecast upward from our early April forecast as it now appears unlikely that El Niño conditions will develop this summer.



(as of 31 May 2005)




This forecast is based on new research by the authors,
along with current meteorological information through late May 2005



By

William M. Gray1 and Philip J. Klotzbach2


with assistance from William Thorson3






This forecast as well as past forecasts and verifications are available via the World Wide
Web: http://hurricane.atmos.colostate.edu/Forecasts - also,


Brad Bohlander, Colorado State University Media Representative, (970-491-6432) is available to answer various questions about this forecast.




Department of Atmospheric Science

Colorado State University

Fort Collins, CO 80523

email: barb@tutt.atmos.colostate.edu




ATLANTIC BASIN SEASONAL HURRICANE FORECAST FOR 2005



Issue Date Issue Date Issue Date
Forecast Parameter and 1950-2000 3 December 1 April 31 May
Climatology (in parentheses) 2004 2005 2005

Named Storms (NS) (9.6) 11 13 15
Named Storm Days (NSD) (49.1) 55 65 75
Hurricanes (H)(5.9) 6 7 8
Hurricane Days (HD)(24.5) 25 35 45
Intense Hurricanes (IH) (2.3) 3 3 4
Intense Hurricane Days (IHD)(5.0) 6 7 11
Net Tropical Cyclone Activity (NTC)(100%) 115 135 170








PROBABILITIES FOR AT LEAST ONE MAJOR (CATEGORY 3-4-5) HURRICANE LANDFALL ON EACH OF THE FOLLOWING COASTAL AREAS:



1) Entire U.S. coastline - 77% (average for last century is 52%)

2) U.S. East Coast Including the Florida Peninsula - 59% (average for last century is 31%)

3) Gulf Coast from the Florida Panhandle westward to Brownsville - 44% (average for last century is 30%)

4) Expected above-average major hurricane landfall risk in the Caribbean and in the Bahamas





Acknowledgment



We are grateful to NSF and AIG - Lexington Insurance Company (a member of the American International Group) 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 project research over many years by former graduate students and now colleagues Chris Landsea, John Knaff and Eric Blake. We thank Professors Paul Mielke and Ken Berry for much statistical analysis and advice over many years.


DEFINITIONS

ABSTRACT

Information obtained through late May 2005 indicates that the 2005 Atlantic hurricane season will be a very active one. We estimate that 2005 will have about 15 named storms (average is 9.6), 8 hurricanes (average is 5.9), 75 named storm days (average is 49.1), 45 hurricane days (average is 24.5), 4 intense (category 3-4-5) hurricanes (average is 2.3) and 11 intense hurricane days (average is 5.0). We expect Atlantic basin Net Tropical Cyclone (NTC) activity in 2005 to be about 170 percent of the long-term average. The probability of U.S. major hurricane landfall is estimated to be 150 percent of the long-period average. We expect this year to continue the past-decade trend of above-average hurricane seasons.

This late May forecast is based on a newly devised extended range statistical forecast procedure which utilizes 55 years of past global reanalysis data. Analog predictors are also utilized. We have increased our forecast from our early April prediction due to continued Atlantic Ocean warming and a decreased likelihood of the development of an El Niño this summer/fall. Conditions in the Atlantic are very favorable for an active hurricane season.

1  Introduction

      This is the 22nd year in which the first author has made forecasts of the coming 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. 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. These evolving forecast techniques are based on a variety of climate-related global and regional predictors previously shown to be associated with 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.

2  Earlier 1 June Statistical Hurricane Forecast Scheme

       Our original early June seasonal hurricane forecast scheme was developed in the early 1990s and demonstrated significant hindcast skill for the period of 1950-1991 (Gray et al. 1994). This scheme included measurements of West African rainfall as an important forecast input.

Since the observed shift of Atlantic Ocean SST patterns in 1995 [and the implied increase in the strength of the Atlantic Thermohaline Circulation (THC)], our original 1 June forecast scheme (1994) has consistently underpredicted Atlantic basin hurricane activity. Our earlier 1 June statistical scheme used West African rainfall data as an important predictor. We do not understand why, but the previously observed (1950-1994) strong association between West Africa rainfall and Atlantic hurricanes has not been reliable since 1994. We have lost confidence in the previous 1 June statistical forecast scheme compared to our newly developed one. We have thus decided to discontinue our earliest 1 June forecast scheme.

Over the past couple of years, we have been using an updated statistical scheme that utilized NOAA/NCEP reanalysis data. However, this scheme used mostly data from the previous fall and winter, and therefore we have recently developed a new early June scheme that makes use of mostly spring data.

2.1  Newly Developed 1 June Forecast Scheme

       Our newly-developed early June forecast scheme also utilizes NOAA/NCEP reanalysis data and was developed on data from 1949-1989. It was then tested on independent data from 1990-2004 to insure that the forecast shows similar skill in this later forecast period.

The pool of four predictors for this new extended range forecast is given and defined in Table 1. The location of each of these new predictors is shown in Fig. 1. Strong statistical relationships can be extracted via combinations of these predictive parameters (which are available by the end of May), and quite skillful Atlantic basin hurricane forecasts for the following summer and fall can be made if the atmosphere and ocean continue to behave in the future as they have during the hindcast period of 1949-2004.
JunePredictors.jpg

Figure 1: Location of predictors for our new 1 June forecast for the 2005 hurricane season.


Table 1: List of our new 1 June 2005 predictors and their anomalous values for this year's hurricane activity. A plus (+) means that positive values of the parameter are associated with increased hurricane activity, and a minus (-) indicates that negative values of the parameter are associated with increased hurricane activity.



Predictor2005 Observed Values

(1) - May SST (5°S-5°N, 90-150°W (-) +0.8 SD
(2) - April-May SST (30-45°N, 10-30°W) (+) +0.7 SD
(3) - March-April SLPA (0-20°N, 0-40°W) (-) +0.7 SD
(4) - Previous November 500 mb Height Anomaly (67.5-85°N, 10°E-50°W) (+) - 0.2 SD

Table 2 shows our statistical forecast for the 2005 hurricane season and the comparison of this forecast with climatology (average season between 1950-2000). Our statistical forecast is calling for about average activity this year.

Table 2: New late May statistical forecast for 2005.



Statistical
Predictands and Climatology Forecast Numbers
Named Storms (NS) - 9.6 9.8
Named Storm Days (NSD) - 49.1 49.3
Hurricanes (H) - 5.9 5.7
Hurricane Days (HD) - 24.5 22.7
Intense Hurricanes (IH) - 2.3 2.3
Intense Hurricane Days (IHD) - 5.0 6.0
Net Tropical Cyclone Activity (NTC) - 100 101

2.2  Physical Associations of Predictors with Hurricane Activity

Brief descriptions of our new late May predictors follow:


Predictor 1. May SST in the Eastern Equatorial Pacific (-)

(5°S-5°N, 90-150°W)

Sea surface temperatures in this area are taken to be a measure of ENSO conditions. When sea surface temperatures are much cooler than normal, La Niña conditions are present, and when sea surface temperatures are much warmer than normal, El Niño conditions are occurring. Although there is some change over during the summer and fall, in general, anomalies in this area tend to persist from the late spring through the summer and fall. El Niño conditions during the summer and fall tend to decrease Atlantic hurricane activity by increasing westerlies at upper levels across the Atlantic (Gray 1984a, Goldenberg and Shapiro 1996). These increased westerlies increase vertical wind shear across the area where Atlantic tropical cyclones develop.


Predictor 2. April-May SST off the Northwestern European Coast (+)

(30-45°N, 10-30°W)

Warm sea surface temperatures in this area indicate that the Atlantic subtropical ridge is weaker than normal, and therefore trade winds across the Atlantic are also weaker than normal. These anomalies in April-May correlate strongly with a generally warm Atlantic Ocean as well as with low sea level pressure throughout the tropical Atlantic during the heart of the hurricane season from August-October. Weaker trade winds and easterly anomalies at upper levels during the summer throughout the tropical Atlantic are also associated with this feature.


Predictor 3. March-April SLP in the Tropical Atlantic (-)

(0-20°N, 0-40°W)

Low sea level pressure in the tropical Atlantic during March-April implies increased instability, reduced trade wind strength and warm sea surface temperatures during the spring. In general, these favorable conditions for tropical cyclone activity tend to persist through the summer and fall, as evidenced by strong lag correlations (r > 0.4) between this feature and warm sea surface temperatures in the tropical Atlantic during the late summer/early fall. Also, reduced vertical wind shear and continued low sea level pressure during the late summer/early fall are associated with low values of this feature in March-April.


Predictor 4. November 500 mb Geopotential Height in the far North Atlantic (+)

(67.5-85°N, 10°E-50°W)

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 blocking 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 winds 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.


2.3  Hindcast Skill of New 1 June Scheme

       Table 3 shows the degree of hindcast variance (r2) explained by our new 1 June forecast scheme based on our 41-year developmental dataset (1949-1989), our skill on the independent dataset (1990-2004), and our skill over the entire dataset (1949-2004). Note that the scheme generally shows comparable or improved skill in the independent dataset, which lends us increased confidence in its use.

Table 3: Variance (r2) explained for our new 1 June forecast scheme in the developmental dataset (1949-1989), in the independent dataset (1990-2004), and over the entire dataset (1949-2004).



Variance (r2) Explained Variance (r2) Explained Variance (r2) Explained
Developmental Dataset Independent Dataset Entire Dataset
Variable (1949-1989) (1990-2004) (1949-2004)

NS 0.27 0.49 0.29
NSD 0.40 0.65 0.37
H 0.31 0.67 0.36
HD 0.51 0.63 0.49
IH 0.45 0.67 0.49
IHD 0.54 0.38 0.48
NTC 0.54 0.70 0.52


3  Analog-Based Predictors for 2005 Hurricane Activity

       Certain years in the historical record have global oceanic and atmospheric trends which are substantially similar to 2005. These years also provide useful clues as to likely trends in activity that the forthcoming 2005 hurricane season may bring. For this 31 May forecast, we project atmospheric and oceanic conditions for August through October 2005 and determine which of the prior years in our database have distinct trends in key environmental conditions which are similar to current late May 2005 conditions. Table 4 lists our analog selections.


Analog Years. We have found five prior hurricane seasons (since 1949) which appear to be similar to current late May 2005 conditions and projected August-October 2005 conditions. Specifically, we expect the North Atlantic (50-60°N, 10-50°W) warm SST anomalies to remain warm for the 2005 hurricane season. An unusually warm north Atlantic is indicative of a strong Atlantic thermohaline circulation. The tropical equatorial Quasi-Biennial Oscillation (QBO) is projected to be from an easterly direction. Easterly QBO conditions typically act to slightly reduce Atlantic major hurricane activity. We anticipate that the recent global atmosphere and ocean circulation regimes which have been present in all but two of the last ten years (the El Niño years of 1997 and 2002) will continue to be present in 2005. In addition, we looked for years that had weak El Niño conditions the previous fall and winter with neutral ENSO conditions in the eastern and central Pacific observed during the summer of the year being selected.

There have been five hurricane seasons since 1949 with characteristics similar to what we observe in late May 2005 and what we anticipate for the summer/fall 2005 period. These best analog years are 1958, 1966, 1995, 2003 and 2004 (Table 4). Thus, based on this analysis, we expect 2005 to be an active hurricane season and in line with the average of eight of the last ten years (1995, 1996; 1998-2001; 2003, 2004).

Table 4: Best analog years for 2005 with the associated hurricane activity listed for each year.



NS NSD H HD IH IHD NTC

1958 10 56 7 30 4 8.25 133
1966 11 64 7 42 3 7.00 134
1995 19 121 11 62 5 11.50 222
2003 16 75 7 33 3 16.75 173
2004 14 90 9 46 6 22.00 229

Mean 14.0 81.2 8.2 42.6 4.2 13.1 176

2005 Forecast 15 75 8 45 4 11 170


4  Comparison of Forecast Techniques

       Table 5 provides a comparison of our statistical and analog forecast techniques along with the final adjusted forecast and climatology. Column 1 is our 1 June statistical scheme, column 2 is our analog scheme, column 3 is our adjusted final forecast, and column 4 is the 1950-2000 climatology. We have chosen to give more weight to our analog prediction and foresee a very active 2005 season.

Table 5: Comparison of our 2005 statistical and analog forecast techniques along with our final adjusted forecast and the 1950-2000 climatology.



(1) (2) (3) (4)
Forecast Statistical Analog 31 May 2005 1950-2000
Parameter Forecast Forecast Final Fcst Climatology

Named Storms (NS) 9.8 14.0 15 9.6
Named Storm Days (NSD) 49.3 81.2 75 49.1
Hurricanes (H) 5.7 8.2 8 5.9
Hurricane Days (HD) 22.7 42.6 45 24.5
Intense Hurricanes (IH) 2.3 4.2 4 2.3
Intense Hurricane Days (IHD) 6.0 13.1 11 5.0
Net Tropical Cyclone Activity
    (NTC) 101 176 170 100


5  Discussion

       We watched closely as a series of eastward-propagating Kelvin waves, triggered by strong MJO events, transported anomalously warm water eastward to the west coast of South America. These Kelvin waves caused considerable warming in the eastern and central Pacific during the latter part of April and the early part of May; however, the trade winds in the east Pacific have been fairly strong over the past few weeks, and there does not appear to be much forcing for additional warming in the coming weeks. Sea surface temperatures have cooled in all Nino regions over the past two weeks, and we expect neutral ENSO conditions to persist throughout this upcoming summer and fall.

This will be an easterly Quasi-Biennial Oscillation (QBO) year. Low-latitude Atlantic hurricane activity is typically reduced in these seasons. But, this association has not worked well since we have entered this new era of enhanced activity that began in 1995. We do not anticipate the QBO exerting a large reducing influence on this year's activity.

The reader will note that we have raised our forecast considerably from what the statistics would indicate. This is due to the fact that we are in a new active era for Atlantic major hurricane activity, and Atlantic SST conditions are close to being the highest on record. We believe these unusually warm Atlantic SST conditions are high enough this year that they will trump most of our usual seasonal statistical predictors shown in Table 1.

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. We have therefore raised our forecast to be in more line with our analog years.

6  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.


Slide4.jpeg

Figure 2: Flow diagram illustrating how forecasts of U.S. hurricane landfall probabilities are made. Forecast NTC values and an observed measure of recent North Atlantic (50-60°N, 10-50°W) SSTA* are used to develop regression equations for U.S. hurricane landfall. Separate equations are derived for the Gulf and for Florida and the East Coast (FL+EC).


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 6) 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 (May 2005) value of SSTA* is 62. Hence, in combination with a prediction of NTC of 170 for 2005, a combination of NTC + SSTA* of (170 + 62) yields a value of 232.

As shown in Table 6, 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 6: NTC activity in any year consists of the seasonal total of the following six parameters expressed in terms of their long-term averages. A season with 10 NS, 50 NSD, 6 H, 25 HD, 3 IH, and 5 IHD, would then be the sum of the following ratios: 10/9.6 = 104, 50/49.1 = 102, 6/5.9 = 102, 25/24.5 = 102, 3/2.3 = 130, 5/5.0 = 100, divided by six, yielding an NTC of 107.


1950-2000 Average

1) Named Storms (NS) 9.6
2) Named Storm Days (NSD) 49.1
3) Hurricanes (H) 5.9
4) Hurricane Days (HD) 24.5
5) Intense Hurricanes (IH) 2.3
6) Intense Hurricane Days (IHD) 5.0


Table 7 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 Atlantic basin NTC activity in 2005 is expected to be greater than the long-term average (170), 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.

Table 7: Estimated probability (expressed in percent) of one or more U.S. landfalling tropical storms (TS), category 1-2 hurricanes (HUR), category 3-4-5 hurricanes, total hurricanes and named storms along the entire U.S. coastline, along the Gulf Coast (region 1-4), and along the Florida and the East coastline (Regions 5-11) for 2005. The long-term mean annual probability of one or more landfalling systems during the last 100 years is given in parentheses.


Coastal Category 1-2 Category 3-4-5 All Named
Region TS HUR HUR HUR Storms

Entire U.S. (Regions 1-11) 89% (80) 85% (68) 77% (52) 97% (84) 99% (97)
Gulf Coast (Regions 1-4) 72% (59) 58% (42) 44% (30) 77% (61) 94% (83)
Florida plus East Coast (5-11) 61% (51) 65% (45) 59% (31) 86% (62) 95% (81)

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. Table 8 summarizes the data currently available on the webpage.

Table 8: Data currently available on our CSU landfalling hurricane probability webpage.


Annual Annual
Landfall Vicinity 50-Year
Probability Probability Probability

NS X X X
H X X X
IH X X X


Landfall_Webpage1.jpg

Figure 3: View of landfalling hurricane webpage centered on Subregion 1E - the Houston/Galveston metropolitan area.


Figure4.jpg

Figure 4: Example of data available from the United States landfalling hurricane 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.

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 of the 2004 season as well as the increased Atlantic major hurricane activity of the previous nine years 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. Even though the 2004 hurricane season was quite active, it was only somewhat more active than seven of the past nine hurricane seasons (1995-1996, 1998-2001, 2003). It was the environmental steering currents that drove four of the six major hurricanes of 2004 on such long, low-latitude westerly tracks that made the 2004 Atlantic basin hurricane season so special. The very damaging 2004 hurricane season was simply a low probability event resulting from unusual natural variability in the ocean-atmosphere system.

Similarly, the ten typhoons that struck Japan in 2004 were also a rare statistical event that was in part a consequence of the anomalously warm tropical central Pacific sea surface temperatures and a weaker-than-normal West Pacific subtropical anticyclone. This caused a high percentage of West Pacific typhoons that formed in the central Pacific to be steered toward the Japanese Islands. Such high U.S. and Japan landfalls events of last year should in no way be associated with the human-induced global warming hypothesis.

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 5 August, Friday 2 September and Monday 3 October 2005. These 5 August, 2 September and 3 October forecasts will include separate forecasts of August-only, September-only and October-only Atlantic basin tropical cyclone activity. Table 9 displays our upcoming forecasts in a tabular format. 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.

Table 9: Upcoming 2005 Atlantic Basin tropical cyclone forecasts and seasonal updates.


Date 5 Aug. 3 Sep. 2 Oct.

Seasonal Forecast X X X
Monthly Forecast X X X

11  Acknowledgments

       Besides the individuals named on page 3, 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 AIG - Lexington Insurance Company (a member of the American International Group) for the last two years. We thank the GeoGraphics Laboratory at Bridgewater State College for their assistance in developing the Landfalling Hurricane Probability Webpage.


12  Citations and Additional Reading


13  Verification of Previous Forecasts


Table 9: Summary verification of the authors' six previous years of seasonal forecasts for Atlantic TC activity between 1999-2004. Verification of our earlier year forecasts for the years 1984-1998 are given in our late November seasonal verifications (on this Web location).

Update Update Update
1999 5 Dec 19987 April 4 June6 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 1010 15
Net Trop. Cyclone (NTC) Activity 160 160 160 160 193

Update Update Update
2000 8 Dec 1999 7 April 7 June4 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

Update Update Update
2001 7 Dec 20006 April 7 June7 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

Update Update Update Update
2002 7 Dec 20015 April 31 May7 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

Update Update Update Update Update
2003 6 Dec 2002 4 April 30 May6 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 16
No. of Hurricane Days 35 35 35 25 25 35 33
No. of Named Storm Days 65 65 70 60 55 70 75
Hurr. Destruction Potential(HDP) 100 100 100 80 80 125 131
Major Hurricanes (Cat. 3-4-5)3 3 3 3 3 2 3
Major Hurr. Days 8 8 8 5 9 15 16.75
Net Trop. Cyclone (NTC) Activity 140 140 145 120 130 155 173

Update Update Update Update Update
2004 5 Dec 2003 2 April 28 May 6 August 3 Sept 1 Oct Obs.

No. of Hurricanes 7 8 8 7 8 9 9
No. of Named Storms 13 14 14 13 16 15 14
No. of Hurricane Days 30 35 35 30 40 52 46
No. of Named Storm Days 55 60 60 55 70 96 90
Major Hurricanes (Cat. 3-4-5) 3 3 3 3 5 6 6
Major Hurr. Days 6 8 8 6 15 23 22
Net Trop. Cyclone (NTC) Activity 125 145 145 125 185 240 229


Footnotes:

1Professor of Atmospheric Science

2Research Associate

3Research Associate


File translated from TEX by TTH, version 3.12.
On 26 May 2005, 13:35.