(An unusual season of unprecedented Florida landfall events and intense long-lived tropical cyclones - the large number of major hurricanes (6) was not anticipated)

(see our website for prior forecasts, verifications, and term definitions)


William M. Gray1 and Philip J. Klotzbach2

with special assistance from William Thorson3

[This forecast as well as past forecasts and verifications are available via the World Wide
Web:] - 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


1 October 2004



Our forecast and verification of August-only hurricane activity made in early August - a significant under-forecast of one of the most active Augusts on record.

Tropical Cyclone August 2004 Adjusted
Parameters and 1950-2000 Statistical August 2004 August 2004
August Average (in parentheses)Forecast Forecast Verification
Named Storms (NS) (2.8) 3.6 4 8
Named Storm Days (NSD) (11.8) 14.3 20 32.25
Hurricanes (H) (1.6) 1.6 3 4
Hurricane Days (HD) (5.7) 2.3 8 15
Intense Hurricanes (IH) (0.6) 0.3 1 3
Intense Hurricane Days (IHD) (1.2) -2.0 1 5.25
Net Tropical Cyclone (NTC) (26.4) 20.1 35 85.6

Our forecast and verification of September-only hurricane activity made in early September - a successful forecast of a very active month; however, the degree of the increased activity was under-forecast.

Tropical Cyclone September 2004 Adjusted
Parameters and 1950-2000 Statistical September 2004 September 2004
August Average (in parentheses)Forecast Forecast Verification
Named Storms (NS) (3.4) 3.8 5 4
Named Storm Days (NSD) (21.7) 24.6 30 51.50
Hurricanes (H) (2.4) 1.7 3 3
Hurricane Days (HD) (12.3) 15.1 20 29.25
Intense Hurricanes (IH) (1.3) 1.22 3
Intense Hurricane Days (IHD) (3.0) 3.1 9 17.75
Net Tropical Cyclone (NTC) (48) 49.5 85 134


Update Update Update Update Update
Forecast Parameter and 1950-2000 5 December 2 April 28 May 6 Aug 3 Sept 1 Oct
Climatology (in parentheses) 2003 2004 2004 2004 2004 2004
Named Storms (NS) (9.6) 14 14 14 13 16 15
Named Storm Days (NSD) (49.1) 60 60 60 55 70 96
Hurricanes (H)(5.9) 8 8 8 7 8 9
Hurricane Days (HD)(24.5) 35 35 35 30 40 52
Intense Hurricanes (IH) (2.3) 3 3 3 3 5 6
Intense Hurricane Days (IHD)(5.0) 8 8 8 6 15 23
Net Tropical Cyclone Activity (NTC)(100%) 145 145 145 125 185 240


Full Season Tropical Observed Updated Full Season
Cyclone Parameters 2004 Activity October-only Adjusted Updated
and their 1950-2000 Through Statistical Oct.-only 1 Oct. 2004
Climatology (in parentheses) September Forecast Forecast Forecast
Named Storms (NS) (9.6) 12 1.8 3 15
Named Storm Days (NSD) (49.1) 83.75 9.5 12 96
Hurricanes (H)(5.9) 7 1.2 2 9
Hurricane Days (HD)(24.5) 44.25 4.7 8 52
Intense Hurricanes (IH) (2.3) 6 0.3 0 6
Intense Hurricane Days (IHD) (5.0)230.9023
Net Tropical Cyclone Activity (NTC)(100%)2201820240


The hurricane season through September 2004 has been one of the most active seasons on record since 1950 with 12 named storms, 7 hurricanes and a Net Tropical Cyclone (NTC)4 activity of 220 percent of the full-season average. As of 1 October, the 2004 hurricane season has been more than twice as active as the full-season average from 1950-2000.

Our new October-only forecast calls for three named storms, two hurricanes, no major hurricanes and an NTC activity value of 20 which is slightly above the mean October-only average value of 17. United States and Caribbean basin landfall probability for the remainder of the 2004 season is now estimated to be slightly above the October-November average. The likelihood of a November cyclone is fairly minimal in any year.


Full Season Tropical Full Season Full Season Observed Updated Full Season
Cyclone Parameters Updated 5 Dec. Updated 2004 Activity October-only Adjusted Updated
and their 1950-2000 2 Apr and 28 May 6 Aug 2004 Through Statistical Oct-only 1 Oct 2004
Climatology (in parentheses) 2004 Forecasts Forecast September Forecast Forecast Forecast
Named Storms (NS) (9.6) 14 13 12 1.8 3 15
Named Storm Days (NSD) (49.1) 60 55 83.75 9.5 12 96
Hurricanes (H)(5.9) 8 7 7 1.2 2 9
Hurricane Days (HD)(24.5) 35 30 44.25 4.7 8 52
Intense Hurricanes (IH) (2.3) 3 3 6 0.3 0 6
Intense Hurricane Days
(IHD)(5.0) 8 6 23 0.9 0 23
Net Tropical Cyclone Activity
(NTC)(100%) 145 125 220 18 20 240


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.


Colorado State University (CSU) has issued seasonal hurricane forecasts for the last 21 years. These forecasts, which are now issued in early December of the prior year, and in the early part of the months of April, June and August of the current year, have steadily improved through continuing research. CSU forecasts now include individual monthly predictions and seasonal updates of Atlantic basin activity and monthly U.S. hurricane landfall probabilities which are issued in early August, early September and early October.

The National Oceanic and Atmospheric Administration (NOAA) has also recently begun to issue Atlantic basin seasonal hurricane forecasts, but they do not issue monthly forecasts or landfall probability forecasts. NOAA issues two forecasts per season and gives a range of numbers. The NOAA forecasts are independent of the CSU forecasts, although they utilize prior CSU research augmented with their own insights. The NOAA and the CSU forecasts will not necessarily be in agreement.

Special Note

Chris Landsea, Eric Blake and John Knaff (former Gray project graduate students) deserve (by virtue of their many contributions to our project climate studies) to be co-authors on this forecast. As each of these individuals now work for NOAA (or a branch of NOAA), they have been directed to remove their names from the CSU forecasts.

1  Atlantic Basin 2004 Tropical Cyclone Activity through

       As of the end of September, 2004 has had twelve named storms, seven hurricanes and six major hurricanes. Three major hurricanes have made U.S. landfall (Charley, Ivan and Jeanne) and one Category 2 hurricane has also made landfall (Frances). This season has been distinguished by persistent tropical cyclone activity with at least one named storm in existence on every day from August 25-present. There have also been two very long-lived major hurricanes (Frances - 7.25 intense hurricane days and Ivan - 10 intense hurricane days). Ivan's 10 intense hurricane days are the most for any single tropical cyclone since 1900. Table 1 and Fig. 1 display the 2004 tropical cyclone statistics for the Atlantic basin through the end of September, and Fig. 1 displays the tracks of these tropical cyclones.

Table 1: Observed 2004 tropical cyclone activity through September.

Highest Peak Sustained Winds
Category Name Dates Knots /lowest SLP in mb NSD HD IHD NTC
IH - 3Alex Aug. 1-6 105 kt/957 mb 5.25 3.25 0.75 18.3
TS Bonnie Aug. 9-12 55 kt/1000 mb 3.00 2.8
IH - 4 Charley Aug. 10-15 125 kt/941 mb 5.25 3.00 0.25 16.5
H - 2 Danielle Aug. 14-20 90 kt/970 mb 6.75 3.50 9.2
TS Earl Aug. 14-16 40 kt/1005 mb 1.75 2.3
IH - 4 Frances Aug. 25-Sep. 7 125 kt/935 mb 12.25 10.00 7.25 46.9
TS Gaston Aug. 28-29, Aug. 31-Sep. 6 60 kt/991 mb 3.00 2.8
TS Hermine Aug. 29-31 45 kt/1000 mb 1.50 2.2
IH - 5 Ivan Sep. 3-17, Sep. 23-24 145 kt/910 mb 14.75 11.50 10.00 67.2
IH - 3 Jeanne Sep. 14-27 110 kt/950 mb 13.25 6.25 0.75 23.1
IH - 4 Karl Sep. 17-24 120 kt/938 mb 8.00 6.75 4.00 32.4
TS Lisa Sep. 20-23, Sep. 25-present 60 kt/990 mb 9.00 5.1
Totals 12 83.75 44.25 23.00 220


Figure 1: 2004 Atlantic basin tropical cyclones through September. Figure courtesy of Brian McNoldy, CSU.

Although September saw the formation of four named storms (only slightly above the average of 3.4), it was much more active than a normal September by virtue of two long lasting and intense hurricanes: Frances (Cat 4, 10 HD and 7.25 IHD) and Ivan (Cat 5, 11.5 HD and 10 IHD). Both of these cyclones formed at low latitudes and had long westward tracks over the Atlantic Ocean.

Ivan became the longest lasting intense hurricane since 1900 with 10 intense hurricane days. September has had more named storm days and intense (Cat 3-4-5) hurricane days than any September since 1950.

1.1  2004 U.S. Landfalling Storms

       Figure 2 displays the U.S. landfalling storms for the 2004 hurricane season as of September 30.


Figure 2: Tropical cyclones making US landfall (TS Bonnie, Category 4 Hurricane Charley, Category 2 Hurricane Frances, TS Gaston, TS Hermine, Category 3 Hurricane Ivan and Category 3 Hurricane Jeanne). Alex did not technically make U.S. landfall but did effect the Outer Banks with hurricane-force winds. A solid line denotes a tropical cyclone of hurricane intensity, and a dashed line indicates a tropical cyclone of tropical storm intensity.

Hurricane Alex brushed the Outer Banks of North Carolina on August 3 as a Category 2 hurricane while traveling northeastward across the Atlantic. Wind gusts to hurricane force were reported on Cape Hatteras. Damage estimates from Alex are approximately 2.5 million dollars. No deaths were reported.

Tropical Storm Bonnie made landfall near Apalachicola, Florida on August 12 with an estimated intensity at landfall of 50 miles per hour. Bonnie was responsible for three deaths in North Carolina from tornadoes spawned from the weakening system. Damage from this system was minimal.

Hurricane Charley made its initial United States landfall on August 13 near Punta Gorda, Florida as a Category 4 hurricane with estimated winds of 145 miles per hour. Charley then traversed northeast through the center of the state, severely affecting Orlando and Daytona Beach before reaching the Atlantic Ocean. Charley made three additional landfalls: first at Cape Romain, SC with estimated winds of 80 miles per hour, a second landfall near Myrtle Beach with estimated winds of 75 miles per hour, and a final landfall on Long Island, NY as a minimal tropical storm with estimated winds of 40 miles per hour. Preliminary damage estimates for this system are between 13-15 billion dollars. Charley is the second most expensive hurricane to hit the United States, behind only Hurricane Andrew of 1992.

Hurricane Frances made landfall as a Category 2 hurricane near Sewall's Point, Florida on September 5 with maximum winds estimated at 105 miles per hour. Frances slowly tracked west-northwest across the state before emerging into the Gulf of Mexico. Frances made a second landfall as a 65 mph tropical storm near St. Marks, Florida on September 6. Insured damage from Frances is estimated at around 4 billion dollars, bringing the total damage estimate to around 8 billion dollars. Frances was responsible for at least 24 deaths.

Tropical Storm Gaston made landfall near McLellanville, SC with maximum winds estimated at 70 miles per hour. Preliminary damage estimates for South Carolina from Gaston are 15 million dollars. No fatalities have been attributed to Gaston.

Tropical Storm Hermine made landfall as a minimal tropical storm near New Bedford, MA with maximum winds estimated at 40 miles per hour. No damage estimates are available, and no loss of life was reported from this system.

Hurricane Ivan made landfall near Palm Shores, AL as a Category 3 hurricane with maximum winds at landfall estimated at 130 miles per hour. Extensive destruction occurred in Pensacola and its suburbs. Ivan was responsible for death and destruction along the entire East Coast of the United States with over 50 deaths attributed to the system in the United States. The remnants of Ivan later tracked southwestward over the Atlantic Ocean, across the state of Florida, and then back into the Gulf of Mexico. Once in the Gulf, the system regenerated and made a second landfall in Cameron Parish, LA as a minimal tropical storm with maximum winds estimated at 40 miles per hour. Initial estimates of insured damage losses from Ivan range from 3-6 billion dollars, bringing the total damage estimate to between 6-12 billion dollars.

Hurricane Jeanne made landfall late on September 25 on the southern part of Hutchinson Island, Florida as a Category 3 hurricane with maximum sustained winds estimated at 120 miles per hour. Jeanne was the fourth hurricane to affect the state of Florida in a span of six weeks. Nine deaths have been reported in the United States from this hurricane, and initial estimates of insured damage range from 4-8 billion dollars according to Risk Management Solutions. This puts estimated total damage between 8-16 billion dollars. In addition, Jeanne was responsible for between 1500-2000 deaths along the northern coast of Haiti.

Table 2 displays total estimated damage from this year's four landfalling hurricanes. Damage from these four tropical cyclones will likely exceed that of Hurricane Andrew.

Table 2: Damage estimates of the four U.S. landfalling hurricanes in 2004 (in billions of dollars).

Storm Insured Total
Charley 7.5 15
Frances 4.5 9
Ivan 4.5 9
Jeanne 6.0 12
Total 22.5 45
Andrew (1992) in 2004 Dollars 20 40

2  Predictions of Individual Monthly Atlantic TC Activity

       A new aspect of our climate research is the development of TC activity predictions for individual months. On average, August, September and October have about 26%, 48%, and 17% or 91% of the total Atlantic basin NTC activity. August-only monthly forecasts have now been made for the last five seasons, and this is our third year making September-only forecasts. This is the second year that we have issued an October-only forecast.

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 TC activity by the beginning of each of these three months. These efforts have been recently documented in CSU project reports by Eric Blake (2002) for our August-only forecast and by Phil Klotzbach (2002) for our September-only forecast - see the last page for references. Klotzbach is presently documenting our new October-only forecast.

These monthly 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 NCEP global reanalysis data. Table 3 gives an outline and timetable of the different forecasts and verifications we issue after the end of July.

Table 3: Timetable of the issuing of our after-July monthly forecasts (in early August, early September, and early October), the times of their verifications, and dates of seasonal updates. Note that we make three separate October-only forecasts; two separate September-only forecasts; and one separate August-only forecast. Seasonal updates are issued in early September and early October.

Times of
Fcts. and

Based on
Data Through

Jul. Fcst.
for Aug.
for Sept.
for Oct.
Full Season
Aug. Aug.
and Seasonal

for Sept.

for Oct.

of Season
Sept. Sept.
and Seasonal

for Oct.

of Season

Table 4 and Fig. 3 list the five October-only predictors with their location and sign for higher TC activity. The physical reasoning behind the use of these predictors is as follows:

  1. July-August SLP (12.5-27.5N, 15-45W) (-)

    Low sea level pressure in July-August in this part of the subtropical Atlantic is the most important predictor for October tropical cyclone activity. Low pressure indicates that a weak subtropical ridge is present, trade winds are weaker, and consequently, due to an evaporation decrease, the tropical Atlantic is warmer than normal. On a climatological average, tropospheric vertical wind shear and sea level pressure are directly related. Lower-than normal sea level pressure indicates that late-season tropical cyclones are more likely to occur due to a combination of reduced wind shear and a warm tropical Atlantic.

  2. July-August 200 mb U (35-47S, 160E-155W) (+)

    Increased upper-level westerlies near New Zealand indicate increased Southern Hemisphere winter baroclinicity which is typically associated with favorable conditions for tropical cyclones in the Atlantic. These conditions tend to persist through October increasing the likelihood of late-season tropical cyclones.

  3. Previous November SLP (45-65N, 115-145W) (-)

    Low sea level pressure in this area during November of the previous year implies a deeper and eastward-shifted Aluetian Low which is typical of a positive Pacific North American Pattern (PNA). A positive PNA is frequently associated with the final year of warm ENSO conditions and therefore a return to cooler conditions in the eastern Tropical Pacific the following year. Cool ENSO conditions provide a more favorable environment for the development of October tropical cyclones.

  4. August SST (22.5-35N, 120-150E) (+)

    Warm waters in the Pacific Ocean south of Japan are well-linked to a cold Pacific Decadal Oscillation (PDO). In general, a cold PDO is associated with blocking over the central Pacific and low pressure and reduced wind shear over the tropical Atlantic.

  5. September 200 mb U (37.5-47.5S, 0-30W) (+)

    Increased westerlies throughout the Southern Hemisphere are commonly associated with active years in the tropical Atlantic. Heightened winter baroclinicity off the coast of Brazil is typically seen during years with reduced wind shear over the tropical Atlantic.

Table 4: Meteorological predictor and its location for the October-only forecast. A plus (+) means that positive values of the predictor are associated with increased hurricane activity, and a minus (-) indicates that negative values of the predictor are associated with increased hurricane activity.

Name of Predictor Location
1) July-August SLP (-) (12.5-27.5N, 15-45W)
2) July-August 200 mb U (+) (35-47S, 160E-155W)
3) Previous November SLP (-) (45-65N, 115-145W)
4) August SST (+) (22.5-35N, 120-150E)
5) September 200 mb U (+) (37.5-47.5S, 0-30W)


Figure 3: Predictors utilized for the October-only forecast which employs meteorological data through September. Numbers of each area are keyed to the predictor description given in Table 4.

Table 5 lists the value of each October-only predictor and whether its 2004 value indicates above or below-average October-only TC activity. Table 6 gives a summary of hurricane activity through September and projected activity for the remainder of the year. We are projecting slightly above-average activity for October.

Table 5: Values of meteorological parameters for the October-only forecast.

Name of Parameter Value in Indication for Above
Predictor Standard Deviation (SD) or Below-Average Activity
1) July-August SLP (-) +0.1 SD Below
2) July-August 200 mb U (+) -0.7 SD Below
3) Previous November SLP (-) +0.4 SD Below
4) August SST (+) +1.3 SD Above
5) September 200 mb U (+) -0.2 SD Below

Table 6: Summary of hurricane activity through September 2004 and projected hurricane activity for the remainder of the year.

Tropical Cyclone Parameters Observed Updated Updated
and 1950-2000 Full Season Climatology TC Activity Oct. Full Season
(in parentheses) Through September Forecast Forecast
Named Storms (NS) (9.6) 12 3 15
Named Storm Days (NSD) (49.1) 83.75 12 96
Hurricanes (H) (5.9) 7 2 9
Hurricane Days (HD) (24.5) 44.25 8 52
Intense Hurricanes (IH)(2.3)6 0 6
Intense Hurricane Days (IHD) (5.0) 23 0 23
Net Tropical Cyclone (NTC) (100)220 20 240

3  U.S. Landfall Probability Forecast for October

       We have recently developed a methodology for calculating probability of landfall along the entire U.S. coastline for the month of October. Table 7 displays the landfall probabilities for October. The probability of U.S. October landfalling tropical cyclones in any particular year is directly related to the NTC forecast. All landfall probabilites are slightly above average based on an NTC forecast of 20 for October.

Table 7: Estimated probability (expressed in percent) of one or more U.S. landfalling tropical storms (TS), category 1-2 hurricanes (HUR) and category 3-4-5 hurricanes making landfall along the entire U.S. coast for October 2004. The long-term mean annual probability of one or more landfalling systems during the last 52 years is given in parentheses for October.

Oct. 2004
Named Storm (29%) 33%
Hurricane (15%) 17%
Intense Hurricane (6%) 8%

4  Formation of Major Hurricanes

       The majority of major hurricanes that develop in the Atlantic basin originate from middle-tropospheric easterly waves that move out of West Africa and, if central Atlantic meteorological conditions are favorable (such as vertical lapse rates, high values of low-level horizontal wind shear small values of tropospheric vertical wind shear and low-level mass convergence), they intensify and grow into strong hurricanes and move west-northwestward. Tropical cyclones forming at higher latitudes typically do not become major hurricanes.

It is not the number of easterly waves emanating from West Africa (approximately 60 per hurricane season) which is important but the conditions at tropical latitudes in the central Atlantic into which they move which determines development. Conditions were unusually favorable during August-September of this year, and eight tropical cyclones formed in the tropical Atlantic east of the Windward Islands (Fig. 4).


Figure 4: Named storm genesis locations of eight tropical cyclones in 2004 that formed in the tropical Atlantic east of the Windward Islands.

5  This Year's Destructive Hurricane Tracks

       Most of this year's hurricanes had long westward tracks that were a-typical of the tracks of most hurricanes during the recent years of 1995-2003. This was due to the differences in middle-latitude westerly wind patterns as compared to most years between 1995-2003. Figure 5 shows the tracks of the four major hurricanes which caused considerable damage in southeast United States.


Figure 5: Tracks of four of the most destructive storms to impact the United States.

Favorable formation conditions as occurred this year are not uncommon and have occurred in many prior years (but not usually with high Atlantic SLPA values and a warm Pacific SSTA pattern that was present this year). What is unique about this year is not the large amount of activity in the tropical Atlantic but having this high number of formation events occur in combination with anomalously high West Atlantic upper-level ridge activity in the latitude belt between 35-50N (Fig. 6). The Bermuda high was stronger than normal and caused anomalously strong upper-level easterly steering current winds in the west Atlantic sub-tropical belt. This caused a high percent of the systems to take long westward tracks up to the longitudes of the United States.

The mean westerly wind conditions of August-September of this year showed a trough along and over the western North American continent and a ridge over the eastern North American continent and western Atlantic. This East Coast ridge protected this year's west-northwest moving hurricanes against the impinging influence of the middle-latitude westerlies that act to turn the westerly moving cyclones to the right and then recurve them. Most of this season's hurricanes kept moving westward and did not recurve until they got in the longitude of the southeast United States. These long-lived and intense hurricanes were the ones that affected the U.S. severely this year. We were thus unlucky in the positioning of the westerly ridge-trough patterns relative to the tropical Atlantic.

The westerly wind pattern this year is generally opposite to the typical middle-latitude ridge-trough patterns that occurred during the years of 1995-2003 when a ridge pattern was typically present over the North American West Coast and a trough near the North American East Coast and the western Atlantic (compare Figs. 6 and 7). During these recent nine years the North American East Coast trough acted to bring about an impingement of middle-latitude westerlies into sub-tropical and tropical latitudes and deflected the westward moving tropical hurricanes to the north and recurved them. Few made it far enough west to impinge on the United States. Figure 8 shows the tracks of all 32 Atlantic Basin major hurricanes during the nine-year period of 1995-2003. Note that only 3 of these 32 major hurricane hit the United States. This year we have had 3 of 6 major hurricanes strike the United States.


Figure 6: Predominant flow pattern in August-September 2004.


Figure 7: Predominant flow pattern for August-September 1995-2003.


Figure 8: Tracks of all intense hurricanes from 1995-2003.

6  Why was This Year so Active With Major Hurricanes?

       Although we forecast an above-average 2004 season (14 NS, 8 H, 3 IH, NTC of 140 for our first three forecasts of 5 December, 4 April, 28 May) we lowered our forecast slightly (13 NS, 7 H, 3 NS, NTC of 125) in early August. There is no way we could have foreseen the number of Atlantic basin major hurricanes (6) that occurred during August-September, or the large impact this activity would have on southeastern U.S. residents. This year did not behave like any other year we have studied.

Special conditions which occurred in the low latitude central and eastern Atlantic which caused so many storms to form were:

  1. very warm sea surface temperatures,

  2. high low-level horizontal wind shear

  3. strong low-level convergence as illustrated by positive rainfall anomalies

  4. low values of tropospheric vertical wind shear.

It is unusual to simultaneously have storm-enhancing values of these four favorable tropical cyclone formation conditions over a continuous seven-week period despite anomalously high values of Atlantic Sea Level Pressure Anomalies (SLPA) (Fig. 12) and Pacific NINO 3.4 Sea Surface Temperature Anomalies (SSTA).

Figure 9 displays the very warm sea surface temperature anomalies across the Atlantic in August-September. Figure 10 displays the increased tropical Atlantic low-level convergence and increased tropical Atlantic horizontal wind shear in August-September. This figure shows how cross-equatorial flow is an important component of the low-level flow which provided more favorable conditions for tropical cyclone genesis and maintenance during August-September. Figure 11 displays satellite rainfall measurements across the tropical Atlantic in August. It illustrates this season's enhanced convergence along the Atlantic Intertropical Convergence Zone (ITCZ). Analysis of tropospheric vertical wind shear during August-September (not shown) indicates relatively low values. All four of these favorable tropical cyclone formation conditions were present in the tropical Atlantic during August-September despite high Atlantic SLPA and Nino 3.4 SSTA.

Reasons for Forecast Underestimate. Long period records indicate that active hurricane seasons almost never occur with such high positive values of Atlantic SLPA or when such high anomalous values of sea surface temperature are present in NINO 3.4 (August-September 2004 values were +0.8C). Table 8 lists the 10 most active hurricane seasons of the last 120 years and shows that none had warm NINO 3.4 SSTA. Table 9 shows the most 12 inactive Atlantic basin tropical cyclone years of the last century. Note that NINO 3.4 SSTA were nearly as warm this year as the average of the 12 most inactive years. The Net Tropical Cyclone (NTC) activity through September was 220. This is 7 times greater than the activity in the 12 most inactive years. High Atlantic SLPAs and tropical Pacific SSTAs were the major reason that we were hesitant to forecast more than a moderately active season.


Figure 9: August 1, 2004 - September 25, 2004 sea surface temperature anomaly (C) for the Atlantic Ocean. Anomalies are taken with respect to the 1968-1996 climatology.


Figure 10: Low-level wind flow in the tropical Atlantic for August-September 2004.


Figure 11: August 2004 precipitation anomaly (mm). Note the very active Atlantic ITCZ.


Figure 12: August 2004 sea level pressure anomaly (in mb). Anomalies are taken with respect to the 1968-1996 climatology.

Table 8: The 10 most active Atlantic basin NTC years of the last 115 years with accompanying August-September NINO 3.4 SSTA values.

Seasonal Aug-Sept NINO 3.4
Year NTC SSTA ()
1893 251 -1.21
1916 205 -0.98
1926 239 +0.16
1933 225 -0.98
1950 240 -0.63
1955 196 -0.62
1961 220 -0.37
1995 231 -0.37
1996 198 -0.08
Mean 215 -0.45
2004 225 +0.80

Table 9: The 12 most inactive Atlantic basin NTC years of the last century with accompanying August-September NINO 3.4 SSTA values.

Seasonal Aug-Sept NINO 3.4
Year NTC SSTA ()
1905 25 + 1.71
1907 13 +0.43
1914 4 +1.22
1925 10 +0.71
1930 37 +1.15
1968 46 +0.31
1972 28 +1.42
1982 37 +1.08
1983 32 +0.09
1986 38 +0.71
1987 47 +1.82
1997 54 +2.36
Mean 32 +0.98
2004 225 +0.80

7  It Could Have Been Worse

       Florida's four destructive hurricanes fortunately came ashore along coastlines that were not very densely populated. Pensacola, FL was the largest Florida community feeling the direct brunt of one of these four damaging hurricanes. The coastal and inland areas around Punta Gorda-Port Charlotte (where Charley came ashore), and Stuart (where Frances and Jeanne came ashore) do not have large coastal populations. The three major Florida coastal population concentrations from Tarpon Springs to Sarasota, West Palm Beach to South Miami, and Daytona Beach to Melbourne (and inland to Orlando) were all removed from the direct brunt of these four hurricanes. Economic loss many times greater could have occurred if the center of any one of these four hurricanes had come into one of these more concentrated Florida population areas. For instance, it has been estimated that if Hurricane Andrew (1992) had come inland just 15-20 miles north of its actual landfall near Homestead that it would have caused two to three times the $40 billion dollars in property loss that resulted.

8  Florida's Unlucky 2004 August-September

       The five named storms (4 hurricanes, 3 major hurricanes and 1 tropical storm) that have effected Florida over a 48 day period are unprecedented in terms of historical records going back 130 years, although they are well within the range of natural climate fluctuations. What makes August-September 2004 so special in regards to landfalling hurricanes in Florida is the unusual combination of high hurricane activity and very favorable surrounding hurricane steering currents that advected the hurricanes from the deep tropics across Florida (see Fig. 6).

9  Florida's Unusually Lucky Last 38 Years

      It is important that Floridians view this terribly damaging landfall season from a longer period perspective. Overall Florida has been extremely fortunate in recent years. Between 1966-2003 (38 years) the Florida Peninsula has experienced the landfall of only one major hurricane (Andrew, 1992) (Fig. 13). But in this long major hurricane lull period since the mid-1960s, Florida's population and coastal development has exploded. Few of the new Floridians have experienced a major hurricane hit. Most Floridians were not prepared for this unusual onslaught of four devastating storms in such a short period of time. But old-timers who lived in Florida in the 1930s through the 1950s well remember that Florida used to be hit by many intense storms. Between 1928-1965 (41 years) the Florida Peninsula experienced 14 major hurricane landfalls (1 per 3 years).


Figure 13: Schematic illustrating just how lucky Florida has been over the past 38 years.

For many years we have been discussing how lucky Florida had been with regards to its few recent landfalling major hurricanes. We said it was inevitable that this period of few major hurricane strikes would end and that the long period climatology would eventually reassert itself. There was no way, however, of knowing that the law of averages would try to catch up to its deficit so rapidly in one year.

10  Is Global Warming Involved?

       Florida residents should not interpret the four damaging hurricane landfalls to their state in August-September to be related, in any way, to the much publicized human-induced global warming hypothesis. Although an unusual event (likely occurring about once every 100 years), these four strong landfall events are a rare combination of an above-average season of major hurricane activity together with unusually favorable broad-scale steering currents that drove mid-Atlantic tropical cyclones westward instead of allowing them to recurve. Such a combination of high tropical cyclone frequency and special westward steering currents is not frequent but well within the range of natural climate fluctuations and might be considered from a statistical point-of-view as a rare 2 1/2 sigma event. There would be little discussion of this year's hurricanes activity if these four major storms had not made U.S. landfall, which could have just as easily occurred

Although the Atlantic basin has been very active this year, as have eight of the last 10 Atlantic basin seasons, the other six global tropical storm basins which account for about 88 percent of the globe's approximately 80 named tropical cyclones per year have not shown a similar increase. In fact, global net tropical cyclone activity has actually shown a small decrease during the last 10 years. If global warming (natural or man-made) were the cause of the increased Atlantic basin activity, we should have seen an increase in the other storm basins as well. This has not occurred.

Atlantic basin major hurricane activity can increase or decrease during periods when the mean global surface temperature is warming or when it is cooling. During the period of the early 1970s to the mid-1990s, when the globe was warming, Atlantic basin major hurricane activity was below average. During the period of the mid-1940s to early 1970s there was a small net cooling of the global surface temperature, and Atlantic basin major hurricane activity was above average. We can explain these multi-decadal variations in Atlantic basin major hurricane activity as resulting from multi-decadal variations in the Atlantic Ocean thermohaline circulation (see our previous forecast discussions). Paleo climatology data has shown that such multi-decadal Atlantic thermohaline circulation changes have occurred many times in the past.

There has been a small global surface warming since the mid-1970s and when averaged over the last 100 years. We believe this global warming is due to a slowing down of the global ocean's sinking (or deep water formation) in the North and South polar regions. The globe undergoes warming when polar region deep-water formation is reduced as it was between the late 1960s to mid-1990s and on average over the last century. By contrast, during the periods between the mid-1940s and the early 1970s, there was enhanced polar deep-water formation, and the global mean surface temperature underwent a modest cooling.

We believe that the global mean surface temperature changes that have been observed over the last 30 years and over the last century are of mostly natural origin (ocean-forced) and likely not a result of any human influence. We do not attribute Florida's four landfalling hurricanes of the last two months to be related in any way to human influences.

11  Florida's Future Hurricane Seasons

       Floridians should view this year's onslaught of hurricane activity as a rare anomaly. This year's landfalling hurricane activity does not by itself represent the beginning or the end of any cycle or trend for landfalling hurricanes. This year will have no bearing on what will occur in future years anymore than the great paucity of Florida landfalling major hurricanes between 1966-2003 had any bearing on this year's landfalling systems. The probability of having hurricane-spawned winds, rain, and storm surge at any spot in any year along the U.S. coastline is very low. We would not recommend that anyone move out of Florida or decide not to move to Florida solely because of the threat of hurricanes. Florida hurricanes must be accepted as one small negative of an otherwise pleasant climate.

12  Imbalance in Hurricane Research Funding

       There is much yet to be known about hurricanes, especially in years like this one where forecast precursor signals did not fit the pattern of previously active seasons. These are natural threats to coastal residents of the U.S. that are not being studied as much as they should be. The amount of research directed to the better understanding and the prediction of hurricanes is miniscule in comparison to the massive research funding being directed to human-induced global warming research, which at best, remains a nebulous hypothesis. We need a better balance in federal research expenditures. The residents of the southeastern U.S. have not been well served by the federal government in its research allocation. Is going to Mars more important than obtaining a better understanding and develping better forecasts of hurricanes?

13  Discussion

       The 2004 season has already seen more than twice the activity of an average tropical cyclone season. We expect the remainder of the 2004 hurricane season to be about as active as the typical after-September hurricane season.

This year is the eighth of the last ten seasons (since 1995) that have had distinctly above-average tropical cyclone activity. This season adds further evidence to our supposition that we are in a new era for Atlantic basin hurricane activity. The only two below-average seasons since 1994 were 1997 and 2002 which were both El Niño years. The Atlantic Basin in the last ten years has had an average of 3.8 major hurricanes per year while the previous 25 years (1970-1994) had an average of only 1.5 major (category 3-4-5) hurricanes per year - or only 40 percent of that experienced over the past ten years.

In terms of intense or major hurricane days (IHD), the last ten years have averaged 10.2 IHD per year while the 25-year period of 1970-1994 had an average of 2.26 IHD per year, or only one-quarter as many. There can be no question that from 1995 onward we have been in a new era for major hurricane activity.

14  Full Season Forecast Verification

      Verification of all of our monthly and full season forecasts for 2004 will be verified in late November 2004. Our first seasonal hurricane forecast for the 2005 season will be issued on 3 December 2004. All past and present forecasts and verifications are available at our web address given on the front cover

15  Papers Which Document Our Monthly Atlantic Basin Forecasts

Blake, E. S., 2002: Prediction of August Atlantic basin hurricane activity. Dept. of Atmos. Sci. Paper No. 719, Colo. State Univ., Ft. Collins, CO, 80 pp.
Blake, E. S., and W. M. Gray, 2004: Prediction of August Atlantic basin hurricane activity. Wea. and Forecasting, 19 (in press).
Klotzbach, P. J., 2002: Forecasting September Atlantic basin tropical cyclone activity at zero and one month lead times. Dept. of Atmos. Sci. Paper No. 723, Colo. State Univ., Ft. Collins, CO, 91 pp.
Klotzbach, P. J., and W. M. Gray, 2003: Forecasting September Atlantic basin tropical cyclone activity. 18, 1109-1128.


1Professor of Atmospheric Science

2Research Associate

3Research Associate

4NTC is a combined measure of the yearly mean of six indices (NS, NSD, H, HD, IH, IHD) of hurricane activity as a percent deviation from the 1950-2000 year average.

File translated from TEX by TTH, version 3.12.
On 30 Sep 2004, 17:35.