The hurricane season through September 2003 has been above average with 11 named storms, 6 hurricanes and a Net Tropical Cyclone (NTC)⁵ activity of 134 percent of the full-season average. As of 1 October the 2003 hurricane season has been about one-and-a-half times more active than the full-season average. Due to the long-lasting intense Hurricane Isabel, 2003's Hurricane Destruction Potential (HDP) of 113 is nearly twice what the average season brings by the end of September.

Our new October-only forecast calls for three named storms, two hurricanes, no major hurricane and an NTC activity value of 21 which is slightly above the mean October-only average value of 17. United States and Caribbean basin landfall probability for the remainder of the 2003 season is now estimated to be above the October average. The likelihood of a November cyclone is fairly minimal in any year.

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.

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 CSU forecasts.

Colorado State University (CSU) has issued seasonal hurricane forecasts for the last 20 years. These forecasts, which are 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.

1  Tropical Cyclone Activity through September

       As of the end of September, 2003 has had eleven named storms, six hurricanes and two major hurricanes. Four tropical cyclones have made landfall along the United States coastline (see Table 1 and Figure 2). This season was distinguished by its very early start in April (TS Ana) and by the three additional named storms (Bill (TS), Claudette (H-1) and Danny (H-1)) that occurred before 20 July. There was then a long 39 day break with only one system (TS Erika) forming until long-lasting Hurricane Fabian (Category 4) was initially named on 28 August.

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: Fabian (Cat 4, 10 HD and 6.75 IHD) and Isabel (Cat 5, 11.5 HD and 8 IHD). Both of these cyclones formed at low latitudes and had long tracks over the Atlantic Ocean.

Isabel became the third longest lasting major hurricane on record since 1950. September had more major (Cat 3-4-5) hurricane days than any September of the last 60 years except for 1961.

Kate was the eleventh storm of the season and the last named storm of September. It formed in the mid-Atlantic and moved north. It was named as a minimal Category 1 hurricane for only one 6-hour period on September 30th and is currently tracking west.

1.1  2003 U.S. Landfalling Storms

Tropical Storm Bill made landfall in southeastern Louisiana on June 30 with an estimated intensity at landfall of 50 knots. Bill was responsible for four deaths and about 30 million dollars in damage.

Hurricane Claudette made landfall on July 15 near Port O'Connor, Texas. Maximum sustained winds at landfall were estimated near 70 knots, and three deaths in the United States were attributed to the hurricane. Preliminary damage estimates were put at 17 million dollars, but these values will likely increase.

Tropical Storm Grace also made landfall near Port O'Connor, Texas on August 31 with an estimated intensity at landfall of 35 knots. So far no damage or deaths have been attributed to this system.

Tropical Storm Henri threatened western Florida as a 45 knot storm but was only a tropical depression when it crossed over central Florida and moved northeastward dissipating 100 miles off the U.S. coast.

Hurricane Isabel made landfall near Ocracoke Island, North Carolina on September 18 as a Category 2 hurricane with maximum sustained winds (at landfall) estimated at 85 knots. Extensive winds and flooding were experienced through the eastern and mid-Atlantic states. At least 40 deaths and over one billion dollars in damage have been attributed to this system.

1.2  Other 2003 Non-U.S. Landfalling Storms

       Hurricane Fabian (cat 4) formed in the tropical Atlantic in late August. It rapidly intensified into a major hurricane and tracked northwest across the Atlantic. As it began to recurve, it passed almost directly over Bermuda causing extensive damage.

Hurricane Juan (cat 2) formed late in September at subtropical latitudes just east of Bermuda. This was a hybrid system with combined baroclinic and tropical processes. Juan then moved rapidly northward and made landfall in central Nova Scotia with significant wind and flooding damage.

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 four seasons, and this is our second year for making September-only forecasts. This is the first 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 2 gives an outline and timetable of the different forecasts and verifications we issue after the end of July.

Table 3 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 (12.5-27.5^°N, 15-45^°W) (-)

   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 (35-47^°S, 160^°E-155^°W) (+)

   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 (45-65^°N, 115-145^°W) (-)

   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 (22.5-35^°N, 120-150^°E) (+)

   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 (37.5-47.5^°S, 0-30^°W) (+)

   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 lists the value of each October-only predictor and whether its 2003 value indicates above or below-average October-only TC activity. Table 5 gives a summary of hurricane activity through September and projected activity for the remainder of the year.

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 6 displays the landfall probabilities for October. The probability of US October landfalling tropical cyclones in any particular year is directly related to the NTC forecast. All landfall probabilites are above average based on an NTC forecast of 21 for October.

4  Discussion

       The 2003 season has already seen more hurricane activity than the average year. We expect the remainder of the 2003 hurricane season to be about as active as the average after-September hurricane season.

This year is the seventh of the last nine 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 last nine years have had an average of 3.5 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 - less than half as much.

In terms of intense or major hurricane days (IHD), the last nine years have averaged 8.72 days 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.

When hurricane-spawned damage is normalized for inflation, wealth per capita and coastal population, it has been found that major hurricanes cause about 80-85 percent of damage even though major hurricanes account for only about one-quarter of all named storms. The Atlantic basin has had 31 major hurricanes since 1995. Only three have crossed the U.S. coastline as major hurricanes (Opan 1995, Fran 1996, and Bret 1999). Over the last century about one in three Atlantic basin major hurricanes made U.S. landfall. Assuming this one in three ratio reasserts itself in coming years (and with the ever growing coastal population and property values), it is inevitable that the U.S. experience hurricane destruction in coming years on a level never before seen.

5  Forecast Verification

      Our forecasts for 2003 will be verified in late November 2003. Our first seasonal hurricane forecast for the 2004 season will be issued on 5 December 2003. All forecasts and verifications are available at our web address given on the front cover
(http://typhoon.atmos.colostate.edu/Forecasts).

6  Papers Which Document Our Monthly Atlantic Basin Forecasts

1 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.

1 Blake, E. S., and W. M. Gray, 2003: Prediction of August Atlantic basin hurricane activity. Submitted for publication to Wea. and Forecasting.

1 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.

1 Klotzbach, P. J., and W. M. Gray, 2003: Forecasting September Atlantic basin tropical cyclone activity. Wea. and Forecasting, 18 (in press).

1 Klotzbach, P. J., 2004: Forecasting October Atlantic basin tropical cyclone activity (in preparation).

Footnotes:

¹Professor of Atmospheric Science

²Research Associate

³Research Associate

⁴Research Associate

⁵NTC 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.