Unusually active for an El Nino year; western basin was relatively quiet. The 2024 season is likely more dangerous than 2023 for the Gulf of Mexico and U.S. East Coast. Executive summary: The North Atlantic basin observed 20 tropical storms, 7 hurricanes, and 3 intense hurricanes with an ACE index of 146 in 2023. The activity level is considered moderately higher than normal and unusually active for an El Nino warm season. The much warmer than normal North Atlantic basin enabled buoyant activity although except for Idalia most activity was out-to-sea. The preliminary outlook for 2024 is a stronger season than 2023 with much greater risk of hurricanes returning to the western North Atlantic basin. The 2023 season discussion: The 2023 North Atlantic basin tropical cyclone season was unusual for several reasons. First, the number of tropical storms was 20 whereas the average number for an El Nino summer/autumn is 9.1. Second, seasonal activity reaching 20 storms is rare. This is only the 4th season on record with that achievement 3 of which are in the past 4 years. The much warmer than normal North Atlantic basin is blamed for the uptick. In fact, the 2023 SSTA across the North Atlantic basin was record warm. Third, almost all the activity (except for Major Hurricane Idalia) was out to sea. The mid-troposphere across the western North Atlantic was dry preventing most activity from developing or traveling to the Western Caribbean or Gulf of Mexico. Fourth, while oceanic El Nino was moderate-to-strong during the warm season, atmospheric El Nino (based on multi-variate ENSO index) was neutral. The neutral atmosphere/El Nino Ocean combination is unique. The 2023 seasonal activity was close to normal number of hurricanes and intense hurricanes although somewhat above normal for number of tropical storms and ACE index. Note that the various climatology is increasing as the years considered are more modern. The Climate Impact Company seasonal activity forecasts were unusually variable. Initial forecasts were on target with the number of hurricanes/intense hurricanes but too low for number of tropical storms. A significant upward adjustment was made in July due to the record warm North Atlantic SSTA but lowered slightly in August as El Nino westerly shear was developing. The preliminary hurricane tracks forecast for 2023 identified Idalia but had one more hurricane affecting the U.S. compared to verification. The other 5 hurricane tracks were forecast off the East Coast. Of the 20 events in 2023, Idalia (115 knots) ranked 3rd strongest trailing Lee (145 knots) and Franklin (130 knots) as most intense. Preliminary 2024 seasonal outlook: The 2023-24 El Nino is expected to peak in intensity over the next 1-2 months and weaken the first half of 2024. Forecast models vary but generally indicate neutral ENSO for the 2024 tropical cyclone season. Other models, such as the NCEP CFS V2 favor a strong La Nina. Consequently, the ENSO regime is likely favorable to produce above normal seasonal activity. Additionally, the recent tendency for a warmer than normal North Atlantic basin is likely to continue further enhancing the above normal seasonal risk (and intensity). Analog years with El Nino to neutral ENSO (or La Nina) transitions from the past 30 years with a warm North Atlantic basin include 2016 and 2010. The seasonal activity average for each year was 17 tropical storms, 10 hurricanes, and 4 intense hurricanes with an ACE index of 153. The entire western North Atlantic basin is at risk of impacts from hurricanes in 2024. Fig. 1: The 2023 North Atlantic basin seasonal tropical cyclone activity. Tropical Storms Hurricanes Intense Hurricanes ACE Index 2023 FINAL 20 7 3 145.6 50-Year 13.0 6.6 2.7 105.7 30-Year 15.4 7.5 3.3 129.9 10-Year 16.9 7.6 3.3 132.6 AUG FCST 17 6 2 114 JUL FCST 20 9 4 171 JUN FCST 13 7 3 136 APR FCST 12 7 3 92 Table 1: Tropical cyclone activity for 2023 compared to 30-year and short-term climatology plus the APR, JUN, JUL, and AUG seasonal forecasts by Climate Impact Company. Fig. 2: Climate Impact Company projection of hurricane tracks issued last spring. Fig. 3: The AUG/SEP/OCT 2023 global SSTA analysis from IRI/LDEO. Identified are the presence of El Nino and a very warm North Atlantic basin which helped shape the North Atlantic tropical cyclone season climate pattern. Fig. 4: July through October 2023 relative humidity anomalies at 600 MB across the tropical/subtropical North Atlantic environment. The dry western North Atlantic basin prevented most activity from forming or moving to that sector. Storm # Storm Name Dates Max Wind (knots) Pressure (MB) ACE Index 1 Unnamed 1/16-17 60 976 1.4 2 Arlene 6/2-3 35 998 0.4 3 Bret 6/19-24 60 996 4.6 4 Cindy 6/23-26 50 1001 2.5 5 Don 7/14-24 65 988 7.2 6 Gert 8/21-9/4 50 998 2.4 7 Emily 8/20-21 45 1001 0.7 8 Franklin 8/20-9/1 130 926 26.6 9 Harold 8/22 34 998 0.5 10 Idalia 8/27-31 115 940 7.4 11 Jose 8/31-9/1 50 997 1.3 12 Katia 9/2-3 50 998 1.2 13 Lee 9/5-16 145 926 36.8 14 Margot 9/7-17 80 970 12.7 15 Nigel 9/17-22 85 971 10.5 16 Ophelia 9/22-23 60 981 1.4 17 Philippe 9/23-10/6 45 998 9.4 18 Rina 9/28-10/1 45 999 1.9 19 Sean 10/11-14 40 1004 1.4 20 Tammy 10/18-29 90 965 15.2 Table 2: A list of tropical cyclone events for 2023 including dates, max wind, and low pressure plus ACE index. Data provided by Colorado State University. Fig. 5: A collection of all dynamic/statistical Nino34 SSTA forecasts by IRI/LDEO indicates neutral ENSO for the 2024 North Atlantic tropical cyclone season.