Highlight: Weak El Nino leads to slightly below normal TC activity in 2023. Executive summary: The Climate Impact Company 2023 North Atlantic basin tropical cyclone season forecast projects 12 tropical storms, 7 hurricanes, and 3 intense hurricanes. The accumulative cyclone energy (ACE) index forecast is 92. The activity forecast is slightly below normal due to the presence of weak El Nino. Despite an El Nino season, at least two hurricanes are forecast to strike the U.S. targeting the north-central Gulf and eastern Florida coastlines (Fig. 1). Fig. 1: The Climate Impact Company seasonal activity forecast and projected formation areas/tracks of hurricanes for the 2023 North Atlantic basin TC season. Introduction: Seasonal forecasts of tropical cyclone activity began in 1984 propelled by research lead by Dr. William Gray at Colorado State University. NOAA began issuing seasonal hurricane forecasts in the late 1990’s. Climate Impact Company (CIC) began issuing seasonal forecasts in 2004 (and in 1999 with a previous company). Many private companies, government organizations, and academia present seasonal forecasts of tropical cyclone activity nowadays. Forecasts are generated in many ways but are primarily dependent on the El Nino southern oscillation (ENSO) phase whereas La Nina and neutral ENSO limit upper-level westerly shear across the tropical North Atlantic basin allowing tropical cyclones to flourish in number and intensity while El Nino produces upper shear suppressing tropical cyclone activity (Fig. 2). Fig. 2: Climate Impact Company climatology of North Atlantic basin tropical cyclone activity since 1950 according to ENSO phase and intensity. Climatology: The Climate Impact Company constructed analog (CIC-CA) forecast is based primarily on the ENSO forecast and SSTA regime in the MDR. ENSO and the North Atlantic basin SSTA regimes have long-term cycles. During the mid-to-late 1990’s, ENSO flipped from the warm cycle to the cool cycle as El Nino episodes are suppressed and La Nina events more frequent (Fig. 3). At the same time, in the North Atlantic basin, the Atlantic Multi-decadal oscillation (AMO) shifted from the cool to warm cycle in which the North Atlantic is warmer than normal most of the time (Fig. 4). In the MDR, the tropical North Atlantic (TNA) index also shifted to the warm phase and is causal to the increased number of hurricanes of recent decades (Fig. 5). Since a more persistent presence of La Nina and warmer than normal SSTA in the North Atlantic favors increased tropical cyclone activity, the climatology of seasonal activity has increased (Fig. 6). The Climate Impact Company seasonal forecast of North Atlantic basin tropical cyclone activity is primarily reliant on analog years within the current cool ENSO/warm AMO climate with similar ENSO/AMO (and TNA) characteristics. Fig. 3: The 1950-2023 monthly plot of the El Nino southern oscillation phase. Fig. 4: The 1950-2023 monthly plot of the Atlantic multi-decadal oscillation. Fig. 5: The 1950-2023 monthly plot of the tropical North Atlantic index. Fig. 6: Climate Impact Company climatology of North Atlantic basin tropical cyclone activity. Other predictors: Given the decades of issuance of seasonal tropical cyclone forecasts, research has given rise to many other aspects of the global climate system relevant to seasonal tropical cyclone activity. Initially, the quasi-biennial oscillation (QBO) was used to assist in seasonal activity forecasts. The QBO is a high-level belt of wind shifting from east-to-west and west-to-east every 1-2 years. Depending on the direction, the QBO controls ventilation in the upper atmosphere which modulates tropical cyclone activity and intensity. Reliance on the QBO has weakened during the past 1-2 decades and generally dropped as a predictor. However, CIC has discovered that QBO phase can dictate whether the deep tropics or subtropics are most active with the 2005 season, the leading example. At the time, a record 28 tropical cyclones formed but most flourished in the subtropics where upper-level ventilation supported development while the negative phase of the QBO suppressed activity in the deep tropics. The North Atlantic oscillation is also a primary predictor. During MAR/APR/MAY if the NAO is in the positive phase, the semi-permanent North Atlantic high-pressure system (“Bermuda High”) strengthens and propels faster trade winds that cool the SST in the deep tropics. The cooler SST pattern lasts through much of the tropical cyclone season suppressing seasonal activity. When the MAR/APR/MAY NAO pattern is negative, the Bermuda High is weaker and trade winds weaker in the deep tropics allowing warmer SST which increases tropical cyclone development. The projected West Africa tropical rainfall pattern is also used as a predictor of seasonal activity in the North Atlantic basin. When the intra-topical convergence zone (ITCZ) is stronger than normal in this region, tropical cyclone activity increases as tropical waves moving into the North Atlantic basin are stronger and more likely to intensify. When the ITCZ is weak, tropical cyclone development in the MDR is diminished. In a related predictor, the Sahara Dust extending into the tropical/subtropical North Atlantic basin propelled by strong trade winds can inhibit tropical cyclone development. CIC has added the low-to-mid-level troposphere (600 MB) relative humidity (RH) forecast (using analog years) as an indicator of where tropical cyclone activity is most likely to form and flourish. This technique is the lead catalyst to the CIC formation area/tracks of hurricanes forecast. Climate forecast: The first formal outlook for the 2023 North Atlantic basin tropical cyclone activity forecast is based on analog years when ENSO transitioned into weak El Nino (Fig. 7) and the TNA index became quite warm (Fig. 8). The ENSO analog forecast is based on four 4-year long La Nina episodes since 1950. The analog years are weighted differently depending on what ENSO cycle. The 1998-2001 analog is weighted 4 times due to the occurrence during the current ENSO cycle. The 1954-57 and 1973-76 La Nina episodes are weight twice each due to their presence in a similar ENSO cycle but also during a period when global oceans were cooler than the past 2 decades. The 1983-86 analog is weighted once due to occurrence during a long-term warm cycle of ENSO. The TNA analogs are purposely taken from the short-term climatology due to the warming character of global and the North Atlantic ocean surface during that time. The International Research Institute (IRI) for Climate and Society collects all dynamic and statistical ENSO phase forecasts (Fig. 9) which vary widely for 2023 and therefore emphasize below average forecast confidence. Interestingly, the consensus of dynamic models, project moderate El Nino ahead. However, statistical models are cautions and render a weaker El Nino later in 2023. The CIC-CA ENSO phase forecast is similar to the IRI statistical ENSO forecast. Other climate factors such as West Africa rainfall are likely wetter than normal indicating increased risk of stronger tropical waves entering the North Atlantic basin during AUG/SEP 2023 and more easily capable of sudden development. The MAR/APR/MAY 2023 NAO pattern is changeable and therefore not an obvious indicator of North Atlantic SSTA for the tropical cyclone season. The QBO is likely weak positive phase for the upcoming tropical cyclone season which lends marginal support for increased development in the tropical North Atlantic Fig. 7: Climate Impact Company ENSO phase analog forecast for 2023 indicates weak El Nino for the second half of the year. Fig. 8: Climate Impact Company TNA phase analog forecast for 2023 indicates another warm SSTA pattern in the MDR for the tropical cyclone season. Fig. 9: The International Research Institute for Climate and Society ENSO phase forecast for 2023. 2023 forecast: The seasonal activity forecast is based on the weighted years of the ENSO analog and non-La Nina years of the TNA analog. Several TNA analog years occurred during La Nina therefore only 2012 is considered in the seasonal activity forecast. The possibilities vary widely from emergence of a straight-forward El Nino year when activity is sharply suppressed (1986) to a year featuring mostly neutral ENSO to borderline El Nino and a warm tropical North Atlantic (2001 and 2012). The average of the weighted analogs yields a reasonable forecast given the climate predictors: 12 tropical storms. 7 hurricanes, and 3 intense hurricanes with an accumulative cyclone energy index of 92 (Table 1). Analog Year Weighting Tropical Storms Hurricanes Intense Hurricanes ACE Index 1957 2 8 3 2 84 1976 2 10 6 3 81 1986 1 6 4 0 36 2001 4 15 9 4 106 2012 1 19 10 2 133 Average 12.1 6.8 2.8 92.3 Forecast 12 7 3 92 Table 1: The weighted analog years and their tropical cyclone activity is averaged to produce the seasonal forecast for 2023. The 2023 seasonal activity forecast has similarities to last year (Table 2). The outlook is somewhat less active than the past 7 years. The outlook is also less than the 15-year and 30-year climatology although reasonably close to the 50-year normal. Tropical Storms Hurricanes Intense Hurricanes ACE Index 2023 12 7 3 92 Last Year 14 8 2 95 Last 7 Years 18.6 8.4 3.7 150.0 15-Year 16.3 7.4 3.2 122.5 30-Year 15.0 7.4 3.2 126.4 50-Year 12.7 6.5 2.6 103.6 Table 2: The 2023 North Atlantic basin seasonal tropical cyclone activity forecast compared to various climatology. Regional forecast: To project the most likely formation areas and potential paths of this year’s 7 hurricanes, the analog is used to generate 600 MB RH for the core months of the season (JUL/AUG/SEP/OCT). In July, the CIC-CA RH forecast indicates a busy ITCZ in the deep tropics (Fig. 10). However, low latitude tropical cyclones (and especially hurricanes) are not likely in July in this location. The most likely location for a tropical cyclone developing is in the western Caribbean Sea, Gulf of Mexico, and Bahamas. The RH forecasts suggests a tropical system forming in the Bahamas during July 2023 likely to track due northeast but potentially reach minimal hurricane status across much warmer than normal waters. In August, the RH forecast indicates low risk of tropical cyclone activity on the Southeast U.S. Coast and Gulf of Mexico (Fig. 11). However, there is risk of two hurricanes moving northwestward out of the ITCZ in August. One system is forecast to weaken substantially striking the high terrain of Hispaniola. A second hurricane stays farther to the north and stays out to sea. Several hurricanes are likely in September. Based on the RH analog, dry conditions dominate the southern Gulf of Mexico, Caribbean Sea, and east of the Bahamas (Fig. 12). A classic El Nino suppressing signature! However, concerning are wet RH anomalies across the Gulf Coast States enabling potential for two hurricanes forming close to shore and moving inland with limited warning. Finally, in October, a late season storm forms in the Bahamas and turns northeastward reaching hurricane strength in anomalous warm water (Fig. 13). Fig. 10: The CIC-CA 600 MB relative humidity forecast across the North Atlantic tropics and subtropics to determine the most likely origin and path of hurricanes in July 2023. Fig. 11: The CIC-CA 600 MB relative humidity forecast across the North Atlantic tropics and subtropics to determine the most likely origin and path of hurricanes in August 2023. Fig. 12: The CIC-CA 600 MB relative humidity forecast across the North Atlantic tropics and subtropics to determine the most likely origin and path of hurricanes in September 2023. Fig. 13: The CIC-CA 600 MB relative humidity forecast across the North Atlantic tropics and subtropics to determine the most likely origin and path of hurricanes in October 2023. Summary: A weak El Nino is expected during the 2023 North Atlantic tropical cyclone season. Typically, weak El Nino keeps seasonal activity beneath climatology. However, the 2023 seasonal forecast elevates climatology slightly due to the expected warmer than normal tropical and subtropical North Atlantic. The 2023 forecast includes 12 tropical storms, 7 hurricanes, and 3 intense hurricanes with an ACE index of 92. The forecast is beneath the 7-year (2016-2022) active period which has brought considerable U.S. coastal strikes. Despite slightly below normal climatology, the 2023 forecast brings a coastal threat of a hurricane to the north-central Gulf of Mexico and East Coast of Florida. Other hurricanes forming east of Florida travel northeastward and out-to-sea. Hurricane risk to the western Gulf of Mexico and Coastal Northeast U.S. Corridor is below normal. The next forecast will be issued by June 1, 2023. Other forecasts: Colorado State University will issue their 2023 forecast on April 8, 2023. Tropical Storm Risk/U.K. will issue their 2023 forecast on April 6, 2023. NOAA/CPC issues their seasonal forecast later in May. Forecast verification: Climate Impact Company has issued seasonal forecasts for North Atlantic tropical cyclones since 2004 and the methodology was used at a major U.S. weather company from 1999-2003. The early seasonal forecasts, issued in April compared with observed results indicate reasonably accurate projections (grade of A or B) in 17 of 24 years with 3 years when the early forecast failed (Table 3). Pre-Season Forecast Actual Error 1999 = A 11/7/4 12/8/5 -1/-1/-1 2000 = A- 12/6/3 14/8/3 -2/-2/0 2001 = A- 13/8/4 15/9/4 -2/-1/0 2002 = B 8/4/1 12/4/2 -4/0/-1 2003= B- 12/6/2 16/7/3 -4/-1/-1 2004 = B- 14/7/2 15/9/6 -1/-2/-4 2005 = F 13/8/3 27/15/7 -14/-7/-4 2006 = F 16/9/4 9/5/2 7/4/2 2007 = B- 14/8/5 15/5/2 -1/3/3 2008 = A 16/8/3 16/8/5 0/0/-2 2009 = C 12/6/2 9/3/2 3/3/0 2010 = A 17/10/5 19/11/5 -2/-1/0 2011 = C 18/6/3 13/7/4 -5/1/1 2012 = D 12/6/3 19/10/1 -7/-4/2 2013 = F 13/8/4 13/2/0 0/6/4 2014 = B 9/4/2 8/6/2 1/-2/0 2015 = A- 12/5/2 11/4/2 1/1/0 2016 = A- 15/9/3 15/7/3 0/2/0 2017 = D 12/7/3 17/10/6 -5/-3/-3 2018 = B 12/7/3 15/8/2 -3/-1/1 2019 = B 12/6/3 18/6/3 -6/0/0 2020 = B- 20/10/4 30/13/6 -10/-3/-2 2021= B- 17/9/2 21/7/4 4/-2/2 2022 = B- 19/9/4 14/8/2 -5/-1/-2 Table 3: Climate Impact Company North Atlantic basin pre-seasonal forecasts compared to observed and the attendant error. Red = El Nino year.