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01/11/2019, 9:46 am EST

North Atlantic Basin 2019 Tropical Cyclone Season Forecast

Executive summary: Since 2005 Climate Impact Company has issued an early season assessment of the following North Atlantic tropical cyclone season activity forecast. In 2019 the outlook indicates 14 tropical storms, 8 hurricanes and 3 intense hurricanes are expected. The activity forecast is at or slightly above both the 20-year and 30-year normal. The accumulated cyclone energy forecast is 99 which is slightly below climatology. The forecast is made with below average forecast confidence due to uncertainty in the lead predictors: El Nino southern oscillation (ENSO) and Atlantic multi-decadal oscillation (AMO). The preliminary forecast indicates both the Gulf of Mexico and East Coast are susceptible to coastal strikes of hurricanes in 2019. The next seasonal assessment will be issued in early April. Discussion: The preliminary Climate Impact Company 2019 North Atlantic basin tropical cyclone season forecast is issued. The leading diagnostic used to project seasonal tropical cyclone activity is phase of El Nino southern oscillation (ENSO). Presence of El Nino (or La Nina) greatly determines how active the tropical cyclone season (Fig. 1). When El Nino is present, seasonal tropical cyclone is reduced while near to above normal activity occurs when ENSO is neutral or a La Nina is present. Fig. 1: Climate Impact Company 1950-2018 North Atlantic basin tropical cyclone season climatology for each phase of ENSO. The Climate Impact Company 2019 ENSO outlook is based on a constructed analog. Constructed analog means the best years regressed 12 months matching the current Nino34 SSTA pattern and projected forward to 1 year. The analogs agree on weak El Nino for quarter 1 of 2019 followed by either rapid weakening to a La Nina by late summer or a continued El Nino that may strengthen during 2019 (Fig. 2). The current trend is weakening of El Nino. The forecast is based on a consensus between an average of all scenarios and the average of just the warm scenario. The warm scenario is favored by most dynamic models therefore weighted more heavily in the forecast. The outlook reveals weak El Nino is likely through the tropical cyclone season. Fig. 2: The Climate Impact Company constructed analog forecast of Nino34 SSTA through DEC/JAN/FEB 2019-20 to determine ENSO phase. The forecast is based on a consensus of all forecasts and just the warm forecasts. Analog forecast: The preliminary Climate Impact Company 2019 North Atlantic basin tropical cyclone season forecast is based on the ENSO analog. The 2007 and 2010 analogs are weighted twice given their presence in the modern-day climate regime featuring similar long-term ENSO/PDO and AMO cycles plus the new fast-rising CO2 atmosphere. There is a lot of variance in the ENSO regimes for the analog years. If the weak El Nino forecast for the 2019 tropical cyclone season is stronger the seasonal forecast amount will be lower. If El Nino ends and is followed by La Nina the seasonal activity will be higher than forecast. The weighted average of the analog years indicates 14 tropical storms, 8 hurricanes and 3 intense hurricanes are likely across the North Atlantic basin in 2019 (Table 1). The activity forecast is similar to the 30-year and 20-year climatology although the accumulated cyclone energy index is moderately lower than climatology. The activity forecast is similar to last year and not quite as active as 2017 while ACE index is markedly lower than the past 2 years. Tropical Storms Hurricanes Intense Hurricanes ACE Index 2010 (2) 19 12 5 165 2007 (2) 15 6 2 72 1987 (1) 7 3 1 34 1977 (1) 6 5 1 25 1969 (1) 18 12 5 158 Forecast 14.1 (14) 8.0 (8) 3.0 (3) 98.71 (99) 30-year cli 13.6 7.0 3.0 115.7 20-year cli 15.0 7.4 3.4 122.4 Last Year 15 8 2 129 2 Years Ago 17 10 6 226 Table 1: Analog calculation for the 2019 North Atlantic basin tropical cyclone activity forecast. The CIC analog forecast indicates slightly higher seasonal activity compared to the Tropical Storm Risk/U.K. outlook. TSR cites a potential stronger El Nino signal holding seasonal activity too slightly below the long-term average.  The Colorado State University Tropical Prediction Project has not issued an early season forecast. CSU also cites uncertainty in the ENSO forecast and adds that the lead predictor, North Atlantic ocean temperatures as defined by the Atlantic multi-decadal oscillation may be changing phase from the warm cycle which started in the late 1990’s and has been persistent through 2014 to a cooler (or less warm) signature. A cooler North Atlantic would also hold down seasonal activity. Tropical Cyclones Hurricanes Intense Hurricanes Accumulated Cyclone Energy CIC 14 8 3 99 TSR 12 5 2 82 Last Year 15 8 2 129 30-Yr Normal 13.6 7.0 3.0 114.8 Table 2: A collection of preliminary operational forecasts of seasonal tropical cyclone activity across the North Atlantic basin in 2019. Forecast tracks: The outlook is made with below normal confidence and is susceptible to significant change when updated the first week of April. To depict the possible variance in seasonal activity for 2019 a look at the extreme upper and lower limit activity (and tracks) from the analogs is provided (Fig. 3-4). If the tropical North Atlantic is warmer than normal and ENSO is neutral to borderline El Nino (which is forecast by CIC) the season will be very active featuring up to 12 hurricanes including a very busy year in the Gulf of Mexico. Whether the tropical North Atlantic is warm enough to support this forecast (or not) is a big question mark. If El Nino is stronger (as most dynamic models indicate) and the recent trend of a cooler North Atlantic holds the seasonal activity would be minimal. Fig. 3: The “most active” scenario for 2019 based on analog years would be similar to 1969. If ENSO is near neutral or even weak El Nino (as forecast by CIC) and the tropical North Atlantic is warmer than normal (uncertain right now) the season will be very active. Fig. 4: The “least active” scenario for 2019 based on analog years would be similar to 1987. If El Nino is stronger this solution is most correct. Summary: There is great uncertainty in the tropical cyclone outlook for the North Atlantic basin in 2019. The consensus of a wide range of possibilities is 14 storms, 8 hurricanes and 3 intense hurricanes with an ACE index of 99. The forecast represents about average activity compared to 20-year and 30-year climatology. The forecast is based on very weak El Nino and favors a warmer than normal North Atlantic basin. However, the El Nino could be stronger as suggested by dynamic models or the current trend of weakening El Nino could hold and drive the cooler analogs as indicated by CIC. Knowing which direction is most likely will be firmly known in the early April update. The North Atlantic SSTA regime is also not forecast with much confidence. Dynamic models maintain the decadal climatology which is warm. However, 3 of the last 4 years have shown the North Atlantic to be cooler suggesting a potential change in the long-term phase of the Atlantic multi-decadal oscillation. The answer to this question may not be firmly known until early June. The official forecast is close to normally active which supports both Gulf of Mexico and East Coast of the U.S. impacts.