North Atlantic 2020 Tropical Cyclone Season Forecast: Busy season; significant coastal threats.

Executive summary: The Climate Impact Company 2020 North Atlantic basin tropical cyclone season projects above normal activity with ominous hurricane tracks indicated including several into the U.S. East Coast and two to the Gulf of Mexico Coast. The evidence is overwhelming in support of this forecast given a very warm North Atlantic and likelihood of weak La Nina.

Table 1: The Climate Impact Company 2020 North Atlantic basin seasonal tropical cyclone activity forecast.

Fig. 1: Based on analog years and the anticipated upper air pattern for 2020 an estimate of where this season’s hurricanes form and where they track is indicated.

Regional forecast discussions: The Climate Impact Company 2020 seasonal tropical cyclone forecast for the North Atlantic basin hurricane development areas and prevailing tracks is ominous. The analog years contain many significant U.S. coastal threats. The very warm North Atlantic basin coupled with a possible weak La Nina and upper ridge pattern over eastern Canada identifies the appropriate set-up for MANY U.S. coastal strikes of hurricanes this season. The development areas are created by a combination of the analog years and forecast SSTA patterns. The Gulf of Mexico is certainly very warm and an area for rapid intensification of storms moving across the very warm Gulf. The forecast does not indicate long distance travelers into the Gulf. The long duration hurricane treks are projected out of the deep tropics in the outer basin west-northwest to North Carolina in August with a second similar path turning toward New England in September. Another hurricane forming late in the season in the Caribbean travels north and into the Mid-Atlantic region. The coastal strikes in the Mid-Atlantic/New England region are generated by the blocking high pressure to the north of the area.

1. Gulf of Mexico: The Gulf of Mexico is very warm this season and should easily support intensification of tropical cyclones to hurricane intensity. Fortunately only two locally generated hurricanes are expected. Despite their short duration either hurricane could become a major hurricane given the abundant upper ocean heat. An early season (June) system is expected.
2. Florida: Florida may enter the tropical cyclone season in a drought condition. Florida droughts are frequently broken by heavy rains from tropical cyclones. This season will be busy and Florida will frequently be on alert. However, the forecast indicates hurricanes miss the state to the west and east this season.
3. Southeast Coast: North Carolina appears as a target for the upcoming season. The projected hurricane strike is in August and a long distance traveler from the outer tropical Atlantic. This system has a chance to be a major hurricane.
4. Mid-Atlantic/New England Coast: The outlook is ominous and features both a September and October coastal strike and at least one of these systems could be a major hurricane. Forecast models indicate very warm waters off the East Coast and with a high pressure block to the north an ideal set-up for coastal strikes is projected.
5. Caribbean: The Caribbean Sea will be normally busy as many storms pass just to the north. Cuba can expect at least one hurricane likely late in the season from the south while at least two hurricanes skirt Puerto Rico and Hispaniola to the north.

Climate discussion: The Climate Impact Company 2020 seasonal tropical cyclone forecast for the North Atlantic basin is based on 2 primary climate regimes: El Nino southern oscillation and the North Atlantic basin sea surface temperature anomaly (SSTA) pattern. There are several other predictors however they are all governed somewhat by the ENSO and North Atlantic SSTA influence on climate. Let’s take a look at the current SSTA pattern (Fig. 2).

Of primary interest is the ENSO regime (Fig. 3). Currently, a borderline weak El Nino persists in the eastern equatorial Pacific Ocean. The borderline weak El Nino has lingered since last October. One key indicator that the weak El Nino may fade is the emergence of a cool phase of the Pacific decadal oscillation in the northeast Pacific. The ocean currents travel from the Gulf of Alaska off the U.S. West Coast and southwestward to the south of Hawaii. The –PDO regime has just formed. If it continues cooler water will be funneled into the tropics south of Hawaii causing the warm El Nino signature present to fade.

In the North Atlantic the basin-wide SSTA is +0.62C and representative of the warm phase of the Atlantic multidecadal oscillation (+AMO). In the tropics from the equator to 23N in the zone between the far eastern Caribbean Sea and northwest coast of Africa the tropical North Atlantic (TNA) index is +0.57C warming significantly the past 30 days. The warm North Atlantic and particularly the TNA index which represents the area where most hurricanes form in the deeper tropics is an essential enhancer of tropical cyclone activity.

Finally, the cool pool of water south of Greenland in an otherwise warm North Atlantic persists. The pool of cool water dubbed the North Atlantic Warm Hole (NAWH) by climate scientists formed early last decade and has persisted since and is linked to a reliable upper trough over the North Atlantic during the warm season which helps to shape other upper air features across the North Atlantic and U.S. East Coast governing tropical cyclone tracks (Fig. 4). This regime returns during summer 2020.

Fig. 3: Current global SSTA and regions of influence affecting the North Atlantic tropical cyclone season forecast.

Fig. 4: During the past decade a persistent cool pool of water has emerged south of Greenland caused by (more) rapid ice melt during the spring season of the Greenland ice sheet. Across the cool pool of water an upper trough has persisted during the warm season of the past decade forcing an upstream ridge over eastern Canada. This pattern is likely to return in summer 2020.

Fig. 5: The GFDL global SSTA forecast for JUL/AUG/SEP 2020 and regions of influence on the North Atlantic basin tropical cyclone seasonal forecast.

Climate forecast: The GFDL global SSTA forecast valid JUL/AUG/SEP 2020 (core of the tropical cyclone season) indicates the weak El Nino departs and a weak La Nina may form (Fig. 5). A weak la Nina is a strong enhancer of tropical cyclone activity across the North Atlantic basin. The forecast indicates the North Atlanic basin is very warm except for the NAWH southeast of Greenland. The waters of the outer tropical North Atlantic also warm for mid-to-late summer supporting hurricane development in the outer North Atlantic basin.

The seasonal forecast is based on analog years taken from the 1995-2020 climatology due to the long-term phase change of ENSO/PDO and AMO during that time and the acclerating CO2 rise evident during that 25-year period. The analogs are chosen based on presence of neutral ENSO or a La Nina pattern and positive phase of the AMO and TNA indices. Analog years from 2011-2019 are weighted more heavily due to the presence of the NAWH-induced upper air pattern.

Table 2: The Climate Impact Company 2020 North Atlantic basin seasonal tropical cyclone activity forecast.

Two sets of analog years are chosen (Table 2). The analog years from the past decade are 2017, 2016, 2014, 2013 and 2012 chosen due to their reasonably close ENSO and AMO/TNA signatures to the 2020 GFDL forecast. These analog years are weighted twice on the infleunce of the 2020 seasonal forecast. The analog years taken from earlier in the 25-year climatology are 2005, 2003 and 1995.

The outlook indicates slightly above normal activity for the 2020 season with 16 tropical storms, 8 hurricanes and 3 major hurricanes. The accumulated cyclone energy (ACE) index is 128. There are more hurricanes forecast for 2020 (8) compared to last year (6). Other than 2017 (10) the 2020 forecast indicates the most hurricanes since 2012 (10). The “caveat” forecast (i.e. possible change) is definitely higher amounts of activity especially if La Nina forms.

The Climate Impact Company 2020 seasonal outlook for the North Atlantic basin is similar to the Colorado State University forecast issued today and last December’s outlook issued by Tropical Storm Risk U.K. (Table 3) which will be updated April 7^th.

 Table 3: The current North Atlantic basin 2020 tropical cyclone seasonal forecasts from leading providers.

Summary: The concern for the 2020 season are projections of a very warm North Atlantic coupled with the enhancing influence of a weak La Nina and a blocking high pressure area over northeastern North America to produce above normal hurricanes this season with tracks that run into the U.S. East Coast. The Gulf of Mexico is also very warm propelling at least two hurricanes.

Verification: In 20 years the forecast verification grade has been “A” on 6 occasions and either an “A” or “B” on 14 occasions (Table 4). There have been 4 forecast failures.

Table 4: Climate Impact Company North Atlantic basin seasonal forecasts issued each April since 1999 (the 1999-2004 forecasts were issued while at Weather Services Corporation) compared to observed. Red = El Nino year.