2020 Outlook Verification

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Summary of 2020 Season

The North Atlantic basin observed one of its most active years on record in 2020 with a record-breaking 30 named storms, 14 hurricanes, a record-tying 7 major hurricanes, and a seasonal Accumulated Cyclone Energy (ACE) value of 180 (*10^4 kt^2) occurring over the span of approximately six months (from May 16 to November 18). 2020 was only the second year since the inception of the World Meteorological Organization’s naming system in 1979 that the Greek alphabet auxiliary naming list was used, the first being 2005. Further, the basin observed a record-pace of named storm formation with every storm except Arthur, Bertha, and Dolly breaking the earliest named storm formation date, respectively. July tied the record (2005), and September broke the record for most named storms in their respective months with 5 in July and 10 in September. Hurricane Laura tied the record for most intense hurricane (130 knots; 150 mph) to make landfall in Louisiana, tying the Last Island hurricane of 1856. The most intense hurricane in the basin was Hurricane Iota which peaked as a high-end Category 4 hurricane with sustained winds of 135 knots (155 mph) and a minimum central pressure of 917 hPa (millibars) on November 16 making it one of the strongest November hurricanes on record (Table 1). A record-breaking 11 named storms made landfall in the United States in 2020 with 4 of these named storms (3 of which were hurricanes) hitting the state of Louisiana which also broke the previous record (Cristobal, Laura, Delta, and Zeta). This barrage of landfalls in the United States made 2020 the fifth-costliest hurricane season on record behind only 2017, 2012, 2005, and 2004.

In this report, the climate teleconnections and subsequent hurricane activity observed in the North Atlantic basin will be analyzed and used to evaluate the performance of the seasonal outlook released on June 1, 2020.

The New Normal

With the completion of 2020, weather and climate data have been reviewed and finalized to provide the final data point in the new 1991-2020 climatological averages that will be used for the next decade and will replace the 1981-2010 averages. The 1980’s featured a frequently quieter North Atlantic basin compared to normal so with its removal and the addition of a decade of generally more active seasons, the new climatological averages are notably higher in the North Atlantic basin (Table 2). The 1991-2020 average for the North Atlantic is 14.4 named storms, 7.2 hurricanes, 3.2 major hurricanes, and a seasonal ACE value of 122.7 (*104 kt2). With this new norm, the perception of what is considered an average or above average season will change (Table 3). The new threshold for an above average (>125% of normal) season based upon seasonal ACE is 153.3 (*104 kt2) which is 20.3 (*104 kt2) higher than the 1981-2010 average (Table 4). By other metrics, the threshold for an above average season is 17.5 (+2.3 from 1981-2010) named storms, 9 (+1) hurricanes, and 4 (+0.6) major hurricanes.

FOrecast discussion verification

I. North Atlantic SSTs

The North Atlantic’s Main Development Region (MDR; 10⁰N-20⁰N, 85⁰W-20⁰W) observed its fourth warmest sea surface temperatures (SST) during peak hurricane season (August-October) per the OISSTv2 dataset. Only 2005, 2010, and 2017 observed warmer SSTs which fueled the record-breaking activity comparable to the activity observed in these years. By May, it had already become increasingly clear that the North Atlantic SSTs would be favorable for hurricane activity as the subtropics and tropics warmed notably through the spring:

“The spring 2020 North Atlantic SST profile maintains some of these irregularities with near average SSTs in the MDR, above average SSTs across a region between 20⁰N to 30⁰N and well-below average SSTs observed in the subtropical Atlantic and near the Canadian Maritime. While not initially apparent, the evolution of the SST profile in the North Atlantic has progressively become more comparable to a traditional positive AMO.”

However, it was noted in the outlook discussion that the SST profile was still somewhat irregular with near average SSTs in the MDR which was more characteristic of the active West African Monsoon (WAM) period of the 1950s and 1960s:

“This pattern is indicative of a particularly strong WAM and was observed in 2018 and 2019, but with much stronger anomalous zonal easterlies which caused more significant cooling of the MDR during those years… …The most noteworthy finding of this analysis was that several of the most active years of the 1950’s and 1960’s featured an 850mb zonal wind pattern like 2020 and observed a migration of the pattern northward as the WAM amplified into peak hurricane season. This behavior yielded further warming of the tropical Atlantic during hurricane season and enhanced positive (cyclonic) relative vorticity in the region. Considering the analogous pattern and enhancement in TC activity observed in recent years during peak hurricane season because of such a regime leads to the conclusion that Atlantic hurricane activity will be enhanced in 2020.”

The uncertainty noted in this text regarding how much warming would occur by peak hurricane season was reduced by July and August when warming of the MDR brought SSTs to near-record values. This gave greater certainty that historic activity would occur in the North Atlantic through the remainder of the season.

II. El Niño Southern Oscillation

The forecast for the El Niño Southern Oscillation (ENSO) was well-predicted in 2020 with the increasing likelihood that La Niña conditions would be present by fall. Given the high confidence that El Niño conditions would not be present to inhibit hurricane activity during peak hurricane season in the North Atlantic, this was another factor that aided in higher confidence of a well-above average season:

“By April, with a strengthening UKW now dominating the subsurface and the remnant DKW meeting its demise in the east Pacific, it became increasingly apparent that such a subsurface profile is analogous to many traditional developing La Niña events. This observation was further strengthened by the apparent lack of Madden-Julian Oscillation (MJO) activity in the Pacific basin which is a mechanism typically associated with enhancing WWBs and El Niño growth... …With the UKW now arriving to the eastern Pacific, further cooling is expected and there is a high likelihood of La Niña conditions being present during the peak of hurricane season. The anticipated strength of this La Niña during hurricane season will be dependent on the emergence of a reinforcing UKW, however, it is plausible that a moderate La Niña (-1.0⁰C to -1.4⁰C) may be present by the fall of 2020 given current trends.”

ENSO Neutral conditions were present for the beginning of hurricane season with the NOAA Climate Prediction Center officially declaring La Niña conditions were present in August. ENSO rapidly cooled into fall with the persistent Easterly Wind Bursts aiding in oceanic upwelling and generation of new Oceanic Upwelling Kelvin Waves (UKW). By peak hurricane season (ASO), the Oceanic Niño Index (ONI) reached -0.9⁰C and moderate La Niña conditions were present by September-October-November (Table 5). This enabled record-breaking activity in the North Atlantic in the months of October and November in which 7 named storms, 6 hurricanes, and 5 major hurricanes developed during this period.

This activity, while primarily focused closer to the Americas, generated approximately 65 ACE and aided in 2020 reaching the above average threshold for ACE as well as tying the record for most major hurricanes observed in the basin with 2005.

III. Pacific Decadal Oscillation & Pacific Meridional Mode

After several years of a positive PDO regime from 2014 to 2019, the tropical and subtropical eastern Pacific cooled substantially in early 2020. With the coldest PDO and PMM since 2013 and the onset of La Niña, the eastern Pacific hurricane season observed its fewest number of named storms (17) since 2012, fewest number of hurricanes (4) since 2010, and lowest seasonal ACE (74) since 2010 (Table 6). This was also a well-anticipated aspect of the seasonal forecast:

“With a cooler (Western Development Region) WDR in combination with the increasing likelihood of La Niña developing by peak hurricane season, it appears that the eastern North Pacific is slated for one of its most inactive years since 2011. In conclusion, the cooling of the PMM and PDO, to their coolest since 2013, would serve to increase the favorability of the MDR for hurricane activity especially as ENSO cools further into peak hurricane season.”

With an unfavorable background state for the eastern Pacific, only MJO activity was enough to spur hurricane activity in the basin. This enabled a brief period of enhanced activity in August as the MJO propagated across the Pacific which briefly dampened hurricane activity in the North Atlantic. This provided a prime example of the more nuanced aspects of intraseasonal (days to weeks) forecasting which is challenging to account for in a seasonal forecast.

IV. African Easterly Jet & African Sahel Precipitation

The active West African Monsoon played a vital role in the well-above average hurricane activity observed in the North Atlantic in 2020. As noted in the forecast discussion, the African Sahel was significantly wetter than normal with only 2019 being wetter in recent years. This was a characteristic indicative of an amplified WAM that would fuel an active Atlantic hurricane season. Further, the West African Monsoon also enhanced zonal westerlies across the MDR which slowed trade winds and enhanced warming of the tropics into peak hurricane season which was another aspect that raised the ceiling on potential hurricane activity.

While hurricane activity was not anomalously more active in the eastern Atlantic, the vigorous tropical waves that exited the African coastline spawned numerous tropical cyclones from July (Gonzalo) to November (Iota). A recurring attribute observed with these tropical waves was their sprawling size that caused them to take longer to consolidate surface lows sufficient for tropical cyclogenesis. This was evident in cases such as Hurricanes Isaias and Laura where both systems were initially anticipated to develop in the central Atlantic but ended up developing closer to the Lesser Antilles instead. With these waves taking longer to develop, storms had less time over water before impacting land which also subsequently limited ACE outputs. Regardless, it was apparent that the amplified WAM was an enhancing component for the 2020 hurricane season.

V. Climate Background State

The climate state began to give clear evidence toward an exceptionally favorable North Atlantic hurricane season in Spring 2020. The standing wave over the African continent that had been apparent since 2017 was competing with a Modoki El Niño which had enhanced upward motion over the central Pacific during 2018 and 2019 which limited the favorability of the Atlantic basin. However, with the termination of warm ENSO in Spring 2020, the African standing wave became the focal point of anomalous upward motion across the tropics through summer. With anomalous easterlies across the Maritime Continent and the central Pacific through much of this period, significant ENSO cooling occurred which resulted in the development of La Niña in the fall which only further aided late-season hurricane activity in the Caribbean Sea.

Seasonal Outlook Critique

Tyler Stanfield's 2020 seasonal outlook verification and forecast errors

The 2020 North Atlantic hurricane season was well-anticipated to be above average given the favorable modes of highly correlated variables evident by June 1. However, despite being the highest seasonal activity predicted since the inception of these forecasts in 2014, the predicted number of named storms, hurricanes, and major hurricanes were well under the observed activity in 2020 (Figure 1). This was remedied to a degree in August when an updated forecast was released to revise these numbers for the high confidence of near-record activity in the basin given the level of activity that had already occurred in June and July. However, this forecast was also still below the observed activity that occurred (Table 8). It was acknowledged that the minor uncertainties in MDR SSTs warming and the rate at which ENSO cooled provided some hesitation from producing a more aggressive June forecast, however, the overall forecast still provided sufficient indication of a well-above average season. Further, the seasonal ACE forecast using the ACE model v2 linear model for the June forecast metrics of 20 named storms, 10 hurricanes, and 5 major hurricanes ended up verifying closer to the observed seasonal ACE for 2020 largely because many of the storms were short-lived and/or developed close to land and did not stay intense for extended periods of time (Table 9). For reference, the ACE model v2 yields a hindcast seasonal ACE value of 245.6 using the blended named storm, hurricane, and major hurricane outputs for observed activity. This is a clear weakness in the ACE model in seasons like 2020 in which ACE per storm is lower than the 1950-2019 mean but helped to verify a lower forecast in this case.

In conclusion, this season provided a renewed example of how even in high-confidence forecasts, it is still a challenge to anticipate just how aggressive a numerical forecast should be in representing the ceiling of potential hurricane activity.

Prior June Outlooks

Categorized forecast skill plots for Tyler Stanfield’s June seasonal hurricane outlooks from 2014-2020 for number of named storms (top left), number of hurricanes (top right), number of major hurricanes (bottom left), and seasonal ACE (bottom right).

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