Mid-April | *2020 Atlantic Basin Tropical Outlook*

Current sea surface temperature (SST) anomalies with La Nina (colder-than-normal) conditions just beginning to appear off the west coast of South America. The water is much warmer-than-normal water in the Gulf of Mexico and generally above-normal across the tropical Atlantic Ocean. Map courtesy NOAA, tropicaltidbits.com

Overview

The overall numbers are likely to be above average this year in terms of the number of tropical storms and hurricanes in the Atlantic Basin (includes the Caribbean Sea and Gulf of Mexico) with around 13 named tropical systems, 7  hurricanes, and 3 majors (i.e., category 3, 4 or 5 on the Saffir-Simpson Hurricane Wind Scale). The average number of named storms in an Atlantic Basin tropical season is 12.1 with 6.4 reaching hurricane status and 2.7 of those becoming major (base period for comparison 1981-2010). 

The major factors involved with this year’s tropical outlook include the development of La Nina in the equatorial Pacific Ocean.  In addition, the Gulf of Mexico and much of the tropical Atlantic Ocean are featuring warmer-than-normal sea surface temperatures and those patterns are likely to continue through the tropical season.  This is generally a favorable type of pattern for the development or intensification of Atlantic Basin tropical systems and it also makes the southern and eastern US quite vulnerable this season; especially, to “home-grown” type storms.

2020 “names” to be used for Atlantic Basin tropical storms/hurricanes (courtesy weather.com)

Executive Summary

• Above-normal tropical activity in the Atlantic Basin

• The development of La Nina in the equatorial Pacific Ocean will play a major role

• Warmer-than-normal water in the Gulf of Mexico and much of the tropical Atlantic Ocean

• Southern and eastern US vulnerable to tropical hits this season

• Analog years that are based on the predicted sea surface temperature anomaly pattern suggest warm and wet conditions for Mid-Atlantic region

2019 Atlantic Basin tropical storm tracks (plot courtesy Wikipedia)

Recap of 2019 tropical season in the Atlantic Basin

The 2019 Atlantic hurricane season was the fourth consecutive year of above-average activity dating back to 2016. It is tied with 1969 as the fourth-most active Atlantic hurricane season on record in terms of named storms with 18 of them although many were weak and short-lived, especially towards the end of the season. The 2019 Atlantic tropical season may long be remembered for Hurricane Dorian which was an extremely powerful and devastating Category 5 Atlantic hurricane.  Dorian became the most intense tropical cyclone on record to strike the Bahamas, and is regarded as the worst natural disaster in the country's history. In addition, Dorian surpassed Hurricane Irma in 2017 to become the most powerful hurricane on record in the open Atlantic region, outside of the Caribbean Sea and Gulf of Mexico.

Compilation of computer model forecasts for El Nino Southern Oscillation (ENSO) state for the rest of 2020 with most of these developing La Nina conditions by the summer tropical season. The graph shows forecasts made by dynamical and statistical models for sea surface temperature (SST) anomalies in the Nino “3.4” region for nine overlapping 3-month periods. Plot courtesy International Research Institute/CPC, NOAA, ECMWF, JMA

The development of La Nina in the tropical Pacific Ocean

What goes on in the tropical Pacific Ocean does indeed have an effect on the tropical Atlantic Ocean. El Nino, which refers to warmer-than-normal waters in the equatorial Pacific Ocean, affects global weather patterns and it tends to be an inhibiting factor for tropical storm formation or intensification in the Atlantic Basin. First, El Nino tends to produce faster-than-usual high-altitude winds over the tropical Atlantic Ocean and this increase in wind shear (change of wind speed with height) is hostile to the development of maintenance of tropical storms.  In addition, El Nino tends to result in sinking motion over parts of the Atlantic Basin which too is hostile to tropical storms.  On the other hand, La Nina conditions in the equatorial part of the Pacific Ocean usually correlate with weaker wind shear over the breeding grounds of the tropical Atlantic Ocean which is a favorable configuration for the development or intensification of tropical activity.  Numerous computer forecast models including NOAA’s CFSv2 develop La Nina conditions in the tropical Pacific Ocean by the summer season and continue that same type of sea surface temperature anomaly pattern this fall. 

NOAA’s CFSv2 sea surface temperature anomaly forecast map for 3-month period from June through August 2020. Map courtesy NOAA/CPC

NOAA’s CFSv2 sea surface temperature anomaly forecast map for 3-month period from July through September 2020. Map courtesy NOAA/CPC

NOAA’s CFSv2 sea surface temperature anomaly forecast map for 3-month period from August through October 2020. Map courtesy NOAA/CPC

Depth-longitude section of the equatorial part of the Pacific Ocean upper-ocean (0-300 meters) temperature anomalies (°C) at the end of March. This plot of sub-surface temperature anomalies provides support to the notion of formation of La Nina in the tropical Pacific Ocean as colder-than-normal water (shown in blue) lies just beneath the surface in the central Pacific. Indeed, there already is colder-than-normal water at surface-levels near the west coast of South America (upper, right). Map courtesy NOAA, IRC/CPC

Favorable SSTs for tropical activity in the Atlantic Basin

Not only are equatorial Pacific Ocean sea surface temperature anomalies important in terms of potential impact on tropical activity in the Atlantic Basin in a given season, but Atlantic Ocean water temperatures can have a major impact on atmospheric circulation and, in fact, more of a direct role in the likelihood for formation and intensification of tropical storms.  Sea surface temperatures of  >80°F are generally considered a requirement for the formation of tropical storms; therefore, above-normal sea surface temperatures are typically more favorable for tropical storm formation and intensification as tropical waves trek westward in the trade winds from Africa’s west coast to the western Atlantic Ocean.  

Indeed, the waters in the Gulf of Mexico are warmer-than-normal as we close in on the tropical season and they range from normal to warmer-than-normal in the tropical Atlantic.  Signs point to these sea surface temperature anomaly patterns continuing during the next several months – quite favorable for the formation or intensification of tropical storm activity.  The warmer-than-normal sea surface temperatures near the southern and eastern US make these areas quite vulnerable to tropical hits this season; especially, when “home-grown” systems tend to dominate.  What I refer to as “home-grown” systems are those which form over the Gulf of Mexico or far western Atlantic as compared with those that trek all the way from the west coast of Africa toward the US.  The “home-grown” type storms generally tend to dominate during the latter stages of an Atlantic Basin tropical season.

The top five analog years which are based on comparisons with the forecasted sea surface temperature anomalies are as follows: 1970, 2010, 1975, 1998 and 2013. These top five analog years featured La Nina conditions in the tropical Pacific Ocean – similar to what is expected during the upcoming tropical season. Maps courtesy NOAA, tropicaltidbits.com

The top five analog years when averaged together generally featured warmer-than-normal conditions in much of the eastern half of the nation and wetter-than-normal weather as well. The western half of the US in these five analog years generally averaged out to near normal in terms of temperature and precipitation anomalies. Maps courtesy NOAA

Analog Years and the Mid-Atlantic Summertime Outlook

Based largely upon the expectation of La Nina conditions later this summer and fall, there are five analog years that were found which featured similar SST anomalies and when averaged together, they featured above-normal tropical activity in the Atlantic Basin.  The analog years are listed here along with the number of named storms, number of hurricanes, and the number of major hurricanes in each particular year: 1970 (10/5/2), 2010 (19/12/5), 1975 (9/6/3), 1998 (14/10/3) and 2013 (14/2/0). 

In the Mid-Atlantic region, these five analog years generally featured weather conditions that were on average warmer-than-normal in the June through September time frame. Specifically, in terms of temperature anomalies, I expect the summer season in the Mid-Atlantic region to average from 2.0-4.0 (°F) above normal.  In fact, there is reason to believe that much of the eastern half of the nation will experience warmer-than-normal conditions this summer based upon comparisons with the analog years while the western half of the US will largely average out to near normal. 

Drought conditions are non-existent in most of the eastern half of the nation and recent heavy rainfall will likely inhibit development during the next few months. Map courtesy NOAA

While I do support the idea of a warmer-than-normal summer in the Mid-Atlantic region, I believe that soil moisture is just too high for an excessively hot season thanks to a long-running relatively wet weather pattern in the eastern US and also a relatively chilly month of April which has limited evaporation in the top layers of soil.  Given the current and long-standing normal-to-wet soil conditions, it is likely that a portion of the sun’s energy this summer will be “used up” in the evaporation of soil moisture rather than in the heating of the ground and, in turn, the lower atmosphere. Excessively hot summers are much more probable when drought conditions are severe and this is not looking too likely for the Mid-Atlantic region.  In terms of precipitation, the Mid-Atlantic region will likely experience wetter-than-normal conditions this summer on the order of 1.0-3.0 inches above-normal based on comparisons with the analog years as will much of the eastern half of the nation while the western half of the US will likely be nearly normal.

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com

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