An above-average Atlantic hurricane season is likely in 2021, the Colorado State University (CSU) hurricane forecasting team says in its latest seasonal forecast issued April 8.

Led by Dr. Phil Klotzbach, with coauthors Dr. Michael Bell and Jhordanne Jones, the CSU team is calling for an Atlantic hurricane season with 17 named storms, 8 hurricanes, 4 major hurricanes, and an Accumulated Cyclone Energy (ACE) of 150. In comparison, the long-term averages for the period 1981-2010 were 12.1 named storms, 6.4 hurricanes, 2.7 major hurricanes, and an ACE of 106.

The CSU outlook predicts the odds of a major hurricane hitting the U.S. to be 69% (long-term average: 52%). It gives a 45% chance for a major hurricane to hit the East Coast or Florida Peninsula (long-term average: 31%), and a 44% chance for the Gulf Coast (long-term average: 30%). The Caribbean is forecast to have a 58% chance of having at least one major hurricane pass through (long-term average: 42%).

The CSU forecast uses a statistical model honed from 38 years of past Atlantic hurricane statistics, plus output from the ECMWF (European) model to augment the statistical technique.

Departure of sea surface temperatures
Figure 1. Departure of sea surface temperature (SST) from average for April 7, 2021. SSTs were well above-average in the subtropical Atlantic. In the hurricane Main Development Region (MDR) between Africa and Central America, SSTs were average or above average in the Caribbean, and below average in the eastern Atlantic. Virtually all African tropical waves move through the MDR, and these tropical waves account for 85% of all Atlantic major hurricanes and 60% of all named storms. Much above average SSTs in the MDR during hurricane season generally lead to a very active season in the absence of an El Niño event. Conversely, when MDR SSTs are cooler than average, a below-average Atlantic hurricane season is more likely. (Image credit: tropicaltidbits.com).

Analogue years

Five years with similar pre-season January, February, and March atmospheric and oceanic conditions were selected as “analogue” years that the 2021 hurricane season may resemble. These years had La Niña conditions the previous winter, and then neutral or weak La Niña conditions during the peak of the Atlantic hurricane season (August-October). The CSU team also selected years that had near- to above-average sea surface temperatures (SSTs) in the tropical Atlantic. The five analogue years were:

1996 (13 named storms, 9 hurricanes, and 6 major hurricanes);
2001 (15 named storms, 9 hurricanes, and 4 major hurricanes);
2008 (16 named storms, 8 hurricanes, and 5 major hurricanes);
2011 (19 named storms, 7 hurricanes, and 4 major hurricanes); and
2017 (17 named storms, 10 hurricanes, and 6 major hurricanes).

The average activity for these years was 16 named storms, 9 hurricanes, 5 major hurricanes, and an ACE of 155 – well above the long-term average. (Let’s hope that 2021 does not resemble one of those analogue years chosen, 2017: That year featured three incredibly destructive major hurricanes – Harvey, Irma, and Maria.)

Neutral or weak La Niña conditions …

The CSU team cited two main reasons, addressed below, that 2021 may be an above-average hurricane season:

1) A significant El Niño seems unlikely. El Niño conditions favor a slower-than-usual Atlantic hurricane season as a result of an increase in the upper-level winds over the tropical Atlantic that can tear storms apart (higher vertical wind shear). If neutral or La Niña conditions are present, instead, an active hurricane season would be more likely.

The current weak La Niña event in the Eastern Pacific appears likely to transition to neutral conditions this summer, and NOAA’s Climate Prediction Center (CPC) predicts in its latest April 8 monthly advisory an 80% chance of neutral conditions during the May-July period. The Australian Bureau of Meteorology, which uses a more stringent threshold than NOAA for defining La Niña, says that neutral conditions have already begun.

For September-November, NOAA gives a 40-50% chance that La Niña conditions might redevelop, a 40-50% chance that neutral conditions will prevail, and only a small chance that El Niño conditions might develop.

Sea surface temperatures (SSTs) were about 0.3 degrees Celsius below average during the past month in the so-called Niño 3.4 region (5°S-5°N, 120°W-170°W), where SSTs must be at least 0.5 degrees Celsius below average for five consecutive months (each month using a three-month centered average for departure of temperature from average) to qualify as a weak La Niña event.

Reviewing the latest predictions from a large number of statistical and dynamical El Niño models for the peak of the Atlantic hurricane season in August-October, more than half call for neutral conditions, two predict El Niño conditions, and four predict La Niña conditions.

… and above-average tropical Atlantic SSTs predicted

2) Anomalously warm SSTs are expected in the tropical Atlantic for the peak of the Atlantic hurricane season. SSTs averaged across the tropical Atlantic have been near average over the past month, and subtropical Atlantic SSTs were warmer than average. The current pattern of SSTs and the output from the European model suggest that SSTs will be above average during the peak of hurricane season, favoring an active season.

As is its practice, the CSU team included this standard disclaimer:

“Coastal residents are reminded that it only takes one hurricane making landfall to make it an active season for them, and they need to prepare the same for every season, regardless of how much activity is predicted.”

Comparison of the percent improvement graphic
Figure 2. Comparison of the percent improvement in mean square error over climatology for seasonal hurricane forecasts for the Atlantic from NOAA, CSU and Tropical Storm Risk (TSR) from 2003-2020, using the Mean Square Skill Score (MSSS). Values less than zero indicate that pure climatology does a better job than the forecast. The figure shows the results using two different climatologies: a fixed 50-year (1951-2000) climatology, and a 10-year 2011-2020 climatology. Skill for forecasts issued in April is close to or below zero, is modest for June forecasts, and is moderate-to-good for August forecasts. Using this methodology, TSR has had the best seasonal forecasts. (Image credit: Tropical Storm Risk, Inc. (TSR))

April hurricane season forecasts have little or no ‘skill’

On average, April forecasts of hurricane season activity have had no “skill,” or even negative skill when computed using the Mean Square Skill Score (Figure 2). A negative skill means that a forecast simply using climatology would do better. April forecasts must deal with the so-called “spring predictability barrier“. In April, the El Niño/La Niña phenomenon commonly undergoes a rapid change from one state to another, making it difficult to predict whether there will be El Niño, La Niña, or neutral conditions in place for the coming hurricane season.

Last year’s CSU April forecast predicted an above-average Atlantic hurricane season for 2020, with 16 named storms, 8 hurricanes, 4 major hurricanes, and an Accumulated Cyclone Energy (ACE) of 150. The 2020 season ended up with record to near-record activity, with 30 named storms, 13 hurricanes, 6 major hurricanes, and an ACE of 180.

The next CSU forecast, due on June 3, is worth paying more attention to, as late May/early June forecasts have shown considerable skill over the years. NOAA issues its first seasonal hurricane forecast for 2021 in late May. The British private forecasting firm Tropical Storm Risk, Inc. (TSR) is to issue its first 2021 Atlantic hurricane season forecast on April 13.

Also see: How to make an evacuation plan

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Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...

25 replies on “Forecasters predict an above-average Atlantic hurricane season in 2021”

  2. Hello
    I heard this morning that the heat affecting the west-southwest is due to the wind and not “man-made” climate change. Has anybody heard or read anything on this?

  3. Not being doom and gloom just a messenger.
    That doom (if humans keep allowing it become an all encompassing reality) has been bought to the forefront by uncaring political policies.

    When fact based science warns of dangers if we follow a certain path, the responsibility to take the “high road” falls on those living at that decision moment, if not THEN the doom and gloom becomes reality.
    (Though i disagree with a statement in the article “Which future we get will depend in large part on technology — AI and automation, clean energy, gene editing and more.”

    TO ME FIRST AND FOREMOST:: “Which future we get will depends on the human conscience”. Those that know of my crazy theories know i place a large chunk on man’s present uncaring heart on a side-effect generated by Electricity.

    The long-range forecast for the future of the world is looking dark and stormy.

  4. I’d be interested in science on the impact of the La Soufriere eruption on the hurricane main development region this season if the eruption and ash clouds continue for many weeks. And the global energy balance if it continues for longer or approaches the impact that Mt. Pinatubo had? Or is that not possible or likely with this volcano?

  6. This is the only article on this web site that I’ve seen which allows comments. Why aren’t comments allowed on all articles? (like the Yale 360 site). I have sent this suggestion to the site but did not get a reply.

  8. For those who have not heard, the CO2 measurement is at another record level, 412.5 ppm. We are so screwed.

    1. This is even with the pandemic. What about the RATE of increase? Is the RATE slowing?

      2. ‘NOAA reported that the global average of atmospheric CO2 hit 412.5 parts per million (ppm) in 2020, a rise of 2.6 ppm from 2019, the fifth-largest increase since they began measuring atmospheric CO2 levels 63 years ago. The rise happened despite an estimated 7% reduction in global emissions due to the pandemic. Pieter Tans, the senior scientist at NOAA’s Global Monitoring Laboratory, estimates that 2020 would have been a record-breaking year had it not been for the pandemic.’
        —-
        ‘Scripps Institution of Oceanography at UC San Diego released similar findings Wednesday, saying their measurements showed atmospheric CO2 levels to be 417.4 ppm at their monitoring station in Hawaii. Scripps noted that this puts atmospheric CO2 levels 50% higher than they were just prior to the industrial revolution.’
        https://www.cbsnews.com/news/climate-change-carbon-dioxide-highest-level-million-years/

  9. So what is the point of these April forecasts? Surely no one relies on them for things like setting insurance rates, planning cargo shipping, or emergency management preparation.
    I live in Florida and I know no one whose storm season preparations change in any meaningful way after the pre-season forecasts suggest the season might be below/above/just about average.

  10. Thanks Doc. Looks like it’s going to be another active year. Any thoughts on any preseason storms this year?

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