• Wed. May 22nd, 2024

    An extremely active Atlantic hurricane season is likely in 2024, the Colorado State University (CSU) hurricane forecasting team says in its latest seasonal forecast, issued April 4. Led by Dr. Phil Klotzbach, with co-authors Dr. Michael Bell, Alexander DesRosiers, and Levi Silvers, the CSU team is calling for 23 named storms, 11 hurricanes, five major hurricanes, and an Accumulated Cyclone Energy (ACE) of 210 (171% of average). In comparison, the long-term averages for the period 1991-2020 were 14.4 named storms, 7.2 hurricanes, 3.2 major hurricanes, and an ACE of 123.

    The forecast is the most aggressive one ever issued in the 30 years that the CSU team has been issuing April forecasts. If the numbers verify, the number of named storms would rank as the third-highest on record; the number of hurricanes, the fifth-highest; the number of major hurricanes, the ninth-highest; and the ACE index, the ninth-highest. The 2024 forecast falls just short of predicting a hyperactive season, defined as having an Accumulated Cyclone Energy (ACE) index of at least 214 (175% above average). Only seven seasons since records began in 1851 have met this definition: 1933 (ACE of 259), 2005 (ACE of 245), 1893 (ACE of 231), 1926 (ACE of 230), 1995 (ACE of 227), 2004 (ACE of 227), and 2017 (ACE of 225).

    The CSU outlook predicts much higher odds of a major hurricane hitting the U.S. than usual: 62% (long-term average: 43%). It gives a 34% chance for a major hurricane to hit the East Coast or Florida Peninsula (long-term average: 21%), and a 42% chance for the Gulf Coast (long-term average: 27%). The Caribbean is forecast to have a 66% chance of having at least one major hurricane pass through (long-term average: 47%).

    The CSU forecast uses a statistical model honed from 40+ years of past Atlantic hurricane statistics, plus dynamical model output from four groups: ECMWF (European model), UKMET, Japan Meteorological Agency, and Centro Euro-Mediterraneo sui Cambiamenti Climatici (CMCC). It’s worth noting that two of these five models (the CSU statistical model and the CMCC dynamical model) predicted that 2024 would tie the record for most major hurricanes (seven) and set a new all-time ACE record (269-280).

    A map of the Mid-Atlantic ocean.
    Figure 1. Departure of sea surface temperature (SST) from average for April 3, 2024. In the hurricane Main Development Region (MDR) between Africa and Central America between 10-20°N, SSTs were about 1°C (1.8F) above average. 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. Above-average SSTs in the MDR during hurricane season generally leads to an 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 preseason January, February, and March atmospheric and oceanic conditions were selected as “analogue” years that the 2024 hurricane season may resemble. These analogue years had El Niño conditions the previous winter, followed by La Niña conditions during the peak of the Atlantic hurricane season (August-October). The analogue years also had much above-average sea surface temperatures (SSTs) in the tropical Atlantic early in the year. The analogue years were:

    1878 (12 named storms, 10 hurricanes, and 2 major hurricanes);
    1926 (11 named storms, 8 hurricanes, and 6 major hurricanes);
    1998 (14 named storms, 10 hurricanes, and 3 major hurricanes);
    2010 (19 named storms, 12 hurricanes, and 5 major hurricanes); and
    2020 (30 named storms, 14 hurricanes, and 7 major hurricanes).

    The average activity for these years was 17 named storms, 11 hurricanes, 5 major hurricanes, and an ACE of 188 – extremely high levels of activity. It’s worth noting that although the 2010 hurricane season was very active, steering currents predominantly favored recurvature, and no hurricanes hit the U.S. (though two hurricanes hit Mexico, one hit Belize, one hit Canada, and one hit the Lesser Antilles).

    Guidance from all five of the dynamical and statistical models is calling for a hyperactive season, but the real-world analog years are not quite as active, so the CSU outlook (which takes all six schemes into account) ended up just short of calling for a hyperactive season.

    The CSU team cited two main reasons, addressed below, for their aggressive 2024 forecast:

    1)  A transition from El Niño to La Niña seems likely 

    The current El Niño event in the Eastern Pacific has been steadily weakening in recent weeks (see tweet by Michael Lowry below), and is 83% likely to dissipate by June, NOAA’s Climate Prediction Center (CPC) said in its latest March 14 monthly advisory. NOAA gave a 62% chance of La Niña conditions developing during June-August (the early portion of hurricane season), with the odds of La Niña increasing through the fall.

    Reviewing the latest predictions from 19 statistical and dynamical El Niño models for the peak of the Atlantic hurricane season in August-October, five call for neutral conditions, none predict El Niño conditions, and 13 predict La Niña conditions.

    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). In contrast, when neutral or La Niña conditions are present, an active hurricane season becomes more likely. Since 1950, U.S. landfalls by major hurricanes have been more than twice as likely during a La Niña year compared to an El Niño year (see Figure 2 by Gallagher Re’s Steve Bowen below).

    A graphic showing landfalling hurricanes in the U.S. are more common in neutral and La Nina years.
    Figure 2. Landfalling U.S. tropical storms and hurricanes by phase of ENSO, 1950-2023. Landfall activity is much higher during La Niña than El Niño, with an average of 0.7 major hurricanes per year making landfall during La Niña, compared to 0.3 per year during El Niño. (Image credit: Steve Bowen, Gallagher Re)

    2) Record-warm ocean temperatures

    The eastern tropical Atlantic, subtropical Atlantic, and Gulf of Mexico have sea surface temperatures (SSTs) that are near-record-to-record warm, and the Atlantic Main Development Region (MDR) is currently above 1 degree Celsius (1.8°F) above average (Figure 1), which is record-warm. What’s more, very warm water extends down to unusual depths, creating a record amount of ocean heat content, according to hurricane scientist Brian McNoldy (see Tweet below). The current amount of ocean heat content is more typical of what is observed in July than in April, as are the sea surface temperatures.

    In a series of tweets, catastrophe modeler Richard Dixon analyzed what the best-case scenario might be for the evolution of sea surface temperatures (SSTs) in the tropical Atlantic going into the August-October peak of hurricane season. He found that even if the ocean warms at the slowest rate ever observed in the past 40 years, the hurricane Main Development Region (MDR) between Africa and Central America will still see the second-highest SSTs on record, behind only 2023. Furthermore, the five years with the warmest MDR SSTs at the end of March were 2010, 2005, 2013, 2020, and 2023; the average activity for the five years was very high – 22 named storms, 10 hurricanes, and 4 major hurricanes.

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

    “As with all hurricane seasons, coastal residents are reminded that it only takes one hurricane making landfall to make it an active season. Thorough preparations should be made every season, regardless of predicted activity.”

    A caveat: 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). This does not mean a particular April forecast will be incorrect — just that, on average, a forecast simply using climatology would do as well or 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 El Niño, La Niña, or neutral conditions will be in place for the coming hurricane season. However, spring forecast confidence is notably higher when El Niño is transitioning to La Niña, as is now predicted for this spring.

    Charts showing that April Hurricane forecasts have only slightly better skill at predicting hurricane season severity than climatology.
    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 2003-2023, 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 2014-2023 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 in general, although NOAA’s June forecasts were slightly more skillful than TSR’s. (Image credit: Tropical Storm Risk, Inc.)

    In April 2023, CSU predicted a near- to slightly-below-average season — 13 named storms, six hurricanes, two major hurricanes, and an ACE of 100. The actually tally was much higher: 20 named storms, seven hurricanes, three major hurricanes, and an ACE of 146. Their April 2022 forecast also missed the mark, though their April forecasts the previous two years did well.

    However, this year’s April forecast is “is of above-normal confidence for an early April outlook”, the forecasters said, emphasizing that “model guidance is unanimously pointing towards a hyperactive season.”

    The next CSU forecast, due June 1, is worth close attention, as late May/early June forecasts have shown considerable skill over the years. Among other outlooks in the works:

    • NOAA is set to issue its first seasonal hurricane forecast for 2024 in late May.

     

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