NOAA 2024 Atlantic Hurricane Season Outlook Issued on August 8, 2024 and Posted on August 10, 2024

 This is an update from the original Outlook issued in May.

The links lead to graphics that are very technical and IMO not too helpful but the links are HERE,  and HERE (and this pertains to the Eastern Pacific).

This is a good summary of the current Outlook compared to what was issued in May.

The above I believe just covers the Atlantic and is not much changed. They do not define the term ACE which is defined on the Colorado State Website as: “ACE is calculated by summing the square of the maximum sustained winds of each tropical cyclone (in knots) every six hours when the system is classified as either tropical or sub-tropical. The resulting value is then divided by 10,000. Details of the calculation are available on Wikipedia’s website:”  It is generally accepted that the impact of wind increases with the square of the wind speed so this measure is to some extent logical. Notice how this year is expected to compare to the 1991-2020 average which is considered climatology.

I will now present to full NOAA Press Release. Any comments by me will be in a box or I will simply highlight in bold type what I consider to be important or insert my thoughts within brackets [ ].

Some may need to click on “Read More” to access the full article.

“The updated 2024 North Atlantic Hurricane Season Outlook is an official product of the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC). The outlook is produced in collaboration with hurricane experts from NOAA’s National Hurricane Center (NHC) and Atlantic Oceanic and Meteorological Laboratory (AOML). The Atlantic hurricane region includes the North Atlantic Ocean, Caribbean Sea, and Gulf of Mexico.

Interpretation of NOAA’s Atlantic hurricane season outlook:
This outlook is a general guide to the expected overall activity during the upcoming hurricane season. It is not a seasonal hurricane landfall forecast, and it does not predict levels of activity for any particular location.

Preparedness for tropical storm and hurricane landfalls:
Hurricane-related disasters can occur during any season, even for years with low overall activity. It only takes one hurricane (or tropical storm) to cause a disaster. It is crucial that residents, businesses, and government agencies of coastal and near-coastal regions prepare for every hurricane season regardless of this, or any other, seasonal outlook. The Federal Emergency Management Agency (FEMA) through Ready.gov and Listo.gov, the National Hurricane Center, the Small Business Administration, and the American Red Cross all provide important hurricane preparedness information on their web sites.

NOAA does not make seasonal hurricane landfall predictions:
NOAA does not make seasonal hurricane landfall predictions. Hurricane landfalls are largely determined by the weather patterns in place as the hurricane approaches, and those patterns are only predictable when the storm is within several days of making landfall.

Nature of this outlook and the “likely” ranges of activity:
This outlook is probabilistic, meaning the stated “likely” ranges of activity have a certain likelihood of occurring. The seasonal activity is expected to fall within these ranges in 7 out of 10 seasons with similar climate conditions and uncertainties to those expected this year. They do not represent the total possible ranges of activity seen in past similar years.

This outlook is based on analyses of 1) predictions of large-scale climate factors known to influence seasonal hurricane activity, and 2) climate forecast models that directly predict seasonal hurricane activity. The outlook also takes into account uncertainties inherent in such climate outlooks.

Sources of uncertainty in the seasonal outlooks:

1. Predicting El Niño and La Niña events (also called the El Niño-Southern Oscillation, or ENSO) and their impacts on North Atlantic basin hurricane activity, is an ongoing scientific challenge facing scientists today. Such forecasts made during the spring generally have limited skill, but that skill increases during the summer. Specific to this outlook, the major sources of uncertainty are rooted in the uncertainty in the onset and intensity of the predicted La Niña and just how long the record or near-record warm Atlantic SSTs can continue.
2. Many combinations of named storms, hurricanes, and major hurricanes can occur for the same general set of climate conditions. For example, one cannot know with certainty whether a given climate signal will be associated with several shorter-lived storms or fewer longer-lived storms with greater intensity.
3. Model predictions of various factors known to influence seasonal hurricane activity in the Atlantic region, such as sea surface temperatures (SSTs), vertical wind shear, moisture, and atmospheric stability are probabilistic and show some spread for August-October (ASO), the peak months of the hurricane season, ​​so it is unclear as to exactly how conducive these conditions will be for tropical cyclone development.
4. Shorter-term weather patterns that are unpredictable on seasonal time scales can sometimes develop and last for weeks or months, possibly affecting seasonal hurricane activity.

2024 North Atlantic Hurricane Season Outlook Summary

a. Predicted Activity

NOAA’s updated outlook for the 2024 Atlantic Hurricane Season indicates that an above-normal season is most likely, with substantially lower odds for a near- or below-normal season. The outlook calls for a 90% chance of above-normal activity, along with a 10% chance for near-normal activity, and neglible odds for below-normal activity. See NOAA definitions of above-, near-, and below-normal seasons. The Atlantic hurricane region includes the North Atlantic Ocean, Caribbean Sea, and Gulf of Mexico.

The updated 2024 outlook calls for a 70% probability for each of the following ranges of activity during the 2024 hurricane season, which officially runs from June 1st through November 30th:

• 17-24 Named Storms, which includes the 4 names storms recorded thus far.
• 8-13 Hurricanes, which includes the 2 hurricanes recorded thus far.
• 4-7 Major Hurricanes, which includes the 1 major hurricane recorded thus far.
• Accumulated Cyclone Energy (ACE) range of 165%-245% of the median, which includes the approximately 40% of median ACE recorded thus far.

The seasonal activity is expected to fall within these ranges in 70% of seasons with similar climate conditions and uncertainties to those expected this year. These ranges do not represent the total possible ranges of activity seen in past similar years. These expected ranges are above the 1991-2020 seasonal averages of 14 named storms, 7 hurricanes, and 3 major hurricanes. Most of the predicted activity is likely to occur during the peak months (August-October, ASO) of the hurricane season.

This August update is almost identical to the May outlook (17-25 named storms, 8-13 hurricanes, 4-7 major hurricanes, and 150-245% ACE). The update has a slight increase in the probability of above-normal activity (increased to 90% from 85%), no change in the odds for a near-normal season, and a decrease in the odds for a below-normal season (from 5% to near zero). Also, the lower end of the predicted ACE range has also been shifted higher (up to 165% from 150%) since the Atlantic Basin has already accumulated 39 units of ACE (approximately 40% of normal for an entire season). This update’s high probability for above-average activity and the high range for ACE are similar to the August updates for 2005 and 2010, both of which produced extremely high levels of seasonal activity, though 2010 was not among the most active years on record.

b. Reasoning behind the outlook

This updated 2024 seasonal hurricane outlook reflects the expectation of complementary/reinforcing large-scale climate factors during ASO. This combination of factors has been associated with high to very high levels of activity. The main climate factors for this outlook are:

1. The set of conditions that have produced the ongoing high-activity era for Atlantic hurricanes which began in 1995 are likely to continue in 2024. These conditions linked to the high-activity era typically include warmer SSTs, weaker trade winds, and with weaker 200-850 hPa vertical wind shear in the Atlantic hurricane Main Development Region (MDR), along with an enhanced West African monsoon. The oceanic component of these conditions is often referred to as the Atlantic Multidecadal Oscillation (AMO), while the ocean/atmosphere combined system is sometimes referred to as Atlantic Multidecadal Variability (AMV) or the Tropical Multidecadal Mode (TMM). The MDR spans the tropical North Atlantic Ocean and Caribbean Sea, and conditions there are highly correlated with overall seasonal activity. SSTs have been near record when averaged over the MDR and 200-850 hPa vertical wind shear has been near record low. The West African Monsoon rains have been above-normal through June and July 2024, supported by enhanced low-level inflow but with the associated upper-level wind pattern closer to normal.
2. The most recent forecast from the NOAA Climate Prediction Center indicates about equal odds for ENSO-Neutral and La Niña conditions for the peak of the hurricane season. The odds for La Niña to develop during ASO are now 49%, increasing to 66% during SON. A recent pause in the cooling has likely delayed the onset of La Niña during Autumn 2024. During a high-activity era, ENSO-neutral is typically associated with above-average levels of activity. A La Niña event would tend to reinforce those high-activity era conditions by reducing vertical wind shear over the MDR and therefore further increasing the likelihood of an above-normal season with activity near the upper ends of the predicted ranges.

DISCUSSION

1. Forecast 2024 activity

NOAA’s updated outlook for the 2024 Atlantic Hurricane Season indicates that an above-normal season is most likely (90% chance). The outlook also includes a 10% chance of a near-normal season, and a negligible chance of a below-normal season.

The 2024 North Atlantic hurricane season is predicted to produce (with 70% probability for each range) 17-24 named storms, of which 8-13 are expected to become hurricanes, and 4-7 of those are expected to become major hurricanes. These ranges are above the 1991-2020 period averages of about 14 named storms, 7 hurricanes, and 3 major hurricanes.

There is a high likelihood that the 2024 North Atlantic hurricane season will be another active year in the current high-activity era. Since the current Atlantic high-activity era began in 1995, 20 of 29 (about 70%) seasons have had above-normal activity, and only 5 (17%) and 4 (14%) have had near- and below-normal activity, respectively, based on the 1951-2020 climatology. Also, 9 (almost half) of the above-normal years (thus 31% of the 29 years) have been hyper-active (a.k.a. “extremely active”; % median ACE ≥ 165%).

An important measure of the total seasonal activity is NOAA’s Accumulated Cyclone Energy (ACE) index, which accounts for the combined intensity and duration of all named storms and hurricanes during the year. This 2024 outlook indicates a 70% chance that the seasonal ACE range will be 165-245% of the median. According to NOAA’s hurricane season classifications, an ACE value between 75.4% and 130% of the 1951-2020 median reflects a near-normal season. Values above (below) this range reflect an above- (below-) normal season. The 2024 predicted ACE range is centered in the above-normal range, and all of this ACE range is above the hyper-active threshold, with a mean of 205%, which is the third highest predicted mean ACE for the August updates, behind 2005 and 2010. This does not imply that 2024 will have record-setting levels of activity, but simply that this update has a very high confidence that this year will be hyper-active.

Predictions of the location, number, timing, and intensity of hurricane landfalls are ultimately related to the daily weather patterns which determine storm genesis locations and steering patterns. These patterns are not predictable weeks or months in advance. As a result, it is not possible to reliably predict the number or intensity of landfalling hurricanes in a seasonal outlook, or whether a given locality will be impacted by a tropical storm or hurricane this season.

2. Science behind the Outlook

NOAA’s North Atlantic Hurricane Season Outlooks are based on predictions of the main climate factors and their associated conditions known to influence seasonal Atlantic hurricane activity. These predictions are based on extensive monitoring, analysis, research activities, a suite of statistical prediction tools, and dynamical models. The dynamical model predictions come from the NOAA Climate Forecast System (CFS), NOAA Geophysical Fluid Dynamics Lab (GFDL) HiFLOR-S and SPEAR-MED models, the North American Multi-Model Ensemble (NMME), the United Kingdom Met Office (UKMET) GloSea6 model, and the European Centre for Medium-Range Weather Forecasting (ECMWF) Seas5 model. ENSO forecasts are also provided from the NMME dynamical models contained in the suite of Niño 3.4 SST forecasts, which is compiled by NOAA’s CPC. NOAA’s AOML continues to contribute and refine a statistical-dynamical hybrid forecast system, based on SSTs in the NMME.

NOAA’s updated 2024 North Atlantic hurricane season outlook reflects the expectation of complementary/reinforcing climate factors during August-October (ASO), as follows:

1. The main climate factor that could act to enhance Atlantic hurricane activity is the current state of the Atlantic Ocean and the associated atmospheric circulation. The expected continuation of the high-activity era for Atlantic hurricanes, which began in 1995 in association with a transition to the warm phase of the AMO. The recently observed and predicted atmospheric conditions for ASO 2024 generally reflect the warm AMO phase, with several factors conducive for higher levels of activity such as weaker trade winds, near record SSTs and weaker vertical wind shear across much of the MDR. SSTs in the Atlantic MDR, as assessed by the ERSSTv5, established a new record for warmth in June. The trade winds over the Atlantic were below average for most of June and July. Vertical wind shear has been quite low across the MDR, and the forecasts are for weaker than normal 200-850 hPa wind shear to continue through the peak of the Atlantic Hurricane season.
2. Additionally, the forecast for the development of a La Niña event could reinforce some of these local conditions, especially below-average vertical wind shear. The most recent NOAA ENSO probability forecast indicates about even odds for La Niña or ENSO-neutral during ASO and a 66% chance that La Niña conditions emerge during SON. The odds for a strong La Niña have been reduced in recent months, but even a weak La Niña could enhance hurricane activity. When considering the Niño 3.4 region versus the global tropics (20°N – 20°S), the east-central Pacific is cooler than normal, which could be a sign that teleconnections from that region may be stronger than implied in many of the models and tools based on the traditional Niño 3.4.
3. The West African Monsoon, which is positively correlated with Atlantic tropical cyclone activity, is providing supportive signals for a more active Atlantic hurricane season. Typically the conditions associated with a warm AMV are coincident with an enhanced West African monsoon circulation, and the June and July rainfall across the Sahel was well above normal. The upper-level outflow is closer to normal with the low-level inflow enhanced.
4. Enhanced June-July activity in the deep tropical Atlantic reinforces the expectation for an above-normal season. During June and July, there were a total of 3 named storms in the North Atlantic hurricane basin, including a major hurricane (Beryl) in July for a sum total of 37% of median ACE. Hurricane Beryl formed in the deep tropical Atlantic. If only the years with named storms developing pre-ASO in the tropics, specifically the southeast portion of Atlantic hurricane basin (~ 9-22°N, 15-77°W) are taken into account, that activity has been shown to be highly correlated with ASO and overall-seasonal activity since the early formation is an indicator that, when the peak months come, the MDR will be conducive to more development. In the current high-activity era, which began in 1995, of the 11 years with at least one named storm developing pre-ASO in that eastern MDR region, all but one (2013) have had above-normal overall activity and 7 of those years (64%) have had hyper-active levels of activity (i.e., % median ACE ≥ 165%). However, almost half of the years since 1995 with above-normal activity and even several with hyper-active levels of activity did not have a pre-ASO storm develop in the eastern MDR.

a. Predicted conditions within the Main Development Region (MDR)

SSTs are currently well above-average across nearly all of the MDR, with an area-averaged anomaly during June of +1.31°C, compared to +1.23°C during June of last year. These record warm SSTs could complement and reinforce the likely impacts from the predicted ENSO state. For the MDR as a whole, both the CFS and NMME models predict above-average SSTs during ASO. The positive difference between MDR SSTs and the global tropics is another predictor favoring an above-normal season, and has been linked to some hyper-active years.

Two inter-related atmospheric features that are typically analyzed and are also related to the warm phase of the AMO/AMV, are anomalous low-level winds across the central and eastern tropical Atlantic and the strength of the West African monsoon system. The 850 hPa winds show anomalously strong inflow into the West African Monsoon, along with weak trade winds over the tropical Atlantic. Weaker trade winds generally contribute to below-average vertical wind shear which leads to heightened activity. Sahel precipitation resulting from the monsoonal circulations has been above normal despite the circulation pattern showing upper-level winds as near normal.

The CFS and the NMME predict below-normal vertical wind shear over the MDR. This shear predicted for much of the Atlantic basin by the NMME this year is further below climatology than the lower shear that was predicted last year. The predicted shear for 2024 is among the lowest values in the historical record of the NMME. The 200-850 hPa vertical wind shear patterns resemble those associated with La Niña conditions, showing high wind shear over the East Pacific and low wind shear over the Caribbean, Gulf of Mexico, and much of the MDR. Below-average shear is always associated with higher levels of Atlantic hurricane activity.

Overall, the conditions local to the Atlantic MDR are conducive for above-average levels of tropical cyclone development. These interrelated conditions include 1) anomalously warm SSTs and decreased vertical wind shear in the MDR, 2) an African Easterly Jet structure closer to climatological position and amplitude that allows for some low-pressure cloud systems (i.e., African easterly waves) to develop, and 3) the combination of increased moisture and decreased atmospheric stability. Because of these conditions, the enhanced African easterly waves can potentially develop more easily into tropical storms and hurricanes.

b. La Niña expected to develop

La Niña represents one phase of the climate phenomenon known as ENSO (El Niño-Southern Oscillation). The three phases of ENSO are El Niño, La Niña, and ENSO-neutral. El Niño tends to suppress Atlantic hurricane activity, while La Niña and ENSO-neutral tend to enhance it. These impacts can be strongly modulated by conditions associated with a low- or high-activity era, and also by short-lived conditions during any specific year. As of Aug 8, 2024, ENSO-neutral conditions are present and a La Niña watch remained ongoing. The weekly SSTs are currently near average across much of the central and eastern equatorial Pacific and the SST index for the Niño 3.4 region is -0.2 °C. The Niño 3.4 index has shown a significant cooling trend since December 2023 and the weekly Niño 3.4 index has decreased from +1.8 °C in February of 2023 to its current value of -0.2 °C . The outgoing longwave radiation pattern over the central Pacific is reflective of ENSO-neutral with the wind anomaly pattern beginning to reflect La Niña conditions. The traditional Oceanic Niño Index (ONI) is still showing ENSO-neutral values, but a relative ONI that removes the average SSTs in the global tropics is much closer to the La Niña threshold at -0.4°C. The removal of the global tropic values is also done when evaluating the MDR SSTs, so this measure could add some consistency. The area of enhanced low-level easterly anomalies over the eastern equatorial Pacific aligns with an emerging La Niña event while the upper-level anomalous wind field shows a less coherent pattern.

Looking forward, NOAA’s Climate Forecast System (CFS) and North American Multi-model Ensemble (NMME) model-predicted SST anomalies in the Niño 3.4 region generally indicate La Niña conditions (Niño 3.4 index less than -0.5 °C) developing for the peak of hurricane season (ASO). The dynamical model average (dashed black line) indicates La Niña conditions developing and continuing through the remainder of 2024. When using a larger pool of models that includes multiple dynamical models, many statistical models, and unique combinations of those models, many of those models (especially the empirical models) indicated ENSO-neutral conditions through the period, ​​so there is still some uncertainty as to exactly what conditions will be present during ASO.

The official NOAA ENSO Outlook from early August indicates about even odds for the the ENSO-neutral to transition to La Niña during ASO. That is lower than the odds for the development of La Niña that coincided with the May seasonal hurricane outlook. So while La Niña is anticipated to develop, there is uncertainty whether it will become established concurrent with ASO to align with the peak of the Atlantic Hurricane Season to enhance the local signals (warm SSTs, weak tradewinds, active West African Monsoon). Many active seasons have occurred during ENSO-neutral conditions.

c. Factors contributing to the uncertainty

Many combinations of named storms, hurricanes, and major hurricanes can occur for the same general set of climate conditions. For example, one cannot know with certainty whether a given climate signal will be associated with several shorter-lived storms or fewer longer-lived storms with greater intensity. A hyper-active year can be a year with only 6-7 hurricanes with long tracks or many more hurricanes that have shorter lifespans. If many of the storms develop in the low shear areas in the Caribbean and Gulf of Mexico, interference from terrain could limit the duration and maximum intensity of each system. That uncertainty is reflected in the ACE range, which is still smaller than the expected range from observations alone. Additionally, if many of the storms form in the western half of the Atlantic Basin, the track could come close to land, limiting these storms’ ability to realize their maximum intensity.

The main climate-related uncertainty in this seasonal hurricane outlook is the timing and intensity of the predicted La Niña. During the initial release, there was a 77% chance of La Niña developing during ASO, which has now dropped to 49% as the cooling of the east-central tropical Pacific slowed during June and July. Since the predictions for the state of ENSO are more certain during the late summer months, the later emergence of La Niña is likely a more reliable signal than what was indicated in the earlier 2024 outlooks. If La Niña develops and has intensifying related impacts (reduced vertical wind shear and favorable vertical motions) through the latter portions of the hurricane season, the season’s activity will likely be near the upper ranges of this updated outlook.

NOAA FORECASTERS

Climate Prediction Center

• Matt Rosencrans, Physical Scientist; Matthew.Rosencrans{at}noaa.gov
• Dr. Hui Wang, Physical Scientist; Hui.Wang{at}noaa.gov
• Dr. Daniel Harnos, Meteorologist; Daniel.Harnos{at}noaa.gov

National Hurricane Center

• Eric Blake, Senior Hurricane Specialist; Eric.S.Blake{at}noaa.gov
• Dr. Chris Landsea, Branch Chief; Chris.Landsea{at}noaa.gov
• Dr. Richard Pasch, Senior Hurricane Specialist; Richard.J.Pasch{at}noaa.gov

Atlantic Oceanographic and Meteorological Laboratory

• Stanley Goldenberg, Meteorologist; Stanley.Goldenberg{at}noaa.gov
• Dr. Hosmay Lopez, Oceanographer; Hosmay.Lopez{at}noaa.gov

REFERENCES

• Bell, G. D., and M. Chelliah, 2006: Leading tropical modes associated with interannual and multi-decadal fluctuations in North Atlantic hurricane activity. J. Climate, 19, 590-612.
• Blake, E. S., P. Klotzbach, and G. D. Bell, 2018: Climate factors causing the extremely active 2017 Atlantic hurricane season. Presented at AMS 33rd Conference on Hurricanes and Tropical Meteorology, April 2018.
• Goldenberg, S. B., C. W. Landsea, A. M. Mestas-Nuñez, and W. M. Gray, 2001: The recent increase in Atlantic hurricane activity: Causes and implications. Science, 293, 474-479.
• Goldenberg, S. B. and L. J. Shapiro, 1996: Physical mechanisms for the association of El Niño and West African rainfall with Atlantic major hurricane activity. J. Climate, 9, 1169-1187.
• Gray, W. M., 1984: Atlantic seasonal hurricane frequency: Part I: El Niño and 30-mb quasi-bienniel oscillation influences. Mon. Wea. Rev., 112, 1649-1668.
• Klotzbach, P.J., and W. M. Gray, 2008: Multi-decadal Variability in North Atlantic Tropical Cyclone Activity. J. Climate, 21, 3929 – 3935.
• L’Heureux, M. L., and Coauthors, 2019: Strength Outlooks for the El NiñoSouthern Oscillation. Wea. Forecasting, 34, 165175, https://doi.org/10.1175/WAF-D-18-0126.1.
• West, R., Lopez, H., Lee, S. K., Mercer, A. E., Kim, D., Foltz, G. R., & Balaguru, K. (2022). Seasonality of interbasin SST contributions to Atlantic tropical cyclone activity. Geophysical Research Letters, 49(4), e2021GL096712.

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I hope you found this article interesting and useful.

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