FXNT20 KWNC 081618 PMDAHU Atlantic Hurricane Season Outlook 2024 NWS Climate Prediction Center College Park MD 1100 AM EDT Thu 08 Aug 2024 Note: figures mentioned in the discussion are available on the internet at http://www.cpc.ncep.noaa.gov NOAA 2024 North Atlantic Hurricane Season Outlook Issued 8 August 2024 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 NOAAs 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 NOAAs North 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 www.ready.gov and www.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 Nino and La Nina events /also called the El Nino-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 Nina 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 NOAAs updated outlook for the 2024 North Atlantic Hurricane Season indicates that above-normal activity is most likely, with substantially lower odds for a near- or below-normal season. The outlook calls for a 90 percent chance of above-normal activity, along with a 10 percent chance for near-normal activity and negligible odds for below-normal activity. See NOAA definitions /https://www.cpc.ncep.noaa.gov/products/outlooks/Background.html/ of above-, near-, and below-normal seasons. The Atlantic hurricane basin includes the North Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. The updated 2024 outlook calls for a 70 percent probability for each of the following ranges of activity: - 17-24 named storms, which includes the 4 named 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 percent-245 percent of the median, which includes the ACE of approximately 40 percent recorded thus far. The seasonal activity is expected to fall within these ranges in 70 percent 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 percent ACE/. The update has a slight increase in the probability of above-normal activity /increased to 90 percent from 85 percent/, no change in the odds for a near-normal season, and a decrease in the odds for a below-normal season /from 5 percent to near zero/. Also, the lower end of the predicted ACE range has been shifted higher /up to 165 percent from 150 percent/ since the Atlantic Basin has already accumulated approximately 39 units of ACE /approximately 40 percent of normal for an entire season/. This updates 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 /https://www.cpc.ncep.noaa.gov/products/hurricane/diagnostics /animation/shear_200-850_atl_31d_45.gif/ has been near record low. The West African Monsoon rains have been above-normal through June 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 Nina conditions for the peak of the hurricane season. The odds for La Nina to develop during ASO are now 49 percent, increasing to 66 percent during SON. A recent pause in the cooling has likely delayed the onset of La Nina during autumn 2024. During a high-activity era, ENSO-neutral is typically associated with above- average levels of activity. A La Nina 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 NOAAs outlook for the 2024 North Atlantic Hurricane Season indicates that above-normal activity is most likely /90 percent chance/. The outlook also includes a 10 percent chance of near-normal activity, and a negligible chance of below-normal activity. The 2024 North Atlantic hurricane season is predicted to produce /with 70 percent 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 the2024 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 percent/ seasons have had above-normal activity, and only 5 /17 percent/ and 4 /14 percent/ 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 percent of the 29 years/ have been hyper-active /a.k.a. "extremely active"; percent median ACE ? 165 percent/. An important measure of the total seasonal activity is NOAAs 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 percent chance that the seasonal ACE range will be 165-245 percent of the median. According to NOAAs hurricane season classifications, an ACE value between 75.4 percent and 130 percent 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 percent, 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 NOAAs 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 Nino 3.4 SST forecasts , which is compiled by NOAAs CPC. NOAAs AOML continues to contribute and refine a statistical-dynamical hybrid forecast system, based on SSTs in the NMME. NOAAs 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 ERSST, established a new record for warmth in June. The trade winds over the Atlantic /Fig 5, right/ 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 Nina 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 Nina or ENSO-neutral during ASO , and a 66 percent chance that La Nina conditions emerge during SON. The odds for a strong La Nina have been reduced in recent months, but even a weak La Nina could enhance hurricane activity. When considering the Nino 3.4 region versus the global tropics /20?N - 20?S/ /https://www.cpc.ncep.noaa.gov/data/indices/RONI.ascii.txt/, 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 Nino 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 /https://www.cpc.ncep.noaa.gov/products/international/africa_rfe/africa_rfe_Jul2024-Jul2024_wa_pnorm.gif/ 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. There have already been 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 percent 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 percent/ have had hyper-active levels of activity /i.e., percent median ACE ? 165 percent/. 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 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 /Fig 11/. 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 Nina 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 Nina expected to develop La Nina represents one phase of the climate phenomenon known as ENSO /El Nino-Southern Oscillation/. The three phases of ENSO are El Nino, La Nina, and ENSO-neutral. El Nino tends to suppress Atlantic hurricane activity, while La Nina 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 Nina watch remain ongoing. The weekly SSTs are currently near average across much of the central and eastern equatorial Pacific /Fig 10/ and the SST index for the Nino 3.4 region is -0.2 ?C. The Nino 3.4 index has shown a significant cooling trend since December 2023 and the weekly Nino 3.4 index has decreased from +1.8 ?C in February of 2023 to its current value of -0.2 ?C /Fig 10/. The outgoing longwave radiation pattern over the central Pacific is reflective of ENSO-neutral with the wind anomaly pattern beginning to reflect La Nina conditions. The traditional Oceanic Nino 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 Nina 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 Nina event while the upper-level anomalous wind field shows a less coherent pattern. Looking forward, NOAAs Climate Forecast System /CFS/ and North American Multi-model Ensemble /NMME/ model-predicted SST anomalies in the Nino 3.4 region generally indicate La Nina conditions /Nino 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 Nina 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 Nina during ASO. That is lower than the odds for the development of La Nina that coincided with the May seasonal hurricane outlook. So while La Nina 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 increasing 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 Nina. During the initial release, there was a 77 percent chance of La Nina developing during ASO, which has now dropped to 49 percent 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 Nina is likely a more reliable signal than what was indicated in the earlier 2024 outlooks. If La Nina develops and has intensifying related impacts /reduced vertical wind shear and favorable vertical motions/ through the latter portions of the hurricane season, the seasons activity will likely be near the upper ranges of this updated outlook. NOAA FORECASTERS Climate Prediction Center Matt Rosencrans, Physical Scientist; Matthew.Rosencrans@noaa.gov Dr. Hui Wang, Physical Scientist; Hui.Wang@noaa.gov Dr. Daniel Harnos, Meteorologist, Daniel.Harnos@noaa.gov National Hurricane Center Eric Blake, Senior Hurricane Specialist; Eric.S.Blake@noaa.gov Dr. Chris Landsea, Meteorologist; Chris.Landsea@noaa.gov Dr. Richard Pasch, Senior Hurricane Specialist; Richard.J.Pasch@noaa.gov Atlantic Oceanographic and Meteorological Laboratory Stanley Goldenberg, Meteorologist; Stanley.Goldenberg@noaa.gov Dr. Hosmay Lopez, Oceanographer; Hosmay.Lopez@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. 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