FXPN30 KWNC 231440 PMDEPH Eastern North Pacific Hurricane Season Outlook 2024 NWS Climate Prediction Center College Park MD 1100 AM EDT Thu 23 May 2024 Note: figures mentioned in the discussion are available on the internet at http://www.cpc.ncep.noaa.gov NOAA 2024 Eastern Pacific Hurricane Season Outlook Issued 23 May 2024 This 2024 eastern Pacific Hurricane Season Outlook is an official product of the National Oceanic and Atmospheric Administration /NOAA/ Climate Prediction Center /CPC/, and is produced in collaboration with hurricane experts from NOAAs National Hurricane Center /NHC/ and the Atlantic Oceanic and Meteorological Laboratorys /AOML/ Hurricane Research Division /HRD/. The eastern Pacific hurricane region covers the eastern North Pacific Ocean east of 140deg W and north of the equator. Interpretation of NOAAs eastern Pacific 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 imply levels of activity for any particular location. Preparedness 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 NHC, 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, which are 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 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 climate model forecasts, and on predictions of large-scale climate factors and conditions that are known to strongly influence seasonal eastern Pacific hurricane activity. The outlook also takes into account uncertainties inherent in such climate outlooks. Sources of uncertainty in this seasonal outlook 1. Predicting El Nino and La Nina events /also called El Nino-Southern Oscillation, ENSO/ and specific impacts on eastern Pacific hurricane activity is an ongoing scientific challenge facing climate scientists today. Such forecasts made during the spring generally have more uncertainty than those made closer to the peak of hurricane season /July-September: JAS/. 2. Uncertainty as to whether the eastern Pacific has shifted to a long-term low-activity era, or that the recent quiescent period will be short lived and there will be a return to the high activity era that lasted from 2014-2019. 3. Predicting the combined impacts associated with the ENSO, Pacific Decadal Oscillation /PDO/, and the Atlantic Multidecadal Oscillation /AMO/ remains a challenge, especially when they have different temporal variability that sometimes results in competing influences. 4. 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 short-lived storms or fewer longer-lived storms with greater intensity. 5. Shorter-term weather patterns that are unpredictable on seasonal time scales can sometimes develop and last for weeks or months within a season, possibly affecting seasonal hurricane activity. 2024 Eastern Pacific Hurricane Outlook Summary a. Predicted Activity NOAAs 2024 eastern Pacific Hurricane Season Outlook indicates a below-normal season is most likely /60 percent chance/. There is a 30 percent chance of a near-normal season and only a 10 percent chance of an above-normal season. See NOAA definitions /https://www.cpc.ncep.noaa.gov/products/Epac_hurr/Background.html/ of above-, near-, and below-normal seasons. The eastern Pacific hurricane region covers the eastern North Pacific Ocean east of 140deg W and north of the equator. The 2024 outlook calls for a 70 percent probability for each of the following ranges of activity: ? 11-17 Named Storms ? 4-9 Hurricanes ? 1-4 Major Hurricanes ? Accumulated Cyclone Energy /ACE/ range of 50 percent-110 percent of the median. The 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 activity seen in past similar years. The predicted ranges are centered below the 1991-2020 averages of 15 named storms, 8 hurricanes, and 4 major hurricanes. The eastern Pacific hurricane season officially runs from May 15th through November 30th. The peak months of the season are July-September /JAS/. There will be no further updates to this outlook. b. Reasoning behind the outlook Two climate factors are expected to contribute to a below-normal 2024 hurricane season across the eastern Pacific hurricane basins, as follows: 1/ The latest monthly sea-surface temperature /SST/ anomalies reflect El Nino conditions, on the path to the predicted La Nina, and a negative PDO structure, while also showing the North Atlantic SSTs as above-normal for much of the basin. SST anomalies in the regions where many of the tropical storms and hurricanes form /110deg W - 140deg W/ are near normal. The predicted SST anomaly patterns indicate near or below-average SSTs across the eastern Pacific hurricane region, though there is some evidence of influence from the negative PDO related circulation pushing cooler waters southward. The cooler waters are expected to occur mainly east of 120deg W. SSTs across the Atlantic Main Development Region /MDR/ are above normal, with near record warmth. Historically, this combination tends to be associated with below-normal hurricane activity in the eastern Pacific, as La Nina does tend to have a larger influence than either the Pacific Decadal Oscillation /PDO/ or Atlantic conditions. La Nina conditions, concurrent with a negative PDO, are common and found to reinforce a negative PDO. 2/ The most recent forecast from the NOAA Climate Prediction Center indicates La Nina conditions are likely through the hurricane season. The ENSO influence on eastern Pacific hurricane activity is highly dependent upon the background SST patterns across the eastern Pacific hurricane region and the Atlantic MDR. The combination of La Nina and above-normal temperatures in the Atlantic MDR tends to favor decreased eastern Pacific hurricane activity, often resulting in a below-normal hurricane season. DISCUSSION 1. Expected 2024 activity NOAAs 2024 eastern Pacific Hurricane Season Outlook indicates a below-normal season is most likely /60 percent chance/. There is a 30 percent chance of a near-normal season and only a 10 percent chance of an above-normal season. The 2024 eastern Pacific hurricane season is predicted to produce /with a 70 percent probability for each range/ 11-17 named storms, of which 4-9 are expected to become hurricanes, and 1-4 of those are expected to become major hurricanes. These ranges are centered below the 1991-2020'seasonal averages of 15 named storms, 8 hurricanes, and 4 major hurricanes. An important measure of the total seasonal activity is NOAAs Accumulated Cyclone Energy /ACE/ index, which accounts for the combined intensity and duration of named storms and hurricanes during the season. This 2024 outlook indicates a 70 percent chance that the ACE range will be 50 percent-110 percent of the median. An ACE value of 80 percent-120 percent of the median indicates a near-normal season. Values above this range reflect an above-normal season, and values below this range reflect a below-normal season. 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 currently not possible to reliably predict the number or intensity of landfalling hurricanes at these extended ranges, or whether a given locality will be impacted by a tropical storm or hurricane this season. 2. Science behind the Outlook NOAAs eastern Pacific Hurricane Season Outlook is based on predictions of the main climate factors and their associated relationships to the hurricane season, as well as direct output from numerical models. The outlook is 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 and SPEAR-MED modes, 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 2024 eastern Pacific Hurricane Season Outlook reflects the following climate factors: 1/ The predicted SST anomaly patterns across the eastern Pacific hurricane region indicate near to below-average SSTs and above-average SSTs across the Atlantic MDR. In the region where most tropical cyclones form in the eastern Pacific, SSTs are currently near normal. For the Pacific, these conditions also project onto the negative phase of the Pacific Decadal Oscillation /PDO/ and also onto the warm /positive/ phase of the Atlantic Multidecadal Oscillation /AMO/ and the Atlantic Meridional Mode /AMM/ . Historically, this combination of climate patterns tends to be associated with below-normal activity in the eastern Pacific hurricane region. The exact interplay and net result of the interbasin relationships is uncertain and still a focus of ongoing research, and that uncertainty is reflected in the relatively moderate probabilities in this outlook. 2/ The most recent forecast from the NOAA Climate Prediction Center indicates La Nina conditions are likely through the hurricane season. The odds are highest for La Nina /69 percent/, with a smaller probability for ENSO-neutral /30 percent/, and a negligible chance of an El Nino event /1 percent/. Historically, La Nina events usually mean less tropical storm/hurricane activity for the East Pacific, and often for the Central Pacific. During low-activity eras in the East Pacific, typically associated with high activity eras in the Atlantic /which we are in now/, La Nina conditions can produce about as many storms as El Nino. a. La Nina favored An El Nino advisory is in place and a La Nina watch has been issued. The weekly SSTs are currently slightly above average across much of the central and eastern equatorial Pacific and the SST index for the Nino3.4 region is +0.5deg 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 +1.9deg C in December of 2023 to its current value of +0.5deg C /Figs 6 and 9/. The wind and outgoing longwave radiation patterns over the central Pacific are also reflecting a breakdown of atmospheric response to last years El Nino forcings, and are currently showing more influence from intraseasonal modes. Looking forward, model-predicted SST anomalies in the Nino 3.4 region generally indicate La Nina /Nino 3.4 index less than -0.5deg C/ conditions throughout the hurricane season. The dynamical model average /dashed black line/ indicates La Nina through the late summer and autumn of 2024, with only one modeling system indicating ENSO-neutral conditions, but still showing negative anomalies. When using a larger pool of models that includes multiple dynamical models, multiple statistical models, and unique combinations of those models, about 30 percent of the 26 models indicate La Nina conditions, with 4 results showing positive Nino 3.4 values. NOAAs Climate Forecast System /CFS/ and the North American Multi-Model Ensemble /NMME/ are predicting La Nina. The CFS and NMME predict above-normal vertical wind shear over the eastern Pacific. The shear predicted by the NMME as used in the hybrid model is positive, indicating above-normal shear, whereas 2022 outlook was for below-normal shear. b. Eastern North Pacific high- and low-activity eras In addition to year-to-year fluctuations, eastern Pacific hurricane activity exhibits strong variability on decadal and multi-decadal time scales. Periods of decreased activity /such as 1971-1981 and 1995-2013/ are called low-activity eras, and periods of increased activity /such as 1982-1994 and 2014-2019/ are called high-activity eras, though its not clear if the lull in 2019 and 2020 were indeed an end to a high-activity era, or simply a pause, given that the 2023 season was above normal and 2022 was near the threshold for an above-normal season. These are different from the high- and low-activity eras in the Atlantic hurricane region. The differences in seasonal activity between these two eras for the eastern Pacific are considerable. High-activity eras average about 4.5 more named storms, 2.8 more hurricanes, 2.3 more major hurricanes, and 56 percent more ACE, than low-activity eras. During high-activity eras, above-normal seasons occur about three times more frequently /63 percent of seasons compared to 20 percent/, and below-normal seasons are about four times less frequent /11 percent compared to 43 percent/. High- and low-activity eras in the eastern Pacific hurricane region are related to global patterns of SST anomalies that change slowly and last for many years. It is upon these patterns that the inter-annual ENSO signal overlays. One such pattern is called the Pacific Decadal Oscillation /PDO/. The PDO spans most of the North Pacific Ocean, and is associated with decadal fluctuations in hurricane activity. The positive /negative/ phase of the PDO tends to be associated with high- /low-/ activity eras. Another SST pattern is the Atlantic Multi-decadal Oscillation /AMO/, and when linked to wind patterns can be more broadly described as Atlantic Multidecadal Variability /AMV/, measured through the Atlantic Meridional Mode /AMM/. The cold /warm/ phase of the AMO increases the likelihood of a high- /low-/ activity era. The AMO helps to explain the inverse relationship in activity between the eastern Pacific and Atlantic basins, with a warm AMO/positive AMM favoring increased Atlantic activity and decreased eastern Pacific activity. The AMO is positive this year with warm SSTs in the North Atlantic. The 1982-1994 high-activity era was associated with a cold AMO and a positive PDO, while the period from 2014-2020 exhibited higher activity and primarily featured a strong positive PDO, there is uncertainty about the classification of the activity regime in the Pacific. Such a short period of years would not define an activity era. The relative lull in activity during 2020 and 2021 could just be more consistent with repeat La Nina events overriding the multi-year signals, which could be the case in 2023. The intervening 1995-2013 low-activity era featured a warm AMO and negative PDO. Of the years when the August PDO was negative, approximately 70 percent of those years were near normal or below normal for activity /17 of 23 since 1971/. The current value of the PDO is -1.03, and the SST patterns from this spring are similar to many low-activity years. The SST patterns in 2022 and 2023 were also similar, but the eastern Pacific experienced near-normal activity and above-normal activity, respectively, during those years. That variation shows that interannual variability can override the decadal signals. There is high confidence that the current negative PDO pattern will persist through the hurricane season. One reason is that the current negative PDO signal partly reflects the synoptic-scale wind and pressure patterns during the past months, influenced by El Nino, which is forecast to completely fade. ENSO changes typically lead PDO changes by months to seasons, with studies showing ENSO leading the PDO by 6 to 24 months. In addition, JAS Nino 3.4 values and JAS PDO values are correlated at about 0.50, and the predictions for Nino 3.4 are well below zero /i.e., La Nina/ in many models. The official ENSO outlook probabilities indicate the relatively high likelihood of La Nina during a low-activity era. During La Nina years in low-activity eras, tropical cyclone activity in the eastern Pacific exhibits a wide range of outcomes. The current outlook, to account for uncertainty in the activity era classification, includes a 10 percent chance for an above-normal season. 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 Atlantic Oceanographic and Meteorological Laboratory Stanley Goldenberg, Meteorologist; Stanley.Goldenberg@noaa.gov BACKGROUND INFORMATION: EASTERN PACIFIC HURRICANE SEASON The eastern Pacific hurricane region covers the eastern Pacific Ocean east of 140deg W north of the equator. This area is one of the most prolific tropical storm formation regions in the world. Eastern Pacific storms most often track westward over open waters, sometimes moving into the central and even western Pacific hurricane regions reaching Hawaii and beyond. However, some storms occasionally head toward the northeast, bringing rainfall to the arid southwestern United States during the late summer months. Some storms can also impact western Mexico or Central America, especially early and late in the season. The official eastern Pacific hurricane season runs from 15 May through 30 November. The peak activity typically occurs during July through September. During the period 1991-2020, the eastern Pacific seasonal averages were 15.4 named storms /maximum 1-minute sustained 10 m wind speeds between 39-73 mph/, with 8.3 of those becoming hurricanes /maximum 1-minute sustained 10 m wind speeds of at least 74 mph/ and 4.2 becoming major hurricanes /maximum 1-minute sustained 10 m wind speeds exceeding 111 mph, categories 3-5 on the Saffir-Simpson hurricane scale/. Eastern Pacific hurricane seasons exhibit long periods of above-normal and below-normal activity in response to large-scale climate patterns. Seasons also exhibit year-to-year variability in response to ENSO. El Nino contributes to decreased easterly vertical wind shear and favors above-normal hurricane activity in this region. Historically, El Nino is not associated with below-normal seasons. Conversely, La Nina contributes to increased vertical shear and less overall activity. Historically, 60 percent of La Nina episodes have been associated with below-normal hurricane seasons, and only 28 percent have produced an above-normal season. However, the ENSO impacts can be strongly influenced by the background climate patterns. As a result, NOAA accounts for the combined influences of both climate factors when making its seasonal hurricane outlooks. Measuring overall activity: The Accumulated Cyclone Energy /ACE/ Index The phrase "total seasonal activity" refers to the combined intensity and duration of all eastern Pacific named storms and hurricanes during a given season. The measure of total seasonal activity used by NOAA is called the Accumulated Cyclone Energy /ACE/ index /Fig 5/. The ACE index is a wind energy index, defined as the sum of the squares of the maximum sustained surface wind speed /knots/ measured every six hours for all eastern Pacific named systems while they are at least tropical storm strength. NOAAs eastern Pacific hurricane season classifications Reliable tropical storm and hurricane data for the eastern Pacific began in 1971. The 1991-2020 mean value of the ACE index is 108.7 x 104 kt2, and the median value is 97.2 x 104 kt2. The following season classifications are based on an approximate 3-way partitioning of seasons based on the ACE value, combined with the seasonal number of tropical storms, hurricanes and major hurricanes. Above-normal season: An ACE index above 115 x 104 kt2 /120 percent of the median/ and at least two of the following three conditions: 17 or more named storms, 9 or more hurricanes, and 5 or more major hurricanes. Near-normal season: An ACE index in the range 80-115 x 104 kt2 /80 percent-120 percent of the median/, or an ACE value higher than 115 x 104 kt2 but with less than two of the following three conditions being met: 17 or more named storms, 9 or more hurricanes, and 5 or more major hurricanes. Below-normal season: An ACE index below 80 x 104 kt2 /80 percent of the median/. Seasonal means and ranges during 1991-2020 of tropical storms, hurricanes, and major hurricanes during above-normal, near-normal, below-normal, and all, eastern Pacific hurricane seasons. Season Type Mean # of Tropical Storms Range of Tropical Storms Mean # of Hurricanes Range of Hurricanes Mean # of Major Hurricanes Range of Major Hurricanes Below- normal 13.1 7-18 6.0 3-11 2.0 0-4 Near- normal 15.1 10-18 8.0 6-10 4.0 2-6 Above- normal 18.6 13-24 11.6 9-16 6.6 4-9 All Seasons 15.4 7-25 8.3 3-16 4.2 0-9 Seasonal means and ranges during 1991-2020 of tropical storms, hurricanes, and major hurricanes forming in the eastern Pacific during La Nina /7 cases/, ENSO-neutral /17 cases/, and El Nino /6 cases/. The ENSO classification is provided by NOAAs Climate Prediction Center, and is valid for the July-September climatological peak of the eastern Pacific hurricane season. ENSO Status Mean # of Tropical Storms Range of Tropical Storms Mean # of Hurricanes Range of Hurricanes Mean # of Major Hurricanes Range of Major Hurricanes La Nina 12.8 7-21 6.7 3-11 3.3 1-6 Neutral 17.1 9-25 9.1 5-14 4.2 0-9 El Nino 15.0 12-18 8.5 6-13 5.5 3-9 $$