NOAA 2021 Eastern Pacific Hurricane Season Outlook

The 2021 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 scientists from the NOAA National Hurricane Center (NHC). The eastern Pacific hurricane region covers the eastern North Pacific Ocean east of 140°W north of the equator.

Interpretation of NOAA's eastern Pacific hurricane season outlook
This outlook is 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 disasters can occur whether the season is active or relatively quiet. It only takes one hurricane (or even a tropical storm) to cause a disaster. Therefore, residents, businesses, and government agencies of coastal and near-coastal regions are urged to prepare for every hurricane season regardless of this, or any other, seasonal outlook. NOAA, the Federal Emergency Management Agency (FEMA), 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 only predictable when the storm is 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 range of activity seen in past similar years.

This outlook is based on predictions of large-scale climate factors and conditions known to strongly influence seasonal eastern Pacific hurricane activity, along with climate model forecasts. The outlook also takes into account uncertainties inherent in such climate outlooks.

Sources of uncertainty in this seasonal outlook

1. Predicting El Niño and La Niña events (also called ENSO) impacts on eastern Pacific hurricane activity is an ongoing scientific challenge facing climate scientists today. Such forecasts made during the spring generally have limited skill.
2. Uncertainty as to whether the eastern Pacific remains in the high-activity era during 2021, that has persisted since 2014, or if it has shifted to a low-activity era.
3. Predicting combined impacts associated with the Pacific-Decadal Oscillation (PDO) and the Atlantic Multi-Decadal Oscillation (AMO) remains a challenge, especially when they have competing phases.
4. Many combinations of named storms and 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, possibly affecting seasonal hurricane activity

2021 Eastern Pacific Hurricane Outlook Summary

a. Predicted Activity

NOAA's 2021 eastern Pacific Hurricane Season outlook indicates a near- or below-normal season most likely (80% combined chance). There is a 45% chance of a near-normal season and a 35% chance of a below-normal season, followed by a 20% chance of an above-normal season. See NOAA definitions of above-, near-, and below-normal seasons. The eastern Pacific hurricane region covers the eastern North Pacific Ocean east of 140°W north of the equator.

The 2021 outlook calls for a 70% probability for each of the following ranges of activity:

• 12-18 Named Storms
• 5-10 Hurricanes
• 2-5 Major Hurricanes
• Accumulated Cyclone Energy (ACE) range of 65%-120% of the median.

The 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 predicted ranges are centered above 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 are no further updates to this outlook.

b. Reasoning behind the outlook

Two climate factors are expected to contribute to a near- or below-normal 2021 hurricane season across the eastern (and central) Pacific hurricane basins, as follows:

1. The current and predicted SST anomaly patterns indicate below- or near-average SSTs across the eastern Pacific hurricane region, and near- to slightly above-average SSTs across the Atlantic Main Development Region (MDR). Historically, this combination tends to be associated with near- or below-normal hurricane activity. The MDR spans the tropical Atlantic Ocean and Caribbean Sea.


2. The most recent forecast from the NOAA Climate Prediction Center indicates ENSO-neutral conditions, or even the redevelopment of La Niña, are likely. 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 above normal temperatures in the Atlantic MDR and La Niña tend to favor reduced eastern Pacific hurricane activity, often resulting in a near- or below-normal hurricane season. ENSO-Neutral with a warm Atlantic MDR can result in a wide range of outcomes, centered just slightly above the 1991-2020 averages.

DISCUSSION

1. Expected 2021 activity

NOAA's 2021 eastern Pacific Hurricane Season outlook indicates a near- or below-normal season most likely (80% combined chance). There is a 45% chance of a near-normal season and a 35% chance of a below-normal season, followed by a 20% chance of an above-normal season. See NOAA definitions of above-, near-, and below-normal seasons.

The 2021 eastern Pacific hurricane season is predicted to produce (with 70% probability for each range) 12-18 named storms, of which 5-10 are expected to become hurricanes, and 2-5 of those are expected to become major hurricanes. These ranges are centered slightly below the official NHC 1991-2020 seasonal averages of 15 named storms, 8 hurricanes, and 4 major hurricanes.

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 named storms and hurricanes during the season. This 2021 outlook indicates a 70% chance that the ACE range will be 65%-120% of the median. An ACE value of 80%-120% 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

NOAA’s eastern Pacific hurricane season outlook is based on predictions of the main climate factors and their associated conditions known to influence the hurricane season. 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) FLOR and SPEAR 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) model. ENSO forecasts are also provided from a suite of statistical and other dynamical models contained in the suite of Niño 3.4 SST forecasts, which is compiled by the IRI (International Research Institute for Climate and Society) and the NOAA CPC.

NOAA's 2021 eastern Pacific hurricane season outlook reflects two main factors:

(1) The predicted SST anomaly patterns indicate near-average SSTs across the eastern Pacific hurricane region, and above-average SSTs across the Atlantic MDR. For the Pacific, these conditions project onto the negative phase of the Pacific Decadal Oscillation (PDO) and also onto the warm phase of the Atlantic Multi-Decadal Oscillation (AMO). Historically, this combination of climate patterns tends to be associated with low-activity eras in the eastern Pacific hurricane region, and with near- or below-normal eastern Pacific hurricane seasons. There is some uncertainty about the persistence of these two patterns through the entirety of the hurricane season.

(2) The most recent forecast from the CPC indicates ENSO-neutral (50%) conditions or even the redevelopment of La Niña (42%), with low odds for the development of El Niño (8%). Historically, during low-activity eras when the negative PDO and warm AMO are present, ENSO-neutral or La Niña tend to favor a near- or below-normal season. However, if the positive PDO pattern emerges, ENSO-neutral conditions could favor activity near or above the upper ends of our predicted ranges.

a. ENSO-neutral or La Niña

ENSO-neutral conditions are present at this time. The latest weekly SSTs are currently 0.4°C below average across the central and eastern equatorial Pacific and the SST index for the Niño 3.4 region is -0.4°C. The weekly Niño 3.4 index had been below -0.5°C since August 2020. The recent decrease in amplitude of the Niño 3.4 index reflects a weakening of the ongoing La Niña. The wind and OLR patterns over the central Pacific are also reflecting a disruption of the atmospheric response to La Niña.

Looking forward, model predicted SST anomalies in the Niño 3.4 region generally indicate ENSO-neutral (Niño 3.4 index between ± 0.5°C) throughout the hurricane season, though mostly on the cool side. The dynamical model average (red line) and statistical model average (green line) are both on the cool side of zero. ENSO-neutral can be associated with either very active or inactive seasons, doing little to enhance or suppress the season. The uncertainty about whether La Niña or ENSO-neutral conditions will be present this season adds to the uncertainty in this outlook, where a redevelopment of La Niña would result in lower activity, near the lower bounds of the outlook.

NOAA’s Climate Forecast system (CFS) and the North American Multi-model Ensemble (NMME) are predicting ENSO-neutral. Some of the constituent models in the NMME are predicting the redevelopment of La Niña during ASO 2021. The CFS and NMME both predict near-normal shear over the East Pacific. The shear predicted this year is closer to climatology than what was predicted last year. A redevelopment of La Niña would increase the likelihood of a below-normal season, while ENSO-neutral would support a wider range of possibilities.

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 increased activity (such as 1982-94 and 2014-2020) are called high-activity eras, and periods of decreased activity (such as 1971-1981 and 1995-2013) are called low-activity eras. These are different from the high- and low-activity 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 more ACE points, than low-activity eras. During high-activity eras, above-normal seasons occur about three times more frequently (63% of seasons compared to 20%), and below-normal seasons are about four times less frequent (11% compared to 43%).

High- and low-activity eras in the eastern Pacific hurricane region are strongly 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 global SST pattern is the Atlantic Multi-decadal Oscillation (AMO). 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 basin, with a warm AMO favoring increased Atlantic activity and decreased eastern Pacific activity.

The 1982-94 high-activity era was associated with a cold AMO and a positive PDO, while the 2014-2020 high-activity era primarily featured a strong positive PDO. Conversely, the intervening 1995-2013 low-activity era featured a warm AMO and negative PDO. Of years when the August PDO was negative, approximately 2/3 of those years were near normal or below normal for activity (23 of 33 since 1971).

Regarding 2021, the current and predicted SST anomalies reflect the ongoing warm phase of the AMO, which has been present since 1995. They also reflect near- or below-average SSTs associated with ENSO-neutral or La Niña, respectively, across the eastern Pacific hurricane region, which projects onto the negative phase of the PDO (currently at -1.17). The predicted SST’s in the Pacific would not have as strong of an impact should they verify as ENSO-neutral, so the low-end of this year’s outlook is slightly higher than last year, as the possibility for both PDO and La Niña to both restrict development is lower.

Confidence is moderate that the current negative PDO pattern will persist through the hurricane season. One reason is that SST forecasts made several months ahead tend to have limited skill. Another reason is that the current negative PDO signal partly reflects the synoptic-scale wind and pressure patterns associated with persistent higher than normal pressures over the North Pacific. In addition, August Niño 3.4 values and August PDO values are highly correlated, and the predictions for Niño 3.4 are close to zero in many models. This recent period of negative PDO might be short-lived and was amplified by the projection of the ongoing La Niña onto the overall pattern in the Pacific. The official probabilities largely reflect ENSO-neutral during a low-activity era, which are typically associated with PDO negative. Negative PDO and ENSO-neutral are currently the most likely outcomes. These observations lead to larger uncertainty about the nature of the background climate signal upon which the ENSO signal will overlay during the peak months of the 2021 eastern Pacific hurricane season.

NOAA FORECASTERS

• Matthew Rosencrans, Physical Scientist, Matthew.Rosencrans@noaa.gov
• Dr. Hui Wang, Physical Scientist; Hui.Wang@noaa.gov

• Eric Blake, Senior Hurricane Specialist; Eric.S.Blake@noaa.gov
• Dr. Christopher Landsea, Meteorologist; Chris.Landsea@noaa.gov

• Stanley Goldenberg, Meteorologist; Stanley.Goldenberg@noaa.gov