Latest NOAA Atlantic and Eastern Pacific Hurricane Season Forecast

National Oceanic and Atmospheric Administration (NOAA) has issued its forecast for the 2017 Atlantic and Pacific Hurricane Seasons. Forecasters within the U.S. governmental agency are forecasting 11-17 named storms, 5-9 hurricanes and 2-4 major (Category 3+) hurricanes between the months of June and November for the Atlantic Basin. This includes April’s Tropical Storm Arlene. The agency expects 14-20 named storms, 6-11 hurricanes and 3-7 major (Category 3+) hurricanes between mid-May and November for the Eastern Pacific Basin. 

ATLANTIC BASIN FORECAST REASONING

NOAA’s report indicates that there are three main factors for the Atlantic hurricane season forecast, which suggest slightly above-normal or near-normal activity:

• Either ENSO-neutral or weak El Niño conditions are expected in the tropical Pacific Ocean
• Near or above-average sea surface temperatures are anticipated across much of the Atlantic’s Main Development Region (MDR)
• Near-average or weaker-than-average vertical wind shear is likely in the MDR

There remains lots of uncertainty at this lead time for oceanic and atmospheric conditions during the upcoming peak months between August and October. This uncertainty is found in the most recent ENSO, sea surface temperature and vertical wind shear forecasts. NOAA indicates that the combination of ENSO-neutral, a warmer MDR and weaker wind shear could lead to tropical cyclone activity at the high end of the projected range. However, a combination of weak El Niño conditions, a cooler MDR and near or above-average wind shear would likely result in activity at the lower end of the range. The statistical and dynamical ENSO forecast models are currently showing large uncertainty with wide spread, which is typical at this time of year as forecast model skill is at its lowest for the peak development months.

Another area of uncertainty surrounds the state of the Atlantic Multi-Decadal Oscillation (AMO). This unknown is leading to additional complications in predicting sea surface temperature anomalies within the MDR and their possible impacts on the season. As a reminder, the AMO is a main climate factor that influences cyclogenesis in the Atlantic basin, and it acts as the backdrop that other climate phenomenon such as El Niño and La Niña overlay. The AMO typically rotates between “high-activity” and “low-activity” eras for Atlantic hurricane seasons during 25-40 year periods. There currently resides a strong unknown as to whether the warm AMO phase (which began in 1995) has ended, and whether a cool AMO phase (similar to what was observed during 1971-1994) has begun. Differing indices are showing conflicting results at this time.

NOAA provides the following probabilities for the 2017 Atlantic Hurricane Season: 45 percent of an above-normal season, a 35 percent chance of a near-normal season, and a 20 percent chance of a below-normal season.

It is important to note that regardless of the overall number of storms that develop in the basin, it only takes one significant land-falling event to entirely change the perception of the season. As always, it is critical to be aware of the inherent risks with any developing tropical cyclone and its potential threat to land.

EASTERN PACIFIC BASIN FORECAST REASONING

NOAA expects an equal chance of an above-normal or near-normal season in 2017, and lists three main factors during the peak development months between July and September:

• Either ENSO-neutral or weak El Niño conditions are expected in the tropical Pacific Ocean
• Near or above-average sea surface temperatures in the main hurricane formation region
• Near-average or weaker-than-average vertical wind shear in the main hurricane formation region

The agency continues to cite that there remains ample uncertainty surrounding the possibility of whether El Niño develops later in the year. The combination of ENSO-neutral, near-normal sea surface temperatures and near-normal wind shear in the eastern Pacific Ocean would likely lead to activity on the lower end of the projected range. Separately, the combination of El Niño, warmer sea surface temperatures and weaker wind shear would likely spawn activity levels towards the upper bound of the projected forecast. It is worth noting that predicting the location, timing, number and strength of hurricanes and any possible numerical landfall outlook is ultimately tied to daily weather patterns, which dictate genesis locations and steering patterns that are not predictable with any level of skill weeks or months in advance.

In terms of ENSO, La Niña officially dissipated in February and ENSO-neutral conditions have been present since that time. NOAA indicates that above-average sea surface temperatures have emerged in the east-central and eastern equatorial Pacific Ocean and near-average temperatures are evident in the central equatorial Pacific. Additionally, sub-surface temperature anomalies remain warmer than average in the central and eastern equatorial Pacific. Such a pattern suggests that the warmth in the Niño-3.4 region is currently confined to the near-surface, which is not often favorable to a significant short-term evolution towards El Niño.

The last three eastern Pacific hurricane seasons have been above normal, which was correlated to above-average sea surface temperatures across the hurricane basin. This warmth was associated with the positive phase of the Pacific Decadal Oscillation (PDO) that usually features above average ocean temperatures in the hurricane region and either a weak warm phase or neutral phase of the AMO.

The recent uptick in hurricane activity is in sharp contrast to the period registered from 1995-2013, in which eastern Pacific hurricane seasons were generally below normal (called a low-activity era); while the Atlantic basin was experiencing a high-activity era. This dipole pattern in activity between the two hurricane basins was associated with a cold phase of the PDO and a warm phase of the AMO.

NOAA provides the following probabilities for the 2016 Eastern Pacific Hurricane Season: 40 percent of an above-normal season, 40 percent chance of a near-normal season, and a 20 percent chance of a below-normal season