Active Atlantic Hurricane Period That Began in 1995 May be Over: NOAA

By Jeff Masters
Published: 17:14 GMT le 27 mai 2015

It should be another quiet Atlantic hurricane season in 2015, and the active hurricane pattern that began in 1995 may now be over, said NOAA in their May 27 seasonal hurricane forecast. They give a 70% chance of a below-normal season, a 20% chance of a near-normal season, and only a 10% chance of an above-normal season. They predict a 70% chance that there will be 6 - 11 named storms, 3 - 6 hurricanes, and 0 - 2 major hurricanes, with an Accumulated Cyclone Energy (ACE) 40% - 85% of the median. If we take the midpoint of these numbers, NOAA is calling for 8.5 named storms, 4.5 hurricanes, 1 major hurricane, and an ACE index 62.5% of normal. This is well below the 1981 - 2010 average of 12 named storms, 6 hurricanes, and 3 major hurricanes. Hurricane seasons during the active hurricane period 1995 - 2014 averaged 14.7 named storms, 7.6 hurricanes, and 3.5 major hurricanes, with an ACE index 142% of the median. Only three seasons since 1995 have been classified by NOAA as being below normal--including two El Niño years (1997 and 2009), and the neutral 2013 season.

The forecasters cited the following main factors that will influence the coming season:

1) The current borderline weak/moderate El Niño event is expected to persist or intensify during the 2015 hurricane season. El Niño events tend to suppress Atlantic hurricane activity in three ways:

- By creating high levels of wind shear over the tropical Atlantic, which tends to tear storms apart.
- By increasing sinking motion and high pressure over the tropical Atlantic.
- By making the air more stable over the tropical Atlantic.

2) Near-average sea surface temperatures (SSTs) are in place over the hurricane Main Development Region (MDR), from the Caribbean to the coast of Africa between between 10°N and 20°N. These SSTs are expected to be near or below average during the peak August - October portion of hurricane season, and are expected to be cooler than SSTs in the remainder of the global tropics (SSTs in the remainder of the global tropics were 0.31°C warmer than SSTs in the MDR in May.) This configuration of SSTs is often quite hostile to Atlantic tropical cyclone development.

3) The active period of hurricane activity that began in 1995 due to a natural decades-long cycle in hurricane activity called the Atlantic Multi-decadal Oscillation (AMO) may now be over. The SST pattern associated with that cycle is absent this year, and NOAA said: "There have been two seasons in a row, 2013 and 2014, with below-normal and near-normal activity respectively and neither had an El Niño event responsible for the reduced activity. The current configuration of SSTs in the Atlantic Ocean, both in the MDR and the entire North Atlantic, are suggestive that the AMO may no longer be in the warm phase."

Figure 1. Hurricane Gonzalo as seen from the International Space Station on October 16, 2014. At the time, Gonzalo was at peak strength, with 145 mph winds, and was the first Atlantic hurricane to reach sustained winds of at least 145 mph since Hurricane Igor of 2010. Gonzalo hit Bermuda just a week after Hurricane Fay hit the island, and Gonzalo's remnants went on to batter the United Kingdom on October 21 with wind gusts exceeding 100 mph, killing three people there. Image credit: Alexander Gerst.

Though hurricane activity in the MDR should be limited this year, we often see storms occurring in a belt extending from the Gulf of Mexico to the central Atlantic north of the MDR during El Niño years, in association with reduced vertical wind shear across the region (Goldenberg and Shapiro 1996). NOAA's long-range CFS model predicts the vertical wind shear in this belt to be weaker than average during the August - October peak of hurricane season, which could allow activity to be near the higher end of their predicted ranges (11 named storms, 6 hurricanes, and 2 intense hurricanes.) At present, SSTs are unusually warm in the Gulf of Mexico (1°C - 3°C above average) and in a belt from the Bahamas eastward into the central Atlantic (1 - 2°C above average). Storms that form in these waters are typically weaker than ones that form in the MDR, since they have less time to intensify over warm water.

Standard disclaimer: “A below-normal season doesn’t mean we’re off the hook. As we’ve seen before, below-normal seasons can still produce catastrophic impacts to communities,” said NOAA Administrator Kathryn Sullivan, Ph.D., referring to the 1992 season in which only seven named storms formed, yet the first was Andrew – a Category 5 Major Hurricane that devastated South Florida.

Figure 2. Forecast skill of the TSR, NOAA (National Oceanic and Atmospheric Administration) and CSU (Colorado State University) for the number of hurricanes in the Atlantic during 1980 - 2014, as a function of lead time. Forecast precision is assessed using the Mean Square Skill Score (MSSS) which is the percentage improvement in mean square error over a climatology forecast (six hurricanes in a given year.) Positive skill indicates that the model performs better than climatology, while a negative skill indicates that it performs worse than climatology. Two different climatologies are used: a fixed 50-year (1950-1999) climatology, and a running prior 10-year climate norm (2005 - 2014). NOAA does not release seasonal outlooks before late May, and CSU stopped providing quantitative extended-range December hurricane outlooks in 2011. Skill climbs as the hurricane season approaches, with modest skill levels by early June, and good skill levels by early August. Using this metric, NOAA's late May forecasts have about 10% skill over chance, while TSR's have about 20% skill. Image credit: Tropical Storm Risk, Inc (TSR).

TSR predicts a below-average Atlantic hurricane season: 11 named storms
The May 27 forecast for the 2015 Atlantic hurricane season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) will be out later today, and I will update this post with their latest numbers. Their April 9 forecast called for 11 named storms, 5 hurricanes, 2 intense hurricanes, and an Accumulated Cyclone Energy (ACE) of 56.

CSU predicts a below-average Atlantic hurricane season: 9 named storms
Another quiet Atlantic hurricane season is likely in 2015, said the hurricane forecasting team of Dr. Phil Klotzbach and Dr. Bill Gray of Colorado State University (CSU) in their latest seasonal forecast issued April 9. They called for an Atlantic hurricane season with 7 named storms, 3 hurricanes, 1 intense hurricane, and an Accumulated Cyclone Energy (ACE) of 40. The next CSU forecast is due on Monday, June 1, and will receive a lot of media attention. My April 9 blog post has more on their forecast.

UKMET office predicts a below-average Atlantic hurricane season: 9 named storms
The UKMET office forecast for the 2015 Atlantic hurricane season, issued May 21, calls for below-normal activity, with 8 named storms, 5 hurricanes, and an ACE index of 74 occurring during the period June - November. If we add in Tropical Storm Ana, these numbers come out to 9 named storms and 5 hurricanes. In contrast to the statistical models relied upon by CSU, TSR, and NOAA, the UKMET forecast is done strictly using a dynamical global seasonal model, the Met Office GloSea5 system.

WSI predicts a below-average Atlantic hurricane season: 9 named storms
The May 27 forecast from the private weather firm WSI (part of The Weather Company, along with The Weather Channel, Weather Central, and The Weather Underground), is calling for a quiet Atlantic hurricane season with 9 named storms, 5 hurricanes, and 1 intense hurricane. WSI cites the expectation of El Niño conditions this fall as a key factor influencing their forecast.

Penn State predicts a below-average Atlantic hurricane season: 8 named storms
The May 11 forecast made using a statistical model by Penn State's Michael Mann and alumnus Michael Kozar is calling for a quiet Atlantic hurricane season with 8 named storms, plus or minus 2.7 storms. Their prediction was made using statistics of how past hurricane seasons have behaved in response to sea surface temperatures (SSTs), the El Niño/La Niña oscillation, the North Atlantic Oscillation (NAO), and other factors. The statistical model assumes that in 2015 the mid-May -0.18°C departure of temperature from average in the MDR will persist throughout hurricane season, the El Niño phase will be moderate, and the North Atlantic Oscillation (NAO) will be near average.

The PSU team has been making Atlantic hurricane season forecasts since 2007, and these predictions have done pretty well, except for in 2012, when an expected El Niño did not materialize:

2007 prediction: 15 named storms, Actual: 15
2009 prediction: 12 named storms, Actual: 9
2010 prediction: 23 named storms, Actual: 19
2011 prediction: 16 named storms, Actual: 19
2012 prediction: 11 named storms, Actual: 19
2013 prediction: 16 named storms, Actual: 14
2014 prediction: 9 named storms, Actual: 8

NCSU predicts a below-average Atlantic hurricane season: 5 named storms
The April 19 forecast from North Carolina State University (NCSU), is calling for a below-average Atlantic hurricane season with 5 named storms, 2 hurricanes, and 1 intense hurricane.

Coastal Carolina University predicts a below-average Atlantic hurricane season: 8 named storms
The April 17 forecast from Coastal Carolina University is calling for a below-average Atlantic hurricane season with 8 named storms, 4 hurricanes, and 2 intense hurricanes.

Cuban Met service predicts a below-average Atlantic hurricane season: 8 named storms
The May 4 forecast from the Cuban Meteorological Service, INSMET, is calling for a below-average Atlantic hurricane season with 9 named storms and 3 hurricanes. The forecast is based on a statistical prediction model developed by Ballester, González and Pérez (2010).

Figure 3. Departure of temperature from average for May 23 for the Northeast Pacific waters showed large areas of SSTs 1 - 2°C (1.8 - 3.6°F) above average. Image credit: NOAA/NHC.

NOAA predicts an above-average Eastern Pacific hurricane season: 18.5 named storms
NOAA's pre-season prediction for the Eastern Pacific hurricane season, issued on May 27, calls for an active season, with 15 - 22 named storms, 7 - 12 hurricanes, 5 - 8 major hurricanes, and an ACE index 110% - 190% of the median. The mid-point of these ranges gives us a forecast for 18.5 named storms, 9.5 hurricanes, and 6.5 major hurricanes, with an ACE index 150% of average. The 1981 - 2010 averages for the Eastern Pacific hurricane season are 15 named storms, 8 hurricanes, and 4 major hurricanes.

NOAA predicts a below-average Central Pacific hurricane season: 5 - 8 tropical cyclones
NOAA's pre-season prediction for the Central Pacific hurricane season, issued on May 27, calls for an above-average season, with 5 - 8 tropical cyclones. An average season has 4 - 5 tropical cyclones, which include tropical depressions, tropical storms, and hurricanes. Hawaii is the primary land area affected by Central Pacific tropical cyclones.

TSR predicts an above-average Northwest Pacific typhoon season: 11 intense typhoons
The May 6 forecast for the 2015 Northwest Pacific season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) calls for an active season with 27 named storms, 17 Category 1 or stronger typhoons, 11 intense Category 3 or stronger typhoons, and an Accumulated Cyclone Energy (ACE) of 400. The long-term averages for the past 50 years are 26 named storms, 16 typhoons, 8 intense hurricanes, and an ACE of 294. TSR rates their skill level as modest for these late May forecasts--13% - 28% higher than a "no-skill" forecast made using climatology. TSR’s main predictor for their typhoon season forecast is sea surface temperature (SST) in the region 5 ̊S - 5 ̊N, 140 ̊W - 180 ̊W, which they expect to be 1.0±0.5°C warmer than normal.

Invest 92E likely to become Tropical Storm Andres off the Pacific Mexican coast
The first named storm of the Northeast Pacific hurricane season usually forms by June 10, but we are likely to see Tropical Storm Andres well before that date this year. Satellite loops show that Invest 92E, located about 800 miles south-southwest of Manzanillo, Mexico, has developed a large area of heavy thunderstorms and a good degree of spin. The GFS and European models both show 92E developing into a tropical depression or tropical storm by Saturday. SSTs are a very warm 30°C (86°F), which is nearly 2°C (3.6°F) above average. The 8 am EDT Wednesday SHIPS model forecast predicted that wind shear would remain light over 92E during the coming five days, and I expect 92E will develop into Tropical Storm Andres by early next week. In their 2 pm EDT Wednesday Tropical Weather Outlook, NHC gave 92E 2-day and 5-day development odds of 80% and 90%, respectively. This system is far enough offshore that it will not spread heavy rains to the coast of Mexico over the coming week as it moves west-northwest or northwest, parallel to the coast.

In the Atlantic, the models are depicting high wind shear through June 3 over the majority of the regions we typically see early-season tropical cyclone development--the Caribbean, Gulf of Mexico, and Bahamas. However, late next week we should see a decrease in wind shear, which would argue for an increased chance of tropical storm development then (though wind shear forecasts more than 7 days in advance are highly unreliable.) The prospects for an early June Tropical Storm Bill developing in the Gulf of Mexico or waters near the Bahamas are above average, given the warmer than average SSTs in these waters, and the fact that the Madden-Julian Oscillation (MJO) is expected to be active in the Western Hemisphere during the first ten days of June.

Jeff Masters

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About The Author
Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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