CSU Projects a Near-Average Atlantic Hurricane Season in 2016

The 2016 Atlantic hurricane season started off with a surprising bang in early January, when Hurricane Alex formed in the far Eastern Atlantic. However, a near-average Atlantic hurricane season is likely in 2016, said the hurricane forecasting team from Colorado State University (CSU) in their latest seasonal forecast issued April 14. Led by Dr. Phil Klotzbach, with special contributions from Dr. Bill Gray, the CSU team is calling for an Atlantic hurricane season with 13 named storms, 6 hurricanes, 2 intense hurricanes, and an Accumulated Cyclone Energy (ACE) of 93 (these numbers all take Alex into account.) The long-term averages for the period 1971 - 2010 were 12 named storms, 6.5 hurricanes, 2 intense hurricanes, and an ACE of 92. The CSU outlook also calls for a 50% chance of a major hurricane hitting the U.S. in 2016, with a 30% chance for the East Coast and Florida Peninsula and a 29% chance for the Gulf Coast. The Caribbean is forecast to have a 40% chance of seeing at least one major hurricane. All of these probabilities are very close to the long-term numbers from the last century.

Six years with similar pre-season February and March atmospheric and oceanic conditions were selected as “analog” years that the 2016 hurricane season may resemble:

1941 (6 named storms, 4 hurricanes, and 3 intense hurricanes)
1973 (8 named storms, 4 hurricanes, and 1 intense hurricane)
1983 (4 named storms, 3 hurricanes, and 1 intense hurricane)
1992 (7 named storms, 4 hurricanes, and 1 intense hurricane)
1998 (14 named storms, 10 hurricanes, and 3 intense hurricanes)
2003 (16 named storms, 7 hurricanes, and 3 intense hurricanes)

These six years all featured El Niño conditions transitioning to neutral or La Niña conditions. The average activity for these years was 9.2 named storms, 5.3 hurricanes, and 2 major hurricanes, all fairly close to the long-term average. However, as shown in the list above and in Figure 1 below, there is a great deal of variation among the six analog years chosen by CSU.


Figure 1. Two of the analog seasons cited in the new CSU hurricane outlook are a study in contrasts. The 1983 season (left) was the quietest post-1970 Atlantic season on record in terms of accumulated cyclone energy, with only 4 named storms and 3 hurricanes. The 1998 season (right) produced 14 named storms, 10 hurricanes, and more than $3 billion in damage, as well as the catastrophic Hurricane Mitch, which killed more than 10,000 people as it decayed over Central America. The 1983 and 1998 seasons followed the two strongest El Niño events on record, which were roughly on par with the 2015-16 El Niño event.


Figure 2. Hurricane Alex approaching the Azores Islands in the far Eastern Atlantic on January 14, 2016. Alex peaked as a Category 1 storm with 85 mph winds on January 14, then weakened to a tropical storm with 70 mph winds when it made landfall on Terceira Island in the Azores on January 15. The storm caused minimal damage and was responsible for one indirect death. Alex was the first Atlantic hurricane in January since Alice in 1955, and the first to form in January since 1938. Image credit: NASA.

A boost from El Niño’s departure
The CSU team cited two main reasons why this may be an average hurricane season:

1) The El Niño event now fading in the eastern tropical Pacific is expected to transition toward neutral conditions this summer and either neutral or La Niña conditions by autumn (see the discussion below). If La Niña conditions are present this fall, this would tend to favor a busier-than-usual Atlantic hurricane season due to a reduction in the upper-level winds over the tropical Atlantic that can tear storms apart. Sea surface temperatures were 1.3°C above average over the past week in the so-called Niño3.4 region (5°S - 5°N, 120°W - 170°W), where SSTs must be at least 0.5°C above average for five consecutive months (each month being a 3-month average) for a weak El Niño event to be declared. By August-October, most dynamical models are calling for either cool-neutral conditions (Niño3.4 anomalies between 0 and -0.5°C) or La Niña conditions (Niño3.4 anomalies of -0.5°C or greater). The European Centre for Medium-Range Weather Forecasts (ECMWF) shows the best prediction skill of the various El Niño/Southern Oscillation (ENSO) models, and the average of the various ECMWF ensemble members is calling for a Nino 3.4 SST anomaly of approximately -0.4°C, just short of weak La Niña conditions. Several other models, including the NOAA Climate Forecast System (CFSv2), are projecting somewhat stronger La Niña conditions by the August-October period. In its monthly ENSO Diagnostic Discussion released on Thursday, NOAA's Climate Prediction Center issued a La Niña Watch, with the new CPC/IRI probabilistic outlook calling for a 65% chance of La Niña during the August-October period.

2) A fairly unusual pattern in sea surface temperature (SST) is present across the North Atlantic, leading to some uncertainty about how this factor will evolve later in the year. SSTs are now above average in the Northwest Atlantic and well below average in the far North Atlantic, a pattern that the CSU group has associated with the negative phase of the Atlantic Multidecadal Oscillation. Typically this pattern also leads to colder-than-normal water in the tropical Atlantic. However, SSTs in the Main Development Region (MDR) for hurricanes, from the Caribbean to the coast of Africa between 10°N and 20°N, were near to slightly above average in March 2016, with the exception of cooler-than-average waters just off the coast of Africa. SSTs have cooled across both the tropical and far northern Atlantic since late October, largely due to a persistent positive phase of the North Atlantic Oscillation (NAO) since November 2014. A positive phase of the NAO is associated with a strengthened Bermuda-Azores High and faster trade winds across the tropical Atlantic. The faster winds increase mixing of cool water to the surface. These cooler SSTs are associated with higher-than-normal sea level pressures, which can create a self-enhancing feedback that relates to higher pressure, stronger trades and cooler SSTs during the hurricane season. Virtually all African tropical waves originate in the MDR, and these tropical waves account for 85% of all Atlantic major hurricanes and 60% of all named storms. When SSTs in the MDR are much above average during hurricane season, a very active season typically results (if there is no El Niño event present.) Conversely, when MDR SSTs are cooler than average, a below-average Atlantic hurricane season is more likely. The April outlook concludes: “There are some hints of [cold water] emerging in the tropical Atlantic, but it remains to be seen if these cold anomalies will push further across the tropical Atlantic.”

As always, the CSU team included this standard disclaimer:

"Coastal residents are reminded that it only takes one hurricane making landfall to make it an active season for them. They should prepare the same for every season, regardless of how much activity is predicted."


Figure 3. Departure of sea surface temperature (SST) from average for March 2016, as computed by NOAA/ESRL. SSTs in the hurricane Main Development Region (MDR) between Africa and Central America (red box) were near average in the eastern Atlantic, and slightly above average in the Caribbean. Image credit: NOAA/ESRL.

How good are the April forecasts?
April forecasts of hurricane season activity are low-skill, since they must deal with the so-called "predictability barrier." April is the time of year when the El Niño/La Niña phenomenon commonly undergoes a rapid change from one state to another, making it difficult to predict whether we will have El Niño, La Niña, or neutral conditions in place for the coming hurricane season (although there is a fairly strong model consensus this year on a transition from El Niño toward La Niña). For now, these April forecasts should simply be viewed as an interesting research effort that has the potential to make skillful forecasts. The next CSU forecast, due on June 1, is the one worth paying attention to. Their early June forecasts have shown considerable skill over the years. NOAA issues its first seasonal hurricane forecast for 2016 in late May.

TSR predicts a below-average Atlantic hurricane season
The April 5 forecast for the 2016 Atlantic hurricane season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) calls for a below-average Atlantic hurricane season about 20% below the long-term (1950-2015) norm and about 15% below the recent 2006-2015 ten-year norm. TSR is predicting 12 named storms, 6 hurricanes, 2 intense hurricanes and an Accumulated Cyclone Energy (ACE) of 80 for the period May though December. The long-term averages for the past 65 years are 11 named storms, 6 hurricanes, 3 intense hurricanes and an ACE of 101. TSR rates their skill level as low for these April forecasts--just 9 - 15% higher than a "no-skill" forecast made using climatology. TSR predicts a 29% chance that U.S. landfalling activity will be above average, a 27% chance it will be near average, and a 44% chance it will be below average. They project that two named storms will hit the U.S., with one of these being a hurricane. The averages from the 1950-2015 climatology are three named storms and one hurricane. They rate their skill at making these April forecasts for U.S. landfalls just 3 - 7% higher than a "no-skill" forecast made using climatology. In the Lesser Antilles Islands of the Caribbean, TSR projects one named storm and no hurricanes. Climatology is one named storm and less than 0.5 hurricanes.

TSR’s two predictors for their statistical model are the forecast July - September trade wind speed over the Caribbean and tropical North Atlantic, and the forecast August - September sea surface temperatures (SSTs) in the tropical North Atlantic. Their model is calling SSTs 0.03°C below average and trade winds 0.15 m/s faster than average during these periods. Both of these factors should act to decrease hurricane and tropical storm activity. The July-September 2016 trade wind prediction is based on an expectation of near-neutral El Niño ENSO conditions in August-September 2016. They add: "Should the TSR forecast for 2016 verify it would mean that the ACE index total for 2013-2016 was easily the lowest 4-year total since 1991-1994 and it would imply that the active phase of Atlantic hurricane activity which began in 1995 has likely ended. However, it should be stressed that the precision of hurricane outlooks issued in April is low and that large uncertainties remain for the 2016 hurricane season." One factor to keep in mind: TSU’s outlook drew in part on NOAA CFSv2 model projections that conflicted with other major models in calling for El Niño to continue this autumn. NOAA has now corrected an initialization error in the CFSv2, and this month the model has made a major switch, now consistently pointing toward La Niña. CSU was able to this switch into account in developing its April outlook. The next TSR forecast will be issued on May 27.


Figure 4. The departure of tropical cyclone activity from average for the five years 2013, 1988, 1973, 1970, and 1954 (tropical cyclones include all tropical depressions, tropical storms, and hurricanes). These five years had ocean temperatures in the Western Hemisphere similar to what is predicted in the summer of 2016 by NOAA's North American Multi-Model Ensemble. During these five analog years, above-average activity was observed in the western Gulf of Mexico, along the U.S. East Coast, and along the Pacific coast of Mexico. Image credit: WU member Levi Cowan (tropicaltidbits.com)

Analog years from Levi Cowan
WU member Levi Cowan (tropicaltidbits.com) has come up with his own list of analog years for Atlantic and Eastern Pacific hurricane activity, based on the predicted pattern of ocean temperatures this summer from NOAA's North American Multi-Model Ensemble (NMME): 2013 (which featured 14 named storms, 2 hurricanes, and 0 intense hurricanes); 1988 (12 named storms, 5 hurricanes, and 3 intense hurricanes); 1973 (8 named storms, 4 hurricanes, and 1 intense hurricane) ; 1970 (10 named storms, 5 hurricanes, and 2 intense hurricanes); and 1954 (16 named storms, 7 hurricanes, and 3 intense hurricanes). Above-average activity was observed in the western Gulf of Mexico, along the U.S. East Coast, and along the Pacific coast of Mexico during these five years. The average activity for these years was 12 named storms, 4.6 hurricanes, and 1.8 major hurricanes.

We’ll have a post by Friday afternoon looking ahead to the major spring storm that's likely to bring severe weather and excessive rain this weekend to parts of the Southern and Central Plains and heavy snow to the Central Rockies.

Jeff Masters and Bob Henson