CSU Calls for Near-Average 2016 Atlantic Hurricane Season; Ex-Bonnie Tries a Comeback

The 2016 Atlantic hurricane season is off to an early start, with two named storms already in the books before the official June 1 start of the season: Hurricane Alex in January, and Tropical Storm Bonnie in May. Despite this early-season activity, a near-average Atlantic hurricane season is likely in 2016, said Dr. Phil Klotzbach of Colorado State University (CSU) in his latest seasonal forecast issued June 1. The CSU forecast is calling for an Atlantic hurricane season with 14 named storms, 6 hurricanes, 2 major hurricanes, and an Accumulated Cyclone Energy (ACE) of 94 (these numbers all take Alex and Bonnie into account.) The long-term averages for the period 1971 - 2010 were 12 named storms, 6.5 hurricanes, 2 major 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, and the forecast as a whole is largely consistent with CSU’s extended-range forecast issued on April 14. This is the 33rd year CSU has issued a seasonal hurricane forecast, but the first forecast done without Dr. Bill Gray as a main author, as he passed away on April 17.

Five years were selected as “analogue” years that the 2016 hurricane season may resemble:

1973 (8 named storms, 4 hurricanes, and 1 major hurricane)
1978 (11 named storms, 5 hurricanes, and 2 major hurricanes)
1983 (4 named storms, 3 hurricanes, and 1 major hurricane)
1992 (7 named storms, 4 hurricanes, and 1 major hurricane)
2003 (16 named storms, 7 hurricanes, and 3 major hurricanes)

These seasons were characterized by El Niño conditions in the previous winter that transitioned to either neutral or La Niña conditions by summer/autumn, and all had generally cool sea surface temperatures (SSTs) in the far North Atlantic and near-average tropical Atlantic SSTs during hurricane season. The average activity for these years was 9.2 named storms, 4.6 hurricanes, and 1.7 major hurricanes--all fairly close to the long-term average.


Figure 1. 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, but uncertainty about tropical Atlantic SSTs
The CSU forecast cited two main reasons why this may be a near-average hurricane season:

1) The El Niño event now drawing to a close in the eastern tropical Pacific is expected to transition toward neutral conditions this summer and 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. SSTs had fallen to 0.1°C below 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. This is the first below-average weekly value in the Niño3.4 area since July 2014. 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). As summarized by CPC/IRI, the mid-May average of dynamical models was calling for a Niño3.4 value in August-October of -0.9°C. 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 -0.7°C for June-August, but rebounding slightly to -0.5°C by August-October, suggesting borderline-to-weak La Niña conditions. Several other models, including the NOAA Climate Forecast System (CFSv2), are projecting stronger La Niña conditions by the August-October period (the August-October anomaly from CFSv2 is -1.2°C, which would correspond to a moderate La Niña). In its monthly ENSO Diagnostic Discussion released on May 12, NOAA's Climate Prediction Center issued a La Niña Watch, with the new CPC/IRI probabilistic outlook calling for a 71% chance of La Niña during the August-October period.


Figure 2. Departures from average in sea surface temperature (in degrees C) during late May 2016. Image credit: CSU and NOAA/ESRL/PSD.


Figure 3. Departures from average in sea level pressure (in millibars/hPa) during May 2016. The Bermuda-Azores high was stronger than average, which was driving stronger trade winds than average over the tropical Atlantic. Image credit: CSU and NOAA/ESRL/PSD.

2) A fairly unusual sea-surface temperature pattern is present across the North Atlantic, leading to some uncertainty about how this factor will evolve later in the year. During 2016 to date, SSTs were above average in the Northwest Atlantic and below average in the far North Atlantic and eastern subtropical Atlantic, a pattern that the CSU group has associated with the negative phase of the Atlantic Multidecadal Oscillation. However, as shown in Figure 3 above, SSTs in the Main Development Region (MDR) for hurricanes, from the Caribbean to the coast of Africa between 10°N and 20°N, were slightly above average in late May (with the exception of cooler-than-average waters just off the coast of Africa). 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. Despite the presence of above-average SSTs in the MDR this May, there is another factor that correlates even more strongly with the amount of Atlantic hurricane activity later in the year: the May SSTs across the eastern subtropical Atlantic and far North Atlantic. When these waters are cooler than average in springtime, as they are now, the CSU group finds that the cool SSTs tend to cause higher surface air pressure and stronger trade winds across the tropical Atlantic as the summer unfolds, which typically pushes the MDR SSTs below average by the peak of the Atlantic season.

While factor (1) suggests an above-average hurricane season, factor (2) would point toward a below-average season. Together with other variables considered in the CSU forecast, the result is the group’s projection of near-average conditions, which is similar to the NOAA outlook issued last week. See our post from May 27 for a roundup of the recent outlooks from NOAA and other groups, some of which are calling for more Atlantic activity than average. One factor in the mix: some long-range climate models are now projecting that slightly above-average SSTs will persist in the tropical Atlantic, as depicted in May output from the North American Multi-Model Ensemble.

As always, the CSU forecast 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 4. GOES satellite image of the remnants of Tropical Storm Bonnie at 1515Z (11:15 am EDT) Wednesday, June 1, 2016. Image credit: NASA Earth Science Office and NOAA.

Hurricane Hunters tasked to investigate ex-Bonnie
Satellite loops show that the remains of Tropical Storm Bonnie, located off the southeast coast of North Carolina on Wednesday morning, have increased in organization. Heavy thunderstorm activity has increased and the system has acquired more spin, thanks to the fact that ex-Bonnie is under a moderate 10 - 20 knots of wind shear and is over the relatively warm 26°C (79°F) waters of the Gulf Stream. The Hurricane Hunters have been tasked to investigate ex-Bonnie on Thursday afternoon as the storm heads slowly east-northeastwards out to sea at about 5 mph. Ex-Bonnie is expected to bring 1 - 2" of rain to extreme eastern North Carolina through Thursday. Wind shear is projected to remain light to moderate during the next 48 hours. Bonnie’s remnants will have an uphill battle fully regrouping, given that the system will remain fairly close to the Carolina coast until the shear rapidly increases by Friday, but the situation bears a close watch.

Where will the Atlantic's Tropical Storm Colin form?
The next named storm for the Atlantic will be named Colin, and there are two areas to watch next week for Colin's potential development. Over the Western Caribbean, a large area of low pressure laden with plenty of tropical moisture is expected to form early next week. This moisture will ride up over the Florida Keys and South Florida by Monday, bringing heavy rains of 2 - 4" through Tuesday. About 10% of the members of the ensemble runs of the 00Z Wednesday GFS and European models showed a tropical depression forming between the Western Caribbean and South Florida early next week, so we should keep an eye on this region. Wind shear will be high over the Central Gulf of Mexico, though, and this high shear will likely interfere with development. The other region to watch is the waters between Puerto Rico and Bermuda, where the GFS model predicts an area of low pressure capable of developing into a tropical cyclone will form on Sunday or Monday. This low would not be a threat to land, as prevailing winds would take it east-northeast away from the United States and the Caribbean.

First tropical depression of the season likely in the Eastern Pacific this week
In the Eastern Pacific, satellite loops show that an area of heavy thunderstorms located about 1000 miles south of the southern tip of the Baja Peninsula has become more concentrated over the past day. This disturbance (91E) is likely to develop into the Eastern Pacific's first tropical cyclone of the year late this week, according to recent runs of the GFS and European models. The disturbance is moving west-northwestward at about 15 mph, and is not a threat to any land areas. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave 91E 2-day and 5-day odds of development of 50% and 90%, respectively. Should 91E become a tropical storm, it would be named Agatha. The first named storm of the year in the Eastern Pacific typically forms on June 10, so we would be very close to climatology if Agatha were to develop later this week.


Jeff Masters and Bob Henson