CSU Predicts a Quiet 2015 Atlantic Hurricane Season Due to Rising El Niño Odds

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 long-term averages for the period 1971 - 2010 were 12 named storms, 6.5 hurricanes, 2 intense hurricanes, and an ACE of 92. The 2015 forecast calls for a below-average chance of a major hurricane hitting the U.S., both along the East Coast (15% chance, 31% chance is average) and the Gulf Coast (15% chance, 30% chance is average). The Caribbean is forecast to have a 22% chance of seeing at least one major hurricane (42% is average.) Five years with similar pre-season February and March atmospheric and oceanic conditions were selected as "analogue" years that the 2015 hurricane season may resemble: 2014 (which featured 8 named storms, 6 hurricanes, and 2 intense hurricanes); 1993 (8 named storms, 6 hurricanes, and 2 intense hurricanes); 1991, featuring Hurricane Bob, which hit Long Island, New York as a Category 2 storm; 1987 (7 named storms, 3 hurricanes, and 1 intense hurricane); and 1957, which featured the deadliest June hurricane on record, Hurricane Audrey, which killed 416 people in Texas and Louisiana. These five years all had at least moderate El Niño conditions and cool Sea Surface Temperatures (SSTs) in the tropical Atlantic. The average activity for these years was 8 named storms, 4 hurricanes, and 2 major hurricanes.


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.

Why the forecast of a quiet season?
The CSU team cited two main reasons why this may be an quiet hurricane season:

1) A weak El Niño event is underway in the Eastern Pacific, and is expected to intensify by this fall (see the discussion below in this post's last paragraph.) If El Niño conditions are present this fall, this will likely bring about a quiet Atlantic hurricane season due to increased upper-level winds over the tropical Atlantic creating wind shear that will tend to tear storms apart. Sea surface temperatures were 0.7°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 a moderate El Niño (Niño 3.4 temperatures at least 1.0°C above average) or strong El Niño (Niño 3.4 temperatures at least 1.5°C above average). 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 strong El Niño event by September (a Nino 3.4 SST anomaly of approximately 1.7°C.)

2) Sea surface temperatures (SSTs) in the Main Development Region (MDR) for hurricanes from the Caribbean to the coast of Africa between 10°N and 20°N were much cooler than average in the eastern tropical Atlantic, and near average in the Caribbean in March 2015. Much of this unusual cooling was 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.

As always, they included the standard disclaimer with any quiet hurricane season outlook:

"Despite the forecast for below-average activity, 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 2. Departure of sea surface temperature (SST) from average for March 2015, as computed by NOAA/ESRL. SSTs in the hurricane Main Development Region (MDR) between Africa and Central America (red box) were well below average in the eastern Atlantic, and near average in the Caribbean.

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. Correctly predicting this is key, since if El Niño conditions are present this fall, this will likely bring about a quiet Atlantic hurricane season due to increased upper-level winds over the tropical Atlantic creating wind shear that will tend to tear storms apart. 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 Monday, 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 2015 on May 27.

TSR also predicts a below-average Atlantic hurricane season
The April 9 forecast for the 2015 Atlantic hurricane season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) also calls for a quiet season, but is not as low as CSU's forecast. TSR is calling for 11 named storms, 5 hurricanes, 2 intense hurricanes, and an Accumulated Cyclone Energy (ACE) of 56. The long-term averages for the past 65 years are 11 named storms, 6 hurricanes, 3 intense hurricanes, and an ACE of 102. TSR rates their skill level as low for these April forecasts--just 12 - 20% higher than a "no-skill" forecast made using climatology. TSR predicts a 25% chance that U.S. landfalling activity will be above average, a 25% chance it will be near average, and a 50% chance it will be below average. They project that 2 named storms will hit the U.S., with 1 of these being a hurricane. The averages from the 1950-2014 climatology are 3 named storms and 1 hurricane. They rate their skill at making these April forecasts for U.S. landfalls just 4 - 7% higher than a "no-skill" forecast made using climatology. In the Lesser Antilles Islands of the Caribbean, TSR projects 1 named storms and no hurricanes. Climatology is 1 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 in the tropical North Atlantic. Their model is calling for cooler than average SSTs and faster than average trade winds during these periods, and both of these factors should act to decrease hurricane and tropical storm activity. Unlike CSU, TSR is not calling for an El Niño event this fall, which is giving them higher levels of activity in the Atlantic. They add: "Should the TSR forecast for 2015 verify it would mean that the ACE total for 2013-2015 was easily the lowest 3-year total since 1992-1994 and it would imply that the active phase of Atlantic hurricane activity which began in 1995 has likely ended." The next TSR forecast will be issued on May 27.

WSI predicts a quiet Atlantic hurricane season
The April 13 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 to be present this fall as the reason for reducing their expected Atlantic hurricane season numbers from last month's forecast, which called for 10 named storms, 5 hurricanes, and 2 intense hurricanes.

NOAA increases El Niño odds to 60% for the fall
NOAA's monthly El Niño update issued on April 9 gives increased odds that the current weak El Niño event in the equatorial Eastern Pacific will stick around into fall. NOAA is now giving a 70% chance of El Niño lasting through summer, up from their 50 - 60% odds they gave a month ago. They give a 60% chance that El Niño will last though the fall. However, in a March 31 update, the Australian Bureau of Meteorology (BOM) cautioned: “Model outlooks spanning February to May . . . have lower confidence than forecasts made at other times of year. Some models currently show some spread in their outlooks for tropical Pacific Ocean temperatures, indicating that while further warming is indeed very likely, there remains some ambiguity about the amount of warming expected.” In his April 9 post, Wunderblogger Steve Gregory gave a 70% El Niño continuing into this winter, with a 30% chance that we would see a strong El Niño event.

Jeff Masters