The 2015 Atlantic Hurricane Begins; New CSU, TSR Forecasts Call For a Quiet Season

The 2015 Atlantic hurricane season is officially underway. We've already had one early season storm, Tropical Storm Ana; will we have an early June Tropical Storm Bill? There are indications that the second tropical depression of the year has a chance to form late this week in the waters near South Florida or the Bahama Islands on Friday or Saturday. We have warmer than average SSTs in these waters, and the Madden-Julian Oscillation (MJO) is expected to be active in the Western Hemisphere late this week, and both of these factors argue for higher than usual odds of an early June tropical depression forming in the Atlantic. Over the past few days, the GFS model has been consistently advertising the possibility that an area of low pressure capable of developing into a tropical depression will form in this region, although the European model (so far) has not gone along with this idea. If we do get something developing, it would potentially be a heavy rain threat for South Florida and the Northern Bahamas over the weekend, but then move northeastwards out to sea without troubling any more land areas.


Figure 1. The strongest Atlantic hurricane of 2014, 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.

Summary of the 2015 Atlantic hurricane season forecasts
The major hurricane forecasting groups are not impressed with this season's potential to be an active one, and are universally calling for 2015 to be a below average to way below-average year for the Atlantic. The highest forecast numbers were from Weather Underground Community Hurricane Forecast, which called for 11 named storms, 5 hurricanes, and 2 major hurricanes. The lowest numbers were from North Carolina State University: 5 named storms, 2 hurricanes, and 1 intense hurricane. The long-term averages for the past 65 years are 11 named storms, 6 hurricanes, 3 intense hurricanes. Here are the forecasts:

NOAA: 8.5 named storms, 4.5 hurricanes, 1 major hurricane, ACE index 62.5% of normal.
Colorado State University (CSU): 8 named storms, 3 hurricanes, 1 major hurricane, ACE index 44% of normal.
Tropical Storm Risk, Inc. (TSR): 10 named storms, 4 hurricanes, 1 intense hurricane, ACE index 36% of normal.
UKMET office: 8 named storms, 5 hurricanes, ACE index 74% of normal (June - November.)
FSU Center for Ocean-Atmospheric Prediction Studies (COAPS):
Weather Underground Community Hurricane Forecast: 11 named storms, 5 hurricanes, 2 major hurricanes
WSI: 9 named storms, 5 hurricanes, 1 major hurricane
Penn State: 8 named storms.
North Carolina State University: 5 named storms, 2 hurricanes, 1 intense hurricane.
Coastal Carolina University: 8 named storms, 4 hurricanes, 2 intense hurricanes.
The Cuban Meteorological Service, INSMET: 9 named storms, 3 hurricanes.

The main reason for the quiet forecasts is the likely intensification of the current moderate strength El Niño. Every 3 - 7 years, variations in tropical winds and pressure shift warm ocean waters eastwards from the Western Pacific to the South American coast, causing an El Niño event. The unusually warm water tends to drive an atmospheric circulation that brings strong upper-level winds to the tropical Atlantic, creating high levels of wind shear that tend to tear hurricanes apart. Another factor leading to lower forecast numbers than in previous years is the fact that sea surface temperatures in the tropical Atlantic are near average to below average this year--quite a bit cooler than we've seen during the typical year during our active hurricane period that began in 1995.


Figure 2. Departure of sea surface temperature (SST) from average for May 30, 2015. SSTs were near average to slightly below over the Atlantic Main Development Region (MDR) for hurricanes, from the coast of Africa to Central America between 10°N and 20°N, including the Caribbean. As of June 1, 2015, SSTs over the region typically used to define El Niño events, 5°N - 5°S to 120°W - 170°W (the Niño 3.4 region) were in the moderate for El Niño conditions, +1.3°C from average, according to the latest weekly NOAA El Niño update. Image credit: NOAA/NESDIS.

Colorado State predicts a below-average hurricane season: 8 named storms
A new Atlantic hurricane season forecast issued June 1 by Dr. Phil Klotzbach and Dr. Bill Gray of Colorado State University (CSU) again calls for a below-average season with 8 named storms, 3 hurricanes, 1 intense hurricane, and an Accumulated Cyclone Energy (ACE) of 40, which is about 44% of average. The 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 risk of a major hurricane in the Caribbean is also below average, at 22% (42% is average.) The June 1 numbers are nearly unchanged from their April 9 forecast, with the exception of an additional named storm (due to the formation of Tropical Storm Ana in May.)

CSU's Analogue years: 1997, 1987, 1982, 1972, 1965, and 1957
The CSU team picked six previous years when atmospheric and oceanic conditions were similar to what they expect for this year: at least moderate El Niño conditions, and generally cool sea surface temperatures in the tropical Atlantic. Those years were 1997 (which featured 8 named storms, 3 hurricanes, and 1 intense hurricane); 1987 (7 named storms, 3 hurricanes, and 1 intense hurricane); 1982 (6 named storms, 2 hurricanes, and 1 intense hurricane); 1972 (7 named storms, 3 hurricanes, and 0 intense hurricanes); 1965, featuring Hurricane Betsy, which hit New Orleans as a Category 3 storm; and 1957, which featured the deadliest June hurricane on record, Hurricane Audrey, which killed 416 people in Texas and Louisiana. The average activity for these years was 7 named storms, 3 hurricanes, and 1 major hurricane.

TSR predicts a below-average hurricane season: 10 named storms
The May 27 forecast for the 2015 Atlantic hurricane season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) calls for a below-average season with 10 named storms, 4 hurricanes, 1 intense hurricane, and a remarkably low Accumulated Cyclone Energy (ACE) of 37. 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 modest for these late May forecasts: 6 - 24% higher than a "no-skill" forecast made using climatology. 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 hitting the United States. TSR rates their skill at making these May forecasts for U.S. landfalls just 4% - 8% 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 in 2014. Climatology is 1.1 named storms and less than 0.5 hurricanes.

TSR's two predictors for their statistical model are the forecast July - September trade wind speeds over the Caribbean and tropical North Atlantic, and the forecast August - September 2015 sea surface temperatures (SSTs) in the tropical North Atlantic Main Development Region (MDR) for hurricanes. Their model is calling for SSTs 0.33°C below average and trade winds 1.36 m/s stronger than average during these periods; both of these factors should act to decrease hurricane and tropical storm activity. The July-September 2015 trade wind prediction is based on an expectation of weak El Niño conditions in August-September 2015. TSR will issue an updated forecast on July 6, 2015.

Even a quiet hurricane season can be devastating
Quiet hurricane seasons with below-average activity can still produce major hurricanes that cause massive devastation. The five seasons that CSU lists as analogue years for 2015 produced four hurricanes that had their names retired, including one that killed 8,000 people in Cuba (Flora of 1963) and one that killed over 400 people in Texas and Louisiana (Audrey of 1957.) Even if this is an El Niño year, that doesn't mean it will be a quiet season. Recall the El Niño year of 2004, when four major hurricanes pounded the U.S.--Ivan, Charlie, Jeanne, and Frances. Those of you in Hurricane Alley should prepare for the 2015 season the same way you would for a predicted hyperactive season, and be ready for the Storm of the Century to hit your location.

Good luck to everyone in Hurricane Alley this hurricane season, and I look forwards to tracking all the activity this year with you!

Jeff Masters