CSU and TSR predict above average 2012 Atlantic hurricane season
Above-average Atlantic hurricane activity is likely for 2012, but there is a 40% chance of an El Niño event that will keep hurricane activity below average, according to the latest seasonal forecast issued today by Dr. Phil Klotzbach and Dr. Bill Gray of Colorado State University (CSU). For the first time in twenty years, the CSU team is not issuing a December forecast with a specific number of tropical storms and hurricanes. Instead, they have issued a more qualitative forecast, which I think is a great idea, since their quantitative December forecasts have shown no skill. Their outlook for the 2012 Atlantic hurricane season:
15% chance: Very active season with 14-17 named storms, 9-11 hurricanes, 4-5 major hurricanes
45% chance: Active season with 12-15 named storms, 7-9 hurricanes, 3-4 major hurricanes
30% chance: Inactive season with 8-11 named storms, 3-5 hurricanes, 1-2 major hurricanes
10% chance: Very inactive season with 5-7 named storms, 2-3 hurricanes, 0-1 major hurricanes
An average season has 11 named storms, 6 hurricanes, and 2 intense hurricanes. The main reason that CSU's December forecasts have shown no skill is because we have no skill predicting El Niño events nine months or more into the future. When an El Niño event occurs, bringing much above average wind shear over the tropical Atlantic, hurricane activity is substantially reduced. Making successful seasonal hurricane forecasts requires that one make a successful El Niño forecast.
Figure 1. Forecasts of El Niño conditions by 20 computer models, made in November 2011. The longest range forecasts for July-August-September (JAS) at the right side of the image show that 3 models predict weak El Niño conditions, 8 predict neutral conditions, and 1 predicts a weak La Niña. El Niño conditions are defined as occurring when sea surface temperatures in the Equatorial Pacific off the coast of South America (the "Niño 3.4 region) rise to 0.5°C above average (top red line). La Niña conditions occur when SSTs in this region fall to 0.5°C below average (blue line.) Image credit: Columbia University.
What will El Niño do in 2012?
We currently have a borderline weak to moderate La Niña episode in the Eastern Pacific, characterized by cooler than average waters off the equatorial coast of South America. While we can say with good confidence that La Niña will continue through the winter and into spring, it is highly uncertain what might happen next summer and fall to La Niña. In April and May, we typically see La Niña fade to neutral, and in many cases, a full-blown El Niño will develop by the fall. As the CSU team notes, there have been fourteen years since 1950 which had La Niña conditions that were similar to what we are experiencing this December. During the following years' hurricane season, an El Niño event developed 36% of the time, in those fourteen years. In 2012, the odds of a fall El Niño may be higher than this, since we have gone three years since the last El Niño, and these events typically occur every 3 - 7 years. Of the 12 El Niño/La Niña computer models that made November predictions for the July-August-September 2012 portion of hurricane season (Figure 1), only 3 (20%) predicted that El Niño would arrive. However, these models have no skill predicting El Niño so far in advance.
2012 Atlantic hurricane season forecast from Tropical Storm Risk, Inc.
The British private forecasting firm Tropical Storm Risk, Inc. (TSR), issued their 2012 Atlantic hurricane season forecast today. TSR is calling for an above-average year, with 14.1 named storms, 6.7 hurricanes, and 3.3 intense hurricanes. TSR predicts a 49% chance of an above-average hurricane season, 30% chance of a near-normal season, and a 21% chance of a below normal season. TSR bases their December forecast on predictions that sea surface temperatures next fall in the tropical Atlantic will be above about 0.1°C above average, and trade wind speeds will be about 0.2 m/s slower than average. The trade wind speed prediction is based on a forecast for neutral El Niño conditions in August - September 2012.
I like how TSR puts their skill level right next to the forecast numbers: 3% skill above chance at forecasting the number of named storms, 0% skill for hurricanes, and 7% skill for intense hurricanes. That's not much skill, and really, we have to wait until the June 1 forecasts by CSU, NOAA, and TSR to get a forecast with reasonable skill.
Figure 2. Forecast skill of the TSR, NOAA (National Oceanic and Atmospheric Administration) and CSU (Colorado State University) seasonal hurricane outlooks 2002-2011 as a function of lead time. NOAA does not release seasonal outlooks before late May. It is clear there is little skill in forecasting the upcoming number of Atlantic hurricanes from the prior December. Skill climbs slowly as the hurricane season approaches. Moderate skill levels are reached by early June and good skill levels are achieved from early August. Image credit: Tropical Storm Risk, Inc (TSR).