Active 2013 Atlantic Hurricane Season Expected by CSU, TSR, and WSI

After three consecutive years with a remarkable 19 named storms in the Atlantic, expect another Atlantic hurricane season with similar levels of activity in 2013, says the hurricane forecasting team of Dr. Phil Klotzbach and Dr. Bill Gray of Colorado State University (CSU) in their latest seasonal forecast issued April 10. They call for an Atlantic hurricane season with 18 named storms, 9 hurricanes, 4 intense hurricanes, and an Accumulated Cyclone Energy (ACE) of 165. The long-term averages for the past 63 years are 11 named storms, 6 hurricanes, 3 intense hurricanes, and an ACE of 103. The active hurricane period that began in 1995 has averaged 15 named storms, 8 hurricanes, and 4 major hurricanes. The 2013 forecast calls for an above-average chance of a major hurricane hitting the U.S., both along the East Coast (48% chance, 31% chance is average) and the Gulf Coast (47% chance, 30% chance is average). The Caribbean is forecast to have a 61% 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 2013 hurricane season may resemble: 2004 (which featured seven major hurricanes, including four that hit the U.S.); 1996 (six major hurricanes, including Cat 3 Hurricane Fran that hit North Carolina); 1966 (three major hurricanes, featuring Cat 4 Inez, which hit Mexico, Hispaniola, Cuba, the Bahamas, and Florida); 1952 (only seven named storms, but three major hurricanes); and 1915, which boasted a Cat 3 hurricane that hit New Orleans and a Cat 4 that hit Galveston. These years all had neutral El Niño conditions, above-average Sea Surface Temperatures (SSTs) in the tropical and North Atlantic, and cool ocean temperatures in the Northeast Pacific (a negative PDO) during February - March. None of the five analogue years had a significant El Niño during the peak of the hurricane season. The average activity for these years was 10 named storms, 7 hurricanes, and 4 major hurricanes.


Figure 1. Hurricane Michael as seen by NASA's Aqua satellite at 12:20 pm EDT Thursday September 6, 2012. At the time, Michael was a major Category 3 hurricane with 115 mph winds. Hurricane Sandy was the only other major Atlantic hurricane of 2012. Image credit: NASA.

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

1) Neutral El Niño conditions are expected during the August - September - October peak of hurricane season. Since the active hurricane period we are in began in 1995, neutral years have seen much above-average activity (remember the neutral El Niño year of 2005?) 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.

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 near-average in the western tropical Atlantic, but unusually warm in the eastern tropical Atlantic, in March 2013. Much of this unusual warming was due to a persistent negative phase of the North Atlantic Oscillation (NAO) since mid-February (which also brought an unusually cold March to the Eastern U.S. and Western Europe.) A negative phase of the NAO is associated with a weakened Bermuda-Azores High and slower trade winds across the tropical Atlantic. The slower winds allow the ocean to heat up more, due to less mixing of cool water to the surface. 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.


Figure 2. Departure of sea surface temperature (SST) from average for April 8, 2013, as computed by NOAA's NESDIS branch. SSTs in the hurricane Main Development Region between Africa and Central America (red box) were well above-average.

How good are the April forecasts?
The forecasters are using a statistical model developed in 2011 for making April forecasts, so we don't have a long enough track record to judge how good the new model is. The new model predicted a below-average year for 2012, with 10 named storms, 4 hurricanes, and 2 intense hurricanes. The actual tally was much higher, with 19 named storms, 10 hurricanes, and 2 intense hurricanes. 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. Currently, ocean temperatures are very close to average in the Eastern Pacific, and the large majority of the El Niño models are predicting a continuation of these neutral conditions for the coming hurricane season.

CSU maintains an Excel spreadsheet of their forecast errors ( expressed as a mathematical correlation coefficient, where positive means a skilled forecast, and negative means they did worse than climatology) for their their April forecasts. 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 3, is the one worth paying attention to. Their early June forecasts have shown considerable skill over the years.

TSR predicts an active hurricane season
The April 5 forecast for the 2013 Atlantic hurricane season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) calls for an active season with 15 named storms, 8 hurricanes, 3 intense hurricanes, and an Accumulated Cyclone Energy (ACE) of 131. The long-term averages for the past 63 years are 11 named storms, 6 hurricanes, 3 intense hurricanes, and an ACE of 103. TSR rates their skill level as low for these April forecasts--just 6 - 15% higher than a "no-skill" forecast made using climatology. TSR predicts a 63% chance that U.S. land falling activity will be above average, a 21% chance it will be near average, and a 16% chance it will be below average. They project that 4.4 named storms will hit the U.S., with 1.9 of these being hurricanes. The averages from the 1950-2012 climatology are 3.1 named storms and 1.4 hurricanes. They rate their skill at making these April forecasts for U.S. landfalls just 7 - 8% higher than a "no-skill" forecast made using climatology. In the Lesser Antilles Islands of the Caribbean, TSR projects 1.5 named storms, 0.6 of these being hurricanes. Climatology is 1.1 named storms and 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 2013 sea surface temperatures in the tropical North Atlantic. Their model is calling for warmer than average SSTs and slower than average trade winds during these periods, and both of these factors should act to increase hurricane and tropical storm activity.

WSI predicts an active hurricane season
The April 8 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 an active season with 16 named storms, 9 hurricanes, and 5 intense hurricanes.



Jeff Masters