NOAA predicts an active Atlantic hurricane season: 15 named storms, 8 hurricanes
by Jeff Masters, wunderblog, May 19, 2011
The National Oceanic and Atmospheric Administration (NOAA) issued its 2011 Atlantic hurricane season forecast today. NOAA forecasts a very active and possibly hyperactive season. They give a 65% chance of an above-normal season, a 25% chance of a near-normal season, and just a 10% chance of a below-normal season. NOAA predicts a 70% chance that there will be 12–18 named storms, 6–10 hurricanes, and 3–6 major hurricanes, with an Accumulated Cyclone Energy (ACE) 105%–200% of the median. If we take the midpoint of these numbers, NOAA is calling for 15 named storms, 8 hurricanes, 4.5 major hurricanes, and an ACE index 152% of normal.
A season with an ACE index over 165% is considered "hyperactive." An average season has 10–11 named storms, 6 hurricanes, and 2 intense hurricanes. Hurricane seasons during 1995–2010 have averaged about 15 named storms, 8 hurricanes, and 4 major hurricanes, with an ACE index 151% of the median. NOAA classifies 11 of the 16 seasons since 1995 as above normal, with eight being hyperactive. Only five seasons since 1995 have not been above normal, which include four El Niño years (1997, 2002, 2006 and 2009) and the 2007 season.
The forecasters cited the following main factors that will influence the coming season:
(1) Above-average sea surface temperatures (SSTs) are expected in the hurricane Main Development Region (MDR), from the Caribbean to the coast of Africa between between 10° N and 20° N. SSTs in the MDR during April were about 0.5 °C above average, the 14th warmest April SSTs in the past 100 years. This is far below last year's record 1.4 °C anomaly, but still plenty warm enough to help drive above-average Atlantic hurricane activity. Long-range computer forecast models are predicting a continuation of these above-average SSTs through the peak part of hurricane season.
(2) We are in an active period of hurricane activity that began in 1995, thanks to a natural decades-long cycle in hurricane activity called the Atlantic Multidecadal Oscillation (AMO): "During 1995–2010, some key aspects of the tropical multi-decadal signal within the MDR have included warmer than average SSTs, reduced vertical wind shear and weaker easterly trade winds, below-average sea-level pressure, and a configuration of the African easterly jet that is more conducive to hurricane development from tropical waves moving off the African coast. Many of these atmospheric features typically become evident during late April and May, as the atmosphere across the tropical Atlantic and Africa begins to transition into its summertime monsoon state."
(3) An El Niño event is not expected this year: "Another climate factor known to significantly impact Atlantic hurricane activity is the El Niño–Southern Oscillation (ENSO.) The three phases of ENSO are El Niño, La Niña and ENSO-Neutral. El Niño events tend to suppress Atlantic hurricane activity, while La Niña events tend to enhance it (Gray 1984). Currently, the 2010–2011 La Niña episode is dissipating. Based on observations and ENSO forecast models, ENSO-Neutral conditions are likely during the 2011 Atlantic hurricane season."
(4) NOAA is increasingly using output from ultra-long range runs of the computer forecast models we rely on to make day-to-day weather forecasts, for their seasonal hurricane forecasts: "The outlook also takes into account dynamical model predictions from the NOAA Climate Forecast System (CFS), the European Centre for Medium Range Weather Forecasting (ECMWF), the United Kingdom Meteorology (UKMET) office, and the EUROpean Seasonal to Inter-annual Prediction (EUROSIP) ensemble. These models are indicating a high likelihood of an above normal season."
How accurate are the NOAA seasonal hurricane forecasts?
A talk presented by NHC's Eric Blake at the 2010 29th Annual AMS Conference on Hurricanes and Tropical Meteorology studied the accuracy of NOAA's late May seasonal Atlantic hurricane forecasts, using the mid-point of the range given for the number of named storms, hurricanes, intense hurricanes, and ACE index. Over the past 12 years, a forecast made using climatology was in error, on average, by 3.6 named storms, 2.5 hurricanes and 1.7 intense hurricanes. NOAA's May forecast was not significantly better than climatology for these quantities, with average errors of 3.5 named storms, 2.3 hurricanes and 1.4 intense hurricanes. Only NOAA's May ACE forecast was significantly better than climatology, averaging 58 ACE units off, compared to the 74 for climatology.
Using another way to measure skill, the Mean Squared Error, May NOAA forecasts for named storms, hurricanes, and intense hurricanes had a skill of between 5% and 21% over a climatology forecast (Figure 2). Not surprisingly, NOAA's August forecasts were much better than the May forecasts and did significantly better than a climatology forecast.
Figure 1. Mean absolute error for the May and August NOAA seasonal hurricane forecasts (1999–2009 for May, 1998–2009 for August), and for forecasts made using climatology from the past five years. A forecast made using climatology was in error, on average, by 3.6 named storms, 2.5 hurricanes and 1.7 intense hurricanes. NOAA's May forecast was not significantly better than climatology for these quantities, with average errors of 3.5 named storms, 2.3 hurricanes and 1.4 intense hurricanes. Only NOAA's May ACE forecast was significantly better than climatology, averaging 58 ACE units off, compared to the 74 for climatology. Image credit: Verification of 12 years of NOAA seasonal hurricane forecasts, National Hurricane Center.
How do NOAA's seasonal hurricane forecasts compare to CSU and TSR?
Two other major seasonal hurricane forecasts will be released over the next two weeks. On June 1, Phil Klotzbach and Bill Gray of Colorado State University (CSU) issue their forecast, and the British firm Tropical Storm Risk (TSR) will issue their outlook on May 24. A three-way comparison of the forecast accuracy of the three groups' forecasts (Figure 2) reveals that all three organizations enjoy some success at making accurate seasonal forecasts, with NOAA and CSU making the best late May–early June forecasts overall. While the skill of these forecasts is low, they are useful for businesses such as the insurance industry.
Figure 2. Comparison of the percent improvement over climatology for May and August seasonal hurricane forecasts for the Atlantic from NOAA, CSU and TSR from 1999–2009 (May) and 1998–2009 (August). using the Mean Squared Error. Image credit: Verification of 12 years of NOAA seasonal hurricane forecasts, National Hurricane Center.
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