| Above: Hurricane Matthew, the strongest Atlantic hurricane of 2016, as seen on September 30, 2016, when it was rapidly intensifying into the first Category 5 storm observed in the Atlantic since 2007. |
Residents living in Hurricane Alley need to prepare for what may be another busy Atlantic hurricane season. In its first outlook for 2017, issued Thursday, NOAA’s Climate Prediction Center predicted a 45% chance for an above-normal Atlantic hurricane season, a 35% chance for a near-normal season and a 20% chance for a below-normal season. NOAA gave a 70 percent likelihood of 11 - 17 named storms, 5 - 9 hurricanes, 2 - 4 major hurricanes (Category 3 or higher on the Saffir-Simpson Hurricane Wind Scale) and an Accumulated Cyclone Energy (ACE) 75%-155% of the median. These numbers include Tropical Storm Arlene, which developed in April over the northeast Atlantic. If we take the midpoint of these ranges, NOAA called for 14 named storms, 7 hurricanes and 3 major hurricanes. This is above the 1981-2010 seasonal averages of 12 named storms, 6 hurricanes and 3 major hurricanes.
NOAA cited four main factors influencing their forecast:
1) Either ENSO-neutral or weak El Niño conditions are expected over the tropical Pacific Ocean [ENSO refers to El Niño/ Southern Oscillation, which has three phases: El Niño, Neutral, and La Niña.] El Niño suppresses hurricane development in the Atlantic by increasing the amount of vertical wind shear and dry, stable air that tends to prevail over the Main Development Region (MDR) for hurricanes, which includes the tropical North Atlantic Ocean and Caribbean Sea between 9.5°N and 21.5°N latitude. The latest monthly NOAA/IRI probabilistic ENSO forecast, issued May 18, calls for approximately a 60% chance of El Niño conditions during the peak August-September-October period of the Atlantic hurricane season.
2) Near- or above-average sea-surface temperatures (SSTs) are expected across much of the MDR.
3) Near-average or weaker-than-average vertical wind shear is expected in the MDR.
4) Near-average sea level pressures are expected in the MDR.
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| Figure 1. The 2017 Atlantic hurricane season got off to a premature start on April 20, with the formation of Tropical Storm Arlene—just the second April tropical storm of record since the satellite era began in the mid-1960s. Arlene’s top sustained winds reached 50 mph on April 21, but the storm remained far from land in the central Atlantic Ocean, and lost its identity as a tropical cyclone later that day. Above, we see a MODIS image of Arlene at 15:12 UTC April 20, 2017. Image credit: NASA. |
As usual, there is some disagreement among the various outlooks issued since April as to how busy a hurricane season the Atlantic will see, though the outlooks issued more recently have tended to call for more activity than the earlier outlooks. The Barcelona Supercomputing Center and Colorado State University have a nice web page summarizing all of the major Atlantic hurricane season forecasts. Here are some of the major forecasts made since April:
TSR predicts a near-average Atlantic hurricane season: 12 named storms
The April 5 forecast for the 2017 Atlantic hurricane season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) called for a near-average Atlantic hurricane season about 30% below the long-term (1950-2016) norm and the recent 2006-2015 ten-year norm. TSR predicted 12 named storms, 6 hurricanes, 2 intense hurricanes and an Accumulated Cyclone Energy (ACE) of 67 for the period May though December. The long-term averages for the past 65 years are 11 named storms, 6 hurricanes, 3 intense hurricanes and an ACE of 101. TSR rates their skill level as low for these April forecasts--just 9 - 15% higher than a "no-skill" forecast made using climatology. TSR predicted a 29% chance that U.S. landfalling activity would be above average, a 23% chance of near-average, and a 48% chance of below-average. They projected that two named storms but no hurricanes would hit the U.S. The averages from the 1950-2016 climatology are three named storms and one hurricane. They rated their skill at making these April forecasts for U.S. landfalls just 3 - 6% higher than a "no-skill" forecast made using climatology. In the Lesser Antilles Islands of the Caribbean, TSR projected one named storm and no hurricanes. Climatology is one named storm and less than 0.5 hurricanes. They added: "It should be stressed that the precision of hurricane outlooks issued in April is low and that large uncertainties remain for the 2017 hurricane season." The next TSR forecast will be issued on May 26.
CSU predicts a slightly below-average Atlantic hurricane season: 4 hurricanes
A slightly below-average Atlantic hurricane season is likely in 2017, said the hurricane forecasting team from Colorado State University (CSU) in their latest seasonal forecast issued April 6. Led by Dr. Phil Klotzbach, with new coauthor Dr. Michael Bell, the CSU team called for an Atlantic hurricane season with 11 named storms, 4 hurricanes, 2 intense hurricanes, and an Accumulated Cyclone Energy (ACE) of 75. The long-term averages for the period 1981 - 2010 were 12 named storms, 6.5 hurricanes, 2 intense hurricanes, and an ACE of 92. The CSU outlook also called for a 42% chance of a major hurricane hitting the U.S. in 2017, with a 24% chance for the East Coast and Florida Peninsula and a 24% chance for the Gulf Coast. The Caribbean was forecast to have a 34% chance of seeing at least one major hurricane. All of these probabilities are slightly below the long-term numbers from the last century. Their next forecast will be released June 1.
The Weather Company predicts an above-average Atlantic hurricane season: 14 named storms
The May 20 forecast from The Weather Company called for an above-average Atlantic hurricane season with 14 named storms, 7 hurricanes, and 3 major hurricanes. The Weather Company increased its forecast compared to the one issued in April due to two factors:
1) Warmer sea-surface temperatures in the North Atlantic, with indications that further warming is likely.
2) Reduced potential for the development and strength of El Niño later this summer.
Penn State predicts an above-average Atlantic hurricane season: 15 named storms
Here's a forecast worth paying attention to: the April 25 forecast made using a statistical model by Penn State's Michael Mann, Sonya Miller, and alumnus Michael Kozar called for an active season with 15.3 named storms (expected range: 11.4 to 19.2). Their prediction was made using statistics of how past hurricane seasons have behaved in response to sea surface temperatures (SSTs), the El Niño/La Niña oscillation, the North Atlantic Oscillation (NAO), and other factors. The statistical model assumed that in 2017 the late-April +0.81°C departure of temperature from average in the Main Development Region (MDR) for hurricanes in the tropical Atlantic would persist throughout hurricane season, ENSO-neutral conditions (no El Niño or La Niña) would continue through fall, and the NAO would be near average. If a weak El Niño develops, their model predicted reduced activity: 12.4 named storms, plus or minus 3.5 named storms.
The PSU team has been making Atlantic hurricane season forecasts since 2007, and these predictions have done well, except for in 2012, when an expected El Niño did not materialize:
2007 prediction: 15 named storms, Actual: 15
2009 prediction: 12 named storms, Actual: 9
2010 prediction: 23 named storms, Actual: 19
2011 prediction: 16 named storms, Actual: 19
2012 prediction: 11 named storms, Actual: 19
2013 prediction: 16 named storms, Actual: 14
2014 prediction: 9 named storms, Actual: 8
2015 prediction: 7 named storms, Actual: 11
2016 prediction: 19 named storms, Actual: 15
NCSU predicts a close to average Atlantic hurricane season: 11 - 15 named storms
The April 18 forecast from North Carolina State University (NCSU), called for a not far from average Atlantic hurricane season with 11 - 15 named storms, 6 hurricanes and 1 - 3 major hurricanes. They used a statistical model encompassing more than 100 years of past Atlantic hurricane activity to make their forecasts.
Cuban Met service predicts a near-average Atlantic hurricane season: 10 named storms
The May 4 forecast from the Cuban Meteorological Service, INSMET, called for a near-average Atlantic hurricane season with 10 named storms and 6 hurricanes. The forecast was based on a statistical prediction model developed by Ballester, González and Pérez (2010).
