|Above: Hurricane Matthew, the strongest Atlantic hurricane of 2016, as seen by the MODIS instrument in the Caribbean north of Colombia at 02:25 UTC October 1, 2016. At the time, Matthew was near peak strength--a Category 5 storm with 165 mph winds. Matthew was the Atlantic’s first Category 5 storm since Felix of 2007, and lasted as a major hurricane for eight days from Sept. 30 to Oct. 7. Image credit: NASA.|
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 is calling 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 calls 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 is 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.
Five years with similar pre-season February and March atmospheric and oceanic conditions were selected as “analog” years that the 2017 hurricane season may resemble. These years were characterized by neutral to weak La Niña conditions the previous year, with a transition to weak or moderate El Niño conditions during the current year:
1957 (8 named storms, 3 hurricanes, and 2 intense hurricanes)
1965 (6 named storms, 4 hurricanes, and 1 intense hurricane)
1972 (7 named storms, 3 hurricanes, and 0 intense hurricane)
1976 (10 named storms, 6 hurricanes, and 2 intense hurricanes)
2002 (12 named storms, 4 hurricanes, and 2 intense hurricanes)
These five years all featured neutral conditions in the Eastern Pacific transitioning to El Niño conditions. The average activity for these years was 8.6 named storms, 4 hurricanes, and 1.4 major hurricanes--below the long-term average. The most notable storms during these years were Hurricane Audrey of 1957, Hurricane Betsy of 1965, and Hurricane Agnes of 1972.
A quiet season expected due to cooling SSTs, and El Niño
The CSU team cited two main reasons why this may be a below-average hurricane season:
1) A weak to moderate El Niño event is expected to develop by autumn. If El Niño conditions are present this fall, this would tend to favor a slower-than-usual Atlantic hurricane season due to an increase in the upper-level winds over the tropical Atlantic that can tear storms apart (higher vertical wind shear.) Sea surface temperatures (SSTs) were 0.3°C above average over the past week in the so-called Niño 3.4 region (5°S - 5°N, 120°W - 170°W), where SSTs must be at least 0.5°C above average for five consecutive months (each month being a 3-month average) for a weak El Niño event to be declared (and atmospheric conditions must also be consistent with El Niño). In their latest March 9 monthly ENSO outlook, NOAA’s Climate Prediction Center (CPC) gave a 50 – 55% chance of an El Niño developing during the July – December 2017 period, which was also reflected in the CPC/IRI probabilistic outlook. The dynamical model average calls for about a 1.0°C warm anomaly in the Niño 3.4 region by August through October, indicative of a moderate El Niño event.
2) The tropical Atlantic has cooled over the past month, and the far North Atlantic is currently colder than normal. These cold anomalies tend to force atmospheric conditions that are less conducive for Atlantic hurricane formation and intensification. Much of this anomalous cooling is due to a persistent positive phase of the North Atlantic Oscillation (NAO) since late last year. While the NAO has generally been positive since January, the anomalous cooling really ramped up in March. The trade winds have been very strong across the tropical Atlantic during March, driving increased evaporation, upwelling and associated sea surface temperature cooling.
As always, the CSU team included this standard disclaimer:
"Coastal residents are reminded that it only takes one hurricane making landfall to make it an active season for them. They should prepare the same for every season, regardless of how much activity is predicted."
|Figure 1. Departure of sea surface temperature (SST) from average for late March 2017, as computed by NOAA/ESRL. SSTs in the hurricane Main Development Region (MDR) between Africa and Central America were below average in the eastern Atlantic, and above average in the Caribbean. 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. The cold anomalies in the far North Atlantic, relatively cool anomalies in the eastern tropical Atlantic and warm anomalies off the East Coast of the United States are typically associated with a negative phase of the Atlantic Multidecadal Oscillation (AMO) pattern, suggesting that we are no longer in the active hurricane period that began in 1995. Image credit: NOAA/ESRL.|
How good are the April forecasts?
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. 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 June 1, is the one worth paying attention to. Their early June forecasts have shown considerable skill over the years. NOAA issues its first seasonal hurricane forecast for 2017 in late May, with an update in August.
Figure 2. Comparison of the percent improvement in mean square error over climatology for seasonal hurricane forecasts for the Atlantic from NOAA, CSU and TSR from 2003-2016, using the Mean Square Skill Score (MSSS). Values less than zero indicate that pure climatology does a better job than the forecast. The figure shows the results using two different climatologies: a fixed 50-year (1950 - 1999) climatology, and a 10-year 2007 - 2016 climatology. Skill is poor for forecasts issued in December and April, modest for June forecasts, and moderate-to-good for August forecasts. Using this methodology, TSR has had the best seasonal forecasts. Image credit: Tropical Storm Risk, Inc. (TSR)
TSR predicts a below-average Atlantic hurricane season
The April 5 forecast for the 2017 Atlantic hurricane season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) calls for a below-average Atlantic hurricane season about 30% below the long-term (1950-2016) norm and the recent 2006-2015 ten-year norm. TSR is predicting 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 predicts a 29% chance that U.S. landfalling activity will be above average, a 23% chance it will be near average, and a 48% chance it will be below average. They project that two named storms but no hurricanes will hit the U.S. The averages from the 1950-2016 climatology are three named storms and one hurricane. They rate 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 projects one named storm and no hurricanes. Climatology is one named storm and less than 0.5 hurricanes.
TSR’s main predictor for their statistical model of Atlantic hurricane activity is the forecast July - September trade wind speed over the Caribbean and tropical North Atlantic. Their model is calling for trade winds 0.81 m/s faster than average, due to the anticipated development of a moderate El Niño by the summer/autumn of 2017. They add: "Should the TSR forecast for 2017 verify it would mean that the ACE index total for 2013-2017 would be easily the lowest 5-year total since 1991-1994, and would be equivalent to a typical 5-year total experienced during the inactive phase of Atlantic hurricane activity between 1970 and 1994. However, 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.