If we’ve learned anything about forecasts and predictions (pick any recent event, sporting, political etc.) they give an indication of the situation, but cannot predict the absolute outcome, and surprises can most definitely happen. We are into the first weeks of the North Atlantic hurricane season, which officially runs for six months from June 1 to November 30, and a variety of forecasting groups and agencies have issued preseason forecasts.
As of the first week of June, most agencies and meteorological organizations forecast average to above-average levels of hurricane activity for this season. Looking across ten different forecasts between April and right up to June 1, depending on the agency, forecasts predict 10–17 named storms, 4–9 hurricanes, and 1–4 major hurricanes. Predictions from three main agencies (NOAA’s Climate Prediction Center, Tropical Storm Risk, and Colorado State University) show an average to slightly above average season in terms of activity, with the Accumulated Cyclone Energy (ACE) index1 predictions near to average.
Some forecasts were issued from as early as December last year, but as the start of the season approaches, the level of forecast skill generally improves. At this stage, it is always timely to examine what the forecasters believe lies in store for the season, and to draw conclusions from their findings. For a full analysis on this year’s season, click on the link to download the RMS 2017 North Atlantic Hurricane Season Outlook report.
Hurricane Forecasting: A Balancing Act
The latest seasonal hurricane forecasts are a balancing act comprising of the competing impacts of three major oceanic and atmospheric influences; the status of the El Niño-Southern Oscillation (ENSO), sea surface temperatures (SSTs) in the Atlantic, and vertical wind shear across the tropical Atlantic and the Main Development Region (MDR).
ENSO is a coupled ocean-atmosphere climate phenomenon characterized by periodic two-to-seven-year fluctuations in SSTs and sea level pressure gradients across the equatorial Pacific Ocean. The evolution of ENSO through the coming months remains somewhat uncertain, though current operational ENSO guidance suggests an equal probability of warm-neutral or weak El Niño conditions through the period August to October. El Niño, the warm phase of ENSO, is associated with increased Atlantic wind shear that historically inhibits tropical cyclone development in the basin.
Should ENSO transition to a weak El Niño phase during the hurricane season, in the absence of any other factors, this would be expected to result in lower-than-average Atlantic hurricane activity, depending on the timing of the transition. The timing of any forecasted transition to El Niño conditions is crucial in influencing the impact that ENSO will have on overall hurricane activity this season. Conversely, if conditions were to remain warm-neutral, again in the absence of other factors, the expected impact on hurricane activity would be minimal, resulting in average activity.
Nino3.4 SST Anomaly Forecast from the NCEP Coupled Forecast System Model version 2 (CFSv2). The threshold for El Niño conditions is a Niño 3.5 SST anomaly greater than 0.5° Celsius, while for La Niña conditions the threshold is a Niño 3.4 SST anomaly of less than -0.5° Celsius. Data: National Weather Service (NWS), National Centers for Environmental Prediction (NCEP), Climate Prediction Center (CPC)
Running alongside the possible impacts of ENSO, Atlantic SSTs could exert a major influence on hurricane activity this season. SSTs in the tropical Atlantic and the main development region are currently above average. Very warm anomalies are present off the East Coast of the U.S., in parts of the MDR, and off the West Coast of Africa, with cool anomalies in the far North Atlantic and north-central Atlantic.
Above-average SSTs in the central tropical Atlantic, especially in the MDR, are typically associated with above-average hurricane activity. Similarly, average, or slightly weaker-than-average vertical wind shear, which is generally associated with slightly increased hurricane activity, is forecast over the tropical Atlantic and Caribbean during the peak months of the hurricane season.
North Atlantic SST anomalies (° Celsius, anomalies with respect to 1981-2010 climatology) for May 2017. Data: National Oceanic and Atmospheric Administration Earth System Research Laboratory (NOAA ESRL) Physical Sciences Division.
Mix in these and other uncertain and competing factors, and forecasting the activity of this year’s North Atlantic hurricane season becomes particularly complex. The ongoing evolution and timing of changes to these key factors over the coming months will determine just how active the 2017 season will be.
Forecasts Changed Since April
What is also striking is that these recent forecasts represent a marked change since April, when below-average activity was forecast. This change primarily reflects two key factors; the probability of El Niño conditions during the peak months of the hurricane season has diminished since April this year, while Atlantic sea-surface temperatures (SSTs) have increased.
Another Early Start to The Season
One tropical storm has already formed this year. On April 20, Tropical Storm Arlene evolved from a subtropical depression to become the first Atlantic named storm to form in the month of April since Tropical Storm Ana in 2003, making this the third consecutive year that a storm has been named before the official start of the Atlantic hurricane season on June 1.
Will This Season End the Eleven Year Wait for a U.S. Major Hurricane Landfall?
The U.S. has gone 11 years without a major hurricane landfall, since Hurricane Wilma made landfall as a Category 3 storm in Florida in 2005. This is the longest period without a major hurricane landfall in the U.S. since records began in 1851. Could 2017 be the year this record ends?
Forecasts of an above-average season do not increase the chances of a major hurricane landfall, and regardless of the final number of named storms this season, it may take only one landfalling hurricane to cause significant loss and damage.
The forecast groups that issue seasonal landfalling probabilities forecast an increased likelihood of an above-average number of hurricane landfalls along the U.S. coastline in 2017. However, there is low skill in seasonal landfall forecasts at these lead times as individual storm tracks are highly sensitive to the location of cyclogeneses, the local atmospheric and oceanic conditions and weather patterns, which cannot be predicted this far in advance.
For more detail and analysis on the factors discussed in this blog, please download the RMS 2017 North Atlantic Hurricane Season Outlook report.
1 The Accumulated Cyclone Energy (ACE) index is calculated as the square of the sum of the maximum sustained wind speed (in knots) at 6-hour intervals for the duration of the storm at tropical storm strength or greater (sustained wind speed of 35 knots or higher).