Tag Archives: North Atlantic Hurricane Season

Early Riser: Does a Busy 2017 Equate to an Early Start for North Atlantic Hurricanes in 2018?

Although my colleague Peter Holland declared the Atlantic hurricane season officially closed, there are reports in the media that environmental conditions in the North Atlantic basin may be favorable enough to sustain tropical cyclones, potentially adding to what has already been a very active year and perhaps foretelling an early start to hurricane activity in 2018.

The official hurricane season for the North Atlantic runs from June 1 to November 30 and encompasses over 97 percent of annual climatological activity. Out-of-season tropical systems — events happening through December to May — are rare, but not unprecedented. The U.S. National Oceanic and Atmospheric Administration (NOAA) Hurricane Research Division (HRD) states that since 1851, 88 storms have been observed in the Atlantic during the off-season — that’s about one out-of-season storm every two years. In theory, this could be an underestimate, as the peer-reviewed literature suggests an undercount of Atlantic tropical cyclones prior to the satellite reconnaissance era.

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Are We Any Closer to Determining What’s Going on in the Atlantic?

It’s not often that you see an Atlantic hurricane making headlines in January. Subtropical Storm Alex was named by the National Hurricane Center on January 13, 2016 and strengthened into a hurricane one day later. Although Alex ultimately exhibited a short lifespan and caused minimal damage, the storm has the scientific and risk management communities talking about what it might mean for the 2016 hurricane season and the near-term state of the basin.

In October, we discussed the below-average rate of landfalling hurricanes in recent Atlantic seasons, the influence of the Atlantic Multidecadal Oscillation (AMO) on basin activity phase shifts, and how shifts are reflected within the RMS Medium Term Rates (MTR) methodology.

In response to recent quiet seasons, scientists hypothesized about a possible shift in Atlantic hurricane frequency, one that would end the observed active Atlantic hurricane regime that began around the mid-1990s. Central to these discussions was commentary published in the October 2015 edition of Nature Geosciences, suggesting that AMO is entering a negative phase detrimental to Atlantic cyclogenesis.

However, recent peer-reviewed research highlights how sensitive the historical record is to the precise definitions used for hurricane activity. An article soon to be published in the Bulletin of the American Meteorological Society argues that the definition of the recent “hurricane drought,” based on the number of U.S. major landfalling hurricanes, may be arbitrary. This research finds that small adjustments to intensity thresholds used to define the drought, as measured by maximum winds or minimum central pressure, would shorten the drought or eliminate it completely.

In its most recent annual review of the Atlantic basin, RMS recognized that the anticipated atmospheric conditions for the upcoming season present a unique challenge. The latest forecasts suggest that the influence of the El Niño-Southern Oscillation (ENSO), another key indicator of hurricane frequency, may oppose the influence of a negative AMO.

ENSO represents fluctuating ocean temperatures in the equatorial Pacific that influence global weather patterns. El Niño, or a warm phase of ENSO, is associated with increased Atlantic wind shear that historically inhibits tropical cyclone development in the basin. La Niña, or a cool phase of ENSO, is associated with decreased Atlantic wind shear that historically supports tropical cyclone development.

Illustrations of the three main phases of the El Niño-Southern Oscillation. Source: Reef Resilience

ENSO has played an important role in influencing tropical cyclone activity in recent Atlantic hurricane seasons, particularly in 2015. Last season, the basin experienced one of the strongest El Niño phases on record, which contributed to below-average activity and well-below normal Accumulated Cyclone Energy (ACE), an index quantifying total seasonal duration and intensity. .

Looking ahead, the latest ENSO forecasts predict a shift out of the current El Niño phase over the next few months towards a more neutral or even a La Niña phase. The extent to which these conditions impact hurricane activity for 2016 is still to be determined; however, these conditions historically support above average activity.

Mid-February 2016 observations and model forecasts of ENSO, based on the NINO3.4 index, through December 2016. Positive values correspond with El Niño, while negative values correspond with La Niña. Sharp shifts from El Niño to La Niña are not unprecedented: La Niña conditions quickly followed the very strong El Niño of 1997-98. Source: International Research Institute for Climate and Society

The concurrence of potential changes in both the AMO and ENSO represent a unique period for 2016:

  • A negative AMO phase may act to suppress Atlantic hurricane activity in 2016.
  • A neutral or La Niña ENSO phase may act to enhance Atlantic hurricane activity in 2016.

These signals also have a range of potential implications on the RMS MTR forecast. Thus, RMS will spend the upcoming months closely engaging both the scientific community and market regarding this unique state of the basin and its potential forward-looking implications on hurricane activity. Modelers will evaluate the influence and sensitivities of new data, new methods, and new science on the MTR forecast. During this time, RMS will communicate results and insights to the broader market across a variety of mediums, including at Exceedance in May.

This post was co-authored by Jeff Waters and Tom Sabbatelli. 

Jeff Waters

Meteorologist and Manager, Model Product Strategy, RMS
Jeff Waters is a meteorologist who specializes in tropical meteorology, climatology, and general atmospheric science. At RMS, Jeff is responsible for guiding the insurance market’s understanding and usage of RMS models including the North American hurricane, severe convective storm, earthquake, winter storm, and terrorism models. In his role he assists the development of RMS model release communications and strategies, and regularly interacts with rating agencies and regulators around RMS model releases, updates, and general model best practices. Jeff is a member of the American Meteorological Society, the International Society of Catastrophe Managers, and the U.S. Reinsurance Under 40s Group, and has co-authored articles for the Journal of Climate. Jeff holds a BS in geography and meteorology from Ohio University and an MS in meteorology from Penn State University. His academic achievements have been recognized by the National Oceanic and Atmospheric Administration (NOAA) and the American Meteorological Society.

We’re Still All Wondering – Where Have All The Hurricanes Gone?

The last major hurricane to make landfall in the U.S. was Hurricane Wilma, which moved onshore at Cape Romano, Florida, as a Category 3 storm on October 24, 2005. Since then, a decade has passed without a single major U.S. hurricane landfall—eclipsing the old record of eight years (1860-1869) and sparking vigorous discussions amongst the scientific community on the state of the Atlantic Basin as a whole.

Research published in Geophysical Research Letters calls the past decade a “hurricane drought,” while RMS modelers point out that this most recent “quiet” period of hurricane activity exhibits different characteristics to past periods of low landfall frequency.

Unlike the last quiet period—between the late 1960s and early 1990s—the number of hurricanes forming during the last decade was above average, despite a below average landfall rate.

According to RMS Lead Modeler Jara Imbers, these two periods could be driven by different physical mechanisms, meaning the current period is not a drought in the strictest sense. Jara also contends that developing a solid understanding of the nature of the last ten years’ “drought” may require many more years of observations. This additional point of view from the scientific community highlights the ongoing uncertainty around governing Atlantic hurricane activity and tracks.

To provide our clients with a rolling five-year, forward-looking outlook of annual hurricane landfall frequency based on the current climate state, RMS issues the Medium-Term Rate (MTR), our reference view of hurricane landfall frequency. The MTR is a product of 13 individual forecast models, weighted according to the skill each demonstrates in predicting the historical time series of hurricane frequency.

Accounting for Cyclical Hurricane Behavior With Shift Models

Among the models contributing to the MTR forecast are “shift” models, which support the theory of cyclical hurricane frequency in the basin. This was recently highlighted by commentary published in the October 2015 edition of Nature Geosciences and in an associated blog post from the Capital Weather Gang, speculating whether or not the active period of Atlantic hurricane frequency, generally accepted as beginning in 1995, has drawn to a close. This work suggests that the Atlantic Multidecadal Oscillation (AMO), an index widely accepted as the driver of historically observed periods of higher and lower hurricane frequency, is entering a phase detrimental to Atlantic cyclogenesis.

Our latest model update for the RMS North Atlantic Hurricane Models advances the MTR methodology by considering that a shift in activity may have already occurred in the last few years, but was missed in the data. This possibility is driven by the uncertainty in identifying a recent shift point: the more time that passes after a shift and the more data that is added to the historical record, the more certain you become that it occurred.

The AMO has its principle expression in the North Atlantic sea surface temperatures (SST) on multidecadal scales. Generally, cool and warm phases last for up to 20-40 years at a time, with a difference of about 1°F between extremes. Sea level pressure and wind shear typically are reduced during positive phases of the AMO, the predominant phase experienced since the mid-1990s, supporting active periods of Atlantic tropical cyclone activity; conversely, pressure and shear typically increase during negative phases and suppress activity.

Monthly AMO index values, 1860-present. Positive (red) values correspond with active periods of Atlantic tropical cyclone activity, while negative (blue) values correspond with inactive periods. Source: NOAA ESRL

The various MTR “shift” models consider Atlantic multidecadal oscillations using two different approaches:

  • Firstly, North Atlantic Category 3-5 hurricane counts determine phases of high and low activity.
  • Secondly, the use of Atlantic Main Development Region (MDR) and Indo-Pacific SSTs (Figure 2) captures the impact of observed SST oscillations on hurricane activity.

As such, low Category 3-5 counts over many consecutive years and recent changes in the internal variability within the SST time series may point to a potential shift in the Atlantic Basin activity cycle.


The boundaries considered by RMS to define the Atlantic MDR (red box) and Indo-Pacific regions (white box) in medium-term rate modeling.

The “shift” models also consider the time since the last shift in activity. As the elapsed time since the last shift increases, the likelihood of a shift over the next few years also increases, which means it is more likely 20 years after a shift than two years after a shift.

Any uncertainty in tropical cyclone processes is considered through the “shift” models and the other RMS component models, based on competing theories related to historical and future states of hurricane frequency.

Given the interest of the market and the continuous influx of new science and seasonal data, RMS reviews its medium-term rates regularly to investigate whether this new information would contribute to a significant change in the forecast.

If we continue to observe below average tropical cyclone formation and landfall frequency, a shift in the multidecadal variability will become more evident, and the forecasts produced by the “shift” models will decrease. However, it is mandatory that the performance and contribution of these models relative to the other component models are considered before the final MTR forecast is determined.

This post was co-authored by Jeff Waters and Tom Sabbatelli. 

Tom Sabbatelli

Product Manager, Model Product Management, RMS
Tom is a Product Manager in the Model Product Management team, focusing on the North Atlantic Hurricane Model suite of products. He joined RMS in 2009 and spent several years in the Client Support Services organization, primarily providing specialist peril model support. Tom joined RMS upon completion of his B.S. and M.S. degrees in meteorology from The Pennsylvania State University, where he studied the statistical influence of climate state variables on tropical cyclone frequency. He is a member of the American Meteorological Society (AMS).

From Arlene to Zeta: Remembering the Record-Breaking 2005 Atlantic Hurricane Season

Few in the insurance industry can forget the Atlantic hurricane season of 2005. For many, it is indelibly linked with Hurricane Katrina and the flooding of New Orleans. But looking beyond these tragic events, the 2005 season was remarkable on many levels, and the facts are just as compelling in 2015 as they were a decade ago.

In the months leading up to June 2005, the insurance industry was still evaluating the impact of a very active season in 2004. Eight named storms made landfall in the United States and the Caribbean (Mexico was spared), including four major hurricanes in Florida over a six-week period. RMS was engaged in a large 2004-season claims evaluation project as the beginning of the 2005 season approached.

An Early Start

The season got off to a relatively early start with the first named storm—Arlene—making landfall on June 8 as a strong tropical storm in the panhandle of Florida. Three weeks later, the second named storm—Bret—made landfall as a weak tropical storm in Mexico. Although higher than the long-term June average of less than one named storm, June 2005 raised no eyebrows.

July was different.

Climatologically speaking, July is usually one of the quietest months of the entire season, with the long-term average number of named storms at less than one. But in July 2005, there were no fewer than five named storms, three of which were hurricanes. Of these, two—Dennis and Emily—were major hurricanes, reaching categories 4 and 5 on the Saffir-Simpson Hurricane Scale. Dennis made landfall on the Florida panhandle, and Emily made landfall in Mexico. This was the busiest July on record for tropical cyclones.

The Season Continued to Rage

In previous years when there was a busy early season, we comforted ourselves by remembering that there was no correlation between early- and late-season activity. Surely, we thought, in August and September things would calm down. But, as it turned out, 10 more named storms occurred by the end of September—five in each month—including the intense Hurricane Rita and the massively destructive Hurricane Katrina.

In terms of the overall number of named storms, the season was approaching record levels of activity—and it was only the end of September! As the industry grappled with the enormity of Hurricane Katrina’s devastation, there were hopes that October would bring relief. However, it was not to be.

Seven more storms developed in October, including Hurricane Wilma, which had the lowest-ever pressure for an Atlantic hurricane (882 mb) and blew though the Yucatan Peninsular as a category 5 hurricane. Wilma then made a remarkable right turn and a second landfall (still as a major hurricane) in southwestern Florida, maintaining hurricane strength as it crossed the state and exited into the Atlantic near Miami and Fort Lauderdale.

We were now firmly in record territory, surpassing the previous most-active season in 1933. The unthinkable had been achieved: The season’s list of names had been exhausted. October’s last two storms were called Alpha and Beta!

Records Smashed

Four more storms were named in November and December, bringing the total for the year to 28 (see Figure 1). By the time the season was over, the Atlantic, Caribbean and Gulf of Mexico had been criss-crossed by storms (see Figure 2), and many long-standing hurricane-season records were shattered: the most named storms, the most hurricanes, the highest number of major hurricanes, and the highest number of category 5 hurricanes (see Table 1). It was also the first time in recorded history that more storms were recorded in the Atlantic than in the western North Pacific basin. In total, the 2005 Atlantic hurricane season caused more than $90 billion in insured losses (adjusted to 2015 dollars).

The 2005 Atlantic Hurricane Season: The Storm Before the Calm

The 2005 season was, in some ways, the storm before the current calm in the Atlantic, particularly as it has affected the U.S. No major hurricane has made landfall in the U.S. since 2005. That’s not to say that major hurricanes have not developed in the Atlantic or that damaging storms haven’t happened—just look at the destruction wreaked by Hurricane Ike in 2008 (over $13 billion in today’s dollars) and by Superstorm Sandy in 2012, which caused more than $20 billion in insured losses. We should not lower our guard.


Figure 1: Number of named storms by month during the 2005 Atlantic hurricane season

Table 1: Summary of the number of named storms in the Atlantic hurricane basin in 2005 and average season activity through 2014
* Accumulated Cyclone Energy (ACE): a measure of the total energy in a hurricane season based on number of storms, duration, and intensity


Figure 2: Tracks of named storms in the 2005 Atlantic hurricane season

2015 North Atlantic Hurricane Season: What’s in Store?

RMS recently released its 2015 North Atlantic Hurricane Season Outlook. So, what can we expect from this season, which is now underway?

2015 season could be the 10th consecutive year without a major landfalling hurricane over the United States.

The 2014 season marked the ninth consecutive year that no major hurricane (Category 3 or higher) made landfall over the United States. Although two named storms have already formed in the basin so far this year, Tropical Storm Ana and Tropical Storm Bill, 2015 looks to be no different. Forecast groups are predicting a below-average probability of a major hurricane making landfall over the U.S. and the Caribbean in the 2015 season.

The RMS 2015 North Atlantic Hurricane Season Outlook highlights 2015 seasonal forecasts and summarizes key meteorological drivers in the Atlantic Basin.

Forecasts for a below-average season can be attributed to a number of interlinked atmospheric and oceanic conditions, including El Niño and cooler sea surface temperatures.

So what factors are driving these predictions? A strong El Niño phase of the El Niño Southern Oscillation (ENSO) is a large factor, as Jeff Waters discussed previously.

Source: NOAA/ESRL Physical Sciences Division

Another key factor in the lower forecast numbers is that sea surface temperatures (SSTs) in the tropical Atlantic are quite a bit cooler than previous years. SSTs higher than 80°F (26.5°C) are required for hurricane development and for sustained hurricane activity, according to NOAA Hurricane Research Division.

Colorado State University (CSU)’s June 1st forecast is calling for 8 named storms, 3 hurricanes, and 1 major hurricane this season, with an Accumulated Cyclone Energy (ACE) index—used to express activity and destructive potential of the season—of 40. This is well below the 65- and 20-year averages, both over 100.

However, all it takes is one significant event to cause significant loss.

Landfalls are difficult to predict more than a few weeks in advance, as complex factors control the development and steering of storms. Despite the below-average number of storms expected in the 2015 season, it only takes one landfalling event to cause significant loss. Even if the activity and destructive energy of the entire season is lower than previous years, factors such as location and storm surge can increase losses.

For example, Hurricane Andrew made landfall as a Category 5 storm over Florida in 1992, a strong El Niño year. Steering currents and lower-than-expected wind shear directed Andrew towards the coastline of Florida, making it the fourth most intense landfalling U.S. hurricane recorded. Hurricane Andrew also holds the record for the fourth costliest U.S. Atlantic hurricane, with an economic loss of $27 billion USD (1992).

Sometimes, a storm doesn’t even need to be classified as a hurricane at landfall to cause damage and loss. Though Superstorm Sandy had Category 1 hurricane force winds when it made landfall in the U.S., it was no longer officially a hurricane, having transitioned to an extratropical storm.  However, the strong offshore hurricane force winds from Sandy generated a large storm surge, which accounted for 65 percent of the $20 billion insured losses.

While seasonal forecasts estimate activity in the Atlantic Basin and help us understand the potential conditions that drive tropical cyclone activity, a degree of uncertainty still surrounds the exact number and paths of storms that will form throughout the season. For this reason, RMS recommends treating seasonal hurricane activity forecasts with a level of caution and to always be prepared for a hurricane to occur.

For clients, RMS has released new resources to prepare for the 2015 hurricane season available on the Event Response area of RMS Owl.