Chris Landsea and Eric Blake 
An Incredibly Busy Hurricane Season
The 2020 Atlantic hurricane season was extremely active and destructive with 30 named storms. (The Hurricane Specialists here at the National Hurricane Center use the designation “named storms” to refer to tropical storms, subtropical storms, hurricanes, and major hurricanes.) We even reached into the Greek alphabet for names for just the second time ever. The United States was affected by a record 13 named storms (six of them directly impacted Louisiana), and a record yearly total of 7 billion-dollar tropical cyclone damage events was recorded by the National Centers for Environmental Information (https://www.ncdc.noaa.gov/billions/time-series/US). Nearly every country surrounding the Gulf of Mexico, Caribbean Sea, and tropical/subtropical North Atlantic was threatened or struck in 2020. Total damage in the United States was around $42 billion with over 240 lives lost in the United States and our neighboring countries in the Caribbean and Central America.
The 30 named storms in 2020 sets a record going back to the 1870s when the U.S. Signal Service (a predecessor to the National Weather Service) began tracking tropical storms and hurricanes. The only year that comes close is 2005 with 28 named storms. It’s also apparent that a very large increase has occurred in the number of observed named storms from an average of 7 to 10 a year in the late 1800s to an average of 15 to 18 a year in the last decade or so – a doubling in the observed numbers over a century! (The black curve in the figure below represents a smoothed representation of the data that filters out the year to year variability in order to focus on time scales of a decade or more).
However, the number of named storms is only one measure of the overall measure of a season’s activity. And indeed, for the 2020 season, other measures of Atlantic tropical storm and hurricane activity were not record breaking. For example, the number of hurricanes (14) was well above average, but fell short of the previous record of 15 hurricanes that occurred in 2005.
For overall monitoring of tropical storm and hurricane activity, tropical meteorologists prefer a metric that combines how strong the peak winds reached in a tropical cyclone, and how long they lasted – called Accumulated Cyclone Energy or ACE. By this measure, 2020 was extremely busy, but not even close to record breaking. In fact, with a total ACE of 180 units, 2020 was only the 13th busiest season on record since 1878 with seasons like 1893, 1933, 1950, and 2005 substantially more active than 2020. One can also see that while there is a long-term increase in recorded ACE since the late 1800s, it’s quite a bit less dramatic than the increase seen with named storms. There also is a pronounced busier/quieter multi-decadal (40- to 60-year) cycle with active conditions in the 1870s to 1890s, late 1920s to 1960s, and again from the mid-1990s onward. Conversely, quiet conditions occurred in the 1900s to early 1920s and 1970s to early 1990s.
Technology Change and Named Storms
So why would the record for named storms be broken in 2020, while the overall activity as measured by ACE is not even be close to setting a record?
The answer is very likely technology change, rather than climate change. Today we have many advanced tools to help monitor tropical and subtropical cyclones across the entire Atlantic basin such as geostationary and low-earth orbiting satellite imagery, the Hurricane Hunter aircraft of the U.S. Air Force Reserve and National Oceanic and Atmospheric Administration (NOAA), coastal weather radars, and scatterometers (radars in space that provide surface wind measurements). In addition, the instrumentation and measuring techniques used by the satellites, aircraft and radars are continually improving. These technological advances allow us at the National Hurricane Center to better identify, track, and forecast tropical and subtropical cyclones with an accuracy and precision never before available. This is great news for coastal residents and mariners, since these tools help us provide the best possible forecasts and warnings to aid in the best preparedness for these life-threatening systems.
Such technology, though, was not available back at the advent of the U.S. Signal Service’s tropical monitoring in the 1870s. Without these sophisticated tools, meteorologists in earlier times not only had difficulty in forecasting tropical cyclones, but they also struggled in even knowing if a system existed over the open ocean. In the late 19th and early 20th Centuries, the only resource hurricane forecasters could use to monitor tropical cyclones were weather station observations provided via telegraph. Such an approach is problematic for observing – much less forecasting – tropical cyclones that develop and spend most of their lifecycle over the open ocean. Here’s a timeline of critical technologies that have dramatically improved tropical meteorologists’ ability to “see” and monitor tropical cyclones:
The upshot of all of these advances in the last century is much better identification of the existence of tropical cyclones and their strongest winds (or what meteorologists call “Intensity”). So, the further one goes back in time, the more tropical cyclones (and portions of their life cycle) were missed, even for systems that may have been a major hurricane. This holds for both counting named storms back in time as well as integrated measures like ACE. Our database is incomplete and has – as statisticians would say – a severe undersampling bias that is much more prominent earlier in the record. HURDAT2 – our Atlantic hurricane database – is an extremely helpful record which is a “by-product” of NHC’s forecasting operations, but it is very deficient for determining real long-term trends. (It’s important to point out that many data entries in HURDAT2 for intensity and even the position of the named storms are educated guesses as opposed to being based on observations before the 1970s advent of regular satellite imagery). To be able to examine questions about any impact from man-made global warming (aka climate change) on long-term changes in the number of named storms, for example, one must first account for the massive technology change over the last century.
Fortunately, to help address this issue, researchers at NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL) – (Gabe Vecchi and Tom Knutson in 2008’s Journal of Climate) have invented a way to estimate how many named storms were missed in the pre-geostationary satellite era (before the 1970s). This was done by comparing the population of tracks and sizes of named storms that have occurred versus the density of observations from ships that were traversing the ocean. If there were ships everywhere all of the time back to the 1870s (and these ships didn’t try to avoid running into tropical cyclones, which they certainly did), there would be very few named storms unaccounted for. But the reality is that much of the Atlantic Ocean, Gulf of Mexico, and Caribbean Sea was sparsely traversed by ships from the late 19th Century until the middle of the 20th Century. (The plots below indicate the amount of shipping traffic and weather observations from those ships – Orange/Red are numerous, Green/Yellow are moderate, Gray are few, and White are no measurements).
In addition to the issue of named storms that were previously missed, due to the lack of ability to observe them, technological improvements also have effectively allowed the standards for naming a storm to be refined resulting in better identification of weak (near the 39-mph/63-kph threshold) systems. Tropical warnings for many of the weak, short-lived named storms in past eras were not issued, and thus these systems were not automatically included into the HURDAT2 database. In the cases when forecasters in earlier years were either 1) not sure that the system possessed the required 39-mph/63-kph winds, 2) assumed that it would be too short in duration, or 3) thought that the system was non-tropical (i.e., with a warm to cold gradient of temperature across the system’s center), they usually did not issue named storm advisories, and therefore these systems did not get added into the historical database.
In research that the lead author had investigated (Chris Landsea and company in 2010’s Journal of Climate), we discovered that weak, short-lived (lasting less than or equal to two days) named storms – aka “Shorties” – had shown a dramatic increase in occurrence over time. There were only about one a year in HURDAT2 up until the 1920s, about 3 per year from the 1930s to the 1990s, and jumping up to around 5 per year since 2000.
Of the 30 named storms in 2020, seven were Shorties and a few more were just longer than two days in duration. Of these seven Shorties, four are very unlikely to have been “named” before around 2000: Dolly, Edouard, Omar, and Alpha. (Of the remaining Shorties, Bertha and Kyle may have been named, while Fay likely would have been named). These and other weak, short-lived systems since 2000 have been observed and recognized as tropical storms due to new tools available to forecasters including scatterometers, Advanced Microwave Sounding Units, the Advanced Dvorak Technique, and the Cyclone Phase Space diagrams. The Hurricane Specialists here at the National Hurricane Center then are able to issue advisories on these named storms in real-time and then include them into the HURDAT2 database at the end of the season.
From a warning perspective for mariners and coastal residents, it is very beneficial that the National Hurricane Center is now naming (and recording) these Shorties. But without accounting for how technology affects our records, one can come to some unfounded conclusions about true long-term changes in named storm activity. In addition, it is worth pointing out, but perhaps not too surprising, that it has been shown by the researchers at Princeton University and at GFDL (Villarini et al. 2011, Journal of Geophysical Research) that the observed increase in Shorties has no association with any environmental factor known to influence named storms including man-made global warming. It is therefore reasonable to conclude that the dramatic increase in the number of these Shorties is simply due to better observational technology.
An “Apples-to-Apples” Comparison of the 2020 Long-Lived Named Storms with the Past
So how can we come up then with a more apples-to-apples comparison of how the number of named storms has actually changed over the last 100 years plus? Here are the steps that were performed in the 2010 Journal of Climate paper, about Shorties, updated for data through the 2020 hurricane season:
(1) Start with the original HURDAT2 database of named storms from 1878 onward:
(2) Remove all of the Shorties from the original database, leaving just the long-lived named storms:
(3) Add in the best estimate of the number of missed long-lived named storms before geostationary satellite imagery and the Dvorak technique became available:
The resulting final time series shows tremendous variability, with highest values of 23 in 2020 and 20 in 1887 and 2005, and lowest values of 2 in 1914, and 3 in 1925, 1982, and 1994. Overall, there remains a modest upward trend in the database over the entire time series superimposed with quasi-cyclic variations seen in the ACE data as was discussed earlier: higher activity in the late 1800s, mid-1900s, and from the mid-1990s onward, but lower activity in the early 1900s, and in the 1970s to early 1990s. These cycles of higher and lower activity have been linked to a natural phenomenon called the Atlantic Multidecadal Oscillation (AMO) (see paper by Stan Goldenberg, Chris Landsea, and colleagues in 2001’s Science). Recent controversial research, however, is calling into question whether the AMO actually exists (see paper by Michael Mann and company in 2021’s Science). Regardless of the validity of the AMO, the bottom line is that the doubling in the number of named storms over a century is very likely due to technology change, not natural or man-made climate change.
(4) And finally, add in the uncertainty to these estimates with the reasonable largest number of missed long-lived named storms. This represents the 95% method uncertainty value, or in layman’s terms, the largest reasonable number of missed systems.
Note that after adding on the uncertainty to the missed number of long-lived named storms (blue coloring), we can conclude that 1887 and 2020 may be just as busy for the number of long-lived named storms.
The New “Normal” for Named Storm Numbers
With the completion of the 2011 to 2020 decade, climatologists are updating records to provide a new “normal” (or average) to compare against new weather. The previous 30-year based climate period to decide if a weather event or season was unusual or expected was 1981-2010. For weather phenomena around the world, we’re now changing the years to compute normal conditions to 1991-2020. (The 30-year normal concept is designed to provide a long enough time period to obtain relatively stable statistics, and to also have the time period reflect the most recent weather experienced over a human generation. Thirty years is a good compromise between these two aspects.) It might seem odd to non-meteorologists to change the definition of “average” every ten years, but meteorologists/climatologists do so because climate is never stationary, i.e., the climate is always changing. The climate has both natural variations (like El Niño/La Niña, effects from volcanic eruptions, and the Atlantic Multidecadal Oscillation) and man-made changes (like urban heat island, land use changes, and greenhouse gas emissions) that affect what’s been observed around the last three decades. These revisions of new averages are done around the world in conjunction with the World Meteorological Organization. Thus NOAA is updating the average of temperature, precipitation, and other meteorological parameters to reflect what has been observed.
This shift in the period used for the 30-year climate standard changes the definitions of average (or “normal”) levels of tropical cyclone activity to the following for the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico (see this report by NOAA for more details):
|System Type||Old 1981-2010 Average||New 1991-2020 Average|
These changes, therefore, reflect that most of the new 1991-2020 climatology period is within an active period that began in 1995 and includes the impact of the technology changes discussed above that have led to the National Hurricane Center more accurately diagnosing and naming more systems in the last couple of decades.
The answers and conclusions to “Was 2020 a Record-Breaking Hurricane Season? Yes, but…”:
- Doubling in the number of named storms over a century is very likely due to technology change, not natural or man-made climate change;
- 2020 set a record for number of named storms, but given the limitations in our records it is possible that other years (such as 1887) were just as active for long-lived named storms; and
- The boost in average or “normal” conditions from 12 to 14 named storms is due to a combination of a busy era that began in 1995 as well as the ability of the National Hurricane Center to observe and accurately diagnose more weak, short-lived named storms than had been done previously, mostly due to technology advancements.
A follow-on blog post, putting these observed changes of the number of named storms into context of what may be expected to occur in the future, is expected to be published in the near future.
 Christopher W. Landsea is the Chief of the Tropical Analysis and Forecast Branch at the National Weather Service’s National Hurricane Center in Miami, Florida. Eric Blake is a Senior Hurricane Specialist at the National Hurricane Center. It should be noted that the following discussion is Chris’ and Eric’s opinions only and does not represent any official position of NHC, NWS or NOAA in general. Various scientists within NOAA have differing opinions about global warming’s impact on hurricanes and there is no official NOAA policy on the topic. Varying ideas on an issue often mean that it is a science in progress with no definitive answers. That is certainly the case with regards to global warming and hurricanes. Helpful comments on an earlier version of this writeup were provided by Neal Dorst, Stan Goldenberg, Robbie Berg, and Mike Brennan.
 Accumulated Cyclone Energy is calculated by squaring the named storm’s intensity – maximum sustained surface winds (expressed in knots) – for every six hours that the system had at least a 39-mph (63-kph) intensity.
 There is on-going research into updating and revising the HURDAT2 database for the seasons of 1851 to 1999 in order to improve and make more complete the records that currently exist. This is done by obtaining the original named storm observations from ships, weather stations, Hurricane Hunter aircraft, radars, and satellites and using today’s best meteorological analyses to revise the positions, intensities, and statuses in the database. This work also adds in newly discovered named storms that were not identified as such at the time. Currently, the reanalysis project has added 35 years (1851 to 1885) to our official records and has revised the 1886 through 1965 hurricane seasons.
Two years ago this month, Tropical Storm Bill made landfall along the central Texas coast, just 17 hours after becoming a tropical cyclone only 145 miles offshore. The precursor disturbance, a broad and ill-defined area of low pressure, had already been producing tropical-storm-force winds, and there was little doubt that the system would soon bring those dangerous winds onshore. Although NHC’s Tropical Weather Outlooks had been talking about the possibility of those conditions two days in advance, and their likelihood one day in advance, some in the media and emergency management communities lamented the lack of earlier formal tropical storm warnings and full advisory products from NHC. A few even suggested that NHC classify the disturbance as a tropical storm when it wasn’t one. By policy and tradition, NHC advisories, track and intensity forecasts, and any associated watches and warnings begin only after a disturbance has become a tropical cyclone; in this case a tropical storm warning was issued as soon as Bill formed, about 12 hours before the hazardous winds reached the coast. For some additional discussion on why warnings couldn’t have been issued any earlier for Bill, please see our blog post written after that event.
This is hardly the only example of a tropical cyclone striking land shortly after genesis, and well within the normal 48-hour watch/warning time frame. In 2010, Tomas struck Barbados as a tropical storm 27 hours after formation, and St. Vincent and St. Lucia as a hurricane 38 hours after formation. In September of 2007, Humberto made landfall as a hurricane along the Texas coast a mere 19 hours after becoming a tropical cyclone. This recurring problem has been on our minds for a long time, and this season we’ve introduced a service enhancement to address the issue.
Starting this year, NHC has the option to issue advisories, track and intensity forecasts, watches, and warnings for disturbances that are not yet a tropical cyclone, but which pose the threat of bringing tropical storm or hurricane conditions to land areas within 48 hours. This substantial change in policy means that we won’t have to wait for a disturbance to meet the technical requirements of a tropical cyclone (such as having a well-defined center of circulation or sufficiently organized thunderstorm activity) to issue forecasts or post warnings. And boy, it didn’t take long for us to employ this new option, with both the pre-Bret and pre-Cindy disturbances requiring the initiation of potential tropical cyclone advisories on two consecutive days! But more on that in a moment.
Although we’ve been working on the technical and administrative changes to bring this about over the past two years, the effort actually began after the Deepwater Horizon disaster in 2010, when NHC was asked to provide enhanced forecast support for the response effort. Since then, NHC has been practicing making track and intensity forecasts for disturbances, and at the same time we’ve been improving our ability to forecast tropical cyclone genesis. We now believe that the science has advanced enough to allow the confident prediction of tropical cyclone impacts while these systems are still in the developmental stage.
So for these land-threatening “potential tropical cyclones” (and that’s the term we’re using in our advisories), NHC can now issue the full suite of text, graphical, and watch/warning products that previously has only been used for ongoing tropical cyclones. This includes the cone graphic, public advisory, discussion, wind speed probabilities – everything – and all the products will look exactly the same as our tropical cyclone products. The only thing that’s different is what we call the “system type”; we’ve added POTENTIAL TROPICAL CYCLONE to the roster of possible system types. And since you asked (or at least were thinking about asking), here’s the complete list:
POTENTIAL TROPICAL CYCLONE
For those who are interested in the definitions of each of these system types, you can find them in National Weather Service Instruction 10-604, Tropical Cyclone Names and Definitions.
We did consider some alternatives to the term potential tropical cyclone. “Tropical disturbance” was a fairly obvious option but we knew that some of these precursor disturbances weren’t going to be tropical in nature (such as a frontal cyclone evolving into a subtropical or tropical cyclone), so that eliminated tropical disturbance. Another option was simply “disturbance”, which aside from evoking Star Wars imagery (I felt a great disturbance in the Gulf), did not in our view adequately convey the appropriate level of threat. In the end, potential tropical cyclone seemed both accurate and appropriate to the threat, although it’ll take a bit of getting used to for some.
Potential tropical cyclones will share the naming rules currently used for depressions, with depressions and potential tropical cyclones being numbered from a single list (e.g., “One”, “Two”, “Three”, …, “Twenty-Three”, etc.). The assigned number will always match the total number of systems we’ve written advisories on within that basin during the season. For example, if three systems requiring advisories have already occurred within a basin in a given year, the next land-threatening disturbance would be designated “Potential Tropical Cyclone Four”. If a potential tropical cyclone becomes a tropical depression, its numerical designation doesn’t change (i.e., Potential Tropical Cyclone Four becomes Tropical Depression Four).
Potential tropical cyclone advisory packages will be issued at the standard advisory times of 5 AM, 11 AM, 5 PM, and 11 PM EDT, with three-hourly Intermediate Public Advisories being issued at 2 AM, 8 AM, 2 PM, and 8 PM EDT when watches or warnings are in effect. The product suite will include a five-day track and intensity forecast, just as is done for ongoing tropical cyclones. In addition, the Potential Storm Surge Flooding Map and Storm Surge Watch/Warning graphic would be issued for these systems when appropriate. We’ll continue issuing advisory packages on a potential tropical cyclone until watches or warnings are discontinued or until the threat of tropical-storm-force winds for land areas sufficiently diminishes, at which point advisories would be discontinued. However, if it seems likely that new watches or warnings would be necessary within a short period of time (say 6-12 hours), then advisories could continue during that brief gap in warnings in the interest of service continuity.
Since the primary issuance trigger is the threat of tropical storm conditions over land, there won’t be any specific threshold of formation likelihood for the initiation of advisories. For example, a fast-moving tropical wave approaching the Lesser Antilles might already have tropical-storm-force winds but no closed wind circulation. In this case, a genesis forecast of 40% – 50% would likely be enough to trigger advisories and warnings. In contrast, a genesis forecast of 70% for a system close to shore might not trigger advisories if the system were not expected to reach tropical storm strength before moving inland.
The issuance of NHC products for potential tropical cyclones is very much analogous to the change that occurred after Hurricane Sandy in 2012, when NHC advisories on post-tropical cyclones became possible. After Sandy, we realized that there was great benefit to users in NHC’s being able to continue writing advisories on systems even after they were no longer a tropical cyclone. That solved the service continuity problem on the “back end”, and now we’re completing the process by ensuring a steady flow of information on the front end of a tropical cyclone’s life cycle. In all cases, we’ll be trying to ensure that warning types (tropical vs. non-tropical) don’t have to change in the middle of an event.
There are some things to be aware of with this new capability. First, potential tropical cyclone advisories will not be issued for systems that threaten only marine areas – largely because this would pose an unmanageable workload/staffing issue for us but also because marine forecast products (the High Seas and Offshore Waters forecasts) already allow the issuance of gale and storm warnings before a tropical cyclone has formed.
Second, because potential tropical cyclones will have a standard five-day forecast track and uncertainty cone, to avoid potential confusion with the cone we’re going to stop drawing potential formation areas for these systems in the Graphical Tropical Weather Outlook.
We’re also concerned that some users may pay too much attention to the longer-range part of these new forecasts (the part beyond 72 hours). We know that forecast errors for weaker and developing systems tend to be larger than those for strong storms and hurricanes, and we even considered only going out to 72 hours with the new potential tropical cyclone advisories (since the primary purpose was to support watches and warnings). But in the end, consistency and technical issues argued for going out to five days, and that’s what we’re doing. So it’s likely that forecast-to-forecast changes in the longer-range portion of our potential tropical cyclone advisories will be larger than what folks are used to. And for those of you who like to look at forecast model intensity guidance, be aware that most of these intensity models assume the system is a tropical cyclone. Since that won’t be the case for these systems, intensity models run on potential tropical cyclones will generally have a high bias. And lastly, since many potential tropical cyclones will not have well-defined centers, there will likely be large jumps in the reported location of these systems from advisory to advisory. But even with all these caveats, we think that the ability to post warnings before a cyclone forms is an important service enhancement – one that will help save lives and protect property, while at the same time allowing NHC to analyze and report on tropical systems as accurately and as honestly as possible.
After our experiences with Bret and Cindy, we’re optimistic about the value of this new capability. Advisories on Potential Tropical Cyclone Two were started 24 hours before Bret officially became a tropical cyclone, giving residents of Trinidad and Tobago, Grenada, and northeastern Venezuela an additional day of warning for tropical storm conditions. If this were still 2016, places like Trinidad may have only had three to six hours between the time of the first advisory and the time when tropical storm force winds began on the island. And for Cindy, advisories on Potential Tropical Cyclone Three were initiated roughly 21 hours before Cindy met the criteria of a tropical cyclone. This allowed Tropical Storm Warnings to be issued for southeastern Louisiana 21 hours earlier than they would have been if the storm had occurred last year.
Just a few weeks into the new season, we’re pretty happy about the way this all worked. We think we successfully demonstrated the ability to provide more advanced warning than we could have in previous years for these developing tropical cyclones. But we’d love to hear feedback from our users, customers, and partners. Were the potential tropical cyclone advisories in advance of Bret and Cindy confusing? Helpful? Maybe both? Or bad puns aside, did the new capability fit the “Bill”?
If you’d like to provide comments on your experiences with the Potential Tropical Cyclone advisories during Bret and Cindy, please feel free to contact Jessica Schauer, the NWS Tropical Cyclone Program Leader, at Jessica.Schauer@noaa.gov.
— James Franklin
Editor’s Note: This post marks James’s last blog contribution as a member of the NHC family. After 35 years of service in the federal government (17 years at NOAA’s Hurricane Research Division and 18 years at the National Hurricane Center), James is retiring at the end of this week. We want to thank James for his contributions to not only the blog, but also for his many contributions to hurricane forecasting and NHC operations over the past several decades. Although James will no longer be “inside the eye” of the sometimes-hectic NHC scene, we know he won’t be too far away cheering on his beloved Miami Hurricanes, Miami Dolphins, and Florida Panthers. Congratulations, James, and happy retirement!
The development of Tropical Storm Bill so close to the Texas coast, with the posting of a formal tropical storm warning only about 12 hours before winds of that intensity came ashore on Tuesday June 16th, highlighted a long-standing and well-known limitation in the tropical cyclone program of the National Weather Service (NWS). An ongoing NHC initiative to improve service for such systems is the subject of today’s blog post.
Although there’s nothing new about a tropical cyclone forming on our doorstep, what is new is an increased ability to anticipate it. NHC has greatly enhanced its forecasts of tropical cyclone formation over the past several years, introducing quantitative 48-hr genesis forecasts to the Tropical Weather Outlook in 2008, and extending those forecasts to 120 hours in 2013. In 2014, we introduced a graphic showing the locations of tropical disturbances and the areas where they could develop into a depression or storm over the subsequent five days. Thirty-six hours in advance of Bill’s formation, NHC gave the precursor disturbance a 60% chance of becoming a tropical cyclone, and increased that probability to 80% about 24 hours in advance. While nothing was guaranteed, we were pretty confident a tropical storm was going to form before the disturbance reached the coast. And although we weren’t issuing specific track forecasts for the disturbance, NHC’s new graphical Tropical Weather Outlook (example below) showed where the system was generally headed.
We wouldn’t have had such confidence 20 years ago, or even 5 years ago. And so our tropical cyclone warning system, developed over several decades, doesn’t allow for a watch or warning until a depression or storm actually forms and NHC’s advisories begin; by both policy and software, warning issuances are tied to cyclone advisories. If we had wanted to issue a tropical storm watch for Bill on the morning of Sunday the 14th (48 hours prior to landfall), or a warning that evening (36 hours ahead of landfall), we would have had to pretend that the disturbance in the Gulf of Mexico was a tropical cyclone. Even during the day on Monday, data from an Air Force Reserve Hurricane Hunter aircraft showed that the disturbance had not yet become a depression or storm.
Couldn’t NHC have called the disturbance a tropical storm anyway, in the interest of enhanced preparedness? Yes, but what if the disturbance never becomes a tropical storm – remember, even an 80% chance of formation means it won’t become a tropical storm at least once every five times. So naming it early risks the credibility of the NWS and NHC, and endangers a trust we’ve worked for decades to establish. In addition, there are legal and financial consequences to an official designation of a tropical cyclone – consequences that obligate us to call it straight. And finally, as custodians of the tropical cyclone historical record, we have a responsibility to ensure the integrity of that record.
When systems that have the potential to become tropical cyclones pose hazards to life and property, NHC’s best avenue for highlighting those hazards currently is the Tropical Weather Outlook. Ahead of Bill’s formation, the possibility of tropical storm conditions along the middle and upper Texas coast was included in the Sunday evening Outlook, and by early Monday afternoon the Outlooks were saying those conditions were likely. Products issued by NWS local forecast offices (WFOs) carried similar statements.
Although most folks seemed to have gotten the message that a tropical storm was coming, it’s widely thought that the Tropical Weather Outlook and local WFO products don’t carry the visibility and weight of an NHC warning, or of an NHC advisory package with its attendant graphics. In addition, some institutions have preparedness plans that are tied to the presence of warnings. We agree that warnings during the disturbance stage could improve community response, and we’ve been working toward that goal since 2011. In that season, NHC initiated an internal experiment in which the Hurricane Specialists prepare track and intensity forecasts for disturbances with a high likelihood of development, and use these forecasts to determine where watches and warnings would have been appropriate. These internal disturbance forecasts have had some successes and failures, but may now be good enough to make public.
With our colleagues across the NWS, we’re now working through the logistics of expanding the tropical cyclone product and warning suite to accommodate disturbances. One plan under consideration calls for NHC to produce a five-day track and intensity forecast for those disturbances having a high chance of becoming a tropical cyclone, and which pose the threat of bringing tropical-storm-force winds to land areas. The forecasts would be publicly issued through the standard NHC advisory products, including the Public Advisory, Discussion, and Wind Speed Probability Product, along with the forecast cone and the other standard graphics. These advisory packages would be issued at the normal advisory times, and continue until the threat of tropical-storm-force winds over land had diminished. If and when the disturbance became a tropical cyclone, advisory packages would simply continue.
We are still evaluating these and other options for getting tropical cyclone warnings out for potential tropical cyclones. If we do begin issuing forecasts for these systems, we know from our experimental forecasts that they won’t be as accurate as our current public forecasts for tropical cyclones are – and we’ll want to make sure users know about those uncertainties. There are many details to iron out and much technical work to do, but we’re hopeful to have this service enhancement in place for the 2017 hurricane season.
— James Franklin
A previous blog entry described the new NHC five-day tropical cyclone formation (or genesis) products. In this blog entry, we discuss the factors that go into these predictions.
The primary tool used at NHC for five-day tropical cyclone genesis forecasts is global numerical modeling. Global models can predict many of the environmental factors that influence tropical cyclone formation, and the skill of these models has been improving with time. More tropical cyclone formations are being forecast with longer lead times, and weather prediction models show fewer “false alarms” than in the past. Recent studies suggest, and forecaster experience seems to confirm, that a consensus of the available model guidance usually outperforms any single model. This “two heads are better than one” approach works as long as the models (or heads) are somewhat independent of one another. In addition, NHC is currently evaluating a few statistical techniques that use the global model output to produce objective guidance designed to assist hurricane specialists in developing the probabilities of formation issued in the Tropical Weather Outlook.
Kelvin Waves and the Madden-Julian Oscillation
Global model guidance is not the only tool available to NHC forecasters, however. Researchers have learned that a majority of lower latitude tropical cyclone formations are associated with waves in the atmosphere moving through the global Tropics from west to east. Two particularly important wave types are the Convectively Coupled Kelvin Wave (CCKW), which circumnavigates the equator in about 15 to 20 days, and the Madden-Julian Oscillation (MJO), which transits the globe in 30 to 60 days. These waves are normally initiated by large areas of thunderstorm activity over tropical regions, especially near India and southeastern Asia. These waves are different in both frequency and direction of motion from the more well-known tropical waves that originate over Africa and often spawn tropical cyclones as they move westward across the Atlantic and eastern North Pacific basins.
Tropical cyclone formation often accompanies the passage of the “active phase” of either the faster-moving CCKWs or the slower-moving MJO. Figure 1 shows tropical cyclone tracks over a 37-year period in active and inactive phases of the MJO as the wave moves around the globe, along with increased or decreased rainfall anomalies associated with the two phases of the MJO (Zhang 2013). In the figure, the active phase of the MJO for the Atlantic occurs in panel (a), while for the eastern Pacific the active phase occurs in panel (d). The less active phases for these two basins fall in panels (c) and (b), respectively.
This concentration of tropical cyclone activity occurs because each type of wave temporarily makes large-scale environmental conditions, such as vertical wind shear or atmospheric moisture, more conducive for tropical cyclone formation. Although not every wave causes a tropical cyclone to form, pre-existing disturbances have a greater likelihood of developing into tropical cyclones after the passage of a CCKW or the MJO. High-activity periods can last as long as a week or more with the MJO, but are generally followed by days to possibly weeks of little to no activity during the inactive phases of these waves, when large-scale conditions become unfavorable for tropical cyclone formation. Forecasters use real-time atmospheric data and other tools to diagnose the location and motion of these important catalysts for tropical cyclone formation.
Here is an example from the 2014 hurricane season of how forecasters used these atmospheric signals. The graphic below, called a Hovmöeller diagram, shows where large areas of rising air (cool colors) and sinking air (warm colors) exist near the equator as a function of time. The dashed black contours depict the active phase of successive CCKWs, and the solid red contours show the inactive phases. In this particular case, forecasters noted that there was a strong CCKW moving through the eastern Pacific in the middle part of October. Extrapolating the wave forward in time, along with numerical models forecasts of the wave’s location and strength, suggested that a tropical cyclone could form within a few days over the far eastern Pacific from a disturbance that was already in the area. The green dot indicates where Tropical Storm Trudy formed, a day or two after the CCKW passed the disturbance. Although CCKW tracking is only a secondary factor in determining a Tropical Weather Outlook forecast, a basic knowledge of this atmospheric phenomenon is an important part of the process.
Forecasters consider many factors when preparing the five-day genesis probabilities for the Tropical Weather Outlook, including explicit forecasts from the global models and knowledge of any ongoing CCKWs or the MJO. In addition, the final NHC forecast also reflects the current trends of the disturbance, which are weighted much more heavily in the two-day outlook, but also can affect the five-day forecast as well. There are several ongoing research projects that will hopefully yield objective probabilities and other tools designed to help better predict tropical cyclone formation. These tools, in combination with the dynamical guidance from numerical models, should improve the quality of genesis forecasts and perhaps in the next five years extend reliable tropical cyclone formation forecasts from five days to one week.
— Eric Blake and Todd Kimberlain
Thanks to Chidong Zhang and David Zermeno, University of Miami RSMAS, for Figure 1.
For many years NHC’s forecasts of tropical cyclone formation extended only 36-48 hours into the future. Recent advances in numerical modeling, however, as well as improved understanding of some of the physical triggers for genesis, prompted NHC to begin an in-house experiment to see whether its genesis forecasts could be extended. The four-year experiment showed, somewhat surprisingly, that a five-day tropical cyclone forecast could be made with nearly the reliability of the existing 48-hour forecasts, and NHC publicly extended the range of its Tropical Weather Outlook (TWO) text product to five days on August 1st of 2013.
In its present form, the text TWO describes areas of disturbed weather and their potential for development into a tropical or subtropical cyclone. This description normally includes discussion of the large-scale factors that could influence development, the general motion of the disturbance and any hazards that might affect land areas, and concludes with a quantitative forecast of formation likelihood for both the next 48 hours and the next five days.
The New 5-Day Graphic Explained
Beginning today, July 1, 2014, at 2 PM EDT (11 AM PDT), the text TWO will be accompanied by an experimental graphical depiction of the five-day potential cyclone genesis areas. These areas will appear as color-coded hatched areas (yellow, orange and red representing low, medium, and high risks of tropical cyclone formation, respectively). Although the areas or swaths don’t explicitly represent a track forecast, they do provide a general indication of where these systems are headed whenever the formation potential extends over several days.
If a hatched formation area is associated with a currently existing disturbance, the location of the disturbance is marked with an ‘X’ on the graphic. Arrows are used to link the location of a disturbance with its potential genesis area if the formation area is displaced from the current location of the disturbance. The overview graphic (above) can occasionally become crowded with disturbances, especially during the peak of the hurricane season, so separate graphics for each disturbance are created to ensure legibility.
The introduction of the five-day graphic on July 1st will be accompanied by an important change to the existing 48-hour graphic. Disturbances on this graphic will no longer be identified with circles or ovals; instead the location of current disturbances will be marked with an “X” for consistency with the five-day graphic.
In a future blog post we’ll be talking about how NHC’s Hurricane Specialists arrive at the formation probabilities appearing in the TWO, as well as some experimental guidance and ongoing research projects that might allow us to extend these genesis forecasts even further in time. In the meantime, we welcome user feedback on the new graphic, which can be provided at http://www.nws.noaa.gov/survey/nws-survey.php?code=FDGTWO
The following video also provides a description of the new 5-Day Graphical Tropical Weather Outlook: