The Advent of Potential Tropical Cyclones – What, When and Why?

21 06 2017

Dale C. S. Destin |

History was made this past Sunday when for the first time ever the U.S. National Hurricane Center (NHC) issued tropical storm (TS) warnings for portions of the southern Caribbean, in the absence of a TS. This they were able to do by designating an approaching tropical disturbance a “potential tropical cyclone” (PTC).


In the past, it was the policy of the NHC to not issue tropical cyclone (the generic term for tropical depressions, TSs and hurricanes) watches and warnings UNLESS there EXISTS a threatening tropical cyclone (TC). This was the case even if there was a 100% certainty that one was going to form and impact land in a short space of time.

This policy served us well. However, just about every year, there would be, at least, that one tropical disturbance that would approach land, and everybody knew it was going to form into a TC just before impact. But the existing policy would get in the way of issuing watches and warnings that were needed to sprung persons into meaningful preparations.

A classic example of this was Hurricane Tomas of 2010. The system approach the southern Caribbean in late October and was not upgraded to a TS until it was less than 12 hours away from Barbados.


Tomas as a potential tropical cyclone on the “doorsteps” of Barbados, less than 9 hours before impact, with no warning issued

So instead of having 36-48 hours lead time to prepare, there was less than 9 hours. The bulletin announcing the warnings not only came late but also late in the day – 5 pm, which means any preparations that could be done, took place mainly after-dark, after it started to rain and after hardware stores were closed.

Tomas cause eight deaths and over US$500 million dollars in damage across the southern Caribbean. Some of the loss could have been avoided if many persons were not caught off guard due to the very short lead time between the formation of Tomas and its impact on the islands.

Hurricane Gonzalo of 2014 similarly caught many Antiguans off guard and unprepared. Many persons did not hear about the system until hours before it arrived. One person told me that she suffered damage to her property because she only found out what was happening when the winds started to pick up. By then, it was to late for her to go outside to close the shutters. Many boats were damage or sunk because of insufficient time to secure them.


Gonzalo as a potential tropical cyclone less than 20 hours before it made landfall in Antigua as a hurricane with no warning issued

To solve this problem, the NHC has revised its policy and instead of just issuing watches and warnings for ONLY existing TCs, they have started this year to issue them for “PTCs”. By definition, according to NHC, a PTC is: ”…a disturbance that is not yet a TC, but which poses the threat of bringing TS or hurricane conditions to land areas within 48 hours.”

So, no longer is a disturbance, with a high chance of becoming a TC, allowed to march up to a country without 24-48 hours of watches and warnings being issued. Such systems can now be declared PTCs and the requisite watches and warnings will be issued, early.

It could be argued that an approaching tropical disturbance with a high chance of becoming a TC should be enough to spring persons into TS or hurricane preparations. However, studies have shown, a great number of persons just don’t prepare until watches or warnings are issued. Hence, the need for them to be issued early.

This policy change may be one of the most important ever by the NHC. Like the previous policy though, this one is not perfect; however, it plugs a huge loophole in the TC warning system. It has the distinct potential benefit of further reducing “surprised” attacks from TC, which translate into saving more lives, more properties and livelihoods.

Notwithstanding, it has the potential to create some unnecessary stress. This will be so when disturbances designated PTCs do not actually become cyclones. I expect, this be a rare occurrence. In any event, preparations for PTCs are deemed low-regret actions at worse.

Happily, we are off to a good start. The historic first PTC became TS Bret less than 6 hours before making landfall in Trinidad. Under the old policy, many persons would have been caught off guard and unprepared, not so this time around.


Earl as a potential tropical cyclone forecast to be near or over Jamaica in a day or two with no watch or warning issued

This new policy may have been hastened into being by the actions of the Jamaica Meteorological Service (JMS) last year, as it related to TS Earl. Whereas the NHC made history by issuing its first TC warnings based on a PTC, they were not the first to do so. The JMS, quite bravely, “took the bull by the horn” and made the tough, unprecedented but right decision to issue TS warnings for Jamaica in the face of TS Earl of 2016, when it was what we are now calling a PTC, approaching the island.

So, well done to Jamaica for setting the stage for the first ever PTC bulletin by NHC – both are success stories – mission accomplished, congratulations!

Here We Go Again – It’s Hurricane Season 2017; What’s the Forecast?

2 06 2017

Dale C. S. Destin |

When you live in “Hurricane Alley” like we do, you often don’t want to think about the hurricane season before it starts – it’s too stressful. But you really should, as preparation is key to survival. If you are now preparing for the 2017 Atlantic hurricane season, you are late, but there is still some time left.

What to prepare for – What’s the forecast?

Key to your preparations is to know what to prepare for. From this vantage point, we will most likely have a near normal hurricane season. Normally, the season has 12 named storms (tropical storms and hurricanes), 6 hurricanes and 3 major hurricanes. This year, our ensemble (mean) forecast is for 14 named storms, 7 hurricanes and 3 major hurricanes.


A better indicator of the activity for the season is the accumulated cyclone energy (ACE) index. This is a measurement of the potential of wind and storm surge destruction of a named storm. Summing together the ACE index of each named storm, provides a more complete picture of activity for a season, outside of just the number of storms. The mean ACE index forecast for this year is 110; the average is 106 (1981-2010).

The latest forecast represents an uptick in the activity for the season. The previous forecast called for a below normal hurricane season – 11 named storms, 6 becoming hurricanes and 3 major hurricanes, with an ACE of 71. This was predicated on the forecast of an El Nino during the heart of the season.

El Nino normally inhibits tropical cyclone (tropical depression, tropical storm and hurricane) activity. However, over the past few months, an El Nino seems less likely, delayed or insignificant, if developed; hence, the uptick in the activity for the season.

Also, contributing to the uptick in the forecast activity for the season is the forecast of warmer than normal sea surface temperatures across the tropical north Atlantic and lighter than normal trade winds. All things relish by tropical cyclones.


Our forecast is the ensemble (mean) of the forecasts from Klotzbach of Colorado State University, the U.S. National Oceanic and Atmospheric Administration (NOAA), Saunders and Lea of Tropical Storm (TSR), the Integrated Forecast System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF), UK Met Office and The Weather Company (TWC).

It must be noted that NOAA is the only group, of the five surveyed, that is forecasting an above normal season.

So, that’s the forecast – what is the probability of Antigua being hit?

The probability of Antigua being hit or brushed by one or more named storm this year is around 63% – up 14% from the average of 49%. The probability of one or more hurricanes is 22% – down 15% from the average of 37%.

These numbers were calculated based ONLY on the likely best similar years (1957, 1969, 1979 and 2006) to the upcoming hurricane season according to Klotzbach. Of these years, we were hit by Tropical Storms Claudette, Frederic and Chris and brushed by Hurricane David.

Notwithstanding, at the end of the day, these numbers don’t mean a whole lot. We could be hit by one or more named storms or none at all. Bottomline – we truly do not know what is going to happen, with any certainty. However, what we do know, with absolute certainty, is that storms and hurricanes form every year, and we could get hit, as we have many times in the past. So, regardless of the probabilities and forecasts, we need to prepare. It only takes one tropical cyclone to set you back for years.

A season can produce many storms, but have little impact, or produce few storms and have one or more hitting Antigua with major impact.

Antigua averages one hurricane every three years and one named storm every two years or every other year.

What happened last season – 2016?

The 2016 season was more active than normal – the first active season since 2012 and the most active since 2010. It spawned 15 named storms, 7 became hurricanes and 3 reached major hurricane status. The strongest hurricane for the season was Matthew, which had peak sustained winds of 258 km/h (160 mph).

Satellite Image of Hurricane Matthew - Sep 30, 2016

Satellite Image of Hurricane Matthew – Sep 30, 2016

Hurricane Matthew also caused the most devastation. In total, up to 600 deaths have been attributed to the system, including over 500 in Haiti, making it one of the deadliest Atlantic hurricanes.

Hurricane Matthew 2016

Rainfall Trail From Hurricane Matthew 2016

The 2016 season is the first year since 2008 that no tropical cyclone hit or brushed Antigua. It was likely the least stressful hurricane season for the island in, at least, eight years.

The 2017 season is forecast to be similar to 2016.

The hurricane season runs from June 1 to November 30.

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