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).

PTC_Bret

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.

TOMASAsTropDisturbance2010

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_AsAPotentialTropCyclone

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.

EarlAsATropDisturbanceAugust2016

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 a success stories – mission accomplished, congratulations!





Weather Expectations from the Present Disturbance

28 10 2014

Dale C. S. Destin |

Disturbance

Disturbance

A weather disturbance has developed around 250 miles east of Antigua. It has a 20% chance of becoming a tropical cyclone (the generic term for depressions, storms and hurricanes) while it is in our area and a 30% chance of become a tropical cyclone after it moves away for the northeast Caribbean. However, this system is not expected to become a tropical cyclone as the winds aloft are not favourable for much further development. Notwithstanding, based on data available up to this morning, there are three weather scenarios looked at below with respect to the impact of the disturbance on Antigua and the northeast Leeward Islands.

Expected

The weather expected from this disturbance is: Cloudy to overcast skies with precipitation (rain and showers); thunderstorms are likely Tuesday night and Wednesday. Further rainfall accumulation of 10 to 25 mm is possible. The winds are going to swing to the southeast at 5 to 12 knots by tonight. The seas will get up to near two (2) metres or 6 feet tonight and will fall back to near 1.5 metres or 5 feet tomorrow Wednesday.

Best Case

For a best case scenario, the disturbance will get decimated by the unfavourable winds aloft. Thus, although cloudy skies will continue, not much more rainfall will take place.

Isaias_Intensity

Worst Case

For the worst case scenario, the disturbance could develop further and impact Antigua and Barbuda as a tropical depression and the rest of the northeast Caribbean as a minimal tropical storm (Isaias). This scenario is very unlikely; however, if this pans out, quick action would be required to protect life and property.

Isaias_Invest

Conclusion

Changes will definitely take place with this disturbance over the next 24 hours; however, something close to the best case scenario is anticipated. Notwithstanding, this system will be monitored and if the chance of development into a tropical cyclone rises to above 50%, a tropical cyclone alert will be issued by the Antigua and Barbuda Met Service. Residents in the Leeward Islands should continue to monitor this system until it has dissipated or out of our area.





Antiguan Storms During July

15 07 2013

Dale C. S. Destin |

What is Antigua’s record with respect to tropical cyclones in July? While tropical cyclones are unlikely to affected Antigua during the month of July, it is not unheard of. For the period 1851 to 2012, Antigua has been affected by six tropical storms, including one hurricane (AntiguanStorms). The last tropical cyclone to have affected Antigua in July was Hurricane Bertha in 1996. The system produced 39 mph sustained winds with gusts near 60 mph.  Total rainfall from it, measured at the airport was 49.1 mm. Based on the active multi-decadal period that we are in, which started around 1995, the probability of a storm or hurricane affecting Antigua is 5% (one every 20 years) as compared to the (1981 – 2010) normal of 3% (one every 33.3 years). Broadly speaking, we a not due another storm in July until sometime in the period 2016 – 2029. Meanwhile, a major hurricane has never affected the country during July.

Two tropical storms formed during the month of June in the North Atlantic Basin. Based on a 30-year (1981 – 2010) average, a tropical storm forms in June in the basin about once every other year.








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