The Flood of Lenny

19 11 2018

Dale C. S. Destin |

Nineteen years ago today, we were in the midst of the greatest known flood of our recent history – November 17-19, 1999. Although pale in comparison, it was our modern-day closest to the Flood of Noah, brought on by Hurricane Lenny.

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At the end of November 18, 1999, most parts of the Island would have seen over 200 mm (8 in) in two days. And by the end of the deluge, over 460 mm (18 in) in just three days. November 18-19, 1999 produced over 430 mm (17 in). At the V. C. Bird International Airport (VCBIA), 22.3 mm (0.90 in), 199.5 mm (7.9 in) and 241.8 mm (9.5 in) fell over the three days respectively – totalling a staggering 463.6 mm (18.25 in). For the same period, Jolly Hill, Bolans amassed even more – 624.8 mm (24.6 in).

LennysRainNov17-19

Needless to say, this caused massive flooding and landslides across Antigua and Barbuda.  Roads were washed away and some beaches were severely eroded. It is estimated that up to 65% of Barbuda was under water, at some point during the flood. Similar types rainfall totals, flooding and landslides were seen across other northeast Caribbean islands, with in excess of 750 mm (30 in) falling across parts of St. Martin. 

Rainfall totals from Lenny. Compliments David Roth of NOAA

Not only is November 17-19 the wettest three-day period on record dating back to 1928, it is wetter than all months on record except for November 1999 and May 1987. It was like having all the rain for the first six months of the year falling in three days – almost unimaginable.

The deluge poured down by Lenny is quite rear for Antigua and Babuda – it has a less than 1% chance of occuring or happening once every 110 years on average.

The area was primed for flooding when less than a month earlier, Hurricane Jose came calling, dumping over 150 mm (over six inches) of rain in 48 hours – October 20 and 21. Further, in excess of a further 130 mm (over 5 in) fell between Jose and Lenny.

Noticed that I am focusing on the rains from Hurricane Lenny and not his winds. This is because, by the time the centre of Lenny reached between Antigua and Barbuda, around 8 pm November 19, it had rapidly weakened to a tropical storm with winds of around 105 km/h (65 mph). Thirty-six hours earlier, it was a Category 3 Hurricane. Peak sustained winds from the system at the VCBIA was 72 km/h (45 mph) with peak gust of 95 km/h (59 mph). There was very little wind damage, most of the damage was water related – flooding and mudslides.

Due to the rainfall from the Flood of Lenny, November 1999 is the wettest month on record with an island-average of 531.1 mm (20.91 in). A few areas, across southwest Antigua, got in excess of 760 mm (over 30 in).

Lenny remains the rarest of hurricanes. It was said he went the wrong way – travelling its entire lifespan (November 13-23) in a generally easterly direction from the western to the eastern Caribbean, then dissipating about 965 km (600 mi) east-northeast of Antigua. This is unprecedented as no other hurricane has ever similarly traverse virtually the entire Caribbean Sea travelling from west to east; normally they travel oppositely – east to west.

LennysTrack

The Track of Hurricane Lenny – November 13-23, 1999

In our history dating back to 1851, Lenny is the only hurricane to affect us in November – passing within 120 miles of our islands. It is also the most powerful, latest forming, Atlantic hurricane on record, with peak winds of 155 mph – Category 4, on November 17. At that its peak, it was near St. Croix, 48 hours before passing between Antigua and Barbuda.  

Lenny is said to be responsible for 17 deaths and damage of around US$686 million dollars. Damge to Antigua and barbuda was estimated to be around US$50 million.

The Flood of Lenny is a once in a lifetime event. It had nothing on the Flood of Noah, but it is the closest distant comparison we have.  

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2nd Wettest Start to November on Record

16 11 2018

Dale C. S. Destin |

Rainfall_Nov1-15_2018

We just had one of our wettest starts to November based on record dating back to 1967. Just how wet was it and what are the chances of it being this wet? What caused it, and could it have been wetter? What are the chances of a repeat during the second half of the month?

For the first time in 44 years, the first 15 days of November yielded a massive island-average rainfall of 194.8 mm (7.67 in) – well over twice the half-month average of 77.7 mm (3.06 in). It is even will over the month’s average of 149.4 mm (5.88 in).

Only once before has more rain fell during November 1-15, 1974 when 360.4 mm (14.2 in) was recorded at the V.C. Bird International Airport (VCBIA) with an even higher island-average. This means that the chance of this happening is around 2% or around once every 50 years. So, it could have been wetter but the chance of it was only around 2%.

The very wet weather was initiated by a stationary front on November 2. Thereafter, the area was kept unstable mainly by a series of surface troughs and tropical waves. It eventually culminated in a tropical disturbance bring down the curtains on this rare wet spell.

The past fortnight plus one day has been wetter than any month since September 2017. The period is also wetter than the combined total of the first six months of 2015, 2001, 1977 and 1939 respectively.

The second half of November has a slightly higher probability of getting deluged by over 190 mm (7.48 in). That much rain has taken place three times over November 16-30, dating back to 1967, at the VCBIA. This means that it has a chance of around 6% of happening annually, in the latter half of the month. However, it has never happened when the first half has gotten more than 105 mm (4.13 in). This mean that there is a very slight chance of the second half of November 2018 getting anything remotely close to the first half.

Going into November, we needed and island-average of 259.1 mm (10.2 in) of rain to end the drought. As of the November 15, we are at 194.8 mm. This has reduced the drought to slight levels. At least, a further 64.3 mm (2.53 in) is required to end the drought.

For very wet November 1-15s, the chance of getting the required rainfall, in the second half of the month, to end the drought is around 28%. Meanwhile, the overall chance of getting 64.3 mm in the latter half of November is around 19%; thus, the drought is unlikely to end this month – the chance is, at most, low.

Overall, at the VCBIA, there have been 12 times when more than 190 mm of rain fell during the first 15 days of any month and 23 times any half of a month has gotten more. The first half of May, August, September and October are tied for the record of the most totals of more than 190 mm – three; and May 16-31 has the record for the latter half of any month – four.

The wettest November on record, dating back to 1928, is that of 1999, when there was a massive deluge caused by Hurricane Lenny. The island-average rainfall was a record 20.91 inches – no other month has been wetter, not even close. And that record is not under threat by this November.

The last time a 15-day period was wetter than November 1-15, 2018, at the VCBIA, was July 26-August 9, 2016 – 26 months ago. In an absolute sense, the wettest 15-day period at the VCBIA (not for the island, which is unavailable), on record, is November 11-26, 1999 with 567.6 mm (22.35 in).

So, how wet was it? Given that the median age of our population is around 31, most persons have never witness such a wet November 1-15. And there is very unlikely to be a repeat during the latter half of the month.

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4th Consecutive Drier Than Normal October for Antigua, Droughts Remain Serious

12 11 2018

Dale C. S. Destin |

The rainfall for October was below normal, for the fourth time in a row and the seventh time since 2009. The total of 59.4 mm (2.34 in) makes this October the second driest since 2001. Never have the rains failed for four consecutive Octobers. Long-term droughts continue at serious intensity, up to the end of October.

Oct2018

The total rainfall for the month – 59.4 mm was paltry 37% of the normally for October – 161.0 mm (6.34 in). Over 60% of the normal rainfall did not occur, for the third consecutive October and for the fourth consecutive October over 40% did not fall.

The last three-month period – August to October, upon which the assessment of the current intensity of the drought is based, was seriously dry. In the last three months, only 269.5 mm (10.61in) of rain fell. This is the 10th driest such period on record dating back to 1928. Only one other October (2015) has been drier since 1993.

Cumulatively, August, September and October normally yield 419.1 mm (16.5 in) of rain; however, a huge 36% of it did not fall. This means that we remain in a severe meteorological drought, the worst category on our drought scale.

So, overall, we remain in a severe drought that is currently at serious intensity. There has virtually been no change from last month – September. Recall that the overall description of the drought is based on the worst intensity achieve during its lifetime; however, over time, the intensity will fluctuate.

Potworks Dam, with a billion-gallon capacity, has gotten some water but the water levels remain below extraction level, up to the end of October, and I am advised that the same is true for the other smaller catchments. These are indicative of the fact that the droughts are at very impactful levels.

The full brunt of the droughts continues to be masked by the presence of the desalination plants, which are virtually still the only source for potable water in the country. Notwithstanding, many impacts are starting to break through. Potable water is still being rationed; places have been left without water for days to weeks, at a time, especially when the sea is stirred up by swells, which negatively impacts the desalination process.

Accumulation_Oct2018

The thirteen-month period – October 2017 to October 2018, the duration of the drought thus far, is deemed severely dry. The total for the last thirteen months of 773.7 mm (30.46 in) is the second lowest on record; only the similar period October 2000 to October 2001 was drier. The period normally gets 1363.2 mm (53.67 in), which means that there is a huge rainfall deficit for the period of 43%.

Based on the last set of rainfall forecasts from regional and especially international sources, the news remains discouraging. Overall, below normal rainfall is likely for the next six months – November 2018 to April 2019. Thus, there is every reason to believe that the droughts will continue and likely worsen. The chance of the droughts ending is, at most 30% or low.

Probabilistic Multi-Model Ensemble Forecast of Rainfall For Nov 2018 - Jan 2019

Probabilistic Multi-Model Ensemble Forecast of Rainfall For Nov 2018 – Jan 2019

On average, our severe meteorological droughts last for around 16 months, but not continuously at severe intensity. Will this one go for another four months? Very likely, given the climate signals.  

I now expected 2018 to be a drier than normal year with a confidence level of 80%. The best forecast for the amount of rainfall for the year is 845 mm (33.3 in) with a 70% chance of the amount ranging between 623 mm (24.5 in) and 1118 mm (44.0 in). Normally, we get 1206.5 mm (47.5 in) annually, 

The month of November has been very wet, thus far. With 125 to 255 mm (5 to 10 in) already fallen across Antigua and Barbuda, the drought has eased to moderate levels or better. With further rains to come, the drought could come to a sudden end by the end of the month.

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