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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