Near Record Driest November for Antigua

7 12 2021

Dale C. S. Destin|

Most Antiguans alive have never endured a November like the just ended one. The month was second driest on record, dating back to 1928. It registered a near record breaking 29.2 mm (1.15 in) of rainfall, the lowest since 25.1 mm (0.99 in) in 1947, almost 75 years ago. This very rare November rainfall event has around a 0.7 percent chance annually or once every 143 years, on average.

November usually logs 152.1 mm (5.99 in) of rainfall annually (1991-2020); this means that only a little over 19 percent of the usual total fell this year. The deficit of 81 percent is the second worst for the year, behind May with 83 percent. November is usually the wettest month of the year.

The absentee rainfall for November has virtually ensured that 2021 will be among the top eight driest, on record. Although unlikely, this year could even break the record for the driest year set by 2015, when just 574.5 mm (22.62 in) rainfall was accumulated.

Thus far, the total for 2021, through November, stood at 537.2 mm (21.15 in). Normally, this period yields 1059.2 mm (41.7 in); hence, there is close to a 50 percent shortfall of rainfall. Less than 37.3 mm (1.47 in) of rainfall for December would see the record fall. Such low annual rainfall is extremely rare. The chance of this meagre annual total is less than 0.2 percent. This translates to a 500-year event or worse, an event with a return period of once every 500 years or more, on average.

November has also virtually sealed the fate of the wet season (July-December). It is certain to be among the driest, if not the driest, on record. The current record is 405.6 mm (15.97 in) for the 1983 wet season. Thus far, July-November, the total for the 2021 wet season is 332.5 mm (13.09 in).

While it is possible that the rainfall for the wet season will be at a record low, the rainfall for the last seven months ending November was in fact record breaking. No other May-November has been drier, on record going back to, at least, 1928. The total of 405.4 mm (15.96 in), for this year, eclipsed that of 406.4 mm (16.00 in) observed in 2015.

It goes without saying that serious meteorological drought continues, and it is highly likely to get worse, in the short-term. Also evident are agriculturalhydrologicalecological and socio-economic droughts, at varying intensities. Potworks Reservoir now has ZERO drop of water. All other surface catchments are in a similar state or below extraction levels.

Potworks Reservoir with zero drop of water – December 2, 2021. Picture courtesy Karen Corbin, Humane Society

Antigua Public Utilities Authority (APUA), the countries water resource manager, informed the public yesterday that 95 percent of all potable water, being produced by the organization, is desalinated water from the sea. We were also informed that the total water production of six million gallons per day is, at least,  a million gallons below the country’s daily requirement; hence, water is being rationed.

With such lean rainfall figures and with the recent conclusion of COP26, the obvious question would be: Was this caused by anthropogenic (human induced) climate change? From my reading of the latest IPCC report and other papers, the answer is no. According to the report, there is a low confidence in the change in agricultural and ecological droughts in the Caribbean, thus far. However, the long-term projection is for an increase in the intensity and frequency of droughts, but this will not be for all regions. Unfortunately ,the Caribbean is likely to be among the regions to bear the negative changes.

Whereas climate change is unlikely to have been a factor in November’s rainfall, or lack thereof; it was noted that suppressing rainfall conditions prevailed across the area for much of the month. Strong positive velocity potential anomalies caused sinking air, which is prohibitive to cloud growth and hence rainfall. The velocity potential is associated with the Madden-Julian Oscillation (MJO) with positive values being indicative of the dry portion of it.

Three-day centred average animation of daily IR and 200-hPa velocity potential anomalies (base period 1991-2020). Velocity potential anomalies are proportional to divergence with green (brown) contours corresponding to regions in which convection tends to be enhanced (suppressed).

This extreme dryness for November was not endured by Antigua only. The unusually low or near record breaking rainfall was observed across much of the Eastern Caribbean. For example, reports out of Trinidad and Tobago indicate that much of the country had its second driest November, on record.

November was the last hope for the replenishment of national surface catchments, this year. With the very disappointing rainfall, the next possible time for replenishing precipitation is May. The issue with insufficient potable water is likely to get worse before getting better.

Please continue to follow me for more on this evolving drought and all things weather and climate via TwitterFacebook and Instagram. Also, share this blog, if you found it useful.

Third Busiest Hurricane Season Ends

1 12 2021

Dale C. S. Destin |

The third busiest Atlantic hurricane season–2021, ended yesterday. The season produced 21 named storms, the third highest on record, behind 2020 with 30 and 2005 with 28. The season also produced 7 hurricanes, tied for 32nd highest on record dating back to 1851. Further, there were four major hurricanes, Category 3 and over, which tied for the 18th highest on record. Thirty-two other seasons had seven hurricanes and 18 others had 4 major hurricanes.

Although the most eye-catching statistic for a given season is the number of storms, this is not the metric used to determine its overall activity. The Accumulated Cyclone Energy (ACE) index remains the internationally accepted metric used to categorize the activity of a season. The ACE takes into consideration not only the number of named cyclones but also their strength and duration. Hence, based on the ACE, 2021 is the 29th most active hurricane season on record, with an ACE index of 142.

The most active hurricane season on record remains 1933 with an ACE of 259, 44% more than 2020, the record busiest season, w.r.t. the number of named storms, NOT ACE. The 1933 season was also over 80% more active than 2021. Thus, notwithstanding the headline-grabbing 21 named storms for 2021, the season was nowhere close to being the third most active. Activity has to do with the ACE, while busyness has to do with the number of named storms.

Based on NOAA’s classification, the 2021 season was above normal. However, NOAA’s classification of season has its challenges, allowing for one season to simultaneously have two classifications. Also, NOAA uses the 1991-2020 period the define an average season but uses the 1951-2020 period to determine if season is normal or not. A better approach is to classify seasons strictly by the ACE index, as does by 268Weather. This approach categorises the 2021 season as near normal; based on the 1991-2020 climate period, the 2021 ACE of 142 falls in the middle tercile. An average season produces 14 named storms, 7 hurricanes, 3 major hurricanes and ACE of 123.

268Weather’s seasonal hurricane forecasts fairly accurately predicted the number of named storms and major hurricanes. However, we did not do as well with the other parameters, particularly the important ACE index. Based on the number of named storms, the ACE is unusually low. This is largely because of the record high nine shorties–storms lasting two days or less. Notwithstanding, tropical cyclone metric forecast by 268Weather fell within the 70% confidence. Interestingly, as little as 30 years ago, most of these shorties would have gone undetected, resulting in the official numbers being significantly less.

As busy as the season was, thankfully, Antigua and Barbuda along with the rest of the Leeward Islands and the British Virgin Islands were spared. Officially, the record will likely say that we were impacted by Tropical Storm Grace; however, this system bought ZERO storm-force winds to our shores.

The season got off to an early start with Tropical Storm Ana on May 22 and closed early with the dissipation of Tropical Storm Wanda on November 7. After flurry of storms from mid-June to mid-July, ending with Elsa (July 1-14), the season went on a bit of a hiatus until August 11, when Tropical Storm Fred formed. In less than one-and-half month, from August 11 to September 29, 15 storms formed, more than the average for a season. October went virtually stormless except for the eleventh-hour development of Tropical Storm Wanda on October 31, exhausting the 21-name Atlantic list.

The most powerful cyclone for the season was Category 4 Major Hurricane Sam, which had peak sustained winds of 250 km/h (155 mph). Sam did not make landfall; hence, it caused minimal impact.

The most deadly and destructive system was  Category 4 Major Hurricane Ida, which had peak sustained winds of 240 km/h (150 mph). It killed 115 persons and caused over US$65 billion in damage, becoming the sixth costliest Atlantic tropical cyclone on record. It impacted mainly the Cayman Islands, Cuba and the Gulf and East Coasts of the United States.

A couple notable records for the season:

  • Elsa became the earliest 5th Atlantic named storm on record when it was named July 1. The previous record was set by Edouard on July 6, 2020.
  • 2021 tied with 2007 for the most shorties (storms lasting <=2 days), on record.  
  • 2021 marks the first time of back-to-back exhaustion of the list of Atlantic named storms.

The 2022 hurricane season will officially begin June 1 and 268Weather will issue monthly forecasts starting early April.

Please continue to follow me for more on this subject and all things weather and climate via TwitterFacebook and Instagram. Also, share this blog, if you found it useful.

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