Drier Than Normal December; Drought to Reintensify

30 01 2019

Dale C. S. Destin |

DCurrentDroughtIntensityecember was drier than normal for Antigua. The rainfall total of 65.8 mm (2.59 in) was below normal – 65% of the average for the month. This was the driest December since 2015. The month had very little positive impact on the droughts; hence, they will likely reintensify in the upcoming months.

The last three-month period – October to December, upon which the assessment of the current intensity of the drought is based, was slight dry. However, the rainfall total fell in the near normal category. The three-month period had 329.2 mm (12.96 in), while the normal amount of rainfall is 413.5 mm (16.28 in).

Rainfall for the past 24 months

Rainfall for the past 24 months for varying time intervals compared to the normal/average and records

We remain in a severe meteorological drought, the worst category on our drought scale. However, at the moment, the current intensity is slight, for the second month in a row. 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, our billion-gallon surface catchment, is again returning to critically low levels. The Authorities will be cutting back on extraction from the Dam and water rationing is imminent, if not already started. This is indicative of the continued drought; notwithstanding the ample rainfall of November.

The fifteen-month period – October 2017 to December 2018, the duration of the drought thus far, is deemed severely dry. The total for the last 15 months of 1043.4 mm (41.08 in) is the second lowest since 1969 and the fourth lowest on record, for such a period, dating back to 1928. This particular interval normally gets 1615.7 mm (63.61 in), which means a rainfall deficit of 35% – more than one-third of the usual rain was absent.

Based on the last set of rainfall forecasts from regional and especially international sources, the news is discouraging with respect to rainfall. Overall, below normal rainfall is likely for the next six months – February to July 2019, with moderate confidence that the period February to April – 2019 having well below normal rainfall. Thus, there is every reason to believe that the droughts will reintensify. The chance of the droughts ending is, at most 20% or slight.

Probabilistic Multi-Model Ensemble Forecast of Rainfall For Jan-Mar 2019, based on 12 global model

Probabilistic Multi-Model Ensemble Forecast of Rainfall For Jan-Mar 2019, based on 12 global models

On average, our severe meteorological droughts last for around 16 months, but not continuously at severe intensity. At current, the drought is in its 16th month; the longest such drought on record lasted 38 months – Jul 2013 to August 2016.

Last year – 2018, was the eighth driest on record, dating back to 1928. It was also the second driest since 1983; only 2015 has been drier since 1983 – 35 years ago.

accumulation_december2018

It is too early to say with any confidence whether the rainfall for 2019 will be below, near or above normal. However, early indicators suggest it could be another tough drought year for Antigua and Barbuda.

Keep following me for more on this story and all things weather and climate.





Strong Winds and Hazardous Seas to Impact Parts of the Caribbean

22 01 2019

As the winds go, so go the seas. As the winds go up the seas will go up and become hazardous across the Bahamas, Cuba and Jamaica, in the western Caribbean, to around Anguilla in the northern Leeward Islands.

Hazardous Seas – Credit UCAR

The winds and wind waves have already started to pick up across the islands mentioned above, particularly the western Caribbean. The winds will eventually get to the range of 34 to 53 kmh (21 to 33 mph) through Thursday. Consequently, seas will become hazardous with wind waves of 2.5 to 3.5 metres (8 to 12 ft), occasionally reaching 4.5 metres (15 ft).

The highest and most dangerous waves will take place across the waters of the western Caribbean, where the winds will be the strongest. Outside of the islands listed above, including Antigua and Barbuda, the wind waves will unlikely reach 2.5 metres (8 ft), as the winds are not expected to get sufficiently high.

Advisories and or warnings to mariners, particularly small craft operators, will be required and already Puerto Rico and the Cayman Islands have issued such.

As a small craft operator, if an advisory is issued – inexperienced mariners, especially those operating smaller vessels should avoid navigating in these conditions. If a small craft warning is issued – you should stay in or very near port.

Potential impacts from this hazardous sea event include injuries or loss of life, damage or loss of boats and fishing equipment and disruption to sea transportation. Other possible impacts comprise of:

  • disruptions to sea search and rescue;
  • scarcity of seafood;
  • disruptions to off shore marine recreation and businesses;
  • business and economic losses.

On the other hand, the strong winds could result in disruptions to air transportation and outdoor sporting activities, soil erosion, vehicular accidents and financial losses.

As the winds go, so goes the seas; however, as the pressure gradient goes so go the winds. The winds will become strong because the pressure gradient will become quite tight across the northern islands. Think of pressure gradient like a hill and the wind as a car. The steeper the hill the faster the car will roll down the hill and vice versa.


Surface Chart for 2 pm (18 UTC) Today. Note the Seven Relatively Close Isobars (Black Lines) on the Left (Western Caribbean) Compared with the Four Widely Spaced one on the Right (Eastern Caribbean). The Pressure Gradient and Winds are Much Higher across the West than the East.

Note, unlike the blog from yesterday that spoke about swell waves, this blog is focusing on wind waves. What is the difference? Swells are waves generated by distant winds and are of danger primarily to users of the near shore, beaches and coastlines. On the other hand, wind waves are locally generated and are mainly of danger to mariners using off shore waters.

The strong winds and hazardous seas will subside to relatively safe levels by late Friday. Keep following my blog and other media – TwitterFacebook and Instagram for more on this event and all things weather and climate.





A Major Swell Event to Impact the Caribbean This Week

21 01 2019

A major swell event is forecast to impact much of the Caribbean this week. Swells are likely to exceed 2.5 metres (8 ft) and occasionally exceeding 3 metres (10 ft), coming out of the north. These swells will result in dangerous surfs for beachgoers; hence, advisories and or warnings will be required.

Spacial Distribution of Swells - Thu 22 Jan 2019

Spacial Distribution of Swells – Thu 24 Jan 2019. Grapic Courtesy windy.com

The swell event will be generated by gale-force/storm-force winds, from a powerful low-pressure system, making its way across the northern North Atlantic. This is the same system that dumped an obscene amount of snow – over 660 mm (26 in) across parts the United States.  Obviously, none of this weather will reach the Caribbean but the sea swells will.

The swells in and of themselves are not the real concern. The greater concern is the large breaking swells or high surfs that these swells will caused when they reach the shorelines across the region. Such long period swells can result in surfs as high as twice their heights i.e. up to 6 metres (20 ft).

High Surf - Fort James, Antigua

High Surf – Fort James, Antigua

The eventual height of the surfs is largely dependent on the bathymetry (shape and depth) of the near shore coastal areas they interact with. Generally, the shallower the near shore areas, the higher the surfs. The greatest impact will be on the north-facing beaches and coastlines.

The event has started across the Bahamas and will reach the western Caribbean by tomorrow – Tuesday. It will then spread to the Eastern Caribbean Wednesday, including Antigua and Barbuda, and the southern Caribbean on Thursday. Moderate swells and associated high surfs are also forecast to reach the coastline of northeast South America, including the Guyanas on Friday.

These high surfs will have the potential impact of injuries or loss of life, beach closures and financial losses. Impacts could also include:

  • disruption to potable water from desalination;
  • salt water intrusion;
  • flooding of low-lying coastal roads;
  • beach erosion;
  • disruptions to near shore marine recreation and businesses;
  • damage to coral reefs and
  • disruptions of marine transportation.

These swells and surfs could result in strong rip currents that can carry even the strongest swimmers out to sea. Rip currents are powerful channels of water flowing quickly away from shore, which occur most often at low spots or breaks in the sandbar and near structures such as groins, jetties and piers.

If caught in a rip current, relax and float. Don`t swim against the current. If able, swim in a direction following the shoreline. If unable to escape, face the shore and call or wave for help.

There is also concern for those who visit non-beach coastal areas. High surfs can knock spectators off exposed rocks and jetties. Those who rock fish need to pay attention and not expose themselves to this hazard. Breaking waves may occasionally impact harbours making navigating the harbour channel dangerous.

With this event happening near a full supermoon, high tides will be higher than usual; hence, coastal flooding and erosion are more likely than usual. Coastal flooding from the sea is largely depended on high tides, onshore wind and swell actions.

The potential impacts listed above are just that – potential/possible impacts. I am not saying that they will all definitely happen, but conditions could result in such and past similar swell events have caused such.

If an high advisory is issued for an area – be extremely cautious; bathe only where lifeguards are present. If a high surf warning is issued – do not enter the water. Relatively safe conditions are likely on the opposite or southern sides of the islands.

Swells and associated surfs will peak across the Bahamas on Wednesday; the Eastern Caribbean on Thursday; the southern Caribbean Friday and northeast South America on Saturday.

In addition, to concerns for those using the beaches and coastlines, there are concerns for mariners of the northern islands, as expected strong winds will, among other things, cause hazardous seas. For more, see my blog – Strong Winds and Hazardous Seas to Impact Parts of the Caribbean, coming out tomorrow.





Why Was It So Cold Night Before Last?

19 01 2019

Night before last was the coldest for Antigua and Barbuda for the year and the coldest for some areas in years. Some areas had temperatures as low as 15 °C (59 °F). Why was it so cold, relatively speaking, and is this unusual?

Min Temp for Antigua and Barbuda
Temperatures recorded January 17/18, 2019

The short answer to the question is radiational/radiative cooling. The long answer involves explaining what such cooling is and how it works. Radiational cooling is the process by which the ground and the adjacent air cool by emitting heat (infrared – IR energy).

The relationship between dew point (atmospheric moisture) and min temperature.
On a calm, clear night, the lower the dew-point temperature, the lower the expected minimum temperature. With the same initial evening air temperature (80ºF) and with no change in weather conditions during the night, as the dew point lowers, the expected minimum temperature lowers. This situation occurs because a lower dew point means that there is less water vapor in the air to absorb and radiate (heat) infrared energy back to the surface. More infrared energy from the surface is able to escape into space, producing more rapid radiational cooling at the surface. (Dots in each diagram represent the amount of water vapor in the air. Red wavy arrows represent infrared (IR) radiation.) Graphic courtesy Meteorology Today.

As we all know, as the sun goes down, the heat from it decreases. Consequently, at some point late in any given day, the ground and the air near it lose more heat that it receives.

The ground being denser than air cools more quickly than the air above it. Hence, after the sun goes down, the ground is cooler than the air directly above it.

For any two objects in contact with each other, heat will flow from the warmer to the cooler. Similarly, the warmer air above gives up heat to the ground, which the ground quickly emits away.

As the night progresses, the ground and the air near it continue to cool more rapidly. Air is a poor conductor (transferrer) of heat. As a result, it takes a while for the air to reach it coolest. However, this is normally reached just before dawn, in the Antigua and Caribbean context.

Now, radiational cooling happens 365 nights a year, so what was different last night? The main difference was the fact that the winds were calm, and the skies were clear – the main ingredients.

Additionally, we are in the Northern Hemisphere’s winter – shorter days, as the sun is away over the Southern Hemisphere; consequently, the coldest time of the year. So, we had almost the perfect recipe for radiational cooling to be at its optimum; thus, our colder than normal weather.

If you live at the bottom of a valley, you may have felt colder than most. This is because cold dense air, which originates from the cold hill tops, slowly flows down the hill slopes and settled in the valley – making for colder weather than non-valley areas.

If you live on a hill top you would have been coldest, as the higher you go, the cold it gets generally – 1 °C (1.8 °F) for every 100 metres (330 ft) you go up. Radiational cooling has a greater impact on hill tops that elsewhere.

There is nothing unusual with us having such cold night. No records were broken, which is indicative of the fact that we have had colder temperatures. Actually, as our climate warms like almost every other place on earth, these “extremes” temperature are occurring less frequently.

When I was a child, I recall that it was very common for my siblings and I to see our breaths in the mornings, at this time of the year – due the cold temperatures. We used to make the mirrors frosty with our breaths and then write stuff on them. That has become a rarity, at least for me – anecdotally indicating that our climate is warming and the reduced frequency of such low temperatures.

Radiational cooling operates at it maximum under clear skies, dry air, calm winds and long nights, which are synonymous with the winter months – December to February. So, we are in the period when radiational cooling operates at it best. The winds have the effect of disrupting the cooling; consequently, with the winds not likely to returning to calm over the next several days, a repeat of night before last is unlikely – fortunately or unfortunately.

You are welcome to follow my blog, so as to not miss another one and also my other social media accounts: TwitterFacebook and Instagram for all things weather and climate.





How Much Wind Did We Get Last Weekend?

7 01 2019

Last weekend – December 27-30, a very steep pressure gradient across the Caribbean resulted in strong and very gusty winds across many of the islands. Just how much wind did we get?

The highest sustained winds were reported by the Luis Muñoz Marín International Airport in Puerto Rico – 31 mph (50 km/h). This occurred at 3:24 am (1924 UTC) on Friday, 28 December and it was raining moderately at the time.

This strongest wind gusts of 46 mph (74 km/h) were reported by two airports – the Princess Juliana International in St. Martin and the Sir Grantley Adams International in Barbados. These took place Friday, 28 December at 9:32 am (1332 UTC) and Saturday, 29 December at 3 pm (1900 UTC) for Juliana and Grantley Adams respectively. At the times, it was showering moderately in Barbados, while it was dry in St. Martin.

Grantley Adams reported the longest stretch of strong winds i.e. winds greater than 24 mph (39 km/h). This occurred for five hours on 29 December – 9 am to 1 pm.

Several islands issued high wind advisories in anticipation of or response to the above normal winds. These include Antigua and Barbuda along with the rest of the Leeward Islands and the British Virgin Islands, Puerto Rico and the US Virgin Islands and Barbados. Some islands got more wind than they got for the 2018 hurricane season.

Please note that this report is based almost solely on winds from airports across the region; hence, it only provides the winds for a very small area of each island. Thus, it is quite likely that higher winds would have occurred – especially at elevations and open waters.

The main impacts of the strong winds were on marine activities. The high winds stirred up the seas, making them very hazardous for most marine events, creating loss or revenue for many.

They were also impacts on air transportation. It was reported that LIAT had to abort landing in Dominica and return to Antigua, as the winds were above the aircraft threshold for landing. They were also reports of LIAT flights being delayed, as they had to wait for up to an hour for conducive conditions to take-off.

These high winds are not unusual for this time of the year. Similar events have taken place in past years. Some people refer to them as the “Christmas Winds”.








%d bloggers like this: