A Break in the Warm Surge for the Western Caribbean

30 04 2015

Dale C. S. Destin |

A cold front is set to dowse the western Caribbean and the Bahamas with heavy showers over the next several days.

2 AM SURFACE PRESSURE ANALYSIS FOR THU APR 30 2015

                               2 AM SURFACE PRESSURE ANALYSIS FOR THU APR 30 2015                               CREDIT NOAA

10:45 AM SATELLITE PICTURE

10:45 AM SATELLITE PICTURE

These showers will be very heavy at times and are expected to will break the warm, dry spell for that part of the region; however, the rest of the area will continue to see scorching weather.

The high pressure ridge, which is the main cause of the parched weather across the region, will block the progress of the front causing it to linger for much longer than normal across the Western Caribbean.  Precipitation from this system could continue well into next weekend, with few breaks.

RAINFALL FORECAST FOR APR 30 TO MAY 7, 2015

ONE WEEK RAINFALL FORECAST: APR 30 TO MAY 6, 2015

Possible rainfall totals over the next seven days across the Bahamas and Cuba are 100-200 mm (4-8 in). This could cause like threatening flash floods and mudslides in some areas. Much lower totals are possible across the Cayman Islands, Jamaica and Hispaniola.

Notwithstanding the potential for flooding, this will be much welcome rainfall as many places across the Bahamas and Cuba are experiencing rainfall deficits. And like the rest of the Caribbean, El Nino is likely to cause significant rainfall deficits for upcoming months.

We will be following this weather as it unfolds.





Unhealthy Saharan Dust Blankets the Caribbean

30 04 2015

Dale C. S. Destin |

In addition to heat, the orientation of Atlantic High is causing winds to transport dry, dusty and unhealthy air from the Sahara Desert, in Africa, to the Caribbean.

Satellite Image Showing the Extent of the Saharan Dust on Apr 29, 2015

Satellite Image Showing the Extent of the Saharan Dust on Apr 29, 2015

The dust streams across the Caribbean all year round, via the generally easterly wind flow between Africa and the our region. The flow of dust peaks in June and decline to a minimum in December. It is most abundant over the period May to September, when it is often times transported by tropical waves, which originate over Africa.

The dust is a major health concern. Research has shown that the dust causes asthmatic flare ups and other respiratory complaints. The impact of the dust on health is said to be caused mainly by the bacterial and fungal spores it contains.

Of much graver concern to health professionals is the fact that the dust also contains chemicals such as pesticides and polycyclic aromatic hydrocarbons, which are known to be very harmful to human health.

Research has also shown that the dust may be harmful to coral reefs. According to retired geologist Gene Shinn, “Our hypothesis is that much of the coral reef decline in the Caribbean is a result of pathogens transported in dust from North Africa”.

The dust has also been credited for algae blooms in a phenomenon called “red tide”. This sought of bloom has been responsible for the death of millions of fish and other marine life, in the past.

Sahara Desert - the Source of the Dust

Sahara Desert – the Source of the Dust

The Saharan dust is not all bad. In addition to being a hurricane blocker, it is good for the environment. The dust is rich in plant nutrients and is believed to be largely responsible for the fertility of the Amazon Rainforest.

This air is notorious for “stifling” shower activity and in the past has triggered or exacerbated droughts in the region and further afield. It’s also a hurricane blocker – it hinders or weakens tropical cyclones (depressions, storms and hurricanes).

Depending on the amount of dust, the reflection and refraction caused can result in magnificent sunsets.

The dust is not predictable beyond days; thus, its impact on the upcoming hurricane season is unknown. However, once present, it could combine with El Nino to produce a very quiet hurricane season. Additionally, it could combine with El Nino to cause well below normal rainfall for the Caribbean.

So the Saharan dust is not a simple matter; it not only causes hazy skies, but among other things, causes disease. It would be wise to take as much precaution, as possible, to avoid inhaling it.

According to Monitoring Atmospheric Composition and Climate, this particular episode of Saharan dust will diminish to negligible levels across Antigua and the rest of the northeast Caribbean by Friday and most of the rest of the area by Saturday. The exception may be some of the southern islands such as Barbados, Grenada and Trinidad and Tobago.

We will be monitoring.





August-like Warmth in April for Antigua

27 04 2015

Dale C. S. Destin|

Unseasonal temperatures have made the past few days feel like August.

Temperatures over the weekend soared as a ridge of high pressure settled over the area and associated light southeasterly winds pumped warmth into the air.

WarmSurge

High temperatures over the weekend rose into the lower 30s °C (near 90 °F). It is expected that most days of the rest of the week will see similar highs as the present wind pattern will persist.

At the Airport, record high temperatures were measured on April 24 and 26, while the highs measured on April 25 and 27 were the third and second highest respectively for the stated days.

The record high on Sunday, April 26, of 31.8 °C (89.2 °F) is also the 13th highest for April and the highest for the year so far. It’s the highest for April since temperature soared to 31.9 °C (89.4 °F) back on April 25, 2008.

The record high for April, at the Airport, is 32.7 °C (90.9 °F), measured on April 7, 1998.

A high in the upper 20s °C (mid 80s °F) is more common this time of the year.

We seldom talk about heatwaves in these parts but they do happen and we may be four days into one.

There are a number of definitions for a heatwave; however, we favour the definition that says a heatwave is to be declared when the daily maximum temperature of at least six consecutive days is ranked in the top 10% of a reference period (1971-2000) for the given days. Temperatures for the past four days satisfy this definition.

If it turns out to be a heatwave, it would only be the second one on record for April. The only other for the month occurred back in 1998, when the record high for the month was set.

For those planning outdoor activities during the upcoming week, dry weather will continue to prevail for most of the island. The exception could be the northwest (parts of St. John’s) where the heat could cause breeze convergence to give rise to showers during the afternoon.

Residents are urged to take it easy during strenuous activities from around midmorning to midafternoon, as well as drink plenty of water to avoid suffering from a heat-related illness. That is especially true for the elderly, children and those sensitive to the heat.

Protection against ultraviolet (UV) radiation to prevent sunburn is also highly recommended. The UV index is generally high all year round in our area; however, with the low cloud coverage expected for the upcoming week, the index will be off the chart.

Although the heat will peak this week, warmer than normal high temperatures will likely continue through the first week of May. In the long run, near to above average temperatures are expected to prevail for the upcoming three months.

So far for the year, the mean daily maximum temperature has been near normal (average).

The warmth is also been felt across much of the rest of the Caribbean, north of St. Lucia, including the Bahamas and Turks and Caicos Islands. High temperatures are running 1-4 °C above average. One of the warmest spots over the weekend was Contramaestre, Cuba with a high of 35.7 °C (96.3 °F).

We will provide updates, as required, during the coming days.





Significant Rainfall Deficits Across Much of the Caribbean

25 04 2015

Dale C. S. Destin|

Rainfall totals across most of the Caribbean has been lower than normal for various time intervals going back a year. For some islands, rainfall deficits have been at or near record levels.

The time interval that shows the greatest deficits is the past 12 months (April 2014 to March 2015); while, over the past three months, there have been some improvements.

One way of expressing rainfall is by way of the standard precipitation index (SPI) and most of the Caribbean has been having negative values which translate to below average rainfall.

Basically, the SPI is an expression of how far away the rainfall was from the average. Positive values of 0.5 or greater (blues on the maps) indicate more rainfall than normal and negative values of -0.5 or less (yellows & reds on the maps) indicate lower than normal rainfall.

By general definition, all areas with an SPI of minus 0.5 or less are experiencing drought. This includes the Virgin Islands to Martinique. Hence, most of the northeast Caribbean is in drought, of some kind. According to the Caribbean Drought Bulletin, drought also exists across Eastern Jamaica and Haiti.

2015 first-quarter rainfall

Lower than normal first-quarter rainfall has not only been experienced by Antigua but also by most of the rest of the Leeward Islands, the Virgin Islands and the Windward Islands north of St. Lucia, according to the graphic (click for larger view) from the Caribbean Institute for Meteorology and Hydrology (CIMH).

SPI for Jan-Mar2015

Credit: @CIMHbb

First-quarter deficits were most extreme across St. Martin and nearby islands such as Saba and St. Barthelemy; the rainfall totals were at record low levels or in the bottom 2% of all totals for January-March.

Surplus rainfall took place across parts of Puerto Rico and the Dominican Republic; rainfall totals were at record high or in the top 5% of all rainfall for this time of the year.

Rainfall for the past six months

Over the past two quarters (October 2014-March 2015), more areas than the first-quarter experienced significant rainfall deficits. The “bullseye” dry spot remained over Dominica, where rainfall totals were in the bottom 5% of the historical record.

SPI Oct2014-Mar2015

Credit: @CIMHbb

Nearby islands – Guadeloupe, Dominica and Martinique, had rainfall totals ranked in the bottom 10% of the historical record.

Meanwhile, Tobago enjoyed surplus rainfall with totals in the top 10% for this period.

Rainfall for the past year

Of the three time scales, the past 12-months saw the greatest area under significant rainfall deficits.  A “bullseye” dry spot is again evident over Dominica; rainfall deficits were near record high levels.

SPI for Apr2014-Mar2015

Credit: @CIMHbb

Rainfall totals were also ranked in the bottom 10 percent for Guadeloupe, Dominica, Martinique and parts of St. Lucia.

Coastal areas of parts of Guyana, Puerto Rico and Jamaica had surplus rainfall.

The Cause

Much of this dry weather is due to the presence of El Nino and a positive North Atlantic Oscillation (NAO). Both phenomena are known to suppress rainfall activity across most of the region.

Outlook

According to CariCOF, the outlook is for most areas for April-June is for above normal rainfall which would ease the deficits. However, with El Nino developing and SSTs cooling in response to the positive NOA, the long term outlook is for the increase and spread of rainfall deficits.

Increasing rainfall extremes, mainly deficits, are likely to increase across the region once El Nino develops and persists. It is likely to be a drought year for much of the Caribbean; how bad would depend on the eventual duration and strength of El Nino and how cool the Atlantic gets.

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One of the Worst First-Quarter Rainfall Deficits for Antigua

20 04 2015

Dale C. S. Destin |

Antigua is having its seventh driest start to a year on record. As of the end of March, the average first-quarter (January-March) rainfall total for the island was 90.4 mm (3.56 in). Normally, by now, we would have received 176.0 mm (6.93 in). Thus, there is a relatively huge rainfall deficit of 85.6 mm (3.37 in).

MonthlyRainfallAccumulationforAntigua

This is the second consecutive year that first-quarter rainfall is much lower than normal. However, this year’s first-quarter rainfall is 20% lower than last year’s.

The rainfall for January-March is 49% lower than normal. In other words, we have only received 51% or just a little over half of what we normally get for this period.

Top10DriestFirst-QuartersForAntigua

Only six other first-quarter rainfall totals have been lower; however, none since 2001, and only one since 1983.

All three months of the first-quarter have been quite dry. The wettest of the three months, February, was drier than normal and the other two months were the driest since 2002.

This kind of dryness to the start of the year can only be expected to occur once in every 15 years, on average. In other words, the chance of the first three months of a year having such low rainfall is less than 7%.

At the Airport, the number of wet days (with 1 or more mm) up to the end of March was 15. Usually we would have 27 for the first-quarter.

The sluggish start to rainfall for the year seems due mainly to a positive North Atlantic Oscillation (NAO) or higher than normal surface pressure across the Atlantic Ocean. A positive NOA has a cooling effect on sea surface temperatures (SSTs), which in turn causes unconducive atmospheric conditions for rainfall.

Outlooks for the next six months are not very encouraging. Based on SSTs across the tropical Atlantic and Pacific Oceans, the main drivers of our climate, lower than normal rainfall is likely through the next six months.

We are in the dry season and it does not normally rain a lot; however, relative to this time of the year, this first-quarter deficit is enormous, especially against the backdrop of the current protracted drought and a dismal rainfall outlook for much of the rest of the year.

The last time we had a drier start to the year back in 2001, we were in the midst of a severe drought. The eventually rainfall total for that year was 850.9 mm (33.50 in), much lower than normal and drier than last year.

A dry first-quarter does not always signal a dry year (below normal rainfall year). However, of years with the top 10 driest first-quarters, 6 are dry, 2 are normal, 1 is wet and 2015 is to be decided. We will keep you posted.

Follow us also @anumetservice, facebook and tumblr to keep updated with weather & climate info for the protection of life, property, livelihood & the enhancement of the economy.





The 2015 Hurricane Season Early Forecast

11 04 2015

By Dale C. S. Destin |

Early forecasts just issued for the upcoming 2015 Atlantic hurricane season indicate another quiet season is likely. The forecasts indicate that the 2015 season could be even quieter than 2014 and perhaps be the quietest since the middle of the 20th century.

Early 2015 consensus forecast

The consensus based on forecasts from Klotzbach and Gray of Colorado State University and Saunders and Lea of Tropical Storm Risk.com (TSR) calls for nine named storms, four becoming hurricanes and two becoming major (Category 3 or higher) hurricanes.

EarlyHurricaneSeasonForecast2015

A better indicator of the activity for the season is the accumulated cyclone energy (ACE) index which is a measurement of the strength and duration of each tropical cyclone. Summing together the ACE of each cyclone, provides a snapshot of how active the season is likely to be outside of just the number of storms.

This year, the consensus forecast calls for an ACE index total of 48. If this forecast pans out, the 2015 hurricane season would be quieter than last year’s and the third quietest since 1995, when the hurricane season went from a quiet phase to an active one.

End of Atlantic active phase?

Around 1995, the Atlantic hurricane season went from a quiet phase, when the average annual number of named storms increased from 9 to 15. Questions are now being raised in the tropical cyclone community as to whether we have come to the end of this active phase.

According to Saunders and Lea, “should the…forecast for 2015 verify, it would mean that the ACE index total for 2013-2015 was easily the lowest 3-year total since 1992-1994, and it would imply that the active phase of Atlantic hurricane activity which began in 1995 has likely ended”.

However, before we burst open the bubblies in celebration of fewer hurricanes for the next few decades, it must be stressed that the ability to accurately forecast an upcoming hurricane season from April, over one and a half months before the start of the hurricane season, is very low. The next set of forecasts, with better skill, will by out by June 1.

Factors pointing to a quiet season

Although the April seasonal forecasts are very low skilled, there are two already occurring phenomena present that are notorious for producing quiet Atlantic hurricane seasons. These are El Nino and a positive North Atlantic Oscillation (NAO). These are the main factors cited for the projected quiet season.

Probability of Antigua being hit by a hurricane

The probability of Antigua being hit by a hurricane annually appears to vary depending on the phase of the Atlantic. Based on the period 1981-2010, the climatological probability of being hit by at least one hurricane is around 28%. However, for the Atlantic hurricane season quiet phase – 1962 to 1994, the probability was around 14%. While for the active phase – 1995 to present, the probability increased to around 36%.

Interestingly, based on ENSO record dating back to 1950, we have never been hit by a hurricane during an El Nino episode that has occurred over the whole or part of a hurricane season.

On the other hand, we have been hit by nine hurricanes during 24 La Nina episodes. By Poisson distribution, this equates to around a 31% probability of us getting hit by at least one hurricane during La Nina.

According to Klotzbach and Gray, the likely best similar/analogue years to the upcoming 2015 hurricane season are 1957, 1987, 1991, 1993 and 2014. Of these years, we were only hit in 2014 by Gonzalo. Thus based on similar years, the probability of Antigua being hit this year is about 18%.

Good news and bad news

There is an Antiguan saying: “de same stick that hit the wild goat will also hit the tame one.” El Nino and the positive NAO will likely “hit” the hurricane season, resulting in suppressed activity. However, these same phenomena will likely “hit” our rainfall activity, resulting in suppressed rainfall and continued drought.

2014 hurricane season and lessons learnt

The 2014 hurricane season produced eight named storms, three hurricanes and two major hurricanes. The ACE index total was 66, the fourth lowest since 1995. It was a quiet year for many but not Antigua as we were hit by Hurricane Gonzalo.

Gonzalo serves as a perfect reminder that notwithstanding a quiet season, it only takes one hurricane to make it an active season for us. Hence, quiet season or not, the same preparations are required.

Our next blog on this topic will be on June 2.

Follow us also @anumetservice, facebook and tumblr to keep updated with weather & climate info for the protection of life, property, livelihood & the enhancement of the economy.








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