What has become of the 2015 Hurricane Season?

8 08 2015

Dale C. S. Destin|

Recently issued updated forecasts for the 2015 hurricane season, which started June 1, reiterated another quiet season is highly likely. The latest set of updated forecasts are similarly calling for this season to be even quieter than 2014 and perhaps be among the top 10 quietest on record.

The updated 2015 ensemble forecast

The updated ensemble or mean forecast is for nine named storms (including Tropical Storm Ana, Bill and Claudette), four becoming hurricanes and one becoming a major (Category 3 or higher) hurricane. On average we get twelve named storms, four hurricanes and three major hurricanes.

2015_HurricaneSeason_Graphic

Our ensemble forecast is based on forecasts from Klotzbach and Gray of Colorado State University (CSU), the National Ocean Atmospheric Administration (NOAA), Saunders and Lea of Tropical Storm Risk.com (TSR) and European Centre for Medium-Range Weather Forecasts (ECMWF).

The indicator of the activity used by meteorologists is the accumulated cyclone energy (ACE) index which is a measurement that takes into consideration the number, strength and duration of tropical cyclones (storms and hurricanes) for the season.

The ensemble ACE index forecast for this season is 44, down from 52 indicated in June and 61.35 below the normal/average. If this forecast verifies, this hurricane season will be the tenth quietest since 1981 and the third quietest since 1995.

The season thus far

TS_Bill_2015-06-16_1955ZWe are two months into the hurricane season and thus far, it has been average, notwithstanding the forecast. May to July has had three tropical storms – Ana, Bill and Claudette.

Damage to property has been minimal; however, there have been eight deaths. The Caribbean including Antigua and Barbuda has been unaffected, thus far (“knock wood”).

Ana was a preseason storm and now holds the record for the earliest tropical cyclone to strike the United States. Antigua has never had a tropical cyclone in May but had a hurricane in January 1954 and March 1908.

Why is a quiet season expected?

El Nino is expected to be the main cause of a quiet season. El Nino has strengthened over the past months and has reached the threshold to be categorized as strong.

History has shown that the stronger an El Nino,, the more difficult it is for tropical cyclones to form over the Atlantic. This year we could see what is referred to as a super (strong) El Nino which would make it super difficult for tropical cyclones to form.

Past times have also shown that cool sea surface temperatures (SSTs) across the tropical North Atlantic (TNA) makes it hard for tropical cyclone formation. As of July, SSTs in the TNA were around 0.5 °C below average; hence, cool unfavourable tropical cyclone conditions exist and should remain this way for much of the next three months.

PA-Atl_SSTs_Jun-Jul_2015

Another limiting factor for tropical cyclone formation is the more than usual flow of very dry, dust Saharan air across the TNA. This almost makes it impossible for tropical cyclone to form in their favourite area – between Africa and the Caribbean. However, it is uncertain if this flow will continue.

500MB_RH1

Combined, we may be witnessing one of those rare occasions when all the ingredients are in place to cause one of the quietest hurricane seasons on record.

Probability of Antigua being hit by a hurricane

Based on the ENSO record dating back to 1950, we have never taken a hit from a tropical storm or hurricane during an El Nino episode that has occurred over any part of the hurricane season, and this year is expected to be no different (“knock wood”).

The probability of Antigua being hit by a hurricane annually appears to vary depending on the phase of the Atlantic. However, the overall probability is 28%, based on the period 1981-2010.

According to Klotzbach and Gray, the best similar/analogue years to June-July 2015 hurricane season are 1965, 1972, 1982, 1987 and 1997. Of these years, only Erika brushed us in 1997. Thus, based on similar years, the probability of Antigua being affected this year, by a storm or hurricane, is around 18%, an increase of 3% from the June and the same as what was issued in April.

Don’t be caught off guard

Quiet season or not, we cannot let our guards down. Recall last year’s season was quiet, yet we were rough up by Hurricane Gonzalo. Stay prepared especially since we are in the peak of the hurricane season, August-October.

The hurricane season runs from June 1 to November 30 each year.

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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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.

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.





Unanimous Hurricane Season Prediction, But!!!

4 06 2014

By Dale C. S. Destin |

Major Hurricane Luis, Sep 3, 1995

Major Hurricane Luis, Sept 3, 1995

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

It is “June too soon”, the start of the Atlantic hurricane season. The predictions this year are unanimously in favour of a normal to below normal season. This is considered good news for the people of the Caribbean, Central America and North America. But, don’t be lulled into a false sense of safety; preparations for the season are still very important, regardless to the forecast.

2014HcaneGraphic

CSU; FSU; TSR; NOAA; WSI; AccuWeather; UK Met; INSMET – Institute of Meteorology, Cuba; EUROSIP; ECMWF; NCSU

Updated Forecast

The consensus forecast calls for 10 named storms (NS) of which five are expected to become hurricanes (H) and two major hurricanes (MH) i.e. winds of 111 mph and greater. These are very normal numbers given the average season, based on the period 1950-2010, usually produces 11 named storms, 6 hurricane and 3 major hurricanes.

A better measure of the activity of a season is the accumulated cyclone energy (ACE) index. In addition to taking into account the number of named storms, the ACE index also takes into account the duration and strength of each storm. For the 2014 Atlantic hurricane season, the consensus is for an ACE of around 72, the fifth lowest since 1995. This is about 30% below the average of 103.5 (1950-2010).

Uncertainties

There are two main reasons why the various models are predicting a normal to possibly below normal season. Firstly, an El Nino is deemed likely. El Nino is the name given to the episode of unusually warm sea surface temperatures (SSTs) in the tropical Pacific. Secondly, the tropical North Atlantic SSTs have been cooler than normal. This configuration of high and low SSTs over the tropical Pacific and Atlantic Oceans respectively is just about the perfect recipe for a quiet hurricane season.

However, it is not clear when this contrast of SSTs will become optimal. If El Nino occurs late in the year, it may not hamper the hurricane season as expected. Hence, the number of storms could be more than indicated by the consensus forecast above. On the other hand, the earlier El Nino develops, the lower the number of storms is likely to be.

There is also the issue of the eventual strength of El Nino. A strong El Nino would have a more suppressing impact on the hurricane season than a weak El Nino or no El Nino, which is probable but highly unlikely.

Additionally, there is also the unknowable of whether it will be a central Pacific or eastern Pacific El Nino. Both suppress hurricane activity but by contrast, the former tends to cause an above normal number of cyclones impacting the western Caribbean, Central America and the United States. Thus, places like Jamaica, Cayman Islands and Cuba have an elevated chance of being struck by a storm or hurricane.

Further, coupled with the inherent difficulties in predicting El Nino during this time of the year (the spring barrier), preseason hurricane forecasting skill is also still quite low. Most forecasting groups have less than 20% skill in forecasting the Atlantic hurricane season during June or before.

What It Does And Does Not Mean?

How is the forecast to be understood? It means that tropical cyclone activity in the Atlantic basin will be normal or possibly below normal with just a slight chance of the season being above normal. It means that many areas will be spared the impact of a storm. However, it does not mean that no place will be impacted. It does not mean that every place or area will be spared of above normal tropical cyclone activity. The forecast speaks to the expected cumulative conditions for the Atlantic basin (Atlantic Ocean, Caribbean Sea and the Gulf of Mexico). It does not say anything about what can be expected for a particular place, country or region.

It is quite possible for there to be a normal or below normal season and your country or region experiences above normal activity. This can happen since the forecast does not say anything about a specific point or country, so while the Atlantic Basin may have a quiet hurricane season as a whole, your country or region may not necessarily escape a disaster. It only takes one hurricane to cause a disaster, ruin your year and cause significant socio-economic setbacks.

Tropical cyclone adaptation and preparation are the only ways to minimize the impacts of tropical cyclones. Do not take any hurricane season lightly. Prepare for every season as if it is going to be the worst you will ever face, because it could well be so, regardless to the forecast.

The Names

The designated storm names for this year’s Atlantic hurricane season – June 1 to November 30, are Arthur, Bertha, Cristobal, Dolly, Edouard, Fay, Gonzalo, Hanna, Isaiah, Josephine, Kyle, Laura, Marco, Nana, Omar, Paulette, Rene, Sally, Teddy, Vicky, Wilfred.

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