The Meaning of the Tropical Cyclone Track Forecast

25 08 2015

Dale C. S. Destin |

Some persons are unhappy with the performance of the Antigua and Barbuda Met Service over the tracking of Tropical Cyclone Danny. One of the main issues is that the forecasts, beyond 24 hours,  had Danny passing over Antigua but instead it passed 60-70 miles south of the island.

So the question is did we, including the U.S. National Hurricane Center get it wrong? The simple answer is no. However, for you to be able to answer the question or make accusations, a better understanding to the meaning for the track forecast is required. I wrote a brief blog on tumblr explaining the inherent errors associated with the forecast; you are invited to have a read.

5yr_ALtkerrdist_sm

From the above graphic: the error for a 12-hour forecast (which takes effect 12 hours from the time issued)  is plus/minus 32 miles, for 24 hours it’s plus/minus 52 miles etc.





Struggling TS Danny

19 08 2015

Dale C. S. Destin |

Recent image of TS Danny

Recent image of TS Danny

Recent images of Tropical Storm (TS) Danny were rather unimpressive in terms of organization (good news for the Caribbean). The system appears to be struggling as dry Saharan air is getting sucked into it. However, strengthening is forecast by the official U.S. National Hurricane Center forecast.

At 11 a.m. this morning, Danny was assessed to have sustained winds of 50 mph about 1400 east of Grenada, in the Windward Islands, and 1435 east-northeast of Antigua, in the Leeward Islands. It was moving west at 12 mph.

Official forecast track

Official forecast track

Danny's track

The models appear to be showing a high degree of certainty with respect to the eventual path of Danny. Almost all the models, I have surveyed, have the system passing over or north of the northeast Caribbean.  This means that Antigua and the rest of the northeast Caribbean could be in the direct line of fire from Danny come Monday.

Danny's intensity guidanceContrastingly, the models are showing considerable uncertainty with the forecast strength of Danny. The official forecast is for it to become a hurricane on Friday. However, the models are split down the middle on the eventual strength of the cyclone with an even chance of it remaining a storm or becoming a hurricane. Becoming a hurricane may become the news worst case scenario.

The models are, however, unanimous on Danny not becoming a Category 2 hurricane, unlike previous forecasts (good news). It could still be of similar strength to Gonzalo, of last year, when it reaches us. There is enough uncertainty to suggest that it may even be a weaker cyclone upon arrival.

The more reliable European Centre for Medium-Range Weather Forecasts (ECMWF) continues to only give Danny a less than 5 percent chance of becoming a hurricane. Further, it gives it a low chance of impacting the islands as a TS. The ECMWF continues to be a source of relatively good news for us.

ECMWF forecast

ECMWF forecast

A hurricane watch will likely come into effect for portions of the northern Eastern Caribbean on Friday followed by a warning Saturday/Sunday. Start or get ready to execute your hurricane plan!

Follow us also on @anumetservice, facebook and tumblr for the latest on the current drought and other weather & climate news.





Assuming the Worst: TD4 becomes Hurricane Danny

18 08 2015

Dale C. S. Destin |

Official_Track_ForecastThe first tropical cyclone (generic term for tropical depressions, storm and hurricanes) to threaten the Caribbean formed this morning. The tropical disturbance over the eastern tropical North Atlantic showed enough organization this morning to be upgraded to Tropical Depression Four (TD4).

The TD4official forecast from the U.S. National Hurricane Centre (NHC) has  the system developing into a Tropical Storm (TS) Danny later today and a Category 2 Hurricane by Sunday about 570 miles due east of Martinique and 650 miles east-northeast of Antigua.

ECMWF model

Notwithstanding the official forecast, arguably the most reliable weather model, the European Centre for Medium Range Weather Forecasting (ECMWF) model, does not seem to agree with it. The ECMWF model was quite early in forecasting the system becoming a TD but at most, so far, only gives it a 40 percent chance of becoming a tropical storm and less than 5 percent chance of becoming a hurricane.

Further, the ECMWF gives the system a less than 5 percent chance of impacting the Caribbean as a TS.

The ECMWF model seems to be expecting the abundance of dry air coming off of Africa and perhaps the return of hostile upper level winds to inhibit the TD.

At this time, I would like to view the Official forecast as nearing the worst case scenario and the ECMWF version of events to be nearing the best case scenario. I could be well wrong; however, given the impeccable track record of the European Centre model, it is difficult to accept that it could be is soo far off its game. It would be an unpleasant surprise to me and many.

Assuming the official forecast to be the more accurate, what would this mean for the Antigua and the Caribbean? We (the Caribbean) would need to prepare for a Category 2 hurricane with winds near 100 mph by Sunday with time to either strengthen further or weaken.

What is expected to be Hurricane Danny by then could impact islands as far south as Trinidad and Tobago or missing all the islands, passing a safe distance northeast of Antigua and Barbuda.

aal96_2015081806_track_gfs

aal96_2015081812_track_early

aal96_2015081812_intensity_earlyAt this time, residents in the Eastern Caribbean should at least be monitoring the potential Hurricane Danny and finalize their hurricane plans. If this goes according to the U.S. NHC forecast, a hurricane watch may be required for portions of the islands around Saturday with a warning around Sunday.

Again, assuming the worst case scenario, parts of the Eastern Caribbean could be in Hurricane Danny around Monday/Tuesday with the most likely path, according to the ECMWF and the U.S. Global Forecasting System (GFS) models, taking it across the northeast Caribbean passing in the vicinity of the Norther Windward Islands, the Leeward Islands, the Virgin Islands and Puerto Rico.

The system has the potential of producing destructive hurricane winds, stronger than those of Gonzalo of last year; flooding rainfall of two to six inches (51-153 mm), rough seas in excess of 12 feet (3.5 metres), significant storm surge flooding, and we cannot rule out tornadoes.

If we were to return to the forecast from the ECMWF as the best case scenario, the system would, at most, become a marginal tropical storm and bring needed rainfall to the Caribbean. Either way, some actions will likely be required this weekend to protect, life, property and livelihood due to hazards brought on by a TC Danny.

A lot can happen over the next several days; it is going to be interesting to see how things pan out; follow us and keep informed.





Antigua is out of Surface Water Again

17 08 2015

Dale C. S. Destin |

Drought stricken Antigua is currently out of surface water once again, according to the Antigua Public Utilities Authority (APUA), our water authority. All surface catchments have fallen below extraction levels or have dried up as of the end of July for the second time in a year.

Potworks Dam, Aug 1, 2015 | Courtesy Karen Corbin, Humane Society

Potworks Dam, Aug 1, 2015 | Courtesy Karen Corbin, Humane Society

The country is now relying enormously on desalinated potable water. Our current daily potable water mix is around 83% desalinated water and 17% groundwater. Under normal conditions, 57% of our potable water comes from the sea, 28% from surface catchments and 15% from the ground.

According to APUA, around 5.5 million imperial gallons (MIG) of potable water are being produced per day, based on recent figures. However, the country requires around 8 MIG per day to satisfy demand. This means that there is a hefty daily deficit of around 2.5 MIG or 31%.

APUA plans to install a new desalination plant later this year; this will reduce the deficit but fall well short of eliminating it. In the interim, the water deficit could increase further as the drought continues.

The country was last out of surface water around this same time last year, 2014, after nearly a year into the current severe and prolonged drought. Unfortunately, we have found ourselves in this position many times in the past, when our surface catchments, which amount to a total capacity of around 1346 MIG, go dry.

Other years of depleted catchments include 2009/2010, 2000-2003, 1991, 1983 and 1973/1974 with perhaps 1983 being one of the most memorable as water had to be barged from Dominica.

A paper by A. J. Berland et al., published in 2013, shows that Antigua’s water woes and insecurities date back to colonial times.

Bethesda Dam, July 31, 2015 | Courtesy Karen Corbin, Humane Society

Bethesda Dam, July 31, 2015 | Courtesy Karen Corbin, Humane Society

Antigua has been in drought for around two years to date. Over the past three months, it has been at severe levels with rainfall in the bottom 1% of the historical records or amounting to less than 28% of the average. Further, year to date, we have had record low rainfall; the driest ever January-July dating back to at least 1928.

During the later part of 2014, we saw a significant recharge of catchments, due mainly to above normal rainfall in November, the only wet month for that year. The rainfall reduced the drought to slight levels but was not enough to end it. Since then, it has been all downhill.

The outlooks for rainfall remain depressing. Lower than normal rainfall is likely for August and August-October, and below to near normal rainfall is likely for November-January. Meanwhile, above normal temperature over much of August-January could exacerbate things.

We rely heavily on the wet season (July-December) rainfall to recharge catchments to take us through the dry season (January-June). If the drought persists, as we expect it to, catchments could remain below extractable levels or dry until the next wet season.

The need for water conservation and efficiency, at this time, cannot be over emphasized.

Follow us also on @anumetservice, facebook and tumblr for the latest on the current drought and other weather & climate news.





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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