Tropical Storms in May Mean Nothing

15 05 2020

Dale C. S. Destin |

The first tropical or subtropical storm for the pre-hurricane season is about to form in the Bahamas. Some think that this is a harbinger (sign) for the upcoming hurricane season – June 1 to November 30, but is it?

Tropical disturbance AL90 across the Bahamas with an 80% chance of becoming Tropical Storm Arthur within 48 hours.

To the “naked eyes”, it is understood why some may take a storm forming in May as a sign of bad news for the upcoming hurricane season; however, the numbers don’t agree.

The numbers say that it usually means nothing in terms of the overall activity of the upcoming season. According to NOAA, there have been 29 named storms in May, spread over 27 seasons from 1842 to 2019. Of the 27 seasons, using NOAA’s definitions, nine were above normal (active or hyperactive), 11 were near normal and 7 were below normal (inactive or quiet).

Overall, most seasons with a storm in May, are near normal – 11 times of 27 – 41%. However, from a statistical standpoint, there are no significant differences between above, near or below normal seasons, when there is a storm in May. Hence, a tropical storm forming in the month has no bearing on the activity of the upcoming hurricane season.

Of the 27 seasons with May storms, 9 or 33% was active or above normal; 11 or 41% was normal and 7 or 26% was quiet or inactive

Notwithstanding the above, this hurricane season is expected to be above normal or likely hyperactive – well above normal, see my latest forecast. Clearly, this has nothing to do with the expected formation of Tropical Storm Arthur over the Bahamas.

Further on the tropical cyclone climatology on May, of the 29 named storms to have formed in the month, 5 became hurricanes with none ever becoming a major hurricane. The strongest tropical cyclone of the month occurred in 1863 – a Category 2 unnamed cyclone.

The only Cat 2 May Hurricane on record – Unnamed Hurricane – 1863

On average, there is one storm forming in May every 7-8 years. However, there have been a storm in May of the last 2 years and 6 in the last 10 years. Twice, two storms formed in May of the same season – 2012 and 1887, the most of any. So much for averages, right!

The Eastern Caribbean has never been impacted by a tropical cyclone in May, “knock wood”, based on available record dating back to 1842.

Storms forming in this part of the region in May are not unusual. Of the 29 May storms, 28% have formed or traverse within 300 miles of Nassau, Bahamas.

Unfortunately for the Bahamas, this year they are still recovering from Super Category 5 Hurricane Dorian of September 2019, which levelled much of Northwest Bahamas. You may say that they can’t “catch a break”, as they are about to deal with potentially strong storm-force winds and flooding rainfall, in addition to dealing with the COVID-19 Pandemic.

Possible tracks of what is expected to become Tropical Storm Arthur in 48 hours

May storms mean nothing in terms of the overall activity of the hurricane season; however, they mean a lot with respect to where they impact. Let us be prepared regardless of omens or forecasts for the season – it only takes one hurricane to ruin your year or life.

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May 2020 Updated Hurricane Season Forecast: Hyperactive Season Likely

11 05 2020

Dale C. S. Destin |

Our updated forecast for the 2020 Atlantic Hurricane Season is out, and it calls for higher than normal activity – an above normal season. The forecast predicts an accumulated cyclone energy (ACE) index of 189 (down 6 from the previous forecast), 19 named storms (down 1), 9 hurricanes (unchanged) and 4 major hurricanes (down 1). With these numbers, it is also likely that the season will be hyperactive i.e. well above normal and possibly unusually destructive.

If this forecast, for which there is very high confidence, pans out, this season would be the most active since 2017, in terms of ACE, and the 10th most active on record dating back to 1851.

The season also has the potential to be record breaking in some areas, as there is 70 percent confidence of

  • 14 to 23 named storms;
  • 6 to 12 becoming hurricanes;
  • 2 to 7 becoming major hurricanes and
  • 112 to 276 ACE.

If the higher end of the ranges were to materialise, records would be equalled or broken for the number of major hurricanes and more importantly ACE, which is the universally accepted metric used to classify the overall activity of a hurricane season.

According to other forecasts surveyed, the consensus is for an ACE of 146 (down 3), 17 named storms (up 1), 8 hurricanes (unchanged) and 4 major hurricanes (unchanged) – above normal season. This is generally consistent with my forecast but with less activity. However, regardless of the forecast, you should always prepare the same each season, as it only takes one hurricane to ruin your year and or life.

Accumulated Cyclone Energy (ACE) based on 10 forecasting entities, including 268Weather.

The main reasons for the above normal forecasts are the likely above normal sea surface temperatures across the tropical North Atlantic and a cold-neutral El Niño Southern Oscillation (ENSO) or a weak cold ENSO, i.e. weak La Niña, during the peak of the hurricane season – August to October.

A typical season has 12 named storms, 6 hurricanes, and 3 major hurricanes. Major hurricanes have sustained wind speeds of at least 178 km/h or 111 miles per hour (e.g., Category 3 or higher), according to the Saffir Simpson Hurricane Wind Scale.

Click for pronounciations

With respect to records, the 2005 season has the highest number of named storms and hurricanes – 28 and 15 respectively. The 2005 season also tied with the 1961 season for the highest number of major hurricanes – 7, and the season with the highest ACE – the most active season on record, based on ACE only, is 1933 with 259.

The last Atlantic hurricane season – 2019, was more active than normal and will long be remembered for Super Category 5 Hurricane Dorian’s destruction of the northwest Bahamas. The season had 18 named storms, 6 became hurricanes – winds of at least 119 km/h or 74 miles per hour, and 3 became major hurricanes.

This forecast will be updated monthly until August. The next update will be issued around June 10.

The Atlantic hurricane season begins in days – June 1 and concludes on November 30, be prepared!

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Driest April for Antigua in Over Two Generations

1 05 2020

Dale C. S. Destin|

Not since 1944 has an April been drier for Antigua than April 2020. It was the third driest April on record – the driest in 76 years or well over two generations. The last time April was drier, V. C. Bird Snr was president of the Antigua Trades and Labour Union and World War II was still raging.

Potworks Dam, May 1, 2020. Picture courtesy Karen Corbin – Humane Society

The island average for the month is 16.1 mm (0.63 in). Only April 1944, with 5.9 mm (0.23 in) and 1939, with 8.4 mm (0.33 in), have been drier in nearly 100 years.

The total for the month represents only 19% of the normal total of 85.6 mm (3.37 in) – a deficit of 81%, based on the current climatological standard period of 1981-2010.

Such a low rainfall total is very rare for April. How rare? It has only around a 4% chance of occurring or once every 25 years on average, based on record: 1928-2019. This would suggest that we were around 50 years overdue for such a parched April.

The reason for us going so many years overdue for such a near record low total is likely due to “positive” climate change. Rainfall for April has been on a steady wetting increase, rising from an average of 53.1 mm (2.09 in) for the period 1928-1957 to 80.8 mm (3.18 in) for the period 1990-2019.

So, if you were to segment the data, there is around an 11% chance of getting 16.1 mm during the years 1928-1957, as compared to just a 0.6% chance for the period 1990-2019. In other words, our current climate for April is over 18 times less likely to produce such a low rainfall total as compared to the past climate of 1928-1957.

Now, based on the segmented data, the return period for April’s rainfall of 16.1 mm is once every 9 years on average for the past climate. However, the return period for the same value in the current climate is once every 167 years on average. Hence, most persons alive along with their grand and great grand children are unlikely to see a similar or drier April in Antigua.

Compared to other months, this April is the driest since June 2018. And of all the 1108 months on record dating back to 1928, it ranks as the 23rd driest. This puts it in the top 2% of all time driest months.

Notwithstanding the miniscule rainfall for April, the rainfall for the year, thus far, is near normal.  However, if not for a wetter than normal first quarter, the situation would be much more dire.

The reason for the absentee rainfall for April looks to be due to a switch from a positive North Atlantic Oscillation (NAO) to a negative NAO. This is manifest in the swing from above normal pressure over the subtropical North Atlantic to below normal pressure. This switch or swing resulted in a reduction of moist unstable air flowing across the area and depositing rainfall.

The pressure over our islands also went from neutral, over the period January-March 2020, to above normal for April. Such a configuration of the pressure would usually also inhibit rainfall, which was evident.

April 2020 was a remarkably dry month. One that most of us have never seen and will likely never see again – happily.

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