Tropical storm Dorian forecast to become second hurricane of season

Tropical storm Dorian is nearing the Caribbean and has strengthened in recent hours, with forecasters now expecting it will intensify into hurricane Dorian as it approaches Puerto Rico and Hispaniola in the coming days.

Dorian became the fourth named tropical storm of the 2019 Atlantic hurricane season over the weekend and while a small storm that some had forecast might dissipate relatively quickly, tropical storm Dorian has persisted and now begun to strengthen as it nears the Caribbean Windward islands.

Tropical storm Dorian currently has maximum sustained winds of 60 mph, with higher gusts and is located just off the Windward islands.

While tropical storm force winds currently only extend roughly 45 miles from the centre of Dorian, making it a relatively small storm, the expectation from NOAA forecasters is that Dorian may near hurricane strength when it passes through the northern Windward Islands on Tuesday and currently is expected to reach hurricane status when it gets closer to Puerto Rico and eastern Hispaniola.

Dorian is expected to produce between 3 to 8 inches of rainfall for the Windward Islands from Martinique south to St. Vincent, including Barbados, with isolated maximum totals of 10 inches possible for northern Windward Islands.

Some weather forecasting teams still feel Dorian may struggle to reach hurricane status, or if it does to maintain them, due to interactions with drier air blown over from the Sahara.

But forecasts appear to now be moving generally towards Dorian becoming a hurricane before it approaches Puerto Rico, the Dominican Republic and Haiti.

Of course Puerto Rico is still recovering from the 2017 hurricanes, events that the insurance, reinsurance and insurance-linked securities (ILS) market remember only too well.

The current forecast then takes Dorian towards the Turks & Caicos Islands and the Bahamas, but NOAA’s forecast suggests the storm will begin to weaken back to tropical storm status.

However, some model runs suggest Dorian may be able to sustain hurricane winds for longer. Here, though, there is considerable uncertainty it seems right now.

Below is the latest intensity model output from TropicalTidbits.com

After the Bahamas, many of the weather models take tropical storm (or hurricane) Dorian towards the east coast of Florida.

How strong Dorian could be at that stage looks a matter of some disagreement currently, with models ranging from a low category hurricane to a relatively weak tropical storm, with the dry African air and wind shear possibly set to make things very difficult for Dorian as it proceeds.

However, warm sea surface temperatures (SST’s) are definitely adequate for Dorian to sustain hurricane strength if conditions became more conducive, so the insurance, reinsurance and ILS industry will need to keep an eye on the tropical storm as it makes progress over the coming days.

Below is the latest hurricane model forecast model tracks for Dorian, from TropicalTidbits.com.

Impact Forecasting, the catastrophe modelling and meteorological division of insurance and reinsurance broker Aon had the below comments on Dorian in its latest update, which explains the uncertainty in the forecast well.

Environmental conditions, except for the abundance of dry mid-level air surrounding and occasionally being entrained into Dorian’s inner-core region, would favor at least steady strengthening due to very low vertical wind shear, warm sea surface temperatures, and the small overall circulation and inner-core wind field. However, until Dorian closes off a solid eyewall, only slow strengthening is likely.

Anticipating when an eye will form is challenging to forecasters, but Dorian could be a hurricane by the time it reaches the Windward Islands. The NHC intensity forecast has been adjusted upward from the previous advisory but is not as high as some of the other available intensity models. Interaction with Hispaniola and possibly Puerto Rico should result in some weakening in 72-96 hours, followed by restrengthening on Day 5 when Dorian will be moving over the very warm waters in the Bahamas in low wind shear conditions.

Given the unknown degree of potential interaction with Hispaniola, the intensity forecast at days 4 and 5 remain highly uncertain at this time.

We will update you on tropical storm Dorian as it proceeds, should it look likely to achieve and sustain hurricane status and pose any threat to global insurance, reinsurance and ILS market interests.

You can always visit our 2019 Atlantic hurricane season page for the latest.