Cape Verde Dorian


Tropical Storm Dorian earlier today, shortly after becoming a tropical storm.
Tropical Storm Dorian earlier today, shortly after becoming a tropical storm to the southwest of the Cape Verde islands.

An early season Cape Verde Tropical Cyclone, Tropical Storm Dorian becomes one of the earliest storms to form so far east in the Atlantic as designated by the National Hurricane Center. The earliest – Hurricane Bertha 2008. However, being so far out in the Atlantic, Dorian has thousands of miles of ocean to cross before reaching the next landmass. That same stretch of ocean contains many obstacles to Dorian’s survival- from marginally warm waters, dry and stable air, and upper level wind shear. Currently, Dorian has maximum sustained winds of 50mph and tropical storm force winds extend 45miles from the center of this storm.

In the near term, Dorian is paralleling sea surface temperatures that are borderline for tropical cyclone support just to the north. Also to the north, dry and stable air on the southeast side of the Bermuda-Azores high could threaten the storm. These two factors could play a role in limiting intensification in the next 1-3days. Once Dorian moves into the warmer waters and more humid air of the central Atlantic after day 3, it then has to contend with stronger upper level winds on the southeast side of the tropical upper-tropospheric trough (TUTT). This semi-permanent feature in the central Atlantic is expected to be positioned such that strong upper level westerly winds could hinder development of Dorian, despite the warmer seas. However, if Dorian can deepen enough despite the current environment that appears only slightly favorable, it may have a strong enough upper level anticyclone to influence the position and intensity of the TUTT in 3-5days.

There is so much uncertainty in how Dorian will interact with its current environment that any long term intensity forecasts should be taken with a grain of salt. In fact, Dorian’s near term intensity has major implications on its future intensity and track – Dorian could dissipate and pass through the Greater Antilles as a tropical wave, or Dorian could become a major hurricane threatening the western Atlantic from Puerto Rico to Canada – including Bermuda. With that in mind, it is advisable to keep an eye on Dorian’s progress over the next 5 days or so as we get a better idea of how it interacts with its current environment and how the steering currents and upper atmosphere sets up around the western edge of the Bermuda-Azores high – we have plenty of time to watch this storm and should follow official updates by the National Hurricane Center and the Bermuda Weather Service.

My Analysis of the Atlantic on July 24th at 20:15z. Red dashed lines indicated the rough position of troughs near Bermuda, while the red circle is Tropical Storm Dorian. The light blue arrows represent rough upper level winds indicating the  location of the TUTT.
My Analysis of the Atlantic on July 24th at 20:15z. Red dashed lines indicated the rough position of troughs near Bermuda, while the red circle is Tropical Storm Dorian with current trajectory. The light blue arrows represent rough upper level winds indicating the location of the TUTT.

Meanwhile, convergent showers and thunderstorms are possible in Bermuda as two separate surface troughs approach from the west and the east producing locally inclement weather and heavy rain. A thunderstorm advisory is in effect for these showers. This activity is slightly enhanced by the TUTT to the south and east of Bermuda. The surface trough the east of Bermuda is being monitored by the National Hurricane Center for tropical development, and they estimate that it has a 20% chance of becoming a tropical cyclone in the next 48hours. However, regardless of development, this system is expected to quickly turn north and track out to sea around the western side of the Bermuda-Azores high; not impacting Bermuda with strong winds or widespread heavy rain.

Bermuda has entered a dry spell, only officially receiving 0.58″ of rain for the last 30 days. While isolated showers have produced locally heavy rain in the last few days, much of the island has remained dry. On the year-to-date precipitation totals, Bermuda has turned 6″ below normal. Only a few isolated showers  are likely in the current five-day forecast with only local improvement in the dry conditions. Additionally, on these southerly and southwesterly winds, warm advection has kept Bermuda a few degrees above normal for the past week in both high and low temerpatures.

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