Tropical Storm Fiona Approaches

Mid-August typically marks the start of the most active period for tropical cyclones in the Atlantic sometimes referred to as the Cape Verde Season. This part of hurricane season is so called because the upswing in activity in the Atlantic this time of year typically occurs from systems that form in the central and eastern Atlantic near the Cape Verde Islands. These type of systems are notorious for their long tracks that can favor stronger hurricanes once they reach the western Atlantic and threaten land. Right on cue, a series of tropical waves has begun tracking into the Atlantic from West Africa.


Early season formation points (June 1-10) compared to formation points this time of year highlighting both the upswing in activity, and the shift into the eastern and central Atlantic, particularly near the Cape Verde Islands off the west coast of Africa. (Link)

Tropical Storm Fiona

201608202045 UTC FIONA IR.gif
GOES Satellite floater un-enhanced infrared centered on Tropical Storm Fiona. (Link)

Fiona is currently a small tropical storm with maximum sustained winds near 35 kts. Its associated cluster of deep convection is supporting tropical storm force winds that only extend out to 70 nm away from the center of the storm. Fiona is currently embedded in the dry and dusty air associated with the Saharan Air Layer which is limiting the amount of deep convection the storm can sustain, while southwesterly vertical wind shear is limiting the organization of the deep convection Fiona does produce.

The Forecast

Bermuda should be monitoring Fiona carefully as the storm tracks generally northwestward, in the general direction of Bermuda. The latest forecast (as of 6pm local) from the National Hurricane Center (NHC) has Fiona continuing northwestward and weakening before degenerating into a remnant low on its final approach to Bermuda by Wednesday. With Fiona’s closest point of approach within the next 72 hours at 312 nm, Fiona is a potential threat to Bermuda (see: Local Advisory).

Fiona’s track to the northwest puts the storm over increasing sea surface temperatures; water near Bermuda is currently around 86°F (30°C) which is plenty warm to support a tropical cyclone. Aside from the increasing sea surface temperatures, Fiona’s environment is not expected to improve enough and re-strengthening is not expected.

Map of a snapshot of sea surface temperatures yesterday – much of the Atlantic is warm enough to support a tropical cyclone using the traditional 79F (26C) threshold for formation (red contour). Warmer waters are generally more favorable for development. (Link)

However, it is important to monitor this system closely for any changes in this forecast as it approaches. In the past, marginal tropical cyclones like Fiona have struggled through dry/sheared environments, only to survive into the western Atlantic, finding it more moist and less sheared. This allowed quick reorganization and strengthening near Bermuda forcing short-notice warnings (eg. Maria (2011) and Gabrielle (2013)).

Otherwise, there are two other significant tropical waves to monitor as of 3pm local time. One such wave is southeast of Fiona, in the central tropical Atlantic. This wave is expected to track westward into the northeastern Lesser Antilles Islands/Caribbean Sea, and the NHC has given it a 60% (medium) chance for becoming a tropical depression or storm in the next 5 days. The second wave is now exiting the west coast of Senegal in Africa. The NHC is giving it a 70% (high) chance of developing in the next 5 days as it too tracks westward into the tropical central Atlantic.

Right now, showers are expected in Bermuda starting Sunday evening as a weak boundary moves into the area from the west. That boundary lingers as an area of troughiness that keeps showers and possibly thunderstorms in the region through Wednesday as Fiona, or Fiona’s remnants approach from the southeast. No adverse weather is expected from Fiona at this time.

As always, the best advice is to exercise constant vigilance with the tropics by following the latest official forecast updates from the Bermuda Weather Service and the National Hurricane Center.

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