Subtropical Storm Andrea Forms and is Not Expected to Last Long

The first named storm of the 2019 Atlantic Hurricane Season was designated this evening in the form of Subtropical Storm Andrea, located between the Bahamas and Bermuda. Andrea formed from Invest 90L over the southwestern Atlantic northeast of the Bahamas. The National Hurricane Center estimates that Andrea has maximum winds of 40mph and a central pressure of 1006mb, as recorded by Aircraft Reconnaissance. The storm is forecast to remain subtropical and weak, and to only survive until Wednesday morning.

NHC Graphical Tropical Weather Outlook, showing Andrea between Bermuda and the Bahamas. (NHC)

Aircraft Reconnaissance flew into the disturbance this morning, initially finding a storm with a broad circulation with winds up to 42kts at the flight level in the eastern quadrant. The plane was able to find a closed mesovortex within the broader circulation, dubbing that as the center of the newly formed subtropical storm. Andrea is still pretty broad and disorganized, but it does have the necessary features to be deemed subtropical, in that it has a broad wind-field and is feeding its energy off of a baroclinic interaction with an upper-level low to the west and the associated instability rather than from the oceanic heat beneath it. The storm contains some centralized convection, causing the surface circulation to consolidate and weaken the embedded mesovortices.

Andrea northeast of the Bahamas at 2350 UTC May 20. (SSEC RealEarth)

The track and intensity forecast remains the same as the models have anticipated, as the storm is still expected to continue north and possibly intensify slightly over the next 24 hours, in that the NHC states that it will likely peak with winds of 45mph. Afterwards, Andrea should take a sharp easterly turn as a frontal boundary dips southward, enhancing vertical wind shear and killing off the cyclone. The chances of Andrea turning tropical are currently unlikely, as the upper-level low is expected to interact with the storm all throughout its lifespan and could potentially stack on top of the storm tomorrow, which could briefly enhance convective coverage and expand the wind-field while also not allowing for anticyclonic outflow. Once the frontal boundary approaches, the upper low will flatten into a trough and eventually fizzle, removing Andrea from its primary energy source. In addition sea surface temperatures, while warm enough to sustain a subtropical cyclone, are not warm enough to sustain a tropical cyclone, so without any baroclinic assistance, Andrea will quickly fizzle out by Wednesday.

Forecast Cone from the NHC for Andrea, showing a track turning east and dissipating to the south of Bermuda by Wednesday morning. (NHC)

Impacts to land from Andrea will be restricted to Bermuda, who should see enhanced rainfall, gusty winds, and rough seas into the middle of the week. Overall impacts should remain minimal. Some swells and rip currents may impact the Bahamas or southeastern United States, but significant impacts are not expected. The formation of Andrea has set 2019 as the fifth year in a row to feature a preseason named storm, the longest stretch on record, beating out 1951-1954, and has set the 2010s decade with more preseason storms than any other decade on record.

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