Tropical Storm Sebastien (2019)

Storm Active: November 19-24

Around November 15, a trough of low pressure formed over the central Atlantic, generating a diffuse area of shower activity well to the east-northeast of the Lesser Antilles. The system moved steadily northwestward over the next few days and developed a weak low pressure center on November 17. Convective activity concentrated some near the center soon after, though upper-level winds and dry air out of the west confined most thunderstorm activity to the disturbance’s eastern side. During the morning of November 19, satellite data indicated a closed circulation was present and the low was upgraded to Tropical Storm Sebastien.

The next day, the cyclone turned northward as it rounded the edge of a subtropical ridge and began to feel the influence of an approaching front from the west. Sebastien faced heavy shear but still managed to strengthen some due to favorable upper-level divergence. That day, it continued its turn to the northeast, accelerating some as it did so. Convection was rather disorganized, but the system maintained its identity as it moved ahead of the advancing frontal boundary.

By November 22, Sebastien was beginning to exhibit some extratropical characteristics, but its inner core was still clearly that of a tropical system as it sped up even further. Under the cyclone, ocean heat content rapidly diminished, but it did not weaken, and in fact reached its peak intensity of 65 mph winds and a pressure of 994 mb the next day. Sebastien was already bringing heavy surf and high winds to the Azores by this time, and it rocketed across the westernmost islands during the evening of November 24. At last, its quick forward speed, coupled with cold ocean waters and frontal interaction, caused the storm to fully transition to extratropical later that night. The remnants of Sebastien merged with another system near Ireland a little over a day later.

The above image shows Tropical Storm Sebastien on November 24 shortly before extratropical transition.

The track and evolution of Sebastien were not well anticipated by computer models. While the general recurvature was agreed upon, initial model runs did not indicate Sebastien would remain tropical for as long as it did, nor that it would be able to maintain strong tropical storm intensity.

Subtropical Storm Rebekah (2019)

Storm Active: October 30-November 1

During the last week of October, another nontropical low over the northern Atlantic slowed to a stop west of the Azores islands. The powerful low weakened some as it drifted generally southeastward over the next couple of days, but it moved over slightly warmer water. By October 30, the low had enough thunderstorm activity to be classified Subtropical Storm Rebekah. Though convection was concentrated in a band wrapping halfway around the center, the center was located under an upper-level low, an indicator of subtropical characteristics. The cyclone moved eastward at a good clip over the next day and changed little in intensity. On October 31, conditions near Rebekah degraded as it encountered cooler waters and strong upper-level winds. Deep convection vanished by that evening, and the storm became post-tropical early on November 1. By this time, the remnants were nearing the Azores, but impacts on the islands were minimal.

Lacking deep convection and colocated with an upper-level low, Rebekah was classified as a subtropical storm.

The above image shows the meandering track of Rebekah over the northern Atlantic. Square points represent times at which Rebekah was subtropical.