Tropical Storm Arlene (2017)

**Tropical Storm Arlene** was an unusually early preseason system that formed over waters typically unable to sustain tropical cyclogenesis. Arlene was first noted as an extratropical low that developed along a frontal system that was draped across the northern Atlantic. By April 18, the system had begun to develop an exposed center of circulation. The next day, convection was able to fire along a shallow convective band to the southeast of the still exposed low level circulation, with the system becoming a subtropical depression at 15:00 UTC on April 19. Continued gradual intensification took place, until a deep convective burst signaled its transition into a purely tropical depression. Continued organisation occurred, and the system took forecasters by surprise and intensified into the first named storm of the 2017 Atlantic hurricane season at 18:00 UTC on April 20. Accelerating to the northwest, Arlene began to interact with a larger frontal system to its northwest, and transitioned back to an extratropical cyclone by 15:00 UTC on April 21, before performing a cyclonic loop and heading west across the Atlantic as an open trough of low pressure, dissipating near the Azores on April 23.

## Meteorological history

The origins of the system can be traced back to a large frontal system that developed on April 14. The large frontal zone began to move towards the northeast, and a low pressure system began to develop within the frontal zone late on April 16, being assessed as having a low chance for subtropical cyclogenesis at 2PM EDT on April 16. Late on April 18, the system began to separate from the aforementioned frontal system, and at 1:30AM EDT,the chances for development were raised to medium after another burst of convection happened as the core of the system began to contract,with the winds occurring closer to the center. Several hours later, the chances for development were raised to high after a banding feature became evident across the southeastern side of the system, albeit being far from the center.

By 1500 UTC, the system had become well enough organised for advisories to be initiated on subtropical depression One about 900 miles southwest of the Azores, or about midway between Bermuda and the Azores. Over the next 12 hours, the convection struggled to organise, until around 03:00 UTC on April 20, when a bust of moderate convection near the center of the cyclones center may have signaled its intensification into a subtropical storm. Over the next 10 hours, the storm's center continued to organise, until a new deep burst of convection over the center of circulation signaled its transition into a purely tropical cyclone at 1500 UTC that day, and the system became a tropical depression.

Continued organisation continued, and besides sea surface temperatures of 18-19C, the tropical depression began exhibiting an eye feature, closed off by a ring of convection, as seen on microwave imagery, and it is estimated that the low intensified into a tropical storm at 1800 UTC that day after turning to the northwest towards the frontal zone that would later absorb it. Although the system began to lose its organisation due to frontal interaction, an ASCAT pass around 01:00 UTC on April 21 showed peak winds of 50 mph near the center of the system, before the convective structure of the storm fell apart, and Arlene became extratropical at 15:00 UTC.

Over the next 2 days, Arlene's remnants did a cyclonic loop, moving southwest, then east as the cyclone moved around another extratropical cyclone. An ASCAT pass early on April 23 confirmed that Arlene's remains had opened up into a trough of low pressure with no surface circulation, before Arlene was absorbed by the extratropical cyclone it had been competing with for several days later that day.

## Records

When subtropical depression One formed on April 19, it marked only the sixth occurrence of an April Atlantic tropical or subtropical cyclone, with the other instances being in 1912, 1915, 1973, 1992 and 2003. When Arlene became a tropical storm, it was one of the highest latitude tropical storms on record, behind only Grace in 2009. In addition to these, the system broke the record set by Ana in 2003 for the strongest recorded Atlantic April storm by 1 mbar.