With yesterday's classification of Tropical Storm 'Arlene,' an extremely rare tropical event in April, lots of discussion has been floating around Facebook and social media among offshore sailors, a lot of it drama-fueled malarkey. I wanted some facts on 'Arlene' - the weather in the Atlantic is top of my mind right now as we plan for our upcoming Trans-At in May - and I had some questions I wanted answered. Jeremy Davis, a lead forecaster at WRI, and Isbjorn's primary weather-router (who is also an extreme tropical weather nerd, dating back to his childhood) - provided me some answers in an email today. As of 1100 today (April 21), the storm officially went 'post-tropical,' so while rare, it was short-lived.

I asked Jeremy to elaborate on 1). how the storm formed; 2). what makes a system 'tropical' anyway, especially these that form outside the actual tropics; 3). could this have been 'missed' in the past prior to the high-resolution imagery weather forecasters have now; 4). how rare it actual is; and 5). if this means we're setting up for a busy hurricane year. Jeremy's discussion follows:

Arlene formed as a “cut-off gale” in the NE Atlantic a few days ago. It was cut off from the prevailing W’lys and as it sat and spun, the temperature/thermal gradient around it weakened. It sat over 20-21C water, which is usually too low for development, but slightly warmer air was being wrapped around the system. Thunderstorms began to develop near the center on the 18th, and on the 19th was classified as a subtropical depression, then tropical depression yesterday morning, and then tropical storm yesterday afternoon into this morning.
By tropical it needs to have a warm core, where temperatures increase as one moves towards the center, and has to have organized, rotating convection (thunderstorms) near the center. Subtropical systems have squalls a bit further removed from the system, with the highest core of winds 50-150nm away from the center, as opposed to tropical cyclones where surface winds are greatest near the center
Modern satellites and surveillance have allowed for greater detection of these “off-season” systems, while a few decades ago they would be easily missed. Regardless, weather conditions are usually extremely unfavorable in April for tropical cyclones to form, so even if these systems were missed in the past, it is unlikely that there were a great quantity of them.
This is only the second April to have a tropical cyclone form. The other one was Ana, 21-24 April 2003, which tracked SE of Bermuda. In 2016, we had Hurricane Alex affect the Azores in January, extremely rare. We also had Tropical Storm Bonnie form in May in 2016. In 2015, Tropical Storm Ana formed in May. In 2012, we had Alberto and Beryl in May as well. So yes, we have had early season systems in the past 5 years. This is a combination of better detection, classification, and some warmer than usual water temperatures in the main development region early in the season (Gulf of Mexico-NW Caribbean – offshore SE US). This system is different in that it was a small tropical system embedded in a larger gale. Now that it has merged, it is a much more typical, complex April gale in the North Atlantic.
It is hard to correlate an early, “freak” system with a busy year – but water temperatures are above normal in the Gulf-SW Atlantic-Carib which could induce more early season storms. With the possibility of a developing El Nino, this may taper down the number of systems, as this results in higher upper level wind shear across the tropical Atlantic. An average season of 10-11 storms is likely. Of course, it only takes one system to cause major damage!