You might have noticed that the first named storm (Arlene) of the 2017 Atlantic Hurricane season has formed in the middle of the North Atlantic Ocean – 815 miles west of the Azores Islands. The biggest impact from this storm will be the discussion in the meteorology community as to whether this system should even be monitored by the National Hurricane Center. Currently Arlene is over relatively cold ocean water (66 F (19 C)) for a tropical system, and according to satellite data, it is questionable if the system has a warm core.
Typically a tropical cyclone is characterized by lack of warm/cold fronts attached, a “warm core” (air is warmer in center of the cyclone than elsewhere), and persistent deep convection wrapped close to the center; these attributes are commonly referred to as “tropical characteristics” of a cyclone.
It would appear that the decision to track Arlene is very subjective: there have likely been dozens of similar systems over the last 100 years (systems that develop in water warmer than 66 F and with a marginally warm core) that have gone unclassified, including systems that have impacted the U.S. (Sept 2008 SC coast, Sept 2009 NJ).
All this matters to the insurance industry because our hurricane catalogs in the catastrophe models are tuned to the historical data, and after decades there is still no objective guidance as to what type of system gets tracked by the NHC. In some cases like Sandy, questionably categorized storms can have large impacts on landfall definitions as well.
Regardless, Arlene is now in our record books as a storm in the North Atlantic, but it will not be around for long as a strong mid-latitude trough will merge with this system later this weekend and make it unrecognizable. These early-season tropical systems should be no surprise (this will be the sixth early-season storm in the past six years). In fact, history suggests that tropical/subtropical systems in April are uncommon, but not necessarily rare. There have been a few, but they are typically short-lived and innocuous.
I am currently tracking the various early season forecasts for the 2017 Atlantic basin season. I should have my views formulated in a couple of weeks. What I can tell you right now there is no correlation for April storms and the rest of the season, partly because of a small sample size.