In the latest advisory from the National Hurricane Center this morning, the first Subtropical Depression of the year is heading north for now, but is expected to curve west over the next day. The center of Subtropical Depression One was located near latitude 34.4 North, longitude 39.3 West and was moving toward the north near 14 mph. The National Hurricane Center is forecasting a turn toward the northwest is expected later today, followed by a motion toward the west-northwest by tonight and on Friday.
While the storm is evolving its form, there isn’t much change with its strength. Maximum sustained winds are near 35 mph with higher gusts. Little change in strength is forecast, and the subtropical depression is expected to be absorbed by an extratropical low in a day or so.The estimated minimum central pressure is 996 mb (29.42 inches).
Data from nearby drifting buoys indicate that the subtropical depression is now moving over sea-surface temperatures of 66 degrees F or less; these cooler waters have resulted in a significant weakening and erosion of the inner core convection during the past few hours. In contrast, curved outer band convection has been increasing in the eastern semicircle. Although it is possible that the system could have reached subtropical storm intensity earlier when a donut-ring of moderate convection completely encircled the low-level circulation center, the recent rapid erosion of the central convection has led the National Hurricane Center to maintaining the storms intensity as is for the morning advisory.
The overnight runs of the major American (GFS) and European (ECMWF) global weather forecast models show the the depression persisting as a well-defined, shallow low pressure system into Friday before it is absorbed by the aforementioned larger extratropical low. The combination of decreasing sea-surface temperatures and increasing southwesterly vertical wind shear ahead of the large extratropical low should act to steadily erode the associated convection, resulting in the depression weakening to a remnant low
pressure system by tonight, if not sooner. By the PM hours on Friday, this storm system should be no more and when that occurs, the National Hurricane Center will cease issuing advisories for it.
The 2017 Atlantic Hurricane Season doesn’t officially start until June 1, but subtropical and tropical cyclones do pop up from time to time during the off-season. Leading hurricane forecasters believe this season will be slightly less active than normal; however, they also stress that it takes just one storm to create significant loss of life and property. Because this Subtropical Depression never became a named storm, the first named storm of the season will continue to be Arlene; the full list of names is here.