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May 2012

Tropical Storm Alberto Forms off of South Carolina

   Posted by Levi at 10:22pm on May 19, 2012

Tropical Storm Alberto has formed off of the Carolina coast, fulfilling the concern we’ve had for a warm-core low to develop over the Gulf Stream beneath the upper-level trough split that has become cut-off near the SE U.S. coast. Alberto currently has tropical storm force winds of 45mph. Moderate convection has been sustained for about 18 hours now, mostly weighted in the northwest quadrant. The southeast quad is devoid of convection due to a lack of strong inflow and wind shear from the jetstream which is just to the south. Directly over the center of the storm, however, wind shear is weaker due to the elongated upper low becoming partially stacked with the surface low, something that some of the big-hitting models did not see happening a few days ago, but was still a concern based on some of their biases, which were discussed in the previous post. A stacked low has allowed convection to warm the mid-upper atmosphere and transition the low into a warm-core system that can now be considered a tropical cyclone. Upper-level anticyclonic outflow to the north of the system is further evidence of its warm-core transition.

Alberto will be meandering within weak steering currents off the SE U.S. coast for the next day or so before really going anywhere due to the blocking ridge over New England which is preventing any movement northward. Later tomorrow and Monday Alberto should begin moving northward or northeastward towards the North Carolina coast, likely giving it a close brush or a brief landfall around Monday before curving out to sea east of the mid-Atlantic coast. Given that most of the weather is weighted on the northwest side of the storm, residents along the North and South Carolina coasts should expect heavy showers and the possibility of tropical storm force winds even if the track keeps Alberto a bit offshore. Alberto is tiny, so the heaviest impacts may be localized to a rather small swath.

Alberto does have some problems to face over the next couple of days that will likely limit intensification. A low to the northeast and a new low forecasted to develop to the southeast coming up from the Bahamas will probably steal low-level inflow from the eastern side, making it difficult to develop thunderstorms there. The northern low moving towards the mid-Atlantic coast will also start shoving dry continental air down at Alberto’s northwestern side, which will likely start choking the system a bit, causing it to weaken when it begins moving northward. Sea surface temperatures are also only around 26C in Alberto’s section of the Gulf Stream, and drop off by several degrees close to the coastline. While this can support a tropical storm and even a minimal hurricane in this thermodynamic environment, the other conditions involved make me believe that Alberto likely can’t exceed a 60mph tropical storm, and a peak intensity between 50mph and 60mph is probable. Peak intensity should occur before any possible landfall, and Alberto will likely be weakening as it recurves.

Overall, Alberto is not a particularly significant or dangerous threat, but could bring tropical storm conditions to the North Carolina coastline as it brushes by or makes landfall there around Monday. This kind of situation was expected with this type of early-season setup, and the pattern delivered. After Alberto, the northwest Caribbean and the Gulf of Mexico will have to be watched for the next focus of disturbed weather later next week and beyond.

We shall see what happens!