Outlook for ArthurUpper-level wind shear will remain light (less than 10 knots) over Arthur through Sunday, giving the storm a window to organize and intensify. Sea surface temperatures of around 25-26°C (84-86°F) are below the usual threshold of 26°C for tropical development, but Arthur’s northward trek over the Gulf Stream will give it access to the warmest possible water, and cold air aloft will help compensate for the reduced instability from the less-than-toasty water. The mid-level air in the vicinity of Arthur will remain moderately moist (relative humidity of 55–65%), although dry air intrusions from land may hinder Arthur's convection at times.
Southwesterly flow ahead of a deep trough in the central U.S. will continue to steer Arthur north-northeastward over the next day or so. This track will bring Arthur over or near the Outer Banks of North Carolina on Monday, with a brief landfall possible. By this point, NHC predicts that Arthur will be a 60-mph tropical storm. Impacts in the Outer Banks be limited by the angle of approach, which will minimize storm surge and keep Arthur’s strongest winds mainly offshore. Squally tropical-storm-force winds and doses of heavy rain will be possible across and near the Outer Banks, mainly in the 1-2” range but up to 4” locally.
Beyond North Carolina, Arthur will likely turn eastward out to sea, as reflected in the official NHC forecast on Sunday, becoming a post-tropical storm as it encounters much higher wind shear and cooler water. However, this standard curving-out-to-sea scenario is complicated somewhat by the approaching upper trough, which is expected to settle into the Southeast as a large cut-off upper low from Tuesday till at least Thursday.
The 0Z Sunday ensemble runs of the European (ECMWF) model are in strong agreement on the turn offshore. A handful of runs from the GFS ensemble run, and the 12Z Sunday operational run of the GFS model show Arthur stalling east of the Delmarva peninsula and perhaps making a half-hearted run back toward the mid-Atlantic coast later next week. The 12Z Sunday HWRF model, though not a top-performing track model, depicts a similar outcome. This is an unlikely scenario, but it can’t yet be ruled out. Even if it were to occur, Arthur would almost certainly be only a minimal tropical storm or tropical depression, after having spent several days over the chilly waters north of the Carolinas.
Whether or not Arthur gets drawn into the picture, the upper low boosts the odds that cool, showery conditions will prevail next week across a large area. Widespread 1-4” rains are possible from South Carolina to Maryland, and local totals could be considerably higher, especially toward the eastern slopes of the Appalachians.
Especially if Arthur slows or stalls well offshore, its presence will boost a long onshore fetch and exacerbate winds, waves, and beach erosion along the coast from Delaware to Massachusetts during much of the coming week.
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