Above: Enhanced infrared satellite image of Tropical Storm Arthur from 0336Z Sunday, May 17, 2020 (11:36 pm EDT Saturday). (NASA/MSFC Earth Science Branch)
The first tropical storm of the Atlantic season, Arthur, is expected to strengthen as it heads toward a potential encounter with the North Carolina coast on Monday. Update: As of 11 am EDT Sunday, a tropical storm warning is in effect from Surf City north to Duck, including the Pamlico and Albemarle sounds. Arthur was centered at 11 am EDT about 345 miles south-southwest of Cape Hatteras, NC, and moving north-northeast at about 9 mph. Top sustained winds in Arthur were 45 mph, according to the NOAA/NWS National Hurricane Center (NHC).
Hurricane hunters found a closed circulation at midday Saturday about 100 miles east of the Florida Space Coast, prompting an upgrade to Tropical Depression 1 at 5 pm EDT. Another reconnaissance flight on Saturday evening found evidence in flight-level winds and microwave radiometer data of surface winds close to minimal tropical storm strength.
On Saturday night, Arthur was not exactly the most elegant-looking tropical storm. The bulk of its vigorous showers and thunderstorms (convection) were located in a band southeast to east of the storm’s center, rather than in a symmetric bundle around the center.
Outlook for Arthur
The usual nighttime peak in convection around a developing tropical cyclone may give Arthur a modest boost as the storm heads into Sunday. Upper-level wind shear will be light to moderate over Arthur through Monday (5 – 15 knots). Sea surface temperatures are marginal for tropical development (about 26°C or 79°F), but Arthur’s northward trek over the Gulf Stream will give it access to the warmest possible water. NHC predicts Arthur will intensify to moderate tropical-storm strength by Monday.
What would typically be a standard curving-out-to-sea forecast for Arthur is complicated by a slow-moving double-barreled upper trough over the Great Plains. This trough includes separate circulations over the Northern and Southern Plains, and as noted by NHC, Arthur’s fate may hinge on whether the southern vortex races ahead of the northern one and digs southeast, which would help pull Arthur closer to the coast.
Over time, the northern vortex should become the main player, gradually settling toward the southern Appalachians by midweek as it becomes detached (cut off) from the polar jet stream. Heavy rains are expected across the Midwest on this upper low’s soggy trek.
Our two leading track models, the ECMWF (Euro) and GFS, were in sharp disagreement Saturday on where Arthur will proceed from Monday onward. Members of the 12Z Saturday ensemble run of the Euro model were near-unanimous (more than 90%) that the system would arc eastward after a close approach to the North Carolina coast. The GFS ensemble had a split verdict: about half of its members nudged Arthur inland as a weakening system around the north side of the upper low, mostly across the Delmarva peninsula, while the other half were similar to the Euro, with a sharp offshore turn beyond North Carolina. The GFS runs also kept Arthur a bit closer to the Southeast coast and implied a better chance of landfall in or near the Outer Banks.
Climatology would favor the offshore right-hand turn predicted by the Euro and many GFS ensemble members and depicted in Saturday night's official NHC forecast. It’s not very common for tropical cyclones to angle inland at the latitude of the Delmarva, though such a track can’t be ruled out. (Hurricane Sandy took an exceptionally sharp left turn and slammed into the New Jersey coast in 2012; however, that mega-extreme outlier is not a good analog for Arthur, a far weaker system.)
Whether or not Arthur gets drawn into the picture, the upper low boosts the odds that cool, showery conditions will prevail next week across much of the mid-Atlantic. Widespread 2-4” rains are possible across Virginia and North Carolina, and local totals could be considerably higher.
Even if Arthur remains offshore, its presence will likely 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. It’s also increasingly likely that Arthur will take on subtropical or extratropical characteristics over time, especially from Monday onward.
For more on the serious threat to eastern India, Bangladesh, and Myanmar in the next several days from fast-strengthening Cyclone Amphan, see our last Category 6 post.