Arthur Strengthening

Today in Bermuda, a dissipating stationary front/trough to the near west has introduced some patchy light rain. Showers and thunderstorms embedded in steadier light to moderate rain exists to our near west along this boundary. Within the last hour, shower activity has waned, but should redevelop later this afternoon or evening, and could make their way eastward into the Marine area or across the island. A Thunderstorm Advisory is in effect for this afternoon. Light to moderate southeasterly winds should hold through the day, but begin to veer southerly tomorrow. Winds may become erratic in and around heavier showers.

Tropical Storm Arthur near advisory time late this morning on RGB enhanced satellite imagery.

Tropical Storm Arthur underwent several structural changes last night. A cluster of deep thunderstorms associated with the storm’s mid-level circulation just to the south of the surface circulation, took over the upper level anticyclone that had been to the west of Arthur producing light-moderate northerly shear. This resulted in a drop in wind shear over the storm and so the mid-level circulation was able to wrap counter-clockwise around the surface circulation. While this occurred, a partial eye-like feature developed on radar and satellite imagery associated with the mid-level circulation. Later last night, and early this morning, the surface circulation became aligned with the mid-level circulation and the eye-like feature became a developing eye as its winds translated to the surface – Arthur strengthened in response. As of the 11am EST advisory from the National Hurricane Center, Arthur has top sustained winds of 60mph – mainly to the east of the center.
While wind shear has decreased over the storm, dry air remains nearby. Mid-level dry air is very near the western side of the center keeping convection relatively shallow there, and deeper layered dry air still exists to the north of the storm as evidenced by low level arc clouds (collapsing showers/thunderstorms due to dry air result in a gust front and rain-cooled air that allows lift for arc clouds to develop). However, Arthur’s center is isolated from the deep layered dry air for now – allowing constant strength/slight strengthening today. The mid-level dry air should be mixed out by tomorrow allowing Arthur to strengthen and become a minimal hurricane before reaching North Carolina Thursday evening.

Same RGB enhanced image as above – but with annotations.

Forecast reasoning for Arthur’s track remains similar to last night – an upper level trough over the US Mid-west and Great Lakes is slowly heading eastwards and deep layered southwesterly flow ahead of it should pick up Arthur and turn and accelerate the storm northeastwards. Arthur is starting to feel the influence of this trough and has begun a steady northward motion at 7mph. It is important to note that because this trajectory is nearly parallel to the coast of North Carolina, if the trough turns Arthur sooner (as is forecast), then it will likely skirt the Outer Banks. Yet, if it is delayed, or takes Arthur on a more north-northeasterly trajectory, then Arthur could make a landfall as far west as Wilmington, NC.  Regardless of these small changes in track, tropical storm conditions are possible across the entire coast of North Carolina from Arthur, but isolated hurricane conditions would only occur near and to the east of Arthur’s center. Tropical Storm Warnings have been issued for the entire North Carolina coast expecting the onset of tropical storm conditions as early as 36hours from 11am EST. A Hurricane Watch has been issued for the Outer Banks of North Carolina. A tropical storm watch extends into bordering parts of South Carolina. Preparations for tropical storm and hurricane conditions should have already begun.

National Hurricane Center Tropical Storm Arthur wind field analysis with watches and warnings valid as of 11am EST.

The National Hurricane Center is expecting a storm surge of 2-4ft in areas of onshore winds in the Hurricane Watch area. Large waves in addition to the surge will likely result in significant beach erosion in coastal North Carolina. Elsewhere, hazardous surf and rip currents are already occurring along the coast of Florida, Georgia, and South Carolina. These sea conditions will spread to North Carolina tomorrow, and could spread north, up the East coast to New England for 4th of July Friday.

Once past North Carolina, Arthur should continue to accelerate northeastwards, weakening slightly as it becomes post- or extra-tropical on Friday. On Saturday, the remnants of Arthur reach Nova Scotia with storm force winds possible. Moisture drawn towards Bermuda on southwest winds ahead of a developing cold front south of the remnants of Arthur may result is scattered showers Saturday. The front  is then drawn near/across Bermuda Sunday with further showers and the potential for thunder.

For the latest official information on Tropical Storm Arthur, see the National Hurricane Center. For information related to Bermuda, see the Bermuda Weather Service.


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