Tropical Storm Arthur Forms


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

Last night at 11pm EST, the National Hurricane Center had determined that a tropical low had become organized enough to be classified as a Tropical Depression over the Gulf Stream just offshore of the east coast of central Florida where tropical storm watches are in effect. By this morning at 11am EST, improved appearance on satellite imagery strongly supported by surface observations from the Northwest Bahamas islands indicated that Tropical Depression One had strengthened to Tropical Storm classification and the National Hurricane Center named the storm Arthur. As of 8pm EST, Tropical Storm Arthur was located 85mile ESE of Cape Canaveral, FL with maximum sustained winds of 50mph. Arthur was moving NW at 2mph.

NCEP 18zGFS 300mb height and winds with shaded isotachs. This is
NCEP 18zGFS 300mb height and winds with shaded isotachs. Thursday at 2am EST, the trough (red dashed line) still inching eastwards, but southwesterly flow (red arrows) already picking up Arthur.

The current forecast (as of 8pm EST) for Tropical Storm Arthur has a continued slow, meandering, northwestward motion through tonight. An deep level trough over the Great Lakes and US Mid-West is slowly moving towards the US east coast. The deep level flow ahead of it, out of the southwest, is expected to then begin to influence the track of Tropical Storm Arthur Wednesday night – this results in Arthur turning to the northeast. As the trough gets closer, Arthur accelerates – brushing North Carolina before continuing out to sea on Friday. Nova Scotia may be threatened by the post-tropical remnants of Arthur this weekend. Additionally, these remnants may push a cold front near/across Bermuda late on Saturday with showers and possibly thunder.

Located over the Gulf Stream, Arthur has plenty of deep, warm water as fuel. However, originating from a frontal system, dry air to the north and some moderate northerly wind shear have worked to slow organization and strengthening of Arthur. This wind shear has gradually lessened as Arthur commandeered the upper anticyclone to its near west. Further, dry air to the north of the storm appears to be moderating as it persists over the warm waters of the Gulf Stream.  These factors indicate the environment around Arthur is gradually becoming more conducive for Tropical Cyclone development. The slow track of Arthur in this improving environment supports the National Hurricane Center’s forecast of Arthur gradually strengthening to Category One Hurricane strength before passing North Carolina. There, tropical storm conditions are likely and hurricane conditions possible.

Earlier, radar from Melbourne, FL evidenced that the surface circulation of Tropical Storm Arthur is a little broad (possibly with multiple smaller circulations), and that it is to the north of a mid-level circulation. A new surface circulation could form beneath that mid-level one, or an existing circulation could get tucked back beneath it to allow quicker strengthening. Dry air resulting in collapsing thunderstorms to the near east of the center likely stretched it to the north, and the moderate wind shear is keeping much of the convection (and any circulations associated therein) to the south of the surface circulation. That being said, if Arthur can isolate a pocket of moist, unstable air near its surface center – rapid intensification is a possibility, but dry air nearby would continue to cause problems if it persists nearby regardless of intensity changes.

Summary

NCEP 18z GFS out to Saturday at 2pm EST.
NCEP 18z GFS Sea-level pressure, 850mb isotherms, and 3hr precipitation accumulation out to Saturday at 2pm EST. Trough/cold front (light blue flagged line west of Bermuda) associated with the now extra-tropical remnants of Arthur approaching Bermuda – shower activity exists far ahead, already near the island.

Arthur lingers off the east coast of Florida as it slowly bends back northeastwards by Thursday. Arthur also slowly strengthens to a minimal hurricane in that period. Thursday into Friday, expect either landfall or a near miss from Hurricane Arthur in North Carolina. In the event of no landfall, the east coast will be on the weak side of the storm and only windward and coastal areas should expect tropical storm conditions. If Arthur makes landfall, expect isolated areas of hurricane conditions and more widespread tropical storm conditions. Regardless of small scale track changes, expect hazardous surf and rip currents for the 4th of July weekend along the US east Coast from Florida to possibly New England as Arthur moves out to sea. As Arthur moves out to sea and becomes post- or extra-tropical, it is expected to drag a cold front southwestward across Bermuda this weekend with showers and thunderstorms possible. Arthur is not expected to produce any other adverse weather conditions in Bermuda at this time.

Follow official updates on Arthur and future tropical cyclones from the National Hurricane Center, and Bermuda related weather information from the Bermuda Weather Service.

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