Ophelia departing, TD 17 arriving?

Storm surge levels observed last night in Bogue Sound, which is the bay between Morehead City and its barrier island, reached seven feet--near the record levels set there from Category 3 Hurricane Hazel in 1954. The storm surge reached 10 feet in some of the smaller creeks in the Neuse River and may have reached 12 feet, a remarkably high storm surge for what was a tropical storm for that area. High storm surges can result from just tropical storm force winds, if they blow over a large area for a really long time, like Ophelia's did.


Figure 1. Storm Surge heights measured in Ophelia.

For those of you who can handle a 1.6Mb animation, the radar loop from Morehead City, NC during the time Ophelia's northern eyewall passed over the city is fascinating. The turbulence created by having part of the eyewall over land and part over water created some smaller vorticies along the inside edge of the eyewall.

While Ophelia did dump it share of heavy rain--around 5 - 7 inches near Wilmington, and over 10 inches around Cape Fear, south of Wilmington--the rain was mostly confined to the coast, and did not cause widespread flooding problems. Ophelia's winds also did relatively light damage--sustained hurricane force winds (74 mph) were only observed at one location, on Cape Lookout near the Outer Banks. The storm surge was what caused the main havoc with Ophelia.


Figure 2. Estimated rainfall from the Morehead City radar for Ophelia's passage.

Ophelia is very slowly lumbering out to sea, and is a mere shell of her former self. The eyewall has disintegrated, and the latest hurricane hunter flight found a rising pressure (987 mb) and winds at flight level (78 knots) that may support downgrading Ophelia to a tropical storm this afternoon. Cooler waters, dry air, and wind shear are all taking their toll on Ophelia, and by the time she races past Cape Cod on Saturday, the worst she will be able to do there is generate wind gusts of 40 mph.

TD 17?
A well-organized tropical wave 900 miles east of the Windward Islands is slowly improving in organization and may become Tropical Depression 17 by tommorow. The wave, located near 9N 47W, is a little too far south to develop quickly, but is moving WNW towards higher latitudes. The shear over the wave is marginal for development, about 10 - 15 knots, but is expected to drop by this evening. Wind measurements from the QuikSCAT satellite show that an elongated surface circulation has already formed, but winds around the disturbance are below 20 knots. The upper level environment is favorable--an anticyclone has formed on top of it, which should provide very favorable outflow for any deep convection that fires up. The disturbance is expected to continue moving west-northwest at 10-15 mph the next few days, then possibly slow down and turn more northwest under the steering influence of a large mid-Atlantic trough.


Figure 4. Early track model runs for the disturbance that may turn into Tropical Depression 17. The 2am GFDL run disippated the disturbance immediately.

Elsewhere in the tropics
No other tropical storm developent is expected elsewhere in tropics through tomorrow. However, conditions for development are expected to improve over much of the tropical Atlantic over the coming week, including the Caribbean and Gulf of Mexico.

Jeff Masters