TD 17 coming soon--and a scientific first!

TD 17?
Substantial deep convection has developed in the past few hours in association with well-organized tropical wave about 500 miles east-southeast of the Windward Islands. Spiral banding is more and more conspicuous with each visible satellite image, and if the present trend continues, NHC wil probably initiate advisories on Tropical Depression 17 tonight or tomorrow morning.

The system is in a favorable environment for intensification, now that it has gotten farther from the Equator and can take advantage of the increased spin a higher Coriolis force offers at higher latitudes. Wind shear has decreased to 5 - 10 knots, and the upper-level winds appear favorable--a small upper-level anticyclone is over the wave, and should provide good outflow. Some weak outflow is apparent to the south, and moderately good outflow to the north. Water temperatures are about 29.5C (85F), and increase to about 30C (86F) near the Lesser Antilles Islands.

The early track models are unreliable. The GFDL disippates the system immediately, and the BAMM has been flip-flopping, alternately taking it west-northwest into the Caribbean or northwest, missing the Leeward Islands entirely. The GFS and UKMET models both take the system to the northernmost Leeward Islands, just east of Puerto Rico. The correct solution will depend upon how quickly the system develops, and how quickly a large mid-Atlantic trough north of the islands lifts out. All interests in the Lesser Antilles Islands need to monitor this storm, it has the potential to grow into a hurricane about the time it reaches the islands on Monday.


Figure 1. Early track model runs for the disturbance approaching the Lesser Antilles.

Blob northeast of Puerto Rico
A disturbance northeast of Puerto Rico continues to generate some impressive clusters of thunderstorms, but is not a threat to develop into a tropical depression until Saturday at the earliest. This disturbance is expected to move westward towards the Cuba and the Bahama Islands the next few days. Strong upper level winds out of the west are creating about 10 - 20 knots of shear over the disturbance, down from 20 knots yesterday. The shear should continue to drop the next few days, and may be low enough by Sunday to allow a tropical depression to form. The system could threaten South Florida and Cuba as it continues to track west. Several computer models indicate that the disturbance is more likely to develop once it reaches the Gulf of Mexico, and pose the greatest threat to Mexico. There are no early computer model track points for this disturbance yet, I will post them when they become available.

ITCZ
The Intertropical Convergence Zone (ITCZ), the band of strong thuderstorms between Africa and South America, has historically been the source of many of the severe hurricanes that affect us in September. These "Cape Verde" type storms, so named because they originate from disturbances in the ITCZ near the Cape Verde Islands, have yet to make an appearance during this peak time of hurricane season. The ITCZ has become very active the past few days, and is forecast to continue to remain active the next two weeks. I expect at least one major Cape Verdes type hurricane to form by the end of September. The main activity is across the eastern and central Atlantic is at about 8N latitude, which is probably too far south to generate a tropical cyclone. If some of this activity works its way to 9N, we may have a better chance of development in this area.

Ophelia: a scientific first
A scientific first was accomplished in Ophelia this afternoon--the first ever remotely-piloted aircraft to do a successful penetration of a tropical cyclone flew through Ophelia at 2,500 foot altitude. The drone measured winds of 74 knots. The project is described in detail on the NOAA Hurricane Research Division's web site. The objective is to use the pilotless aircraft in regions where it is too dangerous for humans to fly:

Simply stated, continuous observation of thermodynamic (temperature and moisture) and kinematic (wind) structure of the near-surface hurricane environment has never been documented in a hurricane. This environment, where the atmosphere meets the sea, is critically important since it is where the ocean's warm water energy is directly transferred to the atmosphere just above it. The tropical cyclone surface layer is also important because it is where we find the strongest winds in a hurricane and coincidentally, the level at which most of us live (i.e. at/near the surface). As such, observing and ultimately better understanding this region of the storm is crucial if we hope to improve our ability to make accurate forecasts of TC intensity change. Enhancing this predictive capability would not only save our economy billions of dollars but more importantly it would save countless lives.

Well done, Aerosonde Corporation and NOAA!

Ophelia has intensified a bit this afternoon, with the pressure falling 3 mb. This will be short-lived, however. In fact, the 5pm EDT hurricane hunter eye report found the pressure had risen 2 mb. Ophelia will pass out of the warm Gulf Stream waters and encounter waters as cold as 70F Saturday. She will still generate some trouble on her trek north; expect a 1 - 3 foot storm surge for southeast Massachusetts and Nova Scotia, 1 - 3 inches of rain, and sustained winds up to 40 mph as Ophelia brushes by.

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 highest wind gusts measured were 92 mph on Cape Lookout and 83 mph at Cape Hatteras. The storm surge was what caused the main havoc with Ophelia--surges heights of up to 10 - 12 feet were observed along the Neuse River north of Wilmington. Preliminary damage estimates put Ophelia's damage to North Carolina over $10 million, but less than $100 million.


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

Jeff Masters