Two tropical depressions formed near the U.S. on Sunday. Tropical Depression Nine formed south of Key West, Florida and Tropical Depression Eight formed southeast of Cape Hatteras, North Carolina. Both depressions could have an impact on the U.S., but Tropical Depression Nine could pose a greater risk to the southeastern U.S.
At 5:00 p.m. EDT on Sunday the center of Tropical Depression Nine was located at latitude 23.7°N and longitude 81.7°W which put it about 60 miles (95 km) south of Key West, Florida. It was moving toward the west at 9 m.p.h. (15 km/h). The maximum sustained wind speed was 35 m.p.h. (55 km/h) and there were wind gusts to 45 m.p.h. (70 km/h). The minimum surface pressure was 1009 mb.
At 5:00 p.m. EDT on Sunday the center of Tropical Depression Eight was located at latitude 31.8°N and longitude 70.9°W which put it about 355 miles (570 km) southeast of Cape Hatteras, North Carolina. It was moving toward the west at 9 m.p.h. (15 km/h). The maximum sustained wind speed was 35 m.p.h. (55 km/h) and there were wind gusts to 45 m.p.h. (70 km/h). The minimum surface pressure was 1010 mb.
The circulation of Tropical Depression Nine is still organizing. A NOAA plane found a distinct center of circulation, but most of the stronger thunderstorms are south and east of the center. There is a broader counterclockwise rotation with numerous showers and thunderstorms indicated by the National Weather Service radar in Key West. An upper level low east of Florida is contributing to easterly winds that are blowing across the northern side of the depression. The vertical wind shear is inhibiting the development of thunderstorms north and west of the center.
The circulation of Tropical Depression Eight was more well organized earlier today. Strong easterly winds from the same upper level low that is affecting Tropical Depression Nine are creating significant vertical wind shear. Those winds and the shear they caused blew the upper portion of the depression’s circulation west of the low level center of circulation. The low level circulation is presently exposed as seen on visible satellite imagery and by an Air Force Reserve Hurricane Hunter aircraft.
The intensity forecast for Tropical Depression Nine is challenging because it is moving through a complex environment. The depression is moving over water where the Sea Surface Temperature is near 31°C. So, there is plenty of energy in the upper ocean. However, as mentioned above, an upper low is causing vertical wind shear which will inhibit intensification. Tropical Depression Nine could move into a region where there is less shear when it moves over the Gulf of Mexico. Tropical Depression Nine has a good chance to intensify into a tropical storm, but it is unclear if it could become a hurricane before it reaches the coast.
There is also significant uncertainty in the track forecast for Tropical Depression Nine. A ridge over the southeastern U.S. is steering the depression toward the west and that general motion should continue for several more days. The ridge is forecast to weaken during the middle of the week and that should allow Tropical Depression Nine to turn toward the north. The timing of that turn and how sharp it will be are still uncertain.
The upper low is expected to continue to generate wind shear over Tropical Depression Eight. If the upper levels winds remain as strong as they are now, the depression will slowly weaken. If the upper level winds weaken, then Tropical Depression Eight could strengthen into a minimal tropical storm. The upper level low is steering the depression west and that general motion is expected to continue in the short term. The depression could stall just off the coast of North Carolina.
Tropical Depression Nine could intensify and bring wind and rain to the Gulf Coast later this week. The Gulf Coast is also vulnerable to storm surges. Given the uncertainty in both the track and intensity, interests along the coast should monitor official sources of information for updates on Tropical Depression Nine. The primary effects of Tropical Depression Eight are likely to be higher than normal surf, rip currents and some beach erosion along the North Carolina coast. People along the Mid-Atlantic coast should pay attention to the depression in case the forecast changes.