Subtropical or Tropical Cyclone Development Possible Off Southeast Coast

Chris Dolce
Published: September 18,2015
As is typical in El Nino years during hurricane season, many areas in the Atlantic basin are dominated by wind shear. This decreases the chance of tropical cyclones developing or surviving as they move across the Atlantic and the Caribbean. Therefore, we often look closer to the U.S. coast for development, where remnant frontal boundaries and other atmospheric features can lead to the formation of subtropical or tropical cyclones.
Such is the case through this weekend and into next week, when there is a chance the large-scale weather pattern could lead to the development of a subtropical or tropical cyclone off the Southeast coast. That said, even if this development does not occur, there will still likely continue to be impacts.
Here's what we know right now and the potential impacts this weekend and early next week.

Area to Watch

Slow Development Possible, But Impacts Regardless

A widespread area of showers and thunderstorms continues to spin off the Southeast coast. On the edge of this stormy weather zone has been Florida, which dealt with drenching downpours on a daily basis this past week, even seeing a few waterspouts.
The setup contributing to this zone of stormy weather includes the remnants of an old frontal boundary and low pressure in the mid-levels of the atmosphere.

Current Radar and Satellite

Upper-level winds are strong over this area of disturbed weather, which is keeping it rather disorganized. However, the National Hurricane Center has highlighted an area off the Southeast coast, Invest 96-L, for a medium chance of development into a subtropical or tropical cyclone in the next five days. A U.S. Air Force Reserve Hurricane Hunters reconnaissance flight is tentatively scheduled to investigate this system Saturday afternoon.
An area of low pressure developed east of the northern Florida peninsula Friday morning and if showers and thunderstorms concentrate near this low pressure center, it could gain enough organization to be classified as a subtropical or tropical depression or storm. (For more information on what a subtropical cyclone is, see the last section below.)
However, no matter whether subtropical or tropical development occurs, there will still be potential impacts. Since the future low may move little into next week, the impacts could last for several days. Here's what could be in store:

1) Carolina Coastal Rain

- A few bands of showers and thunderstorms could affect eastern North Carolina down to coastal South Carolina on Saturday. Most of the heaviest activity should remain offshore, however.

2) Gusty Winds in Coastal Locations

- Breezy to windy conditions are possible for locations near the Southeast coast through Saturday.
- Windy conditions may also spread north to the Mid-Atlantic coast late in the weekend into early next week. This will be due to a increasing pressure gradient between the low and high pressure to its north.

3) Rip Currents and High Surf

- The area of low pressure combined with high pressure to its north could also contribute to an increased risk of dangerous rip currents along parts of the Southeast and Mid-Atlantic coasts this weekend into early next week.
- In addition, the onshore winds would likely contribute to high surf.
Check back with weather.com for updates through this weekend on the latest developments with this story.

What's a Subtropical Storm?

Weather Underground has a full explanation of subtropical cyclones. Basically, a subtropical depression or storm exhibits features of both tropical and non-tropical systems, with a broad wind field, no cold or warm fronts, and generally low-topped thunderstorms spaced some distance from the center.
Subtropical cyclones typically are associated with upper-level lows and have colder temperatures aloft, whereas tropical cyclones are fully warm-core, and upper-level high-pressure systems overhead help facilitate their intensification.
Because of this hybrid nature, the National Hurricane Center still issues advisories and forecasts (i.e. projected path) for subtropical depressions and storms and assigns a number or name much like a regular tropical depression or tropical storm.
Occasionally, if thunderstorms cluster close enough and persist near the center, latent heat given off aloft from the thunderstorms can warm the air enough to make the storm a fully tropical storm. Ana in May 2015 was an example of this transition occurring.

MORE: Hurricanes by the Numbers