Atlantic Hurricane Season May Start Early With Possible Coastal Threat to Carolinas

Jon Erdman
Published: May 6,2015


 

Highlights

• A subtropical or tropical depression or storm is expected to form off the Southeast coast Thursday or Friday.
• The National Hurricane Center has given this area of interest a 60 percent chance of forming in the next 48 hours.
• A reconnaissance aircraft is scheduled to fly into the system late Thursday morning.
• A track toward the coastal Carolinas now looks increasingly possible late this week where most direct impacts may occur (bands of locally heavy rain, some strong wind gusts).
• High surf, rip currents, some coastal flooding possible from Florida to North Carolina regardless.

It appears increasingly possible the Atlantic hurricane season -- which officially runs from June through November -– will get an early start.
(MORE: Hurricane Season Outlook | Hurricane Central | Tropical Update)

Infrared Satellite: Invest 90-L

The Latest

A weak area of low pressure has formed near the northwest Bahamas, designated as Invest 90-L by the National Hurricane Center. Invests are systems of interest the NHC monitors for future development. In essence, a name before a named depression or storm forms.
(MORE: What is an Invest?)
The NHC has tasked a U.S. Air Force Reserve Hurricane Hunters reconnaissance flight to examine the system at 10 a.m. EDT Thursday.
As of now, Invest 90-L does not exhibit sufficiently well-organized convection near a well-defined surface low-pressure center to be designated a depression or storm. However, that could change by the time the plane flies into the system Thursday morning.
Water temperatures generally running above average in the Bahamas and over the Gulf Stream would help fuel thunderstorms near Invest 90-L, helping to increase its organization, according to senior meteorologist Stu Ostro. He emphasizes that whether the system becomes a subtropical or tropical cyclone will ultimately depend more on what's going on in the atmosphere.
For more on what a "subtropical" cyclone is, scroll down the page a bit. First, let's hit on the potential future impacts of this system.

The potentially clogged atmospheric steering pattern for the low late this week.

Where's It Headed?

Eventually, as we alluded to earlier, that low near or north of the Bahamas may sprout enough convection near its circulation to be classified as a subtropical depression or storm either Thursday or Friday. (Again, we have more on what a subtropical system is in the section below.)
If that occurs, the NHC would issue forecast advisories. The first name of the 2015 Atlantic hurricane season is Ana.
This system is expected to drift northward Thursday and Friday, possibly nearing the coast of South Carolina or North Carolina as soon as late Thursday or Friday.
This could bring bands of rain and some strong wind gusts to parts of the coastal Carolinas as soon as Thursday, through at least Friday.
It remains uncertain whether the system moves inland later Friday or stalls out just offshore for awhile this weekend before a southward dip in the polar jet stream finally whisks it away early next week.
The steering flow in the upper atmosphere will be very weak. The northern, or polar jet stream will remain far to the north, while any upper-level high pressure will remain rather weak and centered far to the east.
If the system stalls offshore, it would have more time over the Gulf Stream to sprout convection, so we can't rule out this system transitioning to a fully tropical storm in that case, as was the case with Subtropical, then Tropical Storm Beryl in May 2012. In this case, there would be stronger winds at the coast until the system finally moves inland, after which winds would diminish.
Otherwise, a low inland would maintain a chance of rain showers and breezy conditions at the beaches through the weekend. Even in the quicker inland scenario, south to southeast onshore winds may persist along the southern coast of North Carolina through Saturday, if not Sunday.
Incidentally, this system could become the earliest subtropical or tropical cyclone to produce direct effects in the United States (excluding a tropical storm that made landfall in Florida on Groundhog Day in 1952), according to Ostro.

Potential Impacts

- Bands of rain to move into parts of eastern Carolinas as soon as Thursday. Areas of rain may persist in the eastern Carolinas through Saturday or Sunday.
- Gusty winds pick up in coastal North Carolina (especially south of Hatteras) and coastal South Carolina Thursday, continue into Friday and possibly into the weekend. The magnitude of wind threat remains a bit uncertain at this time, though the threat of damaging winds and power outages appears low.
- Elevated surf, rip current threat spreads from parts of Florida's East Coast to the coastal Carolinas on Wednesday. High surf, rip currents, some coastal flooding and beach erosion from the northern South Carolina coast to the southern North Carolina coast (Hatteras south) continues possibly through Saturday, particularly in areas where onshore flow persists the longest (southern North Carolina coast).
(FORECASTS: Myrtle Beach | Charleston | Wilmington | Hatteras)
If you have plans or live in the coastal Carolinas, keep checking back with us at weather.com and The Weather Channel for the latest.

Water vapor satellite image of Subtropical Storm Andrea in early May 2007. Green shading generally corresponds to areas of convection, with Andrea's center of circulation located in the gray hole surrounded in some distance, by the convection.

Tropical or Not?

This low will start out not as a typical low-pressure system with fronts you may see over, say, the Plains states. Nor will it be a tropical depression or storm like you see in the Atlantic or Pacific Oceans in summer.  It may be a combination of those, called a subtropical cyclone.
Our friends at Weather Underground have 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.
When that happens, the wind field will tighten closer to the center. Of course, this would also open the door to further intensification.

Tracks of May named tropical cyclones near the Southeast U.S. coast from 2007-2014.

It's Happened Recently

Some recent Atlantic hurricane seasons have jumped the June 1 starting gun.
(MORE: When Hurricane Season Starts Early)
Most recently, in May 2012, a pair of tropical storms, Alberto and Beryl, both formed off the coast of the Carolinas, Georgia and north Florida.
Beryl washed out the Memorial Day weekend, and was the strongest tropical cyclone to make a U.S. landfall before June 1 on record, with 70 mph maximum sustained winds.
In fact, Beryl made the transition from a subtropical to tropical storm prior to landfall, exhibiting an eye and eyewall on radar, according to Ostro.
Five years earlier, a cut-off low-pressure system off the Southeast coast morphed into Subtropical Storm Andrea. Before officially gaining the subtropical designation, the wrapped-up low was responsible for a 2-3 foot storm surge in St. Johns and Flagler Counties in Florida. A surfer and four crew members of a sailing vessel lost their lives in high surf from Andrea.
There was even an Atlantic tropical storm in late April 2003, ironically also named Ana.
In all, there have been 39 Atlantic tropical or subtropical cyclones that have formed before June 1 since 1851, according to hurricane specialist Michael Lowry. Thus, you can expect one of these pre-June 1 systems once every four years, or so, on average.
More importantly, there's no link between an early start and an active season. Lowry says only 38 percent of early-starting Atlantic hurricane seasons ended up with above-average numbers of named storms.

MORE: Hurricane Strikes By County, Parish