Published: May 7,2015
Highlights
- A reconnaissance aircraft continues to investigate the area of low pressure for a possible upgrade to a subtropical depression or storm.
- Rain has already begun to wrap into parts of the coastal Carolinas.
- A slow crawl toward the coastal Carolinas is likely into the weekend where the most direct impacts are expected (bands of locally heavy rain, some strong wind gusts, coastal flooding).
- High surf, rip currents possible along parts of Florida's Atlantic beaches and the Georgia coast.
(MORE: Hurricane Season Outlook | Hurricane Central | Tropical Update)
Infrared Satellite: Invest 90-L
The Latest
Low pressure is now well established just over 200 miles south-southeast of the border of South Carolina and North Carolina, 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?)
Bands of rain have already begun to wrap into parts of the coastal Carolinas and some wind gusts over 20 mph have been clocked as of Thursday morning.
(INTERACTIVE: Coastal Carolinas Radar Loop)
A U.S. Air Force Reserve Hurricane Hunters reconnaissance flight is currently examining the low, and has already estimated near-surface winds over 40 mph over a period of 30 seconds.
Invest 90-L is wrapping more convection - translation, thunderstorms - from north and west of the low-level circulation toward the center.
Any further organization and persistence of that convection near the surface low may prompt the NHC to classify this system as a subtropical depression or storm. We expect this to happen soon.
If and when that occurs, the NHC would issue forecast advisories. The first name of the 2015 Atlantic hurricane season is Ana.
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.
Where's It Headed?
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 will bring bands of rain and some strong wind gusts to parts of the coastal Carolinas possibly through much of the upcoming weekend.
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.
Therefore, the low center is expected to stall near or just offshore for awhile this weekend before a southward dip in the polar jet stream finally whisks it away early next week.
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.
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 senior meteorologist, Stu Ostro.
Potential Impacts
- Bands of rain continue to move into parts of eastern Carolinas. Areas of rain may linger in the eastern Carolinas and parts of southern/eastern Virginia through Saturday or Sunday. The greatest potential for locally heavy rain and flash flooding is from the southern Outer Banks to the northeast tip of South Carolina.- 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.
- 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).
- High surf, rip currents may also persist along parts of the Georgia and northeast Florida coasts the next day or so.
(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 started out not as a typical low-pressure system with fronts you may see over, say, the Plains states. Nor was it 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.
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.
Since 1950, eight subtropical depressions or storms have formed before May 7th, according to Dr. Matt Sitkowski, science and weather content coordinator.
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.
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