Could the Atlantic Hurricane Season Start Early?

Jon Erdman
Published: May 5,2015


 

Highlights

• A subtropical depression or storm may form off the Southeast coast later this week.
• Uncertainty continues on ultimate path, intensity of this system (inland or remaining offshore).
• High surf, rip currents, some coastal flooding possible into the weekend from Florida to North Carolina.

The Atlantic hurricane season runs from June through November. Occasionally a storm develops early, and that could happen off the Southeast coast as soon as Wednesday.
(MORE: Hurricane Season Outlook | Hurricane Central | Tropical Update)

Setup For Potential Low

The Setup

The setup for this begins with a leftover, fading frontal boundary over the southwest Atlantic, the Bahamas and Cuba. This is the same frontal boundary that brought a much-needed soaking to South Florida last week and gave Key West its third-heaviest April day of rain on record last Wednesday.
This old frontal boundary is being revived by an infusion of energy from the southern, or subtropical, branch of the jet stream. This will cause an area of low pressure to form near or north of the Bahamas Wednesday or Thursday.
Because water temperatures are generally running above average in the Bahamas and over the Gulf Stream now, according to senior meteorologist Stu Ostro, this would help fuel thunderstorms near the low's circulation, though he emphasizes that whether the system becomes a subtropical cyclone will ultimately depend more on what's going on in the atmosphere.
If necessary - i.e. a sufficient low-pressure center has formed at the time - the U.S. Air Force Reserve "Hurricane Hunters" may investigate the system as soon as late Wednesday morning.
This area of disturbed weather is then expected to linger for awhile.

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 may not be a typical low-pressure system with fronts you may see over, say, the Plains states. It may also not 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 (NHC) 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.

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

What Does This Mean Late Week?

First up, locally heavy rainfall is possible in southern Florida and the Bahamas through early Wednesday. Street flooding is likely in areas where heavy rain persists for an hour or more. There is also a high rip current risk for south Florida's Atlantic beaches.
(FORECASTS: Miami | Freeport, Bahamas)
Eventually, that low near or north of the Bahamas may sprout enough convection near its circulation to be classified as a subtropical depression or storm.
If that occurs, the NHC would issue forecast advisories. The first name of the 2015 Atlantic hurricane season is Ana.
If it does develop 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 Stu Ostro, Senior Director of Weather Communications at The Weather Channel.
Where this low tracks remains quite uncertain this far out.
The problem is the steering flow in the upper atmosphere is likely to 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, this low may loaf and lollygag off the coasts of the Carolinas and Georgia for at least a few days. Doing so over the Gulf Stream could give it more time to sprout more convection, so we can't rule out this system transitioning to a fully tropical storm.
Beyond that, forecast guidance is still split whether the system moves inland this weekend, or simply lingers offshore before a southward dip in the polar jet stream finally whisks it away early next week.
Indirect impacts, such as high surf, rip currents, even some minor coastal flooding and beach erosion look most likely from parts of Florida's East Coast to at least southern North Carolina's beaches, even if the low hovers a bit farther offshore.
(FORECASTS: Myrtle Beach | Charleston | Hatteras)
If, however, the low moves closer to land, bands of rain, possibly heavy, and stronger wind gusts may come into play in the affected areas, particularly if the system becomes a fully tropical storm.
For now, if you have weekend plans or live near the coast from North Carolina to northeast Florida, keep checking back with us at weather.com and The Weather Channel for the latest.

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 senior meteorologist Stu 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.
Most 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