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| Infrared Satellite: Subtropical Storm Ana |
May 8, 2015 - ATLANTIC OCEAN - Late Thursday night, the National Hurricane Center officially deemed the
low off the coast of the Carolinas Subtropical Storm Ana, the first
named storm of the 2015 Atlantic hurricane season.
For more on what a "subtropical" cyclone is, scroll down the page a bit.
U.S. Air Force Reserve Hurricane Hunters reconnaissance missions
measured surface winds from 40-45 mph, justifying going straight to
"storm", rather than "depression" status. Once convection became
sufficiently organized and persistent within the circulation, the system
was named.
Bands of rain already begun to wrap into parts of the coastal Carolinas
Thursday. However, rainbands Friday morning appear less concentrated, as
the circulation is wrapping some drier air around its eastern flank.
Winds gusted above 30 mph Thursday in Wilmington, North Carolina, and a
few coastal Carolina locations experienced gusts over 20 mph.
Where's Ana Headed?
Ana's center of circulation is expected to drift toward the northwest
and finally crossing the northern coast of South Carolina by Sunday.
The reason for this slow forward speed is the steering flow in the upper
atmosphere will be very weak through early Monday. 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.
Bands of rain and some strong wind gusts will pester parts of the Carolinas through the weekend.
Ana remains over the Gulf Stream, and still has a potential to sprout
more convection and transition to a fully tropical storm, as was the
case with Subtropical, then Tropical Storm Beryl in May 2012. However,
due to dry air being pulled into the circulation, we think the chance of
this happening with Ana is small.
Ana's landfall won't be that noteworthy Sunday, compared to a hurricane
landfall with intense eyewall winds and storm surge, with little
intensification expected until that point.
While slowly weakening after landfall later Sunday into Monday, onshore
gusty winds will continue along the southern coast of North Carolina,
spreading northward into the Outer Banks and Virginia Tidewater Monday.
By Monday night, a southward dip in the polar jet stream will catch hold
of what's left of Ana and finally whisk it away into the Atlantic.
Incidentally,
Ana has 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
- Rain/flood threat: Bands of rain continue to move
into parts of eastern Carolinas. Areas of rain may linger in the eastern
Carolinas through Monday, eventually spreading into parts of Virginia
through Monday. The greatest potential for locally heavy rain and flash
flooding is in southeast North Carolina and northeast South Carolina.
- Winds: Gusty
winds pick up in coastal North Carolina (especially south of Hatteras)
and coastal South Carolina Saturday, continuing Sunday, before spreading
from coastal North Carolina to southeast Virginia Monday. Some gusts
over 40 mph are likely, at times. Some downed tree limbs and sporadic
power outages are possible, but the threat of widespread damaging winds
and numerous power outages appears low.
- High Surf/Rip Currents/Coastal flooding: 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 Monday, 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.
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.
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 was 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.
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| Rainfall Potential Ana |
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.
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. -
Weather.
