Earliest subtropical cyclone affecting U.S. forms off coastal Carolinas
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.
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.
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