Could the Atlantic hurricane season start early?
The Atlantic hurricane season runs from June through November. However, the atmosphere doesn't always conform to that. Occasionally a storm develops early, and that could happen off the Southeast coast this week.
The potential 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 will be revived by an infusion of energy from the southern, or subtropcial, branch of the jet stream. This will cause an area of low pressure to form near the northwest Bahamas or off Florida's East Coast midweek.
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 - the U.S. Air Force Reserve "Hurricane Hunters" may investigate the system as soon as late Tuesday morning.
This area of disturbed weather is then expected to move north or northwest into next weekend.
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. (Stu Ostro)
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.
On rare occasions, 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.What Does This Mean Late Week?
First up, locally heavy rainfall is possible in southern Florida and the Bahamas from Monday night through early Wednesday. Street flooding is likely in areas where heavy rain persists for an hour or more.
By mid to late week, that low near the Bahamas or Southeast coast may sprout enough convection near its circulation to be called 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.
Late Sunday morning, the NHC issued a special tropical outlook that gave the area of interest a 30 percent chance of developing into a subtropical cyclone late this week.
Where this low tracks remains highly uncertain this far out.
Unless it remains well offshore and weak, a system like this can produce high surf, rip currents, even some coastal flooding or beach erosion. If it tracks close enough to the Southeast coast, areas of locally heavy rain are possible in those areas.
At this point, the chance of this system becoming a fully tropical system appears to be low, but not zero.
So this isn't something to be overly worried about just yet. We'll continue to monitor the forecast over the next several days, so check back with us at weather.com and The Weather Channel for the latest.
It's Happened Recently
Some recent Atlantic hurricane seasons have jumped the June 1 starting gun.
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.
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.
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.