User blog:PuffleXTREME/2015 atlantic season starting 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 Setup
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 this past week and gave Key West its third-heaviest April day of rain on record 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.

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?
The bottom line is that 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.

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.

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.