We're Watching an Area Off the Southeast Coast for Subtropical or Tropical Development

Chris Dolce
Published: May 23,2016
The official start of the Atlantic hurricane season is just days away, and there's an area to watch for potential development in the western Atlantic or Gulf of Mexico as we head through the final week of May.
To be clear, we are not saying that a tropical or subtropical depression or storm will develop. But this situation is being monitored closely since there are signs it could develop.
Here's what we know right now.
(MORE: Atlantic Hurricane Season Forecast)

Why Are We Watching this Area?

1.) Historically, it’s an area to watch early in the season

This map shows the typical formation areas and tracks for named storms in June.

The area off the Southeast coast, as well as the Gulf of Mexico and northwest Caribbean, is a part of the Atlantic basin where early season tropical or subtropical storm development is most likely.
That said, it's not a very active time of year for storms to form compared to the peak August-October period. For example, only one named storm forms during June every other year, on average.
(MORE: Where June Storms Develop)
Recent seasons have had activity before the start of June.
Last May, Tropical Storm Ana formed off the Southeast coast before making landfall near Myrtle Beach, South Carolina, with minor impacts.
In 2012 two named storms, Alberto and Beryl, formed during May off the Southeast coast. Beryl would go on to make landfall in northeast Florida, while Alberto moved out to sea.
(MORE: When Hurricane Season Starts Early)
Leftover frontal boundaries are one contributor to early-season development. In addition, non-tropical low pressure systems can gain subtropical or tropical characteristics over time, eventually becoming named tropical or subtropical storms.
2.) Computer model guidance hinting at low pressure development

Forecast models are depicting an area of low pressure aloft developing between the southwest Atlantic Ocean and the eastern Gulf of Mexico. That system could help generate the development of a surface low pressure that may gradually become a subtropical or tropical depression or storm. However, this is highly uncertain at this time.

During the last few days, some computer model forecast guidance has been hinting at the development of weak surface low pressure off the Southeast coast or in the eastern Gulf. As mentioned before, this development does not occur until sometime between late this week into early next week.
In this situation, the forecast models show an area of low pressure aloft forming somewhere from the southwest Atlantic Ocean to the eastern Gulf of Mexico. That system could eventually help spawn a surface low pressure system that could gradually aquire either subtropical or tropical characteristics and become a depression or named storm.
However, there is no guarantee that development will occur since the models often struggle with the details in long-range forecasts.
What we typically watch for is consistency and trends in the forecast guidance with each run. That's what we'll be examining in the days ahead.
.) Madden Julian Oscillation (MJO) could provide uptick in rain and thunderstorm activity
A strong Madden Julian Oscillation (MJO) wave is forecast to pass into the Atlantic as we close out May and enter June, according Michael Ventrice, a meteorologist with The Weather Company.
The MJO is essentially a wave of energy in the atmosphere that propagates eastward around the Earth near the equator once every 30-60 days.
By a "wave," we mean the MJO has a phase where upward motion in the atmosphere is strong, helping to boost the formation of clouds and thundershowers, and a suppressive phase, helping to squelch precipitation.
The upward motion phase is the one that would make its way into the Atlantic next week. This could aid in the development of the rain and thunderstorms that are needed for tropical or subtropical storm development.

What is the Upshot of All of This?

Atlantic basin names for 2016.

There is nothing to be overly concerned about at the moment as it's just a possibility and not something to hype.
Even if a tropical or subtropical depression or storm did develop, there is no guarantee that it would ever affect land in a significant way. That said, some increased rainfall chances and the potential for rip currents are in play on the Southeast coast no matter whether the system develops or not.
We will continue to watch this situation closely, and will provide updates over the next few days.
The next named storm of the 2016 Atlantic Hurricane Season will be given the name Bonnie.
If you recall, we already had the “A” named storm this season. Hurricane Alex was a rare January storm that formed in the eastern Atlantic Ocean and impacted the Azores.
(MORE: Florida's Lucky Hurricane Drought)

Difference Between Subtropical and Tropical Storms

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.

MORE: Hurricanes From Space - Satellite Imagery