Invest 91L Could Develop Into a Subtropical Depression or Storm This Week; Would Be First April Named Storm Since 2003

April 19, 2017

A subtropical depression or storm could develop in the Atlantic Ocean this week for the first time in April since Ana in 2003.

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This potential system is associated with an area of low pressure located between Bermuda and the Azores, dubbed Invest 91L by the National Hurricane Center (NHC). This naming convention is used by the NHC to identify features that are monitored for potential future development into a depression or storm.

(MORE: What Is an Invest?

Current Satellite and Pressure

The system is not affecting land at this time but has produced gale-force winds (39 mph or greater) over the last day or so.

Seas as high as 40 feet have been analyzed by NOAA's Ocean Prediction Center during the last couple of days as the low developed.

There has been some increase in persistent shower and thunderstorm activity in association with this low since Tuesday. If that activity persists and becomes better organized, the NHC could classify it as a subtropical depression or storm. For more on what a subtropical system is, scroll to the bottom of this article.

That said, the window of opportunity to become a subtropical cyclone is closing since this low will get absorbed by another low-pressure system that is forecast to develop Thursday.

(MORE: New Hurricane Season Forecast)

As of Wednesday morning, the NHC gave the system a high (70 percent) chance of development in the next 24 hours. They added that only a small increase in the organization is needed for the system to become a subtropical depression on Wednesday.

Should this system develop and then attain sustained winds of 39 mph or greater, it would receive a name and become Subtropical Storm Arlene.

(MORE: 5 Changes Coming to Hurricane Season Forecasts)

Regardless of development, this area of low pressure is expected to have no direct impacts on any land areas. Some swells generated by the low may impact the northeast Caribbean Islands.

How Rare Would This Be?

You'd be right to think it's very early for possible tropical development. The Atlantic hurricane season, which officially begins June 1 and goes through Nov. 30, accounts for about 97 percent of tropical cyclone activity in the basin.

However, a tropical or subtropical cyclone developing in April is a rare, but possible, event.

As mentioned before, Ana in 2003 was the last April named storm to roam the Atlantic Basin. Ana started out as a subtropical storm on April 20, 2003 and soon gained full tropical characteristics. That made Ana the only tropical storm on record to form in the Atlantic Basin in April.

Visible satellite image of Subtropical Storm Ana taken by the OrbView-2 satellite on April 20, 2003. Ana would later become a tropical storm. (SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE)

While Ana remains the only April Atlantic tropical storm in records dating to 1851, an April 1992 subtropical storm was found in post-analysis by the NHC. Since it was not classified as such at the time, a later hurricane that year, Andrew, got the "A" name.

(RECAP: April 2003's Tropical Storm Ana)

There have been numerous seasons that started early. On a long-term average, a tropical system forms prior to June about once every 10 years, and these storms tend to be relatively weak, due in part to cooler sea-surface temperatures.

There has been a recent trend in early starts to the Atlantic hurricane season, with 2012, 2015 and 2016 all reporting tropical cyclone formation before June 1.

(MORE: When Hurricane Season Starts Early)

Just last year, two tropical cyclones formed before the official start date. Hurricane Alex developed Jan. 13, 2016 and made landfall in the Azores. Then, Tropical Storm Bonnie formed on May 28 and made landfall in South Carolina over the Memorial Day weekend.

Areas off the Southeast coast, as well as the northwest Caribbean and the Gulf of Mexico, are common locations for early-season development, especially in May.

Difference Between Tropical And Subtropical Storms

When an area of low pressure forms over waters with sea-surface temperatures of at least 70 degrees, a subtropical low can form. This is due to the core of the storm becoming warm, deriving some of its energy from latent heat, or energy released when water vapor that evaporated from the warm water is condensed into liquid.

A subtropical depression or storm exhibits features of both tropical and non-tropical systems. This includes no cold or warm fronts, a broad wind field and thunderstorms removed some distance from the center.

Subtropical storms also tend to have a large, cloud-free center and a less symmetric wind field. Maximum sustained winds are also much farther from the center, while the strongest winds in a tropical storm are close to the center.

Subtropical Low

Subtropical cyclones typically are associated with upper-level lows and have colder temperatures aloft, whereas tropical cyclones are completely warm-core and upper-level high-pressure systems overhead help facilitate their intensification.

The NHC still issues advisories and forecasts for subtropical depressions and storms. They are assigned a number or name, just like a tropical depression or storm.

Tropical Low

If the subtropical storm remains over warm water, thunderstorms can build close enough to the center of circulation, and latent heat given off aloft from the thunderstorms can warm the air enough to make the storm a fully tropical storm.

As a result, the strongest winds and rain become closer to the center and, with time, further intensification becomes possible.

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