Invest 90-L Could Develop Into a Subtropical Storm This Week; Would Be Second Time In March Since Records Began

Linda Lam
Published: March 27,2017
A subtropical storm could develop in the Atlantic Ocean this week – something that has only occurred once before in March since the pre-Civil War era.
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This potential system is associated with an area of low pressure that has developed well north of the Dominican Republic and Haiti, which has been dubbed Invest 90-L by the National Hurricane Center (NHC). This naming convention is used by the NHC to identify features they are monitoring for potential future development into a depression or storm.
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Current Satellite and Pressure

The system brought locally heavy rainfall to Puerto Rico during the weekend, along with rip currents and high surf. Conditions should improve there as the low moves away.
If the right ingredients come together this week, a subtropical depression or storm could form from the aforementioned area of low pressure.
Persistent shower and thunderstorm activity closer to the center of circulation is necessary to allow the system to develop and become a subtropical cyclone, which hasn't happened yet.
Sea-surface temperatures are generally greater than 75 degrees in the region where the low pressure – one of the ingredients necessary for subtropical development – is expected to develop. This increases the chance that this low will at least have some tropical characteristics.
However, current water vapor satellite imagery shows very dry air over the low-pressure center, a factor that is not favorable for development.

Current Water Vapor Satellite Image

Wind shear is currently too high over the system, but models open up a short window where shear may become low enough, and if there's enough moisture in the region, the low could continue to organize and subtropical development could take place early this week.
(MORE: 5 Changes Coming to Hurricane Season Forecasts)

Current Satellite, Wind Shear

If sustained winds reach 39 mph or greater, it would receive a name and become Subtropical Storm Arlene.
This area of low pressure is expected to move north and then east through the central Atlantic into midweek and could pass south of Bermuda with at least some rain and wind.

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, for a tropical cyclone to develop in March is a rare, but possible, event. There has been only one Atlantic tropical or subtropical cyclone to form in March since records began in 1851. This occurred over 100 years ago on March 6, 1908, northeast of the Leeward Islands.
The 1908 tropical cyclone strengthened into a Category 2 hurricane and impacted the U.S. Virgin Islands. This hurricane was also unusual in that it tracked toward the southwest through the Caribbean.
There have been numerous seasons that started early. On a long-term average, about once every 10 years, a tropical system forms prior to June, 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 National Hurricane Center 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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