Friday, March 24, 2017

Atlantic Tropical or Subtropical Storm May Form Next Week For Just the Second Time In March Since Records Began

Linda Lam
Published: March 24,2017

A subtropical storm could develop in the Atlantic Ocean next week – something that has only occurred once before in March since before the Civil War.
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Currently, a broad trough of low pressure along an old frontal boundary is located north of the Dominican Republic and Haiti. This will allow a moist, southerly flow to bring rainfall to the region into this weekend.

Current Satellite and Pressure
The setup may bring locally heavy rainfall and create urban and small stream flooding in the area, particularly across eastern Puerto Rico through the weekend.
There is also a moderate risk of rip currents and waves up to 10 feet are possible.
This disturbance will likely develop into a low-pressure center by early next week, and if the right ingredients come together, a subtropical storm could form.
A tropical or subtropical low may develop early next week.
After the low-pressure center forms, persistent shower and thunderstorm activity is also necessary to allow the system to develop.
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.
If wind shear is low enough, and there's enough moisture in the region, the low will continue to organize and subtropical development could take place early next week.
(MORE: 5 Changes Coming to Hurricane Season Forecasts)
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 toward the central Atlantic into midweek and could pass near 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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