July 26, 2021

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Subtropical Depression Four Forms in Atlantic but Poses No Direct Threat to U.S.


Current Storm Status and Projected Path

(The red-shaded area denotes the potential path of the center of the subtropical cyclone. It’s important to note that impacts (heavy rain, high surf, coastal flooding and winds) with any subtropical cyclone usually spread beyond its forecast path.)

  • Subtropical Depression Four became better organized and formed Monday afternoon.
  • It is located far off the East Coast and is not expected to impact land.
  • If it strengthened, it would be named Dolly.

Subtropical Depression Four has formed in the Atlantic Ocean, but it will remain well off the East Coast of the United States.

The National Hurricane Center initiated advisories on Subtropical Depression Four on Monday as it moved over an area of warmer water in the Gulf Stream. However, it is expected to be short-lived and become absorbed by a larger area of low pressure or dissipate midweek.

This system is moving toward the east-northeast and then will turn northeastward on Tuesday. It might strengthen briefly into a subtropical storm, and if it does, it would be named Dolly.

The main impact from this system will likely be some minor swells toward Nova Scotia and other parts of Atlantic Canada, but it poses no direct threat to the U.S. East Coast.

Difference Between Tropical and Subtropical Storms

When an area of low pressure forms over water with sea-surface temperatures of at least 70 degrees, a subtropical low can form. This is due to the storm’s core 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 storm or depression 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 of circulation.

Mature subtropical systems also often 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 cyclones are typically 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.

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

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

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

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.



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