Threat From Former TS Arlene Has Passed: National Hurricane Center

FLORIDA — After briefly developing into Tropical Storm Arlene, the hurricane season's first storm has weakened back into a tropical depression, leaving behind some torrential downpours but few other impacts on the Florida Gulf Coast.

During its 4 p.m. update, the National Hurricane Center said the tropical depression is about 135 miles west-southwest of the Dry Tortugas, which is located 70 miles south of Key West, moving at 7 mph with maximum sustained winds of 3 mph with higher gusts.

There are no land-based coastal watches or warnings in effect, however, marine warnings have been issued due to 12-foot seas.

The Air Force Hurricane Hunters flew into the storm Saturday morning and found that the weakening trend is due to strong vertical wind shear and dry air that has wrapped into the
circulation.

A turn to the east is expected by Saturday night, and that motion should continue until the system dissipates on Sunday, said the NHC.

Before downgrading, Tropical Storm Arlene produced 1 to 2 inches of localized rainfall throughout South Florida with up to 5 inches in some areas, according to NHC forecaster John Cangialosi.

The NHC issues weather advisories every six hours during the hurricane season, June 1 to Nov. 30. They are issued at 4 a.m., 10 a.m., 4 p.m. and 1o p.m. Eastern Standard Time and can be found here in the "Atlantic" category under "public advisory."

This webpage also offers the latest forecast advisory and discussion, wind speed probabilities, the estimated arrival times of wind speeds, warnings maps, rainfall potential and flash flooding potential.

A Lesson On Tropical Cyclone Climatology

Patch readers and longtime Florida residents noted that they've never seen a storm move to the south and asked why this storm deviated from the norm.

According to the hurricane center, in general, hurricanes are steered by global winds. The prevailing winds that surround a storm, also known as the environmental wind field, are what guide a storm along its path.

In the tropics, where hurricanes form, easterly winds called the trade winds typically steer a storm toward the west. In the Atlantic basin, storms are carried by these trade winds from the coast of Africa, where they often develop, westward towards the Caribbean Sea and the North American coasts.

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Embedded within the global winds are large-scale high- and low-pressure systems. The typical clockwise rotation of air associated with high-pressure systems often cause tropical storms to stray from their initially east-west movement and curve northward.

However, if the high is positioned to the west and extends far enough to the south, storms can be blocked from curving north and forced to continue southwest.

It's unusual but has occurred several times, although a search by Patch of all hurricane movements since the 1900 Galveston Hurricane has never shown a hurricane to move southeast, although southeast movement has occurred during tropical storms and depressions.

In 1998, Hurricane Mitch crossed Florida and moved slowly southward near the coastal Islands of Honduras. It made landfall over northern Honduras as a Category 1 hurricane and dissipated near the Guatemala-Honduras border.

In 2001, Hurricane Iris first tracked west-northwestward into the eastern Caribbean as a tropical depression, where it developed into a hurricane and moved quickly west-southwest toward the coast of Belize as a powerful Category 4 hurricane.

In 2005, Hurricane Wilma formed from a broad area of disturbed weather that stretched across much of the Caribbean Sea. The cyclone moved erratically westward and southward for two days while slowly strengthening into a tropical storm. It made landfall over Cozumel and the Yucatan Peninsula as a Category 4 hurricane.

Then, in an unusual deviation, Wilma moved into the Gulf of Mexico and accelerated in a northeast direction toward southern Florida, making landfall near Cape Romano as a Category 3 hurricane. It then moved into the Atlantic near Palm Beach as a Category 2 hurricane and traveled 230 miles up the Atlantic coast to Halifax, Nova Scotia.

However, tropical cyclones forming between 5 and 30 degrees North latitude typically move toward the west. Sometimes the winds in the middle and upper levels of the atmosphere change and steer the cyclone toward the north and northwest. When tropical cyclones reach latitudes near 30 degrees North, they usually move northeast.

Depressions, Storms And Hurricanes

A tropical cyclone is a rotating, organized system of clouds and thunderstorms that originates over tropical or subtropical waters and has a closed low-level circulation. Tropical cyclones rotate counterclockwise in the Northern Hemisphere, according to the NHC. They are classified as follows:

• Tropical Depression: A tropical cyclone with maximum sustained winds of 38 mph or less.
• Tropical Storm: A tropical cyclone with maximum sustained winds of 39 to 73 mph.
• Hurricane: A tropical cyclone with maximum sustained winds of 74 mph or higher. In the western North Pacific, hurricanes are called typhoons. Similar storms in the Indian Ocean and South Pacific Ocean are called cyclones.
• Major Hurricane: A tropical cyclone with maximum sustained winds of 111 mph or higher, corresponding to a Category 3, 4 or 5 on the Saffir-Simpson Hurricane Wind Scale.