Tropical Storm Isaac has suddenly developed over the ocean, becoming the ninth named storm of the hurricane season.
Isaac is expected to become a hurricane in the coming days, traveling eastwards across the Atlantic towards the Iberian Peninsula.
Meanwhile, Hurricane Helene is making a beeline for the Florida coast, and may be a major hurricane by the time it makes landfall, with "catastrophic and deadly storm surges" forecast.
Tropical Storm Isaac formed late on Wednesday night over the central Atlantic, developing from an area of low pressure.
"Isaac is moving toward the east near 12 mph and a general eastward to east-northeastward motion is expected to continue for the next several days. Maximum sustained winds are near 50 mph with higher gusts. Some strengthening is forecast during the next 48 hours and Isaac could become a hurricane by Friday over the open Subtropical Atlantic," the National Hurricane Center said in a public advisory.
The storm formed rapidly from a tropical disturbance, and may become a hurricane by early Saturday morning, the NHC predicted. The storm is expected to weaken again into a post-tropical storm by the time it reaches the vicinity of the Azores islands.
"A tropical disturbance is called a tropical depression whenever the winds are between 25 and 38 mph. When the wind speeds are between 39 and 73 mph it is classified as a tropical storm. Above 74 mph it is called a hurricane or typhoon," Annalisa Bracco, a professor of ocean and climate dynamics at the Georgia Institute of Technology, told Newsweek.
Hurricane Helene, which is currently Category 1, formed from an area of low pressure in the Caribbean, having strengthened rapidly over the last few days. The hurricane is expected to be the "strongest hurricane to make landfall in the U.S. this season," possibly hitting Category 3 or 4 strength.
"Two main conditions behind intensification are warm surface waters (the cyclone can extract energy from it and the warmer the water, the more energy it can extract to grow), and low wind shear (high wind shear tends to disrupt vortices, both cyclones and anticyclones)," Bracco said.
Helene has maximum sustained winds of around 90 mph, and is expected to get more powerful before it makes landfall in Florida's Big Bend region this evening, possibly becoming a major hurricane. Hurricane Warnings, Hurricane Watches, Tropical Storm Warnings and Storm Surge Warnings are in effect along much of the Florida coast.
Hurricane-force winds are expected in the Big Bend region and surrounding areas.
"Potentially catastrophic hurricane-force winds are expected within the eyewall of Helene when it makes landfall in the Florida Big Bend region later today. Preparations to protect life and property should be completed this morning before tropical storm conditions arrive. Damaging and life-threatening hurricane-force winds, especially in gusts, will penetrate well inland over portions of northern Florida and southern Georgia later today and tonight where Hurricane Warnings are in effect," the NHC said in a forecast discussion.
Huge volumes of rain are expected to fall as the hurricane arrives, with as many as 18 inches of rainfall being forecast in some areas of the Southeast. This is expected to trigger extremely dangerous flash flooding and landslides.
"Catastrophic and life-threatening flash and urban flooding, including numerous significant landslides, is expected across portions of the southern Appalachians through Friday. Considerable to locally catastrophic flash and urban flooding is likely for northwestern and northern Florida and the Southeast through Friday. Widespread significant river flooding and isolated major river flooding are likely," the NHC said.
Additionally, 15 to 20 feet of storm surge is forecast in some areas of the Florida coast, due to interactions between the storm's winds and the high tide.
"A catastrophic and deadly storm surge is likely along portions of the Florida Big Bend coast, where inundation could reach as high as 20 feet above ground level, along with destructive waves. There is also a danger of life-threatening storm surge along the remainder of the west coast of the Florida Peninsula," the NHC said.
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