90% chance tropical depression could form off Atlantic coast

Forecasters have been keeping a close eye on a low-pressure system all week as it's slowly moved over the southeastern U.S. and spawned an area of showers and thunderstorms off the coast of the Carolinas. The National Hurricane Center has designated the disturbance Invest 98L.

AccuWeather meteorologists are giving the developing system an 90% chance of developing into a tropical depression and possibly Tropical Storm Fay, which would be the sixth named storm of the 2020 Atlantic hurricane season.

On Thursday morning, AccuWeather meteorologists were giving the developing system an 80% chance of developing into a tropical depression and possibly Tropical Storm Fay, which would be the sixth named storm of the 2020 Atlantic hurricane season. By Thursday afternoon, the system was teetering on the edge of becoming a depression or more, with chances of development at 90% and Hurricane Hunters sent to investigate.

This image, captured during Thursday afternoon, July 9, 2020, shows some banding structure developing in the cloud cover near Cape Hatteras, North Carolina, indicative of a developing tropical depression. (NOAA/GOES-East)

"Development of a tropical depression can occur at any time now on Thursday, now that the disturbance that has been lingering over the Southeastern states has meandered just off the North Carolina coast," said AccuWeather's top hurricane expert Dan Kottlowski.

"Winds have been gusting between 20 and 30 mph in thunderstorms on the eastern side of the disturbance during Thursday morning," Kottlowski said. "When winds begin to wrap around the center in a circular motion, we should have a tropical depression."

There are no other significant candidates for a tropical depression or storm over the Atlantic basin into this weekend.

Waters around the coast and offshore from North Carolina through the lower part of the mid-Atlantic are sufficiently warm to support a tropical system, while wind shear in the region is not strong.

Wind shear is the change in the flow of air at different levels of the atmosphere and the change in direction of the flow of air in the horizontal above the sea surface. Strong wind shear can prevent a tropical storm or hurricane from strengthening.

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The system is forecast to drift on a general north to northeast track through this weekend. This path will take the feature right along the mid-Atlantic coast and inland over New England and Atlantic Canada.

"Exactly how close to the coast this system tracks as it moves northward will generally dictate how much strengthening takes place," AccuWeather Senior Meteorologist Randy Adkins said.

"A path 100 miles off the mid-Atlantic coast increases the likelihood of strengthening to a moderate tropical storm as the system would be over warm water for a longer period of time as opposed to hugging the coast or just onshore of the coast," Adkins explained.

While a "hug-the-coast" scenario might still allow a depression to form and maintain itself or reach minimal tropical storm status, immediate coastal waters are significantly cooler than those over the Gulf Stream and the land will offer a lot of wind resistance. That frictional resistance would limit strengthening.

Strengthening to a hurricane is not anticipated at this time no matter whether or not the system hugs the coast or parallels the coast just offshore.

Heavy rainfall with the feature will tend to be limited to the immediate mid-Atlantic coast from Thursday into Friday night. Downpours can be heavy enough to lead to urban flooding problems.

Mid-Atlantic cities that are most likely to receive downpours from the feature include Norfolk, Virginia; Salisbury, Maryland; Dover, Delaware; Atlantic City, New Jersey; and New York City.

Those with plans at the beach may have them interrupted by downpours and locally gusty winds for about a 12- to 24-hour period until the storm passes.

Generally, 1-3 inches of rain is expected from the tropical system along the mid-Atlantic coast, but higher amounts are possible from northern New Jersey to New England at the local level.

As the storm moves northward this weekend, some heavy rain is likely to wrap westward. This, combined with the geography of New England that extends farther to the east into the Atlantic, should produce drenching rainfall for a time over much of the region through the end of the week and into Saturday.

This means that much of New England, which has been in a worsening drought situation could be quenched by the tropical system, assuming it tracks just inland.

A general 2-3 inches of rain seems likely over New England with An AccuWeather Local StormMax™ of 6 inches anticipated. However, that may change on the track and strength of the tropical system.

Depending on the strength of the storm, winds blowing in from the Atlantic will cause above-normal tide levels. Minor coastal flooding at times of high tide can occur as the feature moves northward.

Regardless, seas will build to moderate levels, and rip currents will increase in strength and number until after the storm has passed well to the north this weekend.

In lieu of direct tropical impact, areas farther west will be under the influence of an approaching non-tropical system this weekend. That feature will produce a swath of drenching showers and thunderstorms over the eastern Great Lakes, the Ohio Valley and the central Appalachians first, then the Atlantic coast later in the weekend.

While areas west of the coast and New England may dodge the tropical system, rain will move in from outside of the tropics regardless.

In the wake of the tropical feature along the Atlantic coast this week, a similar setup could allow yet another tropical system to slowly brew in roughly the same area next week.

AccuWeather is projecting a busy season ahead with 14-20 named tropical storms, seven to 11 hurricanes and four to six major hurricanes. Five tropical storms are already in the books for the season, with two U.S. landfalls.

If the feature along the East Coast becomes Fay over the next couple of days, then it would unseat another entry in a long list of 2005 tropical storms that held early-season records since the satellite era of the 1960s. The earliest sixth-named storm on record was Franklin during the record-breaking 2005 Atlantic hurricane season, the same season which produced Hurricane Katrina in late August. Franklin formed on July 21, near the central Bahamas, and traveled northeastward, well to the east of the U.S. coast. The storm did not reach hurricane strength.

This past weekend, Edouard became the earliest fifth tropical storm in any Atlantic hurricane season since the satellite era of the 1960s. On July 5, Edouard beat out Emily, which formed on July 12 and went on to become a Category 5 hurricane in the Caribbean Sea in 2005, by a week.

Cristobal became the earliest "C" named storm in recorded history for the Atlantic on June 2, a feat that typically does not occur until around the middle of August. The storm went on to crash ashore along the Gulf Coast, where it unleashed flooding. Dolly was the second-earliest "D" named storm ever in the basin, but it moved out to sea without impacting land.

Arthur and Bertha, as well as Dolly, formed in the same near-coast waters of the eastern U.S. this season. Both Arthur and Bertha formed in May.

Beyond Fay, the next two named storms on the list for 2020 are Gonzalo and Hanna.

The 2005 hurricane season also holds the earliest dates for tropical storms through the letter "K" when Katrina formed on Aug. 24. Gert formed on July 24, 2005, followed by Harvey on Aug. 3. The name Harvey would eventually be retired after the 2017 season when a different Harvey brought devastating flooding to Texas.

Since the area along the Atlantic coast will remain unsettled through the middle of July, it is not out of the question for another tropical feature to take shape in that area. Odds are against any major tropical system in that area next week. Forecasters say it's more likely systems similar to what have been forming thus far this year in the Atlantic, such as tropical depression and storms, will develop.

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