Ida’s remnants bring threat of flooding, tornadoes to Eastern U.S.

 

 

 

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Ida’s remnants bring threat of flooding, tornadoes to Eastern U.S.

Matthew Cappucci  4 hrs ago

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It has been more than 48 hours since Hurricane Ida slammed ashore in southern Louisiana on the verge of Category 5 strength, bringing calamitous storm surge inundation, destructive winds and overall disaster to areas near and south of New Orleans. Now, the relentless storm, which remains a tropical depression as it swirls through the Deep South toward the Tennessee Valley, is slated to drop a slug of serious rainfall, bringing widespread flash flooding and the risk of a few tornadoes.

The National Weather Service's forecast for how much rain may accompany Ida's remnants as it streams through the Eastern U.S. (WeatherBell)

Flash flood watches stretch more than 1,200 miles, spanning an area from the Florida Panhandle to Cape Cod, Mass. The National Weather Service has included much of that zone in a level 3 out of 4 “moderate risk” of excessive rainfall and flash flooding, with major cities such as Washington, Baltimore, New York and Boston in line Wednesday.

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A growing risk for tornadoes will also accompany Ida’s remnants as they move up the East Coast, with the potential for scattered quick-hitting funnels that could touch down with relatively little warning. Tornado watches will probably be issued Tuesday afternoon and much of Wednesday for areas east of Ida’s disintegrating center.

Ida now

The remnants of Ida as seen from the GOES East weather satellite early Tuesday morning. (College of DuPage)

On paper, Ida was a tropical depression Tuesday morning, meaning its strong winds had largely diminished. Its sprawling swirl of heavy precipitation, however, had not. A juiced-up tropical atmosphere is lending itself to high rainfall rates locally topping an inch per hour.

Ida’s center was spinning through northern Alabama north of Birmingham, but its heaviest arc of rain curved from Middle Tennessee southeast through Georgia. At 11 a.m., it was 60 miles west-northwest of Huntsville as it headed northeast at 15 mph.

Through Tuesday night, it will trek up the Appalachians. The heaviest rain will occur near and east of its center. In most areas, a 12- to 18-hour stretch of precipitation can be expected. By Wednesday, Ida will begin to develop its own fronts and interact with preexisting ones, establishing a warm front near the Mason-Dixon Line and, on Thursday, dragging a wedge of cooler, drier air southeast in its wake.

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Mass flooding, fallen power lines seen in Louisiana day after Hurricane Ida

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Heavy rainfall

A high-resolution model simulation of Ida's remnants sauntering up the East Coast. (WeatherBell)

Peak rainfall totals in Alabama and Mississippi have reached the double digits. Gulfport-Biloxi International Airport posted 10.72 inches. Wilmer, a town west of Mobile, Ala., reported 11.24 inches, while Mobile Regional Airport registered 9.41 inches.

Amounts in Tennessee generally ranged from 2 to 4 inches, but the rainfall was only halfway done — an additional 1 to 3 inches is likely to be there by midnight Tuesday night.

From there, Ida’s remnants shift north and east, drenching Kentucky with 1 to 3 inches, West Virginia with 2 to 4 and the Panhandle of Maryland with 3 to 5. Then, by Wednesday, a serious deluge will unfold in southern Pennsylvania and New Jersey, where 4 to 7 inches are expected.

“A [level 4 out of 4] High Risk area may ultimately be needed” for that rainfall, the National Weather Service warned, but subtle track shifts of even 30 or 40 miles could spell all the difference in where that rainfall bull’s eye ultimately ends up. Northern parts of the Delmarva Peninsula could see plentiful rainfall, too.

Confidence is also growing in a significant flood event in New York City late Wednesday into Thursday, where more than 10 inches have already fallen in August. That’s more than two and a half times the average for the month; simply stated, the ground is saturated in the region and can’t handle much more rainfall.

Long Island, southern Connecticut, Rhode Island and southeastern Massachusetts, including the Cape and the Islands, area also likely to see a broad swath of 4 inches of rainfall with amounts locally exceeding 6 inches Thursday; rain totals will diminish quickly to the north and west, meaning it will be a game of feast or famine in Hartford, Providence and Boston. It’s still unclear which it will be.

Tornado threat

A high-resolution model simulation of low-level spin accompanying Ida's remnants. (WeatherBell)

Equally noteworthy is a growing risk for tropical tornadoes along much of the Interstate 95 corridor as Ida’s remnants work northeastward. A tornado struck Monday near Saraland, Ala., by Mobile, flipping an 18-wheeler and injuring the driver. At least a half-dozen other reports of tornadoes were also received.

Ida is trucking along with it a surge in wind shear, or a change of wind speed and/or direction with height. That means any clouds that become tall enough to span multiple layers of the atmosphere will feel a twisting force and, resultantly, begin to rotate.

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Tornadoes are possible east of Interstate 65 and west of Interstate 85 along the Alabama-Georgia border Tuesday, with a risk zone that includes cities such as Dothan, Columbus, Phenix City and the greater Atlanta area. Tropical tornadoes are often difficult to detect on radar because of their shallow and quick-forming circulations, which radar beams often “overshoot.” That means that any spin-ups that occur may come with little warning.

Scattered to numerous thunderstorms, many of which may rotate with the potential to produce tornadoes, will swing through the Mid-Atlantic on Wednesday afternoon and evening. (WeatherBell)

A greater risk of tornadoes will brew Wednesday, when Ida begins to acquire mid-latitude characteristics and develop more pronounced fronts. A cold front will trail behind the system as southerly winds ahead of it lift a warm front through the Mid-Atlantic region. That will place Maryland, the Delmarva Peninsula, Virginia and North Carolina in the “warm sector” of the storm as a sultry, tropical air mass overspreads the area.

Storms firing by shortly after lunchtime along the encroaching cold front will have the propensity to rotate, particularly as they move east of the Blue Ridge and Interstate 81 corridor. They’ll make it to the Richmond to D.C. to Baltimore corridor by mid- to late afternoon.

There are signs a few storms could linger into the night.

It’s not impossible that one or two cells that form Thursday along the south coast of New England also rotate, yielding a low-end tornado or waterspout threat.