Wednesday night update: An extreme flooding event is unfolding in New York City. See: Ida’s remnants trigger first ever ‘flash flood emergency’ in New York City

Original article

It’s been four days since Hurricane Ida laid siege to Louisiana and demolished parts of the Gulf Coast, the borderline Category 5 storm tying as the eighth-most intense to ever hit the Lower 48 and the strongest on record in Louisiana. Now, the remnants of Ida are moving through the eastern United States, expected to drop very heavy flooding and a few tornadoes that could bring a day of high-impact weather to the Interstate 95 corridor.

An extremely rare high risk for heavy rain and flash flooding has been declared for parts of the Northeast and Mid-Atlantic, the most severe outlook category the National Weather Service can hoist ahead of an anticipated flood event. They’re calling for “widespread and potentially life-threatening flooding,” with totals of three to eight inches likely in a broad swath hundreds of miles long.

Cities such as New York and Hartford, Conn., are included in the outlook bull’s eye, with other places including Baltimore, Philadelphia and Providence, R.I., bracing for major disruptions to travel, too. Flash-flood watches stretch from the Blue Ridge in North Carolina to Maine.

In some areas, 24-hour rainfall totals could represent 1-in-100-year events, or have just about a 1 percent chance of happening in any given year.

“Signals remain strong for potentially deadly and damaging flash flooding,” the Weather Service wrote Wednesday.

Twenty million people are in the high-risk zone for flooding, with an additional 32 million in the surrounding moderate risk area, according to Alex Lamers, a meteorologist at the Weather Service.

The Big Apple ended August with more than 10 inches of rainfall, about a half-foot more than what’s typical. Soils are saturated and can’t handle much additional rainfall, exacerbating the risk of additional serious flash flooding. Many areas just to the south of the Tri-State area are still running well ahead of normal following the passage of Henri just two weeks ago, which helped drop up to nine inches of rain in Monmouth and Middlesex counties in New Jersey.

“With recent wet antecedent conditions, excessive rain will easily become runoff and lead to the potential for flash flooding in poor drainage and low lying areas,” wrote the Weather Service in New York.

High-resolution model simulations are projecting a 75 percent chance of at least eight inches falling in south-central Pennsylvania in the vicinity of Harrisburg and just to the northeast, which will probably be among the hardest-hit areas.

That all comes beneath the core of the expected track of Ida’s waterlogged remnants; a surge of warm, moist air to the east could brew severe weather and potential tornadoes. At least one tornado struck Blacksburg in southwestern Virginia on Tuesday, prompting tornado sirens to blare on the campus of Virginia Tech.

Heavy rain so far

Since Tuesday, widespread rain totals of several inches were reported in western North Carolina, the Appalachian Foothills of Georgia and the Cumberland Plateau of East Tennessee on Tuesday. Areas just northwest of Washington, D.C., in Montgomery County, Md., and Fairfax County, Va., also received up to three inches from overnight thunderstorms associated with Ida.

The heaviest amounts have focused in south central Pennsylvania, where up to 4 to 6 inches have fallen through Wednesday afternoon.

Ida’s remnants have been acquiring mid-latitude characteristics, meaning the storm is no longer a perfectly blended swirl of tropical air and uniform spin. Instead, an insurgence of cool, dry air on the backside of Ida has helped develop fronts. It’s along a stalled cold front draped along the Appalachians that Ida will unleash most of its moisture.

A driver rode through standing water caused by remnants of Hurricane Ida in D.C.'s Rock Creek Park on Sept. 1. (Jim Pekar)

Rainfall rates of more than an inch per hour could accompany downpours that will increase in areal coverage and intensity throughout the day. Some places may see totals approaching two inches in an hour or less, increasing the risk of drainage systems becoming overwhelmed — especially in major urban areas.

Flash flooding is likely and could be widespread and may coincide with the first half of the overnight, making it extremely difficult to see flooded areas. It’s worth noting that “high-risk” flooding days only occur about 4 percent of the time but account for 40 percent of all U.S. flood-related fatalities.

Some rivers are predicted to rise to historically high levels in parts of southern Pennsylvania due to the rainfall.

The timeline

Heavy rain was already falling across most of Pennsylvania on Wednesday into Wednesday afternoon, prompting flash-flood warnings from near Altoona and State College to Scranton. A rare flash flood emergency, the most dire type of flood alert, was issued early Wednesday afternoon in Wilmore, Pa., about 16.5 miles northeast of Johnstown, until 7 p.m. due to “uncontrolled release from the Wilmore Dam.” Residents downstream of the dam were being evacuated, according to WJAC, the NBC affiliate out of Johnstown.

The responsible rainfall will shift northeast with time, orienting itself from northern Pennsylvania through the Hudson Valley to roughly the Connecticut-Massachusetts border by midday.

Around the same time, an additional batch of more cellular rainfall, or a conglomerate of individual thunderstorms and downpours, will begin to cook up along the cold front near Interstate 81 in Virginia. Those cells will present the risk of very heavy rainfall and tornadoes.

A damaging tornado tracked from around Edgewater to Annapolis, Md., Monday afternoon.

That second round will become dominant into the afternoon, eventually merging and tracking up the Interstate 95 corridor during the evening hours. New York City could see a window of torrential tropical downpours between 8 p.m. Wednesday and 2 a.m. Thursday, with up to five inches or more falling during that span of time.

After 8 p.m., Long Island, southern Connecticut and parts of Rhode Island and southeast Massachusetts will be in the midst of a deluge. Boston will probably see moderate to heavy rain, but the jackpot totals might remain just south of the city over Plymouth, Bristol and Norfolk counties.

The rainfall will finally push offshore of southeastern New England by about 9 a.m. Thursday. Much cooler, drier and more refreshing air will arrive in the wake of Ida’s tropical remnants.

Predicted rainfall totals

A broad 3 to 5 inches of rain is expected north of the Mason-Dixon Line over the eastern two-thirds of Pennsylvania, southeastern New York State, northern New Jersey and most of southern New England. A few spots in southern Pennsylvania might exceed eight inches, while half-foot totals can’t be ruled out from New York City eastward to roughly Providence.

Boston proper will probably end up with 3 or 4 inches; flash flooding is likely there, as well as in the other major metropolitan areas of southern New England and the Tri-State.

It’s not out of the question that a few communities somewhere in the Northeast see double-digit rain totals, the result of extremely high rainfall rates contained within storms that may train over the same area.

Rain totals are more uncertain in Baltimore, Washington and Philadelphia, where thunderstorms will be more sporadic, but a greater tornado risk exists there. They’re under a level 3 out of 5 “enhanced risk” for tornadoes.

Behind the rain, drier air will bring more tranquil conditions for the weekend.