An unusual high-risk area for extreme rainfall and flash flooding covered much of the Northeast U.S. on Wednesday as the remnants of Hurricane Ida joined forces with a frontal zone. Ida was declared post-tropical as of 11 a.m. EDT Wednesday, September 1, but the remnant surface low is actually expected to strengthen as it moves from western Virginia to the Northeast coast by Wednesday night. Ex-Ida will merge with a stationary front beneath a strong jet stream, a set of factors that will lead to widespread dangerous weather from Wednesday into early Thursday, including torrential downpours and the chance of tornadic supercells.

At midday Wednesday, heavy rains had already engulfed much of Pennsylvania, northern Virginia and West Virginia, and western Maryland, and rains were intensifying across southeast New York and southern New England. Flash flood warnings covered much of southern Pennsylvania by early afternoon, with 2-to-3-inch rainfall totals already observed. A flash flood emergency was issued downstream from Wilmore Dam on the North Fork of the Little Conemaugh River, including the town of Wilmore, as water was flowing uncontrolled from the dam.

Many parts of the Northeast were soaked by tropical cyclones Fred and Henri over the last several weeks, heightening the risk of flash flooding with ex-Ida. In New York City, Central Park recorded 24.03 inches of rain from June through August, making it the city’s second wettest meteorological summer in 153 years of recordkeeping; the 9.06 inches from August 16 to 30 was Central Park’s second-largest total on record for the last half of August. Widespread urban flooding is possible across the New York metro area on Wednesday night. As winds increase from New York across southern New England toward Thursday, the soggy soil may allow trees to be uprooted, leading to downed power lines and power outages.

A broad band of 3-to-8-inch rainfall is expected from northernmost Maryland to southern New England through early Thursday. Some localized totals could exceed 8 inches, and it’s not out of the question that a few spots will set all-time records for 24-hour totals that might be expected only once in a century or more.

A high risk for excessive rainfall leading to flash flooding was in effect for much of the Northeast U.S., including the New York City area, through Thursday morning, September 2, 2021. (Image credit: NOAA/NWS/WPC)

“Signals remain strong for potentially deadly and damaging flash flooding from parts of the Mid-Atlantic region across southern New York into the southern portion of New England today and tonight … especially across highly urbanized metropolitan areas and areas of steep terrain,” warned the NWS Weather Prediction Center.

Tornadoes may pepper the mid-Atlantic

A warm, moist, unstable air mass will be streaming from the Atlantic toward ex-Ida, setting the stage for intense rainbands and a few supercell thunderstorms that could spawn tornadoes Wednesday afternoon and evening. Such twisters associated with hurricane remnants are often short-lived but can still be destructive. The area most at risk lies east of the Appalachians, just to the south of the stationary front and the heaviest rainfall. The NWS Storm Prediction Center placed a wedge of the mid-Atlantic, including Washington, D.C. and Philadelphia, in an enhanced risk of severe weather, the third-highest of the center’s five risk categories. One tornado was captured on video approaching Annapolis, Maryland, where damage has been reported.

A heat advisory for a powerless New Orleans

Power was restored to a small portion of New Orleans overnight, but almost 1 million customers in Louisiana remained without power on Wednesday, according to poweroutage.us. Temperatures are expected to climb into the 90s with a heat index of 108 degrees Fahrenheit on Wednesday and 106 on Thursday, and a heat advisory has been issued for the city. The lack of air conditioning in the regions that lost power will be hazardous for many. Louisiana’s largest electricity provider, Entergy, said some customers would be without power for over three weeks.

New Orleans airport and Mississippi River remain closed to travel

The Louis Armstrong New Orleans International Airport remained closed and running on backup generators Wednesday and was struggling with low water pressure. However, no significant damage to airport buildings, runways, or taxiways was reported.

A 300-mile stretch of the Mississippi River, from the Gulf of Mexico to Baton Rouge, remains closed to all shipping traffic, with no word on when it might reopen. The Port of New Orleans reports no major damage to its facilities but has no power. The Port of South Louisiana, the world’s largest bulk cargo port, has yet to complete a damage assessment, but likely suffered heavy damage. Ida pounded the port as a category 3 hurricane with 115-120 mph winds.

Waterford 3 nuclear generating station back up

Some very good Hurricane Ida news: the Waterford 3 nuclear generating station, which suffered minor damage to some of its non-nuclear buildings when Ida was pounding it as a category 3 hurricane with 115-120 mph winds, is back up after being on backup generators for just over two days. The plant’s survival of this extreme event was not without incident, though: The reactor got too hot after it switched over to backup generators, and an emergency feedwater pump had to be started. Fortunately, the grounds of the plant were not flooded by Ida’s storm surge. The plant is vulnerable to the storm surge from a major hurricane, as detailed in our August 28 post.

Visible satellite image of Tropical Storm Larry southwest of the Cabo Verde Islands (upper right) at 1605Z (12:05 p.m. EDT) Wednesday, September 1, 2021. (Image credit: tropicaltidbits.com)

Tropical Storm Larry expected to be a long-lived, powerful hurricane

A disturbance that moved off Africa late Monday wasted no time in developing into Tropical Depression 12 on Tuesday and Tropical Storm Larry early Wednesday. Already packing sustained winds of 50 mph as of 2 p.m. EDT Wednesday, Larry was racing westward at 22 mph across the far eastern tropical Atlantic about 300 miles southwest of the Cabo Verde Islands.

Larry was already a well-structured tropical storm at midday Wednesday, with impressive upper-level outflow and intense showers and thunderstorms spiraling around the low-level center and hints of a large eye already taking shape. The 06Z Wednesday runs of the high-resolution HMON and HWRF models both depict Larry becoming a hurricane by Friday, and the 12Z Wednesday SHIPS statistical model gives Larry a 29% chance of rapid strengthening through Thursday (25 knots in 24 hours). Given Larry’s breakneck growth on satellite and the favorable setting, the National Hurricane Center (NHC) forecast issued Wednesday morning was on the bullish side, making Larry a hurricane by Thursday and a major category 3 hurricane with 120-mph winds by Saturday.

Larry will be steered mainly by the Azores-Bermuda High until the storm moves into a weakness in the ridge and angles northwest later in the week. For the time being, the long-range Euro and GFS ensembles are emphatic on an overall recurvature to the north and northeast away from North America, as a strong upper-level trough is likely to set up on the U.S. East Coast next week. The ensembles have been shifting westward over time, and it’s possible that Larry will angle far enough northwest to affect Bermuda just before its expected recurvature.

If nothing else, we can expect Larry to churn for days as a strong hurricane in the open Atlantic, generating large swells that will bring high surf and the risk of rip currents to the U.S. East Coast and the Canadian Maritime Provinces next week. Larry’s expected longevity will also bulk up this season’s total accumulated cyclone energy.

Larry’s formation date of September 1 is very early for the appearance of the season’s 12th named storm; during the 1991-2020 period, the average arrival date of the season’s 12th storm was October 10. The record earliest appearance of the season’s 12th storm was on August 21, 2020 (Laura). According to Phil Klotzbach of Colorado State University, only five other seasons since accurate satellite records began in 1966 have had as many as 12 named storms by August 30: 2020, 2012, 2011, 2005, and 1995.

GeoColor visible satellite image of 91L in the southwest Caribbean at 1635Z (12:35 p.m. EDT) Wednesday, September 1, 2021. (Image credit: tropicaltidbits.com)

Disturbance 91L brings heavy rains to Central America

In the southwest Caribbean, an area of disturbed weather designated 91L was bringing heavy rains to northeast Nicaragua and northeast Honduras on Wednesday afternoon. Satellite imagery showed that 91L had a modest area of heavy thunderstorms that were poorly organized. Dry air and strong wind shear of 20-30 knots, especially on the north side of 91L, were inhibiting development.

91L was moving west-northwest at about 5-10 mph, and close proximity to and/or landfall over Nicaragua/Honduras will inhibit development over the next few days. The disturbance is predicted to cross the Yucatan Peninsula and enter the southwestern Gulf of Mexico by Sunday, when 91L will have the opportunity for development over the Gulf’s warm waters. 91L has only lukewarm model support for development, and in its 2 p.m. EDT Wednesday Tropical Weather Outlook, the NHC gave the system 2-day and 5-day odds of development of 30%. A hurricane hunter aircraft is on call to investigate 91L on Thursday afternoon, if necessary.

Tropical Depression Kate weakening

Tropical Depression Kate, located in the central Atlantic far from any land areas, is struggling to survive as it heads north-northwest at 12 mph. High wind shear and dry air are expected to reduce Kate to a remnant low by Thursday night.

How to help hurricane recovery efforts

The Partnership for Inclusive Disaster Strategies (formerly portlight.org), co-founded by members of the Weather Underground community, is responding to the Hurricane Ida disaster. Also, the Weather Channel has put together a list of many excellent charities that will be active in Hurricane Ida recovery.

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Bob Henson

Bob Henson is a meteorologist and journalist based in Boulder, Colorado. He has written on weather and climate for the National Center for Atmospheric Research, Weather Underground, and many freelance...

Jeff Masters

Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...