September 1, 2021

Hurricane Ida's Remnants Unleash Tornadoes, Flash Flood Emergencies In Northeast

Predictions of widespread flash flooding and an enhanced risk for tornadoes came to pass on Wednesday as the remnants of Hurricane Ida passed through the Mid-Atlantic and Northeast. Flash flood emergencies were issued across the region, including the entire New York City metro area. The system also spawned multiple strong tornadoes in Maryland and New Jersey.

This was a well-predicted event. Forecasters sounded the alarm a few days ago that this system would produce widespread and intense flash flooding in parts of the northeast, and the Storm Prediction Center had a risk for tornadoes highlighted across the affected regions for the past couple of days.

Widespread Flash Flooding

The WPC's excessive rainfall outlook for Wednesday, issued Tuesday afternoon. || SOURCE: WPC

Tropical moisture associated with the remnants of Ida flowed north on Tuesday and overran a stationary front parked over the region. This initial batch of precipitation produced drenching rains over parts of Pennsylvania, New Jersey, New York, and Connecticut, overnight Tuesday into Wednesday.

Ida's remnants then caught up with this boundary as the erstwhile tropical cyclone transitioned into a non-tropical low-pressure system.

The lift from the low and its associated fronts provided a new focal point for additional heavy showers and thunderstorms to develop on Wednesday afternoon. The intense rains continued through early Thursday morning.

These thunderstorms trained over the same areas for hours at a time, tapping into the deep reserve of tropical moisture to produce incredible, rarely seen rainfall rates for this part of the country. Central Park recorded 3.15 inches of rain in one hour between 8:51 PM and 9:51 PM.

Portions of Pennsylvania, New Jersey, and the New York City area witnessed double-digit rainfall totals by midnight on Thursday.

The torrential rains led to widespread flash flood warnings for almost the entire region, including the New York City metro area.
Many of the flash flood warnings were flash flood emergencies, enhanced wording (similar to a tornado emergency) that forecasters can use to describe widespread, life-threatening flash flooding.

This was the first time New York City had ever been included in a flash flood emergency, highlighting the high-end, historic potential of this flash flood event.

It's likely sunrise on Thursday will reveal significant damage from the flash floods across a large swath of the region.

Multiple Strong Tornadoes

The flash floods completely eclipsed the Oklahoma-style tornadoes we saw from some of the storms earlier on Wednesday afternoon.

The remnants of hurricanes are infamous for producing tornadoes as they push inland, especially systems that parallel the Appalachians after hitting the northern Gulf Coast.

Strong wind shear that's favorable for the development of tornadoes is common in the "right-front quadrant" of the storm. This is the eastern side of the storm for most systems that hit the United States.
A model-simulated sounding of the atmosphere near Wilmington, Delaware, on Wednesday afternoon. The clockwise line on the top-right graph shows strong vertical wind shear in the atmosphere, favorable for supercells that could produce tornadoes. || SOURCE: Tropical Tidbits

Winds in this part of the storm veer clockwise between the lower and mid-levels of the atmosphere, providing the strong wind shear necessary for a thunderstorm to begin rotating. As a result, we often see small supercells form in the outer bands of tropical cyclones.

The remnants of tropical cyclones (such as Ida on Wednesday) often act like a low-pressure system we'd see in the middle of the spring. Warm, humid air on the eastern side of the system provides the instability needed for thunderstorms to bubble up and thrive. These storms then tap into that rotation and go on to produce tornadoes.

We saw that situation play out as Ida passed through the region. Multiple tornadoes touched down in Virginia, Maryland, and New Jersey on Tuesday and Wednesday. 

Some of the tornadoes were downright scary, looking more like something we'd see in Oklahoma or Alabama than the Mid-Atlantic.

Annapolis, Maryland:
Mullica Hill, New Jersey:
Burlington, New Jersey:

It'll be a day or two before the National Weather Service is able to get out there and survey the damage, but it wouldn't surprise me if one or two of the tornadoes today was "significant," with a rating of EF-2 or higher.

Ida's remnants now join a long list of strong hurricanes that spawned tornado outbreaks in the days after they made landfall in the southeast.

The most infamous tropical-induced tornado outbreak in recent memory was Hurricane Ivan in 2004, which spawned more than 100 tornadoes as the system and its remnants traversed the southeast and Mid-Atlantic. One of Ivan's tornadoes produced F3 damage in Fauquier County, Virginia, just southwest of Washington, D.C.

Hurricanes Isaias, Florence, and Frances also spawned significant and memorable tornado outbreaks as they moved over the eastern states.

[Satellite Image: NOAA]

Note: I updated this post multiple times on Wednesday night to update the rainfall map and add new information.

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I graduated from the University of South Alabama in 2014 with a degree in political science and a minor in meteorology. I contribute to The Weather Network as a digital writer, and I've written for Forbes, the Washington Post's Capital Weather Gang, Popular Science, Mental Floss, and Gawker's The Vane. My latest book, The Skies Above, is now available. My first book, The Extreme Weather Survival Manual, arrived in October 2015.