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NYC Extreme Flash Flooding Event: Remnants of Ida – Sept 1, 2021

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It’s been a long hiatus for me posting content to this blog. Been a bit preoccupied with everyday life, but getting back into the swing of things with a detailed meteorological summary of the tragic and deadly extreme flash flooding event that took place on the evening of Sept 1, 2021. Credit for this goes in part to my friend and colleague Phil Lutzak, a fellow graduate of the PSU World Campus Weather Forecasting Certificate Program.

Synopsis

In this overview of the NYC flooding event of September 1st, 2021, the main goal is to ascertain why such severe flooding occurred in light of the preceding and concurrent weather conditions for that day. While it was quite well forecast by the NHC and NWS office forecasts in terms of a significant flood threat, clearly no national office could pinpoint the areas that would suffer the worst flooding until the day of the event when it was much clearer where the heaviest rains would be falling, nor could they possibly have gotten the exact severity correct given current forecasting capabilities. In hindsight we will examine the existing conditions during and just before the most intense period of the flooding and try to determine which developing synoptic and mesoscale parameters gave the most clues as to why much of the worst flooding conditions would be directly within the New York City metropolitan area.  

Forecasts Leading Up to the Event

There was a misconception after the event that the meteorological community failed to forecast the possibility of this event occurring. In fact, National Weather Service Weather Prediction Center had a solid grasp of the risks associated with the post-tropical remnants of Ida, having issued a high risk for flash flooding for areas of the Mid-Atlantic and Southern New England in the early morning hours of Wednesday, 01 Sept 2021 (4:20 AM EDT). 

The forecast discussion behind this excessive rainfall outlook was chilling in its prescience, specifically citing the possibility of deadly flash flooding and 3-8” rainfall totals. 

At 3:22 PM EDT, WPC issued Mesoscale Precipitation Discussion #0925, with the accompanying image:

In MPD #0925, WPC forecasters referenced 2-3+” per hour rainfall rates being possible due to a combination of factors, most notably very strong low-mid level forcing via frontogenetical influences and highly efficient, deep layer warm/tropical rain processes – a characteristic of Ida’s hybrid tropical/extratropical nature. 

The importance of Ida’s unique blend of tropical transitioning to extratropical character is also underscored in MPD #0923, in which WPC forecasters discuss the presence of a developing south-southwesterly low-level jet. This feature would reach peak strengths of 35-55+ knots at the height of the event, and became a critical ingredient in creating the exceptionally efficient moisture transport into the NYC area, shown in the following sections.  

By 9:28 PM that evening, the worst-case scenario envisaged by forecasters leading up to the event came to fruition and the NWS OKX office issued its first ever Flash Flood Emergency for NYC, containing the emphatic “THIS IS A PARTICULARLY DANGEROUS SITUATION” wording…

Meteorological History

Surface Conditions 

The following chart shows the radar representation and surface charts of the 48-hour progress of tropical depression Ida from the lower Mississippi valley into the Tennessee and Ohio River valleys and then into the mid-Atlantic and northeast regions. In the middle image it is clear that the heaviest precipitation was reduced as its inflow was becoming detached from the very moist Gulf of Mexico. The third, rightmost image shows how the tropical low, now merging with the warm front, was elongating north to south but at the same time tapping into a very moist airflow from the Atlantic Ocean, strongly enhancing the area of heaviest precipitation near the center of lowest pressure.     

Low and Mid-Level Conditions

While the high precipitation producing ability of tropical depression Ida was still in evidence, the position of its southerly flow in relation to the fairly stationary warm front allowed for maximum isentropic lift to focus right over NYC and surrounding areas, with much of the convection moving in from the south still having high precipitation producing energy powered by high theta-e tropical inflow. 

Here’s the MUCAPE analysis from 0200 UTC showing 1,000-2,000 J/kg positioned over NYC,

… and the following showing the extremely high 850 mb level moisture transport over the NYC area at 0200 UTC.

The 925-850 mb frontogenesis chart shows clearly where maximum frontogenesis was occurring

and the surface moisture convergence and DMC charts show, not surprisingly, that a maximum for surface and deep moist convergence was occurring over NYC and surrounding areas.

Mid to Upper Levels

This situation was one of the clearest examples of how baroclinic forcing can convert a weak or decaying tropical cyclone into an extremely prolific rainmaker. The 500 and 300 mb conditions were critical in allowing this approaching tropical low to make a fairly quick transition to a strong extratropical low. The result was the previously discussed extremely high moisture convergence right over the city. 

The 300 mb chart from 8PM EDT shows the right rear entrance region of a quite strong jet streak for late summer (100-120 knots) moving into position almost directly over the low which was just southeast of the NYC area, allowing for maximum divergence above the low center as it was merging with the warm/stationary front. This allowed for very high divergence to position over an equally strong low level convergence zone. 

In addition, the 500 mb analysis shows an ideal placement of the area just east of the trough base over the lower-level low circulation to allow strong upper-level divergence, resulting in intensification of the transitioning surface low.

All levels

For one final view that ties activity at the previous levels together, here’s the ARL simulated skewT diagrams for 0100 and 0200 UTC at New York City that evening. Clearly the atmosphere was saturated throughout with very high theta-e values at the lowest levels. 

Conclusion

From the NY Times, September 2nd, 2021: 

The rain broke records set just 11 days before by Tropical Storm Henri, underscoring warnings from climate scientists of a new normal on a warmed planet: Hotter air holds more water and allows storms to gather strength more quickly and grow ever larger. 

In New York City, the dead ranged in age from a 2-year-old boy to an 86-year-old woman, the police said. Some drowned in basement apartments in Queens, where a system of makeshift and mostly illegally converted living spaces has sprung up. 

Below are two dramatic images of the flooding in NYC. On the left is severe subway flooding that occurred in lower Manhattan (courtesy NY times). At right are multiple stranded vehicles on the Major Deegan Expressway in the Bronx borough of NYC (courtesy Craig Ruttle/AP).

It’s likely not possible at this time, nor in the near future, to use RAP or any of other quickest analytical tools to anticipate imminent flooding at the smallest mesoscale any more than we are currently able to. At this stage of the science, the most skilled forecasters use the best modeling tools currently available along with their own invaluable experience in watching unfolding precipitation patterns on radar loops and, combined with mesoscale graphics such as we have displayed here, anticipate where the heaviest precip is moving and whether it will intensify, weaken or remain stable.      

In light of this, a critical takeaway is to encourage better education of the public about what an “Emergency” and what a “Particularly Dangerous Situation” mean in practical terms. In other words, when the public receives an EAS alert with these terms in it, they need to be acutely aware of what preventive actions to take immediately rather than just dismiss this as another alert that probably won’t affect them. One other challenge with this system is that these alerts are often only translated to only one or two other languages. This is a problem for a city with hundreds of thousands of immigrants speaking myriad languages, many of whom are not literate in English, as this renders these vital, lifesaving alerts essentially useless to a wide range of the public, with predictably tragic consequences. 

NYC Weather Update – Jul 20, 2021

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A brief warmup Tuesday as highs rise into the upper-80s to near 90ºF before a cold front swings through Wednesday bringing showers and thunderstorms. Seasonable and drier conditions behind the front in the mid-80s to end off the week with plenty of sun.

Rest of today – mostly sunny with high pressure briefly building and temperatures rising to 90ºF. Overnight lows dropping into the low-70s.

Weather Prediction Center surface forecast for 8 AM Wednesday, a cold front will be approaching from the north and west.

Wednesday – partly cloud with a cold front crossing the region during the day. High temperatures in the mid to upper 80s. Showers and thunderstorms possible in the evening. Overnight lows in the upper-60s.

Thursday – drier, cooler, with high temperatures in the mid-80s and mostly sunny skies. Overnight lows in the upper-60s.

GFS 500 mb temperature, height, and wind for Friday at 8 AM. Heights will drop around a closed low centered over the Canadian Maritimes, leading to seasonable temperatures. Some of the energy rotating around this low could bring scattered showers and thunderstorms to the area.

Friday – high temperatures again in the mid-80s with partly cloudy skies and chance of showers and thunderstorms. Overnight lows in the upper-60s.

NYC Weather Update – June 14, 2021

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A wet, cool start to the week with an early round of thunderstorms that moved through this morning. Another is likely to follow early in the overnight tonight. Lingering showers Tuesday with a weak surface low in the vicinity. Highs below normal in mid-70s Monday, then warming up to average by midweek with sunny weather returning. The warm up continues into the weekend – Father’s Day looking good weather-wise so far!

Rest of today – high temperatures in the mid-70s with mostly cloudy skies. A second frontal boundary will push towards the area later this evening, with showers and thunderstorms again possible into the early overnight hours. Temperatures falling into the mid-60s.

HRRR simulated 1-kim radar valid for 9:30 PM tonight. An area of moderate to heavy rain accompanying some thunderstorms is depicted.

Tuesday – mostly cloudy still with a chance for showers early. High temperature near 80ºF. Overnight lows in the low-60s with clearing skies.

Wednesday – high pressure builds in behind what should be a dry cold frontal passage and resulting in a mostly sunny day with high temperatures around 80ºF. Overnight lows in the upper-50s given that the high pressure will usher in cooler, drier Canadian air.

GFS 500 mb temperature, height, and wind for Thursday at 8 PM. Heights will be building with a longwave trough exiting to our east. The result will be pleasant, sunny weather.

Thursday – mostly sunny with high temperatures in the upper-70s. Pleasant conditions with the high pressure above still in control. Overnight lows in the low-60s.

NYC Weather Update – June 8, 2021

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Upper-80s heat and humidity will spawn scattered heavy rain and thunderstorms over the next couple of days. Low vertical wind shear and weak overall steering currents mean the primary threat from these storms will be possible localized flooding. A backdoor cold front drops in from Canada Wednesday night leading to much cooler temperatures Thursday in the low-80s. Rain and even cooler temperatures arrive Fri when we may only just hit 70ºF.

Rest of today – partly sunny with high temperatures in the upper-80s, and a muggy feel to the day. Scattered thunderstorms bring the potential for heavy downpours later in the evening, perhaps starting around 7PM. Overnight lows in the low-70s as showers roll through early.

HRRR simulated 1-kim radar valid for 8 PM tonight. An area of moderate to heavy rain accompanying some thunderstorms is depicted.

Wednesday – high temperatures again in the upper-80s. Mostly cloudy with yet another shot for some heavy downpours especially later in the afternoon as a cold front draws nearer. Overnight lows around 70ºF as a backdoor cold front eventually pushes through.

Thursday – with the backdoor cold front already south of us by Tuesday morning, high temperatures should cool off to around 80ºF. Chance for afternoon showers and thunderstorms. Overnight lows around 60ºF.

Weather Prediction Center surface forecast for valid 12 AM Thursday. A backdoor cold front is forecast to drop in from the north and east. The result will be a much cooler air mass than we’ve seen the beginning of this week.

Friday – much cooler with persistent clouds and showers possible through the day, high temperatures perhaps only around 70ºF. Overnight lows around 60ºF with rain chances continuing.

NYC Weekend Weather – June 4, 2021

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Muggy with thunderstorms rolling through later this afternoon and highs in the upper-70s. The story this weekend is about building heat and humidity. A Bermuda high becomes the dominant feature bringing an extended period of 90ºF+ heat into next week. While this high is anchored in the western Atlantic, we won’t see any appreciable chances for rain at least until later in the week next week.

Rest of today – mostly cloudy with high temperatures in the upper-70s. HRRR simulated radar runs this morning consistently signalling the arrival of a line of thunderstorms around 3-4PM this afternoon. Behind these storms, temperatures overnight expected to be in the mid-60s.

HRRR rapid refresh simulated 1-km radar valid 3:30 PM later today showing a cluster/line of thunderstorms moving through the NYC area.

Saturday – sunny with high temperatures much warmer in the upper-80s. Overnight lows in the low-70s. Humidity will be building as well due to the influence of southwesterly/westerly flow bringing in a warmer and moisture-laden air mass.

Sunday – high temperatures around 90ºF with mostly sunny skies. Overnight lows in the low-70s.

GFS model 1000-500 mb thickness and surface conditions valid 8 PM Sunday. You can see a sprawling high pressure center in the Atlantic. Clockwise anticyclonic flow from the south/southwesterly flow around the periphery of this high will advect hot, humid air towards our area.

Monday – with a Bermuda high still in place, expect high temperatures to once again reach around 90ºF with higher heat index values due to the humidity. Overnight lows in the low-70s.

NYC Weather Update – May 24, 2021

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Welcome relief from the heat today with high temperatures below normal in the 60s as cool, onshore flow sets up. The warmth returns by mid-week with highs again touching 90ºF by Wednesday. A strong thunderstorm is possible with a cold front passing through that day. Afterwards, temperatures cool but still reach above average highs in the low-80s on Thursday.

Rest of today – partly sunny with gradually increasing sun, high temperatures in the mid-60s with easterly and northeasterly winds shifting towards the southeast. Overnight lows in the mid-50s.

Weather Prediction Center surface analysis for valid 5AM Monday. Overnight, a cold front pushed southwest of the area, allowing for a much cooler air mass to take hold, with easterly onshore flow off the still as yet cold waters of the Atlantic.

Tuesday – warmer with high temperatures in the low-70s and partly sunny skies. Overnight lows in the mid-60s.

Wednesday – a warm front will pass through and put us in the warm sector of a strong low centered well to our north over Canada. Temperatures are expected to reach around the 90ºF in the city. Later in the day, as a cold front approaches, scattered but possibly strong thunderstorms could roll through. Overnight lows in the upper-60s.

GFS 850 mb temperature anomalies for Wednesday at 5PM. We are forecast to see +7ºC anomalies at this level of the atmosphere, corresponding to much above normal temperatures at the surface.

Thursday – skies clear out with high temperatures in the low-80s. Overnight lows around 60ºF.

Subtropical Storm Ana

Over the weekend, the first named storm of the 2021 Atlantic hurricane season, Subtropical Storm Ana, formed over the open waters of the Atlantic northeast of Bermuda. While the storm itself was never a threat to land, it does mark the seventh year in a row that a named storm formed ahead of the formal start of the hurricane season.

GOES satellite imagery showing the circulation of Subtropical Storm Ana

We can’t draw any conclusions from a single storm, but it is worth noting that nearly all major forecast sources including NOAA are calling for a season of above average hurricane activity.

NYC Weather Update – Oct 6, 2020

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This week, we see slightly above normal temperatures in the low-70s to start off, dropping to below normal temperatures in low-60s later in the week following the passage of a cold front mid-week. Overall sunny conditions punctuated by this frontal boundary tomorrow. Gusty winds and a stray thunderstorm could accompany this front as its parent low deepens over the Canadian Maritimes.

Rest of today – mostly sunny with high pressure exiting to our south, yielding light southwesterly flow and high temperatures in the low-70s. Overnight lows in the upper-50s.

Weather Prediction Center surface forecast for 8AM Wednesday

Wednesday – slightly warmer with warm southwesterly flow ahead of the cold front, highs in the mid-70s. Overnight lows in the mid-50s behind this front, which could bring isolated showers and thunderstorms to the area. Gusty winds could also accompany this front due to the large pressure gradient between us and its parent low moving over the Canadian Maritimes.

GFS model surface pressure and 10 meter above ground winds for Wednesday at 8PM

Thursday – Canadian high pressure sweeps in behind the cold front bringing sunny skies and cooler temperatures in mid-60s. Overnight lows much cooler, in the upper-40s due to this change in air masses.

Friday – with a cold start, high temperatures will likely top out in the mid-60s again under sunny skies. Overnight lows warm up into the low-50s.

NYC Weather Update – Sept 29, 2020

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Apologies for the lack of updates lately. Have had a few things going on keeping me busy. We have a wet beginning to October on the way with tropical moisture accompanying a slow moving cold front. Heavy rain is possible overnight Tuesday, with isolated flash flooding a concern. Temperatures should be around normal (low-70s) until this weekend when a reinforcing cold front brings us down into the mid-60s.

Rest of today – cloudy with chances for rain increasing later in the day as a cold front approaches from the west. High temperatures in the mid-70s. This latter frontal boundary may also spark some showers overnight, with temperatures dropping into the mid-60s. A strong low level jet will be in place overnight, enabling efficient transport of tropical moisture from the south. This will lead to the potential for rounds of moderate to heavy rain, where some training is possible as well. Flash flooding is a concern as places could see 1-2″ of total rainfall overnight. Winds could also be gusty in some of the heavier downpours, which could also produce some thunder. Temperatures generally in the mid-60s.

  • Weather Prediction Center surface forecast for 2AM Wednesday
  • GFS model output for precipitable water valid 2AM Wednesday, showing the potential for heavy rain.

Wednesday – conditions should improve during the day with showers diminishing in the morning. Mostly cloudy with highs in the low-70s. Subsidence (sinking air) with westerly winds could produce gusty winds. Overnight lows in the upper-50s with mostly clear skies.

Thursday – partly sunny with highs in the low-70s. Overnight lows in the mid-50s.

Friday – partly sunny with highs in the mid-60s. Chance for rain with another cold front forecast to move through. Overnight lows in the low-50s as temperatures start to cool off behind this frontal passage.

Looking ahead, behind this cold front, a period of below normal temperatures is forecast to hit much of the Eastern US next week.

NYC Weather Update – Aug 17, 2020

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This week we will see a possibility of an extended period of cloudy weather, punctuated by periodic storms. This regime will bring cooler than normal temperatures mostly in the low-80s. Good news: Labor Day weekend looks to be dry and cool so far! Looking to the tropics, we’re about to enter the peak of hurricane season, and it’s no surprise in this very active season to date to see that National Hurricane Center is monitoring four disturbances that could become tropical storms.

Rest of today – cool, generally cloudy with possible peaks of sun, high temperatures in the upper-70s. Temperatures will be cool due to a persistent onshore flow with an area of high pressure over the Canadian Maritimes and a slow-moving warm front/stationary front to our south. This latter frontal boundary may also spark some showers overnight, with temperatures dropping into the mid-60s.

Weather Prediction Center surface forecast for 8AM Tuesday

Tuesday – the cool, cloudy trend continues with high temperatures again topping out in the upper-70s. Showers may occur periodically during the day. Overnight lows in the upper-60s with chances of showers continuing.

Wednesday – the cloudy, rainy weather continues, though temperatures may be a touch warmer around 80ºF. Chance for showers and thunderstorms. Overnight lows in the low-70s as the warm front finally pushes through.

Thursday – cloudy, with a chance for showers, and warmer temperatures in the mid-80s. Overnight lows in the mid-70s. Shower chances persist overnight.

Watching the tropics

Keeping eyes on the tropics, given that we’re about to enter the peak of the season (September) when oceanic heat content tends to be the highest, providing the greatest potential fuel for storms. There are four disturbances in the Atlantic that could become tropical systems, however, none of them are an immediate threat to the US mainland.

National Hurricane Center 5 day graphical tropical weather outlook.

NYC Weekend Weather – Aug 28, 2020

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Moisture associated with the remnants of Laura are set to bring soaking rain starting the weekend. A cold front eventually clears things out by Sunday, which will bring a markedly drier air mass into the region. Temperatures during this period will be generally cooler than at most points this week, in the low-80s. Next week, we could see a return to hotter weather towards mid-week.

Rest of today – increasing clouds with high temperatures in the upper-80s. Chances for rain and thunderstorms later in the day along a warm front that’s forecast to lift through during the day. Overnight lows in the low-70s with rain chances continuing.

Weather Prediction Center surface forecast for 8AM Saturday. By this time, a warm front will have lifted through and the remnants of Laura will be contributing moisture to rains over the region.

Saturday – Laura, which wrought serious devastation over the Gulf Coast, will make itself felt on Saturday, contributing tropical moisture to rain that will impact the area. This moisture will contribute to very high precipitable water content > 2″, which could lead to localized flooding. Mostly cloudy with rain possible throughout the day. Stronger storms are possible in the afternoon. High temperatures in the low-80s. Overnight lows in the upper-60s with rain chances diminishing as a cold front sweeps through.

  • GFS model precipitable water content valid 8AM Saturday. Very moist values of > 2.00″ are possible.
  • Weather Prediction Center’s analysis of possible rainfall totals Saturday. Parts of the region could see easily see 0.75-1.00″.

Sunday – by the morning, the trailing cold front associated with the primary low bringing this rain should’ve already pushed through. This will lead to a much drier, cooler day, with high temperatures peaking around 80ºF and plenty of sun. Winds may be gusty at times from the northwest. Overnight lows in the low-60s due to good radiational cooling with clear skies and winds calming.

Monday – even cooler than Sunday with high temperatures in the upper-70s, and sunny skies. Overnight lows in the mid-60s.