Category Archives: Tropical/Hurricanes

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 – 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 – Dec 14, 2020

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A quick hitting storm passes south of the city today bringing some rain and mixed precipitation, with no accumulating snow expected. Colder temperatures then take hold in the mid-30s behind this storm Tuesday as a strong area of high pressure builds over Quebec. This sets the stage for a potentially significant snowstorm developing Wednesday into Thursday with 8″+ totals possible along the I-95 corridor (with higher amounts north and west). However, there is still a good deal of uncertainty with the eventual track of this storm that could result in large differences in outcomes.

Rest of today – temperatures dropping through the low-40s into the 30s with mostly cloudy skies and a steady, chilly rain, possibly mixing with some wintry precipitation. No accumulation is expected. Overnight lows around the freezing mark with precipitation ending as high pressure begins building in.

Weather Prediction Center surface forecast for 1PM Monday. An area of low pressure is seen exiting the east coast, bringing us a period of rain and possible mixed precipitation, with snow further inland.

Tuesday – proverbial calm before the storm as high pressure dominates, giving us a mostly sunny day with high temperatures around 40ºF. Overnight lows again in the upper-20s with cold northerly winds flowing around the east side of this high.

Wednesday – high pressure yields to a developing coastal storm. Partly sunny to start with increasing clouds. High temperatures in the mid-30s. Overnight lows drop into the mid-20s. The big story is the potential for a significant snowstorm with accumulations possibly over 8″. GFS and the Euro model have come into agreement for much higher totals, 14-15″. However, NAM shows a warmer scenario with warmer air working further north, thus cutting into overall totals (only 7″). It’s also possible that the low tracks close enough to shore that we end up with a good deal of rain mixing in at the coast, cutting snow totals even further.

Part of the energy fueling this storm is coming all the way from a shortwave with origins over the Pacific Ocean. This has only just made landfall in the US today. As this energy gets sampled by upstream soundings (weather balloon launches) and other means, the forecast models should get a better sense for how this storm will evolve. At this time, there’s enough uncertainty in the forecast to still not buy into any specific totals. If you look below, you can see there’s a lot of spread in the models in terms of the track of this storm. This then translates to a wide range in ensemble forecast totals. In addition, with a coastal storm like this, where there’s a lack of deep, Arctic cold air ahead of the storm, there’s a sharp gradient between areas where the event would be all snow vs. a mixed event. If you see the 24 hour probabilities of > 8″ accumulation, you can see that a 50 mile shift in the storm track further south and east would give us much higher chances for big snowfall totals, while a shift the other way could result in a bust on snow totals. Suffice to say, this storm will bear close monitoring today and tomorrow as more data comes in and models have a chance to digest it.

  • ECMWF “Euro” model surface pressure, precipitation and type for 1AM Thursday
  • ECMWF total snow accumulation at 7AM Thursday, showing 14″ of snow in NYC.
  • GFS model surface pressure, precipitation and type for 1AM Thursday
  • GFS total snow accumulation at 7AM Thursday, showing 15″ of snow in NYC.
  • NAM model surface pressure, precipitation and type for 1AM Thursday
  • NAM total snow accumulation at 7AM Thursday, showing 7″ of snow in NYC.
  • Weather Prediction Center probabilities of > 8″ total accumulation in 24 hours leading up to 7AM Thursday
  • Weather Prediction Center storm track forecasts, note the cluster of blue crosses associated with a surface low near the Delmarva Peninsula. This is the storm that could bring us snow.
  • GEFs ensemble model plumes showing a huge range of possible snowfall totals at LGA during this storm. The mean is 7.83″.

Thursday – depending on the speed and track of the storm, we could see precipitation lingering into the morning hours, with additional accumulation possible. Highs should hover around the freezing mark. Overnight lows drop into the mid-20s with reinforcing cold air advection on the backside of the departing storm.

NYC Weather Update – Dec 7, 2020

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A week of contrasts is in store for us with colder than average temps in the low-40s to start things off. However, a warming trend will take hold as a more zonal flow evolves at the upper levels, moderating heights and temperatures. This will result in milder conditions later on in the low-50s.High pressure and dry northerly flow should keep things rain-free until closer to the weekend.

Rest of today – below normal high temperatures around 40ºF with increasing clouds as a low pressure center passes well off to our southeast over the Atlantic. The presence of this storm offshore will result in persistent northerly winds, though less potent than over the weekend. Overnight lows in the low-30s.

Weather Prediction Center surface forecast for 7PM Monday. Note the presence of a low pressure center east of North Carolina.

Tuesday – high pressure briefly builds in and provides a mostly sunny day but with temperatures still only peaking around 40ºF. Overnight lows again in the low-30s.

Wednesday – high temperature start to rebound towards normal in the mid-40s with partly sunny skies as a storm passes well to our north, and we end up in the storm’s warm sector. Overnight lows drop into the upper-30s.

GFS model 500 mb temperature, height and winds forecast for 1PM Thursday. Flow at this level is forecast to become more zonal (flatter) than earlier this week.

Thursday – temperatures jump into the low-50s with sunny skies as high pressure briefly moves in again and upper level flow starts to flatten out resulting in the colder air mass in place to start the week getting pushed north. Overnight lows in upper-30s.

NYC Weather Update – Nov 16, 2020

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What a big contrast in sensible weather this week vs. last! We were looking at record-breaking warmth in the upper-70s this time last week. Now, in the wake of a strong cold front and severe storms yesterday, temperatures cool off into the low-50s to start the week. A reinforcing cold front and high pressure brings much below normal highs only in the low-40s mid-week. A round of quick snow showers is possible going into Tuesday morning!

Rest of today – seasonably cool with westerly winds starting to die down. Sunny with high temperatures around 50ºF. Overnight lows in the upper-40s.

Weather Prediction Center surface forecast for 7AM Tuesday

Tuesday – clouds increasing as a cold front moves in from the northwest. High temperatures steady in the upper-40s. Chance of rain then rain and snow overnight. Overnight lows below average in the low-30s.

Wednesday – due to a cold start, and high pressure to the southwest, northwesterly winds will keep things cold on Wednesday with highs only around 40ºF despite sunny skies. Overnight lows drop into the low-30s.

Thursday – temperatures increase to the upper-40s with sunny skies. Overnight lows warming into the low-40s.

Hurricane Iota – A Major Disaster for Central America

A catastrophic disaster could be unfolding for Nicaragua and Honduras, in the same regions that were pummeled by Hurricane Eta barely two weeks ago. Hurricane Iota appears to be on its way to becoming a rare November Category 5 storm, and is looking very impressive on satellite images this morning. The torrential rain this storm brings could result in landslides, and inland flooding, exacerbated by soil that’s still quite damp from Eta’s onslaught. With recovery efforts ongoing, and people still without permanent shelter, Iota could be a major, life-threatening storm for this area.

GFS model 850 mb relative humidity and winds at 1AM Thursday, depicting some decently fast winds and ample moisture accompanying the cold front.

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 Weather Update – Aug 17, 2020

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After a rainy, cool weekend, temperatures will rebound into the low-80s as high pressure takes over. This will bring about a week of pleasant, mostly dry weather, with plenty of sun all the way through end of the week. Further afield, we’re watching the tropics for 2 possible tropical storms brewing.

Rest of today – sunny with high temperatures in the low-80s. Overnight lows in the upper-60s with some potential for a scattered showers and thunderstorms as a cold front pushes through the region later this evening. Timing of these storms hitting the area appears to be around 8-10PM.

  • Weather Prediction Center surface forecast for 2AM Tuesday
  • High resolution rapid refresh simulated radar valid at 9:30PM tonight

Tuesday – behind this cold front, westerly downsloping winds will actually help temperatures rebound into the mid-80s under mostly sunny skies. Overnight lows in the mid-60s.

Wednesday – mostly sunny with highs in the low-80s. Overnight lows continue in the upper-60s.

Thursday – another great day with lots of sun and highs in the low-80s. Overnight lows in the upper-60s.

Watching the tropics

Way out in the tropical Atlantic, we have two tropical waves that the National Hurricane Center is keeping tabs on. These two waves could develop into the 12th and 13th named storms of the 2020 Atlantic Hurricane season, which thus far has been breaking records for the fastest forming named storms, beating out the horrific 2005 season that produced Katrina, Wilma, and some storms that had Greek alphabet names.

National Hurricane Center 5 day graphical tropical weather outlook.

NYC Weekend, Tropical Storm Fay Weather – Jul 10, 2020

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Tropical Storm Fay will bring heavy rain, strong winds, flash flooding to start the weekend. A Tropical Storm Warning is in effect for all of NYC as well as coastal New Jersey and Connecticut, including all of Long Island. Conditions should start to improve Saturday as Fay continues tracking north and inland. We may even see some sun by Sunday. Temperatures start in the low-80s with the rain and clouds from Fay, but should be warming up upper-80s later on this weekend.

Rest of today – cooler with high temperatures in the low-80s. Windy and rain, with bouts of heavy, tropical downpours. 2-4″ of rain are possible with this storm. Sustained winds could approach tropical storm force (30-40 mph) near Fay’s core if it tracks over us. Otherwise, gusts up to 50 mph are possible. The main threat from Fay is from the heavy rains that could produce flash flooding. Overnight lows in the low-70s with tropical storm conditions still possible as Fay lifts north.

  • National Hurricane Center forecast for Tropical Storm Fay.
  • Weather Prediction Center rainfall forecast for Tropical Storm Fay. Note the widespread totals of 2-4″ possible in our region.

Saturday – as Fay lifts north and inland, it will weaken rapidly. Conditions should improve with a spell of dry weather possible before another non-tropical frontal system impacts the area with thunderstorms possible. Temperatures should be warm, in the upper-80s. Overnight lows in the mid-70s with rain chances diminishing behind the passage of this cold front.

Sunday – we may start off dry with some sun, which could allow temperatures to climb to the low-90s. However, scattered thunderstorms are possible later in the day. Overnight lows in the mid-70s.

Monday – similar day to Sunday, with high temperatures in the upper-80s. Partly sunny skies to start and isolated thunderstorms possible later in the day. Overnight lows in the mid-70s.

Weather Prediction Center surface forecast for 8AM Sunday

NYC Weather Update – Jul 5, 2020

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A hot, humid, and stormy start to the week with highs in the low-90s. A backdoor cold front moves through, granting a reprieve from heat mid-week with highs cooling off to the mid-80s Tuesday. The heat comes back after this. Later this week, and into the weekend, a storm with possible tropical characteristics could bring heavy rain to the region.

Rest of today – sunny to start with highs quickly climbing into the low-90s. Clouds increase later this afternoon. A shortwave trough will spark the potential for some showers and thunderstorms later this afternoon, around 4PM. These storms could bring heavy rain and gusty winds. Overnight lows in the low-70s with rain chances quickly dying off.

Weather Prediction Center surface forecast for 8PM Monday.

Tuesday – a backdoor cold front will push through overnight. The onshore flow from the northeast behind this front should result in mostly cloudy skies and cooler temperatures in the mid-80s. Showers and thunderstorms could still impact the area in the afternoon. Overnight lows in the low-70s.

Wednesday – the backdoor cold front will return north as a warm front Wednesday. Temperatures will remain stable with high temperatures in the mid-80s with mostly cloudy skies. Isolated showers and thunderstorms will again be possible in the afternoon hours. Overnight lows in the mid-70s.

Thursday – high temperatures rebound into the upper-80s and low-90s possibly with fewer clouds as high pressure briefly builds in. Overnight lows in the mid-70s. Looking towards the end of the week, we’ll be monitoring the progress of a low pressure center that the National Hurricane Center has identified as having the potential to develop tropical characteristics. This storm could bring some heavy rain this weekend.

National Hurricane Center 5 day graphical tropical weather outlook.

NYC Weather Update – Jun 8, 2020

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High pressure will be the main story this week, providing plenty of sun and summery weather. A chance for rain comes later with the extratropical remnants of Tropical Depression Cristobal. The timing of this is rain not entirely certain, but it looks like this will be Thursday for now. Temperatures this week should be ranging slightly above normal with high temperatures in the low-mid 80s.

Rest of today – mostly sunny with high temperatures around 80ºF. Just a beautiful day with comfortable temperatures and low humidity. Overnight lows in the mid-60s.

Weather Prediction Center surface forecast for 8AM Tuesday.

Tuesday – temperatures warm up into the mid-80s with lots of sun as the high pressure center shifts gradually off to the southeast and we see southerly flow take hold. Humidity still overall not that bad, so it shouldn’t feel muggy. Overnight lows in the upper-60s.

Wednesday – high temperatures back down into the low-80s though clouds could be on the increase. Overnight lows in the upper-60s with rain chances possibly increasing.

GOES visible satellite loop of Tropical Depression Cristobal the morning of June 8, 2020, after it made landfall as a tropical storm this weekend over Louisiana

Thursday – a cold front associated with the extratropical remnants of Tropical Depression Cristobal could bring some rain and thunderstorms to the area. Otherwise, mostly cloudy with high temperatures in the low-80s. Overnight lows in the upper-60s.