Partly sunny start to the week, highs around 60ºF. Cold front coming tomorrow bringing showers with highs in the mid-60s. Behind this front, cooler temps in the mid-50s through mid-week as Canadian high pressure builds in. Cooler than normal trend set to continue into the weekend and early next week.
Rest of today – partly sunny with high temperatures around 60ºF. Mostly cloudy overnight with lows around 50ºF.
Weather Prediction Center surface forecast for Tuesday at 8 AM. An approaching cold front will bring a chance for showers. Behind this front, cooler temperatures will prevail.
Tuesday – mostly cloudy, chance of showers, with high temperatures in the mid-60s ahead of an approaching cold front. Following the cold front, lows will drop back to the upper-40s.
Wednesday – mostly sunny with high pressure building behind the aforementioned cold front. Cooler, with high temperatures in mid-50s. Overnight lows in the upper-30s!
Climate Prediction Center’s 6-10 temperature outlook valid for early next week. The cooler than normal trend looks somewhat likely to continue into this time frame.
Thursday– another mostly sunny but cooler than average day with high temps in the mid-50s and overnight lows only around 40ºF.
The holiday weekend starts of with sunny skies and mid-60s temps. A cold front Saturday brings rain, and much cooler temps by Easter Sunday only in the low-50s. This cooler trend will continue into Monday as clouds increase ahead of the next storm system. Temps the second half of the weekend will be 8-10ºF below normal, a big contrast from this week!
Rest of today (Good Friday, Passover) – sunny with high temperatures in the mid-60s. This will be the best day of the weekend. Overnight lows in the low-50s with clouds building.
Weather Prediction Center surface forecast for Saturday at 8 AM. A cold front will be approaching during the day.
Saturday – cloudy with rain showers developing later in the day, with high temperatures in the low-60s. Temperatures will fall into the low-40s after a cold front passes through. Shower activity should taper off overnight.
Sunday (Easter) – mostly sunny but much cooler, with high temperatures only hitting the low-50s. This is 8-10ºF below normal for this time of year. Overnight lows dropping back into the low-40s.
GFS model 850 mb temperature anomalies valid for 5 PM Monday. Unlike this week, when we had strong positive anomalies, we’ll be seeing a -6ºC anomaly Monday, leading to much cooler temperatures at the surface.
Monday– clouds increasing ahead of the next storm system affecting us, with high temperatures again below normal in the low-50s. Overnight lows in the mid-40s with rain developing.
Rain clearing early today, highs in the low-70s. Slight drop to the mid-60s tomorrow, then a big warm up Thursday ahead of a cold front, highs in the mid-70s. Some thunderstorms possible with this front. Temps back off into the mid-60s to end the week with mostly sunny skies.
Rest of today – showers passing through the next couple of hours. Skies should clear out steadily during the rest of the day. Temperatures should warm up nicely into the low-70s as a result. Overnight lows around 50ºF
Weather Prediction Center surface forecast for Wednesday at 2 AM. The cold front bringing us rain today will return north as a warm front.
Wednesday – partly sunny with high temperatures in the mid-60s as the cold front that passes through today returns north as a warm front. Temps could go higher if the warm front passes early in the day. Little moisture or lift is available with this warm front so not expecting rain at this time. Overnight lows in the mid-50s.
Thursday – with the region in the warm sector of a parent low moving into eastern Canada, temperatures are set to soar into the mid-70s. Inland areas could see 80s. These temperatures would be nearly 20ºF above normal for this time of year. A cold front will sweep through later in the day and could bring some thunderstorms. Overnight lows around 50ºF with showers passing.
GFS model 850 mb temperature anomalies at 5 PM on Thursday, showing a strong +8ºC anomaly at that time. This should translate to much above average high temperatures at the surface.
Friday– high temperatures moderating back down to the mid-60s with sunny skies after the cold front. These temps are still about 10ºF above normal. Overnight lows around 50ºF.
Sunny and mild Friday in the low-60s followed by a seasonable weekend with highs in the mid to upper 50s. This will be punctuated by passing showers Saturday. Return to sunnier skies Sunday, though isolated showers still possible due to shortwaves/disturbances rotating around a closed upper low moving slowly into Quebec. Warm up next week into the mid-60s.
Rest of today – above average warmth with high temps topping around or just above 60ºF. Sunny skies giving way to increasing clouds overnight. Lows in the upper-40s with scattered showers developing.
GFS model 500 mb height and wind, showing a closed low at this level centered over Ontario/Northern Michigan. This feature will only progress northeastwards slowly over the weekend. Shortwaves rotating around its base will bring us chances for showers.
Saturday – cloudy with high temperatures in the upper-50s and chances for scattered showers, esp. early in the day. Overnight lows in the low-40s with skies clearing up somewhat.
Sunday – mostly sunny skies but with scattered showers possible at times. High temps in the mid-50s. Overnight lows in the low-40s.
Weather Prediction Center surface forecast for Monday at 8 AM. A ridge of surface high pressure is forecast to lie over us.
Monday– warming up with high temperatures back in the low-60s under mostly sunny skies. Overnight lows in the upper-40s.
A major winter storm is expected to hit the Northeast primarily Saturday with a strong nor’easter developing and deepening as it tracks offshore. This will bring a swath of > 8″ accumulations from NYC through points north and east, with the bullseye likely over southern New England where snowfall totals could exceed 2 feet locally. Gusty north winds will accompany this storm, leading to some blowing and drifting snow. In line with most other forecasts this morning, my own forecast is for 7-10″ of snowfall in NYC but with the potential for a sharp gradient possibly cutting through the city itself, where the western part of the city could see below 6″ while eastern sections see closer to 10″ or more.
As with the last round of accumulating snow we had, antecedent conditions support enough cold air in place to result in an all snow event. With a storm track offshore enough so there should be no mixing, the primary uncertainty in snow totals arises from storm track. Model runs today should help us determine if the storm will track on/near the 40ºN 70ºW benchmark (more snow), or to the east (less snow). Overnight, model consensus has shifted west, supporting higher snow totals than were forecast at this time yesterday. If we see some consistency across runs today supporting this solution, confidence will grow in a significant snowfall for NYC. Mesoscale banding will bring bouts of intense snow with this storm, and the exact placement / duration of these features will determine where the highest snow totals occur.
GEFS ensemble plumes for total snow accumulation at LGA. The mean here is about 7″ with a significant spread between 3″-14″ between ensemble members.SREF ensemble plumes for total snow accumulation at KLGA. The mean here is much higher than GEFS, 18″, with similarly large ensemble spread between 2-35″!NWS Blend of Models total snowfall forecast ending 1 AM Sunday, showing about 11″ for NYC.NAM model total snowfall forecast ending 1 AM Sunday, showing 8″ for NYC.GFS model total snowfall forecast ending 1 AM Sunday, showing 7″ for NYC.ECMWF model total snowfall forecast ending 1 AM Sunday, showing 12″.WPC probabilities of > 6″ of snow accumulation by 1 AM SundayWPC probabilities of > 12″ of snow accumulation by 1 AM SundayNWS New York Office official forecast as of this morning, mainly calling for 6-10″ in the city proper.
Forecast Details
At the Surface
Models are projecting a couple shortwaves merging and developing as a coastal low off of the Carolinas later today and tonight. This storm is forecast to intensify and track a little east of the 40ºN 70ºW benchmark overnight, a position that historically signals the best snowmaking potential for NYC during coastal winter storms. The forward speed of this storm has picked up in model runs recently, but it is expected to be a strong storm with minimum pressures falling to the 960s by Sunday. A tight pressure gradient is expected between the intensifying low and high pressure to the north, which should cause gusty north winds to help advect cold air down to the coast.
Weather Prediction Center surface forecast valid for 7 AM SaturdayWeather Prediction Center storm track forecast.ECMWF “Euro” model surface forecast for 7 AM SaturdayGFS model surface forecast for 7 AM Saturday
850 mb – 700 mb Level
A robust 850 mb low-level jet flowing onshore from the northeast is forecast. This should provide the storm with good moisture content, aiding the formation of snow. Strong 850 mb and 700 mb temperature advection and frontogenesis is also evident in all models, occuring during the morning Saturday. This should induce heavy snow bands to develop for a few hours early on Saturday. These bands could produce 1-3″ per hour snowfall rates, and as always, areas where these bands set up and persist the longest will ultimately pick up the higher end totals.
GFS model 850 mb relative humidity & winds at 7 AM Saturday. A strong LLJ flowing onshore from the northeast is visible.GFS moidel 700 mb temperature advection at 7 AM Saturday.ECMWF “Euro” model 500 mb height and vorticity at 11 AM SaturdayECMWF “Euro” model 300 mb winds. Two jet streaks are evident in the vicinity of this storm, north and south of the storm’s projected center.
500 mb Level
This storm will benefit from a deep, synoptic scale trough that will acquire a slight negative tilt. This trough will eventually form a closed low at this level. This will aid the low in intensifying by increasing divergence at this level, facilitating upward movement of air over the surface low, leading to lower surface pressures.
300 mb Level
The storm’s center is expected to lie downstream of an exit region of a jet streak, and also upstream of an entrance region of another jet streak to its north. This is ideal placement for maximizing divergence at this level of the atmosphere over/near the low’s center, and supports the development of a strong surface low.
A relatively fast moving offshore low is expected to bring this winter’s first round of accumulating snow in NYC. Fairly good agreement among different models suggests decent confidence of totals NYC in the range of 3-5″, and it is worth noting that trends have been towards a higher total in overnight model runs. In fact, both the Euro and GFS agree on about 5″ of snow for NYC by the end of this event, and I tend to agree with this end of the forecast range at this time.
Antecedent conditions with a shallow but decently cold air mass in place means the primary uncertainty arises from storm track. Subsequent model runs should help us determine if the storm will track on/near the 40ºN 70ºW benchmark, slightly inside (more snow), or to the east (less snow). The timing of the snow during the overnight hours into the early AM rush will help boost totals since we will have enough cold air in place. Moderate and potentially heavy snow is likely during a 3-4 hour window starting around 4 AM.
GEFS ensemble plumes for total snow accumulation at LGA. The mean here is about 3.2″ with a significant spread among ensemble members both higher and lower than this.SREF ensemble plumes for total snow accumulation at KLGA. The mean is also about 3.2″, with similarly large ensemble spread..NWS New York Office official forecast as of this morning, mainly calling for 4″ in the city proper.
Forecast Details
At the Surface
Models are projecting a storm to develop off of Cape Hatteras and move close to the 40ºN 70ºW benchmark overnight, a position that historically signals the best snowmaking potential for NYC during coastal winter storms. This storm isn’t projected to be particularly strong, perhaps only dipping to 996-998 mb at its closest approach to the city, and it will be a relatively fast moving storm, both of which are factors that limit its potential to deliver large snowfall totals.
Weather Prediction Center surface forecast valid for 7 AM FridayWeather Prediction Center storm track forecast.ECMWF “Euro” model snowfall total for NYC from this event.GFS model snowfall total for NYC from this event.High resolution rapid refresh simulated radar valid at 4 AM Friday, depicting the strengthening storm hitting a swath of the Mid-Atlantic (including areas already walloped by this weekend’s major storm) and New England with a band of potentially heavy snow.
850 mb Level
There isn’t a strong sign of a robust 850 mb low-level jet with this storm. That’s in line with the profile of a weaker storm with less intense precipitation. However, most models are depicting strong 850 mb and 700 mb temperature advection and frontogenesis occurring for a few hours just south of the city, which could signal the possibility for some heavy snow bands to develop for a few hours overnight. It’s within these bands that we could see up to 1″ per hour snowfall rates, and as always, areas where these bands set up and persist the longest will ultimately pick up the higher end totals.
GFS model 850 mb temperature advection and frotogenesis as modeled by GFS valid at 4 AM Friday.GFS model 500 mb height & vorticity at 4 AM FridayECMWF “Euro” model 300 mb height and wind at 10 PM Thursday
500 mb Level
Another supporting factor for a decent round of snow with this storm is that there will be a negatively tilted 500 mb through developing upstream of the surface low. This should aid the low in intensifying, though as stated above, it still won’t be a particularly potent one at the surface when it’s nearest to us.
300 mb Level
The 300 mb level shows a an exit region upstream of the surface cyclone preceding the event. The WPC does discuss a coupled jet scenario with the jet streak at this level eventually helping induce a lower-level jet which would boost snowfall totals, but the this might happen after the storm really impacts us.
Surge of spring-like warmth today ahead of a cold front. Showers and gusty winds accompany the passage of the front and usher in a much colder air mass Tuesday. High temps dropping from mid-60s today to low-40s tomorrow. Wed, we could get clipped by outer precip bands from an offshore low, with enough cold air in place to cause some snow flurries.
Rest of today – unseasonably warm and mostly cloudy with highs rising to the low-mid 60s. Breezy southerly winds 10-20 mph with higher gusts. Overnight lows dropping rapidly into the mid-30s behind the passage of a cold front, bringing showers around 7-9 PM tonight.
High Resolution Rapid Refresh simulated radar valid at 7 PM this evening, depicting a narrow band of fast moving showers approaching the city attendant to the passing cold front.
Tuesday – much colder with high temperatures topping out in the low-40s. Clearing skies, partly cloudy. Overnight lows in the low-30s.
Wednesday – snow showers likely, with possible light accumulations up to 1″ in the city (more likely on unpaved areas) due to the impact of an offshore storm. The GFS model seems more bullish on colder air/heavier precip while the Euro shows more of a dusting. High temperatures in the mid-30s. Overnight lows in upper-20s.
GFS 24 hour snowfall accumulation ending 7 AM Thursday.
Thursday– mostly sunny with high pressure building briefly in. High temperatures in the low-40s. Overnight lows in the upper-30s with rain chances increasing going into Friday as another storm system approaches from the west.
Thanksgiving Day 2021 for NYC shaping up to be the best weather of the week. High pressure will keep an approaching cold front at bay and provide warm, westerly downsloping winds that should push temperatures into the low-50s. Enjoy it while you can, because gusty rains will follow Thursday night with the cold front, then we’re looking at cold temps going into the weekend in the low-40s.
At the surface, high pressure will still predominate our local weather. This will keep rains at bay, while the shallow pressure gradient around the area will lead to light winds. The Thanksgiving Day Parade balloons will surely be flying high over the first spectators to view them since the pandemic started!
Weather Prediction Center surface forecast for 8 AM Thursday. High pressure will be to our south while a cold front approaches from the west.
Looking at the forecast soundings at 8 AM Thursday, around parade jump off time, we can see that the boundary layer below 3000 feet should be relatively dry and cloud free, but it does appear that a deck of clouds will be in place from that 3000 feet up. Light winds of 5 knots or less are forecast in this case.
GFS forecast soundings for NYC at 8 AM Thursday
Once the cold front makes its passage overnight Thursday and into Friday morning, the mild weather will come to an abrupt end. Winds will pick up due to an increasing pressure gradient between the strengthening parent low associated with the front and high pressure building over central Canada. The winds should be effective at bringing about cold air advection, transporting this colder, drier continental polar airmass towards us over the weekend. This will result in high temperatures only in the low-40s, clearing skies, and wind chills in the 30s.
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.
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.
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.
