Category Archives: Flooding

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 Weekend Weather – Apr 19, 2019

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A slow-moving storm system will impact the area with potentially heavy rain that could trigger flash flooding overnight into the first part of Saturday. Precipitation chances die down significantly with the passage of this round of rain as a dry slot works into the area. However, lingering chances for showers still exist as this storm makes a slow exit to the east. Temperatures during this period are forecast to be at or above normal.

Rest of today – cloudy, but still warm with temperatures rising to about 70°F. We will be in the warm sector with the influence of southerly winds during the day. Rain chances increasing during the evening hours. Steady rain is forecast late into the overnight hours with temperatures. Some of the rain could be heavy. Thunderstorms are also possible, and these would increase the risk of flash flooding. Overnight lows around 60°F.

Weather Prediction Center surface forecast for 8AM Saturday

Saturday – the north-south orientation of the cold front that will provide lift for these storms overnight will align well with the prevailing southerly flow through most of the atmosphere. The slow eastward progression of this front will allow storm cells to train leading to more flooding potential as rain continues Saturday morning. The cold front associated with this slow-moving low should push through by the afternoon. A dry slot should cut off any precipitation for a while during this time frame. High temperatures should be in the low-60s. Overnight lows should fall into the low-50s behind the cold front.

GFS model output of 850 mb winds and relative humidity for 2PM Saturday, notice the pronounced area of browns, indicating very dry air. This is what is referred to as a “dry slot“, which can often follow on the heels of cold fronts associated with mature mid-latitude cyclones.

Sunday – the upper low associated with this storm system is expected to cutoff (become detached from primary westerly steering currents), resulting in a system that will meander and take a long while to exit the region. As such, Sunday is expected to remain cloudy, with a chance for some showers. High temperatures should be similar to Saturday in the low-60s. Overnight lows will again be around the low-50s.

Monday – temperatures warming up into the upper-60s, but yes, that cutoff low still lingers! Chance for showers continues in the morning. Clouds finally decreasing later and overnight lows in the mid-50s.

NYC Detailed Forecast for Apr 19, 2019

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It’s been four months (!!) since I’ve done one of these detailed forecasts. I’m a bit out of practice but trying to get back into the groove. Lucky for me, I picked a day to forecast that has a decent bust potential for precipitation. I’ll discuss that below.

Friday is going to start off decently enough. It should end up being the warmest day of the week. However, it’s also going to be the start of a protracted period of unsettled weather that will affect the weekend. A flash flood watch is in effect late overnight Friday going into Saturday for this reason.

My Forecast
High: 70°F | Low: 58°F | Max sustained winds: 30 mph | Total precipitation: 0.10″ – verification for temperatures and precipitation will come from METAR data for the period between 2AM Friday and 2AM Saturday (06Z Friday to 06Z Saturday) at LGA. Wind speed verification will draw on the daily climate summary from the National Weather Service.

Verification
High: 76°F | Low: 56°F | Max sustained winds: 24 mph | Total precipitation: 0.16″ – I did decently forecasting the low, and the total precipitation. Hedging way down on precipitation totals towards the NAM solution on a slower moving storm was a good idea. On the low temperature, I think a 2°F margin is acceptable here. Being off by 6°F on the high temperature here constitutes a forecast bust that I’ll put a post-mortem together for – as it turns out the bust happened here and not with precipitation. Winds were also not as strong as anticipated. I should have hedged a bit down too with less intense precipitation anticipated.

Weather Prediction Center surface forecast for 8PM Friday

Synoptic Set Up
We begin the period in the warm sector of a low with a center forecast to sit over southwestern Quebec. A secondary surface low is forecast to form over the OH valley and strengthens as it briefly moves northeast. This low is then forecast to stall out and retrograde northwest as it matures and occludes. This will lead to a protracted period of precipitation, with the primary focus being along a cold front moving through Sat morning. This may result in the best chance for heavy rain being outside the forecast period.

At the 850 mb level, a strong low-level jet will provide efficient moisture transport. At 500 mb we’ll be downstream of a longwave trough, with an embedded closed low that becomes cutoff (this mid-level cutoff low will be tied to why the secondary surface low retrogrades). The best vorticity and divergence (lift) associated with this upper low seems to move through after forecast period according to NAM forecast model. GFS has a bit earlier timing, and hence brings precipitation in earlier Friday. Finally, at the 300 mb, there’s not much evidence of enhanced divergence and lift at this level since, we’ll be east of any favorable entrance/exit regions of a 300 mb jet streak.

NAM forecast model’s depiction of the 500 mb layer at 11PM Friday. The cut off low referenced above is circled in green. The shades of yellow-purple indicate increasingly strong vorticity, which is linked to divergence and enhanced lift.

High Temperatures
GFS and NAM MOS are in pretty close agreement here. It should be warmer than average, since we’ll be in the breezy warm sector of a low as mentioned above. Limitations on warmth will be evaporational cooling due to precipitation, but this doesn’t appear to be a factor until late in the forecast period, past normal times for peak temperatures. The other factor is thick cloud cover. However, temperatures will largely be driven by warm advection tomorrow and not necessarily solar heating. EKDMOS suggests a temperature over 70ºF. In deference to this ensemble model, I’ve sided with the warmer GFS and EKDMOS (as well as National Blend of Models) and call for a high temperature of 70ºF.

  • GFS MOS output
  • NAM MOS output

Low Temperatures
NAM and GFS MOS are again in close agreement. With warm advection happening during the course of the overnight hours (strong winds at large angles to isotherms, southerly flow), have no reason to disagree that temperatures could actually go up early then back down a touch before rising again with daybreak, and I’m going with an overnight “low” of 58ºF that is well above average for this time of year.

NAM showing temperature contours and wind direction/speed for 2AM Friday. Note the direction of the wind barbs (pointing from south) is crossing at large angles of relatively closely packed isotherms. This is a set up favorable for warm advection (transport of warm air).

Max Sustained Winds
NAM is lower on winds than GFS, but the ensemble EKDMOS skews towards faster GFS solution. This makes sense with heavy rain possible and downward momentum transfer. Winds are forecast to back from south-southwest to south-southeast during the period. These directions don’t typically produce the strongest winds climatologically. However, a strong 45-60 knot low-level jet aloft mixing down with precipitation could easily top 20 knots, especially if elevated convection and thunderstorms materialize. I’m going with the EKDMOS 50th percentile range and going with 25 knots.

  • The NAM forecast sounding at 5PM tomorrow shows considerably drier low-mid levels.
  • In contrast, the GFS forecast sounding shows a deep saturated layer indicative of ongoing precipitation by 5PM tomorrow.

Total Precipitation
There’s a large spread between NAM and GFS on forecast totals (more than 0.50″) because the GFS is coming in with precipitation starting much earlier in the forecast period than the NAM. The local forecast office seems to buy more into the NAM solution due to its consistency with slower onset of heaviest precipitation. There is some indication as well of MUCAPE present in SREF plumes, could result in some elevated convection that would boost precipitation totals. Any faster timing and/or burst of elevated convection could cause heavier rain to fall earlier, resulting in a forecast bust.

Counting against that, I don’t see any strong signal for frontogenesis at the 850 mb level enhancing lift. SREF doesn’t show too much in the way of strong lift either. Still, the fact that SREF ensemble shows much heavier precipitation totals are not far off to the west during this forecast period makes me a bit nervous.

Lastly, moisture isn’t a total lock, and there’s some evidence of drier air offshore and pockets of dry air moving through from the west at times. NAM is notably drier than GFS at both outset of the forecast and throughout the forecast period, as seen in the comparison forecast soundings above. Training of storm cells possible with steering currents set up parallel to the forecast orientation of the cold front.


Cyclone Idai Synopsis and Aftermath

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Tropical weather is not often on our minds at this time of the year, since the Atlantic Hurricane season is still months away. But there are other active tropical basins at this time – last week Cyclone Idai spawned in the Southwest Indian Ocean and brought devastation to vast swathes of Mozambique, Zimbabwe, and Malawi. With 700 deaths already attributed to it, Cyclone Idai is one of the deadliest tropical cyclones to have impacted southern Africa on record. Sadly, that death toll could continue to increase as floodwaters recede, revealing the full extent of damage from the storm. In this post, I’ll take a look at why Cyclone Idai was able to grow so powerful, and why it caused such serious flooding.

Meteorological Synopsis

Idai formed as a tropical depression in the Mozambique Channel on March 4, 2019. It moved onshore shortly thereafter and spent several days over Mozambique as it executed a cyclonic loop. During this time, it managed to retain tropical characteristics (keeping a warm core), before it exited again over the open waters of the Mozambique Channel. As it continued moving east, it encountered increasingly favorable conditions for intensification with very warm sea surface temperatures and decreasing vertical wind shear.

  • Visible satellite image of Cyclone Idai close to when it made landfall on March 14, 2019
  • Track map of Cyclone Idai. The purple dot indicates where Idai first formed, and the green dot is where it lost tropical characteristics

For more details on the images above see the Wikipedia article about Idai.

A subtropical ridge began forming and strengthening over eastern South Africa, causing the cyclone to start turning west and heading towards the coast of Mozambique again.

As Idai moved west, it underwent 2 distinct periods of rapid intensification in response to the favorable conditions found over the Mozambique Channel during this time. In between, it experienced an eyewall replacement cycle, where it temporarily weakened as concentric eyewalls formed, and the inner one eventually collapsed as the outer one took over.

  • Upper level divergence at 18 UTC on March 14, 2019
  • Low level convergence at 18 UTC on March 14, 2019
  • Vertical wind shear analysis at 18 UTC on March 14, 2019

Unfortunately for those in Idai’s path, it regained strength after a second eyewall replacement cycle as it approached landfall because conditions continued to be quite favorable. The analyses above with satellite overlays valid for 18:00 UTC March 14, 2019 show the cyclone near its peak intensity under nearly ideal conditions (lower left of the image). The first two analyses show strong upper level divergence and low level convergence around Idai respectively. This indicates that upper level outflow was well established, which is also visible from the symmetrical presentation of the cyclone visually, with outflow channels evident. Upper level outflow is required for “ventilating” intensifying cyclones and helping them grow/maintain their intensity. That’s because as the storm gathers strength, low level convergence intensifies as winds in the storm pull air from surrounding areas inwards towards the eye of the storm. This convergence is what powers the intense thunderstorms in the eyewall. Without upper level divergence, the air converging at the storm’s core would continue building up, eventually resulting in rising pressure and a weakening storm – pressure, after all, is a measure of the mass of air over a given area.

The last image shows that Idai was also in a zone of very low vertical wind shear, between 5-10 knots during this time before landfall. Low vertical wind shear helps preserve the structure of a tropical cyclone. Unlike with severe thunderstorms, wind shear can actually shift the core of the strongest thunderstorms away from the center of a tropical cyclone’s center of circulation. That often marks the beginning of the end for a tropical cyclone. In the case of Idai, low wind shear let it strengthen considerably close to landfall, although it did weaken a bit right before landfall due to increased shear and interaction with land.

Why Such Devastating Flooding from Idai

While powerful storm surges accompanied Idai, the worst impact from Cyclone Idai was widespread catastrophic flooding in Mozambique and in Zimbabwe. Part of this comes down to the geography of the area impacted, with a broad, flat flood plain between Pungwe and Buzi Rivers, and with parts of Beira, the largest city in the area, lying below sea level.

European Space Agency satellite image, with areas in red indicating inundation taken earlier last week

As Dr. Jeff Masters pointed out in a post on Weather Underground, another big reason why Idai caused such serious flooding was because of the storm’s slow forward progress. This came down to the storm being embedded in an environment with weak steering currents.

CIMSS deep layer mean steering (250-850 mb) analysis loop of the Indian Ocean. Cyclone Idai is visible in the lower left corner.

In the animated loop of 250-850 mb deep layer mean steering winds, we see that as Cyclone Idai traversed the Mozambique Channel, it was in an area of very light steering winds (very few streamlines, few arrows on those streamlines). Mature tropical cyclone motion is influenced by winds in this layer of the atmosphere because these winds impact the tall thunderstorms in the cyclone’s core. When steering currents break down as in the loop above (a ridge with anticyclonic flow dissipated north of Idai), a tropical cyclone will start to slow down and sometimes can meander. In this case, with Cyclone Idai, a slower moving storm led to a prolonged period of heavy rain over the impacted areas. Had steering currents remained stronger, Idai would have produced less heavy rain over the same areas during a shorter window, likely reducing flooding impacts considerably.

Consider Donating to Support Recovery Efforts

The affected countries of Mozambique, Malawi, and ZImbabwe simply do not have the financial and physical resources to respond to a disaster of this magnitude. There are many worthwhile NGOs on scene providing relief aid. Please consider making a contribution to support these efforts, as the recovery effort will take years to complete. International Rescue Committee, Doctors Without Borders

NYC Detailed Forecast for Jan 20, 2019

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A major winter storm is set to bring immediate impacts to the entire Tri-State region with a mixed bag of wintry precipitation ranging from snow to sleet, freezing rain, and rain. Coastal flooding and even urban street flooding could be a concern. Temperatures will plummet in the wake of this storm, with Arctic winds blowing from the northwest bringing the coldest air of the season and the potential for a flash freeze. The track of this storm has bounced around a bit throughout the week, though at the time of this writing, a warmer scenario with a storm track further north closer to the coast looks more likely.

My Forecast
High: 48°F | Low: 17°F | Max sustained winds: 35 mph | Total QPF: 1.40″ | Total snow accumulation: 1.00″ – verification for temperatures and precipitation will come from METAR data for the period between 1AM Sunday and 1AM Monday (06Z Sunday to 06Z Monday) at LGA. Wind speed verification will draw on the daily climate summary from the National Weather Service.

Verification
High: 40°F | Low: 15°F | Max sustained wind: 38 mph | Total precipitation: 0.80″ | Total snow: 0.00″ – Decent job of handling max sustained wind and the low, however, I missed the mark on high temperatures and overall precipitation by a large margin. I’ll post a more in-depth post-mortem analysis on this later this weekend. However, I do want to provide an initial assessment of the reasons behind my forecast bust on high temperatures and precipitation. These both have to do with the progression of the coastal and warm fronts associated with this storm. The warm front never made it further north than offshore of Long Island. As a result, we didn’t really get into the warm sector as I thought we would. This resulted in more northeasterly flow, keeping temperatures suppressed. The timing of the heaviest precipitation happened to be in the immediate six hour period preceding the start of my forecast window, when another 0.64″ of precipitation fell. I’ll look a bit more into this in the post-mortem, but I suspect the storm progressed a bit faster than forecast model data suggested on Saturday afternoon.

Weather Prediction Center surface forecast valid 7AM Sunday.

Synoptic Set Up
A large scale storm system with origins from the Pacific has been marching through the Ohio Valley and will eventually move offshore right in our vicinity. As this surface low continues to intensify, a warm front will first push into the coastal areas of the region. Ahead of this, snow should develop late Saturday, heavy at times. Eventually, as the warm front moves north of us, we’ll experience a transition over to rain. As the surface low progresses east, precipitation will end in the afternoon Sunday. This low will then continue to deepen, causing strengthening winds from the northwest to usher in an Arctic airmass and the coldest temperatures of the year.

Weather Prediction Center’s most recent storm track map. There’s still some uncertainty at this point with the track. However, trends have been pushing the storm track north since yesterday and even within the cluster of orange dots near us denoting the storm’s forecast position at 7AM Sunday, not too many are far south enough to see a colder scenario pan out.

High Temperatures
National Weather Service only recently released a warmer forecast in the mid-40s, before which they were showing highs in the mid-30s. However, most other sources are suggesting a substantially warmer high temperature in the upper-40s to near 50°F. In this case, I tend to agree with the consensus shaping up on the warmer side, with MOS guidance being on this warm side, and the generally reliable National Blend of Models similarly warm. The current most likely storm track places the area in the warm sector of this low, with MOS guidance showing an extended period of east-southeasterly winds veering to the south and southwest and blowing at a good clip, I can buy into the idea there will be decent warm advection and a surge of warm air to push temperatures up in to the upper-40s. Furthermore, I don’t see a lot of potential for evaporational cooling occurring, since the atmosphere will start off saturated during the morning hours (it’ll be raining).

  • NAM MOS for KLGA
  • GFS MOS for KLGA

Low Temperatures
With strong northwest winds expected to develop on the cold flank of the surface low as it exits the area, low temperatures are likely to actually occur late overnight going into Monday. This is because warm advection will proceed the bulk of the rain Sunday morning. I’m siding with a blend of low temperatures from NAM, GFS MOS, and National Blend of Models.

NAM forecast sounding for 7PM Sunday, note the environmental temperature line (red/yellow) from the surface upwards to about 875 mb is well-mixed, parallel to the nearest line of dry adiabatic ascent (grey lines slanted to the left at ~45 ° . This is a set up that favors strong winds.

Max Sustained Winds
Strong northwest winds are expected to develop behind the exiting low. A well-mixed layer is visible in forecast soundings, and this suggests the potential for the strong winds aloft at 850 mb to mix down. Sustained wind speeds could easily be in the 25-30 mph range with higher gusts. The intensifying low should also lead to an isallobaric component (winds becoming stronger in response to a deepening low) to the wind, which I think could push wind speeds to 35 mph. I believe the strongest winds of the day are actually coming after the precipitation since there’s going to be a warm nose above the surface, putting an inversion in place that would diminish downward momentum transfer while rain is coming down.

NAM forecast model showing 850 mb frontogenesis early Sunday morning at 4AM. These values of frontogenesis entail potential for bands of heavy rain and snow, with plenty of lift possible to propel precipitation.

Total Precipitation
Several of the key ingredients needed for a heavy precipitation look to be coming together for this storm. This includes several strong vorticity maxima at 500 mb moving through. At the 300 mb level, a jet streak is forecast to be in the vicinity, but I don’t see this area sitting under a favorable jet streak quadrant that would support additional lift. Most of the lift here I think will come from the combination of 500 mb vorticity maxima and 850 mb isentropic lift depicted above. I actually see some low potential for elevated convection that could spur bouts of this heavy precipitation to fall. Moisture looks robust, with an 850 mb low-level jet forecast to develop as well. This should support deep moisture and lends further credence to the potential for heavy rain. Given the temperature profiles, it appears the risk of freezing rain and sleet is low at the coast, though more significant north of the city.

NYC Weekend Weather – Dec 28, 2018

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The last weekend of the year gets off to a mild, rainy start as a storm system moves through. Things cool off and dry out the remainder of the weekend but it looks like another rain event and warm spell hits for New Year’s Eve.

Rest of today – rain through most of the day. Gusty winds around 30 mph possible. High temperatures well above normal in the mid-50s due to a surge of warm advection accompanying the warm front initially responsible for widespread rain. At the 850 mb level, a low-level jet develops, helping enhance moisture convergence and allow for heavier rains. A flood watch is actually in effect for areas of New Jersey in the metro NYC region. Lows going into Saturday will be quite warm in the mid-40s.

Storm Prediction Center mesoanalysis of 850 mb moisture transport. The maroon vectors indicate the magnitude and direction of moisture transport. The long vectors over the NYC region show efficient moisture transport due fast moving winds of a low-level jet at the 850 mb level. The shaded contours show values of theta-e (equivalent potential temperature), while the details of this are quite complex, the easiest way to understand theta-e is that high values indicate very moist airmasses that can be prime for development of convective activity, as an example. In this case, we can tell that the potential for heavy rain exists within areas of high theta-e (dark-green to red shaded areas).

Saturday – a strong cold front will move through during the afternoon which should result in high temperatures peaking early in the day around 50°F, even though it will remain mostly sunny all day. Behind this cold front, Canadian high pressure builds yielding much colder overnight lows into Sunday in the low-30s.

Weather Prediction Center surface forecast for Sunday at 7AM EST.

Sunday – much colder day on tap with Canadian high pressure in control. High temperatures around normal for this time of year near 40ºF even with plenty of sun. Overnight lows into Monday are forecast in the mid-30s.

Monday (New Year’s Eve) – clockwise return flow from the southwest on the western side of the exiting high pressure warms temperatures up into the upper-40s ahead of the next storm. The timing of this storm looks like it will result in a rainy New Year’s Eve in the city. This may sound less than ideal, but recall that we rang 2018 in with near record cold. It’ll be downright balmy in comparison this time with overnight lows forecast to barely budge in the mid-40s.

NYC Weekend Weather – Aug 10, 2018

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Enjoy the pleasant weather today if you can, because conditions will deteriorate going into the weekend. Wet weather will impact the NYC region starting tonight, lingering well into early next week. Rain could be heavy at times, and flash flooding is a possibility. Temperatures during this time will be considerably cooler than this past week with the influence of clouds and rain. We will be stuck under the influence of multiple shortwaves moving through at the upper level during this time. Starting next week, forecast models suggest that a closed 500 mb low will form, then linger in the area due to strong downstream ridging (high pressure) over the western North Atlantic. This cut-off upper level low will provide for persistent unstable conditions conducive for multiple rounds of rain and thunderstorms. I will be off the next two weeks on vacation and will not be posting updates unless there’s severe weather forecast for the area.

Rest of today – increasing clouds as a cold front approaches the area from the northwest but stalls out and dissipates overnight. High temperatures in the upper-80s. Chances for showers and thunderstorms increase overnight.

Saturday – multiple chances for showers and thunderstorms, starting early in the day, then again in the afternoon and evening hours. Much cooler with mostly cloudy skies and high temperatures only in the mid-70s. The cold front that passed through yesterday will return north as a warm front. Ahead of this, winds will be easterly and onshore, bringing the cool, damp influence of the Atlantic over the area.

Sunday – warmer with high temperatures in the low-80s but still mostly cloudy. Chances for rain throughout the day as the frontal boundary above remains in the area.

Monday – yet another impulse of shortwave energy aloft brings a surface low towards the area, so rain chances persist with a shot for thunderstorms as well. Mostly cloudy with highs in the low-80s.

NYC Weather Update – Jul 30, 2018

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Yesterday and today have been a welcome relief from the recent spell of tropical humidity. However, the comfortable weather won’t last this week. A warm front will sweep through and bring us right back into an unsettled, tropical weather pattern. There will be chances for showers and thunderstorms every day this week. Humidity will be back on just as last week.

Rest of today – cloudy, with high temperatures topping out in the low-80s then dropping to the mid-70s by end of the day.

Tuesday – as a warm front nears from the south, there will be chances for showers and thunderstorms throughout the day. High temperatures will be capped in the low-80s due to southeasterly onshore flow off the Atlantic ahead of the warm front.

Wednesday – temperatures should be in the mid-upper 80s, depending on the timing of the warm front passing through. Chances for showers and thunderstorms yet again. Heavy downpours from slow moving storms could cause localized flooding. Dew points will increase into the low-70s resulting in an uncomfortably muggy day.

Thursday – behind the warm front, we’ll get into even warmer and more humid air due to the presence of a Bermuda High. A prefrontal trough associated with a cold front will approach the area and once again give us a chance for thunderstorms. High temperatures should be in the upper-80s.

 

NYC Weather Update – Jul 23, 2018

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Wet weather this weekend continues into much of this week as we settle into a pattern favoring rain chances every day. Tropical moisture remains firmly entrenched in the region due to an anomalously strong upper trough and deep layered southerly flow. A strong high pressure center over the western North Atlantic will block the progress of frontal boundaries until next weekend when we finally get a break from the wet weather.

Rest of today – mostly cloudy, high temperatures in the low-80s. Chances for scattered showers throughout the remainder of the day and into the overnight hours.

Tuesday – mostly cloudy, high temperatures again in the low-80s. Lower chances for rain than the rest of the week due to the high pressure mentioned above retrograding west. This should keep things largely dry.

Wednesday – best chance for sustained bouts of heavy rain as a cold front approaches from the west but stalls out and dissipates. High temperatures in the low-80s with mostly cloudy skies again. Flash flooding could be a concern given the atmospheric water content and the southerly steering currents parallel to the frontal boundary, favoring training of storms hitting the same area repeatedly.

Thursday – yet another day with the chance for showers and thunderstorms, high temperatures in the mid-80s and mostly cloudy.

NYC Weekend Update – Jul 20, 2018

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Last weekend’s beautiful, sunny weather will be but a fleeting memory this weekend as a coastal storm hits the area late Saturday going into Sunday. Periods of heavy rain and even some severe weather are possible Saturday night and Sunday, with the potential for isolated flash flooding. Unsettled weather continues into next week with a tropical weather pattern setting up providing daily chances for showers and thunderstorms.

Rest of today – a great start to the weekend, with high temperatures in the low-80s and sunny skies.

Saturday – cool, with high temperatures only in the upper-70s due to the influence of easterly winds ahead of a warm front attached to the advancing coastal storm. Clouds increasing late in the day and rain chances picking up in the evening hours going into overnight. Chance for heavy rain, scattered thunderstorms with gusty winds (even an isolated tornado possible). Gusty east winds will make the overnight hours feel particularly cool and miserable.

Sunday – rain chances continue into the day and even Sunday night. Temperatures increase behind the passage of the warm front mentioned above and winds shifting to the south – high temperatures should be in the mid-80s with mostly cloudy skies. All told, we could be looking at widespread rainfall totals of 1-3″ across the area.

Monday – showers likely again as temperatures remain in the mid-80s under mostly cloudy skies. A strong upper-level trough will be wedged between two areas of high pressure. The one off to our east over the western North Atlantic will continue to strengthen over the course of the week, blocking the eastward advance of this upper trough. This effectively will result in an extended period of southerly flow, with moist, tropical air coming our way. Multiple impulses of energy are expected to travel along this strong upper-level trough. Each one will bring with it a chance for showers and thunderstorms. This pattern does not appear to break until next weekend, although it’s not as though each day will be a washout either.