Saturday, June 29, 2024

Sunday, June 30, 2024 southern New England Thunderstorm Discussion

 Currently, a northwest to southeast oriented warm front resides from the northern Great Lakes region into Canada southeast across Pennsylvania and into portions of the mid-Atlantic. This warm front accelerates northeast through the remainder of Saturday, advecting in a moisture rich, high theta-e airmass into New England with dewpoints likely into the lower 70's by early Sunday morning. 

Warm front position - early Saturday afternoon

Through the day today, a shortwave trough dives into the upper-Midwest and amplifies as it progresses across the northern Great Lakes region towards the northeast. The result will be unseasonably strong wind shear overspreading the region characterized by 40-50+ knots of bulk shear. In addition, winds in the lowest levels of the atmosphere in the wake of the warm front may remain more southerly with winds more west-southerly aloft. Where low-level winds remain more backed, large helicity values in the 0-1km and 0-3km are likely with as much as 150 m2s2 in the 0-1km layer and 150-200 m2s2 in the 0-3km layer. 

Forecast bulk shear values of 40-50+ knots. More than sufficient for thunderstorm organization.

At the surface, temperatures are expected to climb into the upper 70's to lower 80's with dewpoints into the lower 70's (even as high as 73-75°F should dewpoint pooling result). Depending on the extent of morning cloud cover and degree of heating, temperatures, especially in the valley, could soar as high as 83-85. Despite marginal mid-level lapse rates (~6 C/KM), the combination of surface temperatures into the lower 80's and dewpoints into the lower 70's may yield moderate instability characterized by mixed-layer CAPE values in the 1500-2000 J/KG range and surfaced-based CAPE values around 2000-2500 J/KG. It should be interesting to note some forecast model guidance is suggesting mixed-layer CAPE values could approach 2500+ J/KG with surface-based CAPE values exceeding 3000 J/KG. These values are difficult to obtain without mid-level lapse rates ~7 C/KM and greater, however, if surface temperatures can near 85 and dewpoints climb into the 74-75°F range, these values could become obtained. 

NAM forecast model developing 1500-2000+ J/KG of mixed-layer CAPE Sunday afternoon.

The combination of moderate instability and strong wind shear ahead of an approaching cold front and likely pre-frontal trough development will be more than favorable for scattered-to-numerous thunderstorms developing by early afternoon and organizing into one or multiple convective lines. This combination of ingredients is also supportive for the strongest thunderstorms to produce locally damaging wind gusts and perhaps even a tornado. 

There are some caveats, however, which may reduce a more organized severe weather episode from occurring. In addition to the marginal mid-level lapse rates, the strongest shortwave forcing remains displaced well to the west of the best overlap of shear/instability with forecast model guidance showing the best shortwave forcing still in southeast Canada between the mid-to-late afternoon. 

All in all, the expectation is scattered-to-numerous thunderstorms develop across interior southeast New York by early afternoon with activity quickly organizing into one or more convective lines. Should the more extreme instability scenarios verify (mixed-layer CAPE > 2000-2500 J/KG) potential for a more widespread damaging wind producing event will increase. Given the larger helicity values, there would even be potential for an embedded tornado or two. Any tornado potential would be highest with any discrete cells which form initially or out ahead of the line. Localized flash flooding will be a concern as well given PWAT values approaching and exceeding 2''. 

Saturday, June 22, 2024

Sunday, June 23, 2024 New England Thunderstorm Discussion

Synopsis

Scattered showers and thunderstorms are expected to develop within New York and Pennsylvania by early Sunday afternoon with this activity spreading east through the day with additional thunderstorm development within New England. The potential will exist for some of these thunderstorms to become strong-to-severe and capable of producing localized damaging wind gusts and/or hail. There is also potential for a few tornadoes within the region. In addition, torrential downpours with any thunderstorms may contribute to flash flooding within typical, low-lying, flood-prone areas.

As of Saturday morning, an elongated stationary front stretched from off the New England coast westwards through the Great Lakes region and into the upper-Midwest. Across New England, the north of the boundary is characterized by a much cooler and less humid airmass than what is in place south of the boundary.

Also as of Saturday morning, multiple pieces of shortwave energy were embedded within the jet stream across the north-central Plains and upper-Midwest with multiple weak areas of low pressure at the surface.


Forecast model guidance is in general agreement this shortwave energy will amplify (strengthen) some as it traverses the Great Lakes region and advances towards New England. This will result in a deepening low pressure at the surface and help to lift the stationary front across New England northwards as a warm front, advecting much warmer and more humid air into northern New England on Sunday morning. This will help set the stage for scattered afternoon and evening thunderstorms.

Dynamics

With a strengthening weather system, we’ll see an increase in winds throughout the troposphere. Forecast model guidance overspreads 40-50 knots of bulk shear by Saturday night through Sunday. These values are more than sufficient for thunderstorms to become organized, and these values are more than sufficient for the more mature storms to take on supercell characteristics.


In addition to winds increasing with height through the troposphere (speed shear) winds will also change direction through the atmosphere (directional shear) with the greatest turning of the winds occurring within the lowest 3 km thanks to the presence of a surface warm front. Forecast model guidance suggests 0-1km helicity (a measure of how winds are changing with speed and height) values on the order of 150-200+ m2s2 with 0-3km helicity values on order of 200-250+ m2s2. These values are more than sufficient for any mature, discrete thunderstorms to become supercells.


Thermodynamics

The combination of temperatures climbing into the 80’s with dewpoints into at least the lower 70’s will contribute to a moderately unstable airmass characterized by mixed-layer CAPE values ranging between 1500-2500 J/KG. What will hold back from an even more volatile atmosphere will be the lack of stepper mid-level lapse rates (change of temperature with height) in the 700-500mb level (typically associated with an elevated mixed-layer). One thing to watch for, however, is the potential for dewpoint pooling, especially within the Connecticut River Valley. Should this occur, a narrow corridor of dewpoints rising into the 74-76°F range would be likely, this could further boost mixed-layer CAPE values (in the 2500-3000 J/KG range) and compensate some for the weaker mid-level lapse rates.

Expectation

Combination of approaching shortwave energy, moderate instability, and sufficient wind shear will result in scattered-to-numerous showers and thunderstorms develop by early afternoon across New York and Pennsylvania with this activity spreading east through the day with showers and thunderstorms firing across New England by early-to-mid afternoon. While there is some potential for widespread strong-to-severe thunderstorms, the overall potential will be held back due to;

1)        The lack of an elevated mixed-layer and associated very steep mid-level lapse rates (> 7-7.5 C.KM).

2)        Assessing some forecast soundings within the region, it is noted there are some subtle warm layers aloft. These subtle warm layers (contributing to weaker lapse rates) will have a negative impact on parcel upward acceleration which can make it difficult for thunderstorms to become explosive and very intense, at least on a widespread scale.


The greatest potential for any thunderstorms to become strong-to-severe will be eastern eastern New York, Vermont, New Hampshire, western and central Massachusetts, and northwestern Connecticut. This is where the strongest thunderstorms will be capable of producing localized damaging wind gusts and hail. There will also be potential for a few tornadoes and some localized flash flooding. 

Saturday, March 30, 2024

Significant Early April Winter Storm Potential

 Forecast model guidance remains in strong agreement in an anomalous evolution and structure of the 500mb pattern moving through the week of April characterized by strong ridging building into the Pacific-Northwest region during the early part of the week and shifting into the Inter-mountain West region along with significant trough amplification within the Northeast as the ridging within the Inter-mountain West continues building northeast into the Arctic domain, it will connect with the ridging within the Arctic domain, characterizing a highly negative Arctic Oscillation and North Atlantic Oscillation. This enhances the amplification of the trough across the Northeast:

12z March 30, 2024 run of the GEFS valid for 12z Tuesday:


12z March 30, 2024 run of the GEFS valid for 0z Thursday:


Highly anomalous weather patterns tend to produce anomalous weather and should this evolution verify, which is becoming increasingly likely, there will surely be some anomalous weather within New England. That anomalous weather would be in the form of accumulating snow and potential for significant accumulations. The period of interest is Wednesday, April 3 - Thursday, April 4, 2024.

Accumulating snow within New England during April can and does happen, however, climatologically speaking it is typically the higher terrain (generally above 1,000 feet in elevation), especially the farther north into New England one goes. In the case of this setup, these areas would stand the greatest potential for significant snowfall accumulations and winter storm impacts. However, given the anomalous nature of this setup, plus plenty of available low-level cold air, The 12z March 30, 2024 run of the GFS shows temperatures across New England at 925mb (~2,500 feet above ground level) just below freezing Wednesday morning:


Now it is important to understand where we are in the calendar. We are moving into the first week of April, average temperatures are much warmer than they are in the previous few months. In fact, during early April the average high temperature across many of the climatological stations is pushing into the 50's. So clearly, there has to be one heck of a cold anomaly in the lowest levels and a strong source of low-level cold advection to obtain temperatures cold enough to support snow for elevations below 1,000 feet and towards the coastal Plain. 

There is another factor to consider with this system as well. The premise of this post focused on the 500mb pattern, however, we did not divulge into the surface. Associated with this anomalous 500mb pattern will be a low pressure system at the surface which slowly progresses eastwards across the mid-western states during the first few days of the week. There is potential for secondary low pressure develop and rapid strengthening. If this occurs, and where it occurs, will have great implications on the sensible within the region. Development and strengthening around or south of Long Island would result in colder low-level airmass to the coast and dynamic cooling could result in even colder temperatures down to the surface. This scenario would increase the potential for snow, even down to the coast and increase the potential for accumulating snow for elevations below 1,000 feet. 

This is going to be a wild few days of storm tracking and this one has potential to yield some major surprises. As of now, the highest potential for a significant winter storm will be for elevations above 1,000 feet and within northern New England:



Wednesday, February 14, 2024

Tuesday, February 13, 2024 Storm Recap - Uncertainty, Mayhem, and Communication

 Well - where do I begin and how do I begin? If you were following the forecasts for this storm you may have noticed a theme; an increase in snowfall forecasts moving through the weekend with some significant increases in some spots Sunday and even Monday morning. Then Monday afternoon came...the 12z European forecast model made a significant jump south, then subsequently various other forecast models followed this theme. At one point, it was becoming a concern that even northern Connecticut may see nothing. This was a stunning turn of events. We've seen forecast models make big jumps 36-48 hours out, even 24 hours out, but 12 hours out, almost unheard of in this day in age of advanced modeling. During the day Monday, forecasts drastically started to change with significantly lowering snowfall totals across northern areas and increasing totals towards the south coast. Then Monday evening and overnight came...and well the rest is history. 

So, what happened? Why did we see forecast model guidance make a near 180 Monday afternoon, make a significant shift south, only for the storm to end up a bit farther north? Is this a case of a horrific performance by our forecast models or was something just overlooked? The easy answer here is a mixture of both. But the answer to this is much more complicated and complex than just a one word answer. Below, we will revisit the storm from the perspective of our forecast models and then apply good ole meteorology. There is a phrase which sometimes gets tossed around in the field of meteorology, "Meteorology not modelology". What does this mean? In a nutshell, forecast is much more than just taking the output of a model(s) and saying, "this is my forecast". Forecasting is an art - its is a skill and the skill level is highest when a forecaster takes ample time to digest all the information and apply their knowledge of meteorology to create a forecast. A forecaster who is skilled in this department can develop a great understanding of what forecast models may have a better handle, where some forecast models may struggle more than others, and can sniff out trends before they even happen.

Below, we will look at the NAM, GFS, and European forecast models. Yes, there are many other models out there but I don't want to make this post any longer than it will turn out to be and the context of what is being discussed can be applied to other models. For a large part, this is more for visual referencing and I am using College of DuPage as my data source, therefore, I am able to be consistent with the outputs. Once inside 24 hours the HRRR will be incorporated as well.

Forecast Models - Initialized 12z Sunday, February 11, 2024

 Here is the forecast 500mb vorticity field from Sunday morning, valid for Monday evening. We want to focus on the northern stream/southern stream energies (shown in the reference just below).


Notice there are some large differences between each model in the strength, structure, and placement of the southern stream energy with some structural differences within the northern stream energy. We can also note some differences within the Pacific-Northwest and northern Inter-mountain West region which likely had some influence on the downstream energies. 


What is the importance with these northern stream and southern stream energies and why are these differences noteworthy? In the event of a stronger southern stream energy, this would increase the likelihood for the storm to track north, passing just south of southern New England, favoring heavy precipitation across much of the region. However, with the northern stream and associated confluence, there would be an extent as to how far north this storm could get. In the event of a weaker southern stream energy, the storm would be more likely to track well south of the region, with much lighter, if any precipitation. The northern stream was a bit more complex. In the event the two energies phased (which was becoming unlikely) or the northern stream had at least some interaction, this would help drive a more north track, increasing the potential for heavier precipitation across the region. 

So, how did these differences at the 500mb level translate to the surface? While there are differences between all three models, notice how the European model is south and east with the western edge of the precipitation shield. This becomes important as we get closer to the event:


Let's fast forward to Tuesday morning and see what the 12z runs of the NAM, GFS, and Euro were forecasting in terms of QPF for early Tuesday morning. While there are certainly some differences, the one striking aspect of this is heavy precipitation across all of southern New England. This signal (amongst other factors) was increasing confidence for a significant region-wide storm:


This intense precipitation had substantial merit based on the following:

1. Surface low pressure passing south and east of the region (shown just above).

2. Mid-level/low -level pressure developing just south of the region and tracking northeast (shown just below).

3. Entrance region of the upper-level jet stream favoring upper-level divergence (not shown yet).

Each of these forecast models was developing a low pressure at the 700mb level around or just south of Long Island and tracking it east-northeast. This is nearly textbook for a band of extremely heavy precipitation to move right across the region:


Forecast Models - Initialized 12z Monday, February 12, 2024

Fast-forwarding to Monday's 12z runs the NAM and GFS were continuing to advertise heavy precipitation across much of southern New England in similar fashion to Sunday's guidance. The Euro, however, made a stride south. When this occurred, this started to raise some eyebrows:


 Forecast Models - Initialized 18z Monday, February 12, 2024

Forecast model guidance Monday afternoon continued with the theme of shunting the heaviest precipitation south, with some guidance barely getting heavier precipitation to the Connecticut-Massachusetts border. The Euro was leading the way with the south trend:


Forecast Models - Initialized 0z Tuesday, February 13, 2024 (Monday evening)

As Monday evening forecast model guidance was rolling in, a mere several hours before the onset of precipitation, forecast models continued with the more south track and a south shunt of heaviest precipitation. It was now becoming apparent along and north/west of I-84 very little snow was going to occur with the highest totals across southern Connecticut and into Rhode Island and perhaps Southeast Massachusetts:


The 0z NAM, GFS, and European forecast models had cut back significantly on the amount of precipitation which was expected to fall across southern New England. Based on these forecast totals, it was difficult to believe northern Connecticut (along and north/west of I-84) would see more than 3-4 inches with totals upwards of 6-10 inches across southern Connecticut into portions of Rhode Island and southeast Massachusetts. This was turning into one of the largest model busts inside of 24 hours in recent memory:


The Result

So, what was the result? How did the storm fare. Below, is a compilation of snowfall totals received by the National Weather service:


 What...in...the...heck...? What happened across central Connecticut? Forecast model guidance, just hours beforehand, had cut back on forecast precipitation so much, it was starting to become clear northern Connecticut may not even see a few inches with barely 3-4 inches for the Hartford area, yet over a FOOT occurred in a narrow swath from northeast Connecticut southwest through Hartford into Danbury with 7 inches as far north as Windsor Locks. What an absolute wild, bizarre turn of events. Within 24 hours we go from what looked to be a crushing from the Massachusetts Turnpike into central Connecticut to nothing from along the Massachusetts turnpike into northern Connecticut and barely more than a few inches into north-central Connecticut to a narrow strip of a foot-plus from northeast Connecticut through Hartford southwest into Danbury. How does this happen?

Theory

Did our computer forecast models let us down? Did they just perform one of the greatest busts in recent memory, and perhaps one of the greatest busts inside of 12 hours ever? Was something overlooked by forecasters? As stated in the opening, there is a combination of these two factors. There is no denying forecast models struggled, however, there were hints and signals this scenario was still likely to occur. So why were these missed? Below, I will present some of my theories and ideas. I want to state that this is just my opinion as a professional meteorologist and just because this is my opinion does not mean it is correct. Others may disagree and have a different viewpoint. That is totally fine, differing viewpoints is what drives this filed and is it what drives a forecaster to want to better themselves. 

1) The use of model snowfall maps. 

Don't get me started on these. One of these days I am going to dedicate a blog post to these so I won't provide much specifics about this right now. Since modeled snowfall maps (which aren't produced by forecast models, they're produced by vendors) have been introduced, I believe they have led to lazy winter weather forecasting. If you follow weather on social media, these maps are so widely used and tossed around, especially when they're spitting out large numbers. 

For this storm, there was a lot of analysis going on by using trends in the snowfall output to justify changes within the storm. My simple answer here is - that is a terrible idea. If a forecast model (or models) are showing a decrease (or increase) in precipitation, a forecaster should be asking themselves, why that is and whether that makes sense. 

2) Solely focusing on QPF maps.

QPF maps are an incredible forecasting tool. While forecast models can struggle greatly with QPF, assessing QPF output and combining this with an assessment of the situation is a major part of forecasting snowfall accumulations or rainfall accumulations. Like the snowfall maps, however, QPF trends were being analyzed to visualize the trends but again, there needs to be a, "does this make sense" thought process behind this. 

3) The importance of Fundamentals 

Forecasting is an art. Being a good or great forecaster requires knowledge of forecast models, knowledge of historical weather and patterns, and an understanding of how the atmosphere works. A great forecaster isn't afraid to be wrong, learns from their mistakes, and a great forecaster understands there is much more to a forecast than being right or wrong. A forecast that is correct, but for the wrong reasons is not correct. Forecasting is much more than looking at the output of a forecast model and then rolling out some thoughts. Creating a great, accurate requires a great deal of time. 

I saved this point for last because I wanted to elaborate into greater detail on this aspect. Ultimately, applying fundamentals to the forecasting process, even when forecast model guidance was decreasing the precipitation totals may have prevented the panic of reducing snowfall totals across much of the region (outside of along and north of the Massachusetts turnpike). Above, we looked at how the NAM, GFS, and European forecast models were cutting down precipitation totals and shifting the highest totals southwards. But what we didn't look at was whether this made sense. 

Anyone who understands southern New England snowfall climatology understands that our heaviest snows and biggest storms tend to occur when low pressure at the surface takes a northeast trajectory and passes through the latitude/longitude of 40°N/70°W. Additionally, when low pressure centers develop just southwest of Long Island and strengthen as they move northeast this favors the heaviest banding of snow to impact the region. What was just described here within this paragraph is exactly what forecast models were indicating multiple days out. This is why, especially as we got closer, the storm was being hit harder and forecasts for snow were increasing. There was increasing confidence and consistency in this exact scenario occurring. 

So, what happened in these last 24 hours. Did something drastically happen with the surface low pressure track and development/track of the low pressure centers at 850mb and 700mb? Let's take a look.

0z NAM - Initialized Tuesday, February 13, 2024 (Monday Evening) 

850mb Vertical Velocity and Height - 15z Tuesday (Left) 18z Tuesday (right)

Vertical velocity is a measure of how quickly air is rising or sinking. The more positive the value, the more rapidly air is rising, the more negative the value, the more air is rapidly sinking. This chart also displays the height level (in meters) the 850mb level can be found. I've outlined where the NAM had developed the 850mb low and where it was tracking. If you look at the vertical velocity values, you'll notice extreme upward vertical motion (rising air) right within the center of the low pressure. Scrolling back up to the precipitation field, you'll see this matches very closely with where the 0z NAM had the heaviest precipitation. Going back to fundamentals, we would expect or anticipate the strongest upward vertical motion to be northwest of the low. I've outlined where this would be expected in purple:


700mb Vertical Velocity and Height - 15z Tuesday (Left) 18z Tuesday (right)

At the 700mb level, the NAM was not forecasting a closed off low pressure at 15z, however, by 18z a low was closing off just south of Cape Cod. We continue to see the highest vertical velocities very close to the low center. I've once again outlined where you would expect the strongest vertical motion to be in purple:


0z GFS - Initialized Tuesday, February 13, 2024 (Monday Evening) 

850mb Vertical Velocity and Height - 15z Tuesday (Left) 18z Tuesday (right)

The 0z GFS was similar to the NAM at the 850mb level with 850 low development south of Long Island and tracking northeast. Like the NAM it was pegging the greatest vertical velocities towards the center of the low. Based on 850 low development and track you would expect the greatest lift to be north and to the west:


700mb Vertical Velocity and Height - 15z Tuesday (Left) 18z Tuesday (right)

At the 700mb level some noticeable differences versus the NAM and if you recall, even during the 0z run the GFS was more north with the heaviest precipitation and that matches up better with what you'd expect based on the low tracks:

0z Euro - Initialized Tuesday, February 13, 2024 (Monday Evening) 

850mb Winds and Height- 15z Tuesday (Left) 18z Tuesday (right)

Unfortunately, there is no vertical velocity plot for the European forecast model on College of DuPage, however, we can still view 850mb and 700mb low via height field overlaid with wind field. While the Euro is farther south with the 850mb low, based on the track you would still expect heavy precipitation at least back to the I-84 corridor:


700mb Winds and Height - 15z Tuesday (Left) 18z Tuesday (right)

Similar to the 0z NAM, the European model was not developing a closed low pressure at 700mb by late Tuesday morning, however, it was by early Tuesday afternoon. Based on the development and track, you would expect the strong upward vertical motion north and west of the low and right across southern New England. 

Assessment 

While forecast model guidance continue to shift the axis of heaviest precipitation south and subsequently lower precipitation totals, based on how forecast models were continuing to evolve the low-levels and mid-levels of the storm, this did not make sense. Forecast models were placing the greatest vertical velocity values close to the low pressure centers, which was a vast change from earlier runs where forecast models had the strongest vertical velocity values moving across southern New England and northwest of the low pressure tracks. 

There was another signal in place for enhanced upward vertical motion across southern New England. Forecast models had southern New England in the right entrance region of the upper-level jet, a favorable quadrant for rising motion:


Verification

As Tuesday morning arrived, many were likely surprised as majority of Connecticut was experiencing not only heavy snow, but portions of central Connecticut were into some prolific snow with rates exceeding 2 inches per hour! What the heck happened? SPC mesoanalysis showed intense 850-700mb frontogenesis traversing across Connecticut, which is exactly what you'd expect based on the forecast low tracks:


So the question now remains, why was forecast model guidance continuing to shift south with the axis of heaviest precipitation and why did they back off the the heaviest precipitation north and west of these low pressure tracks? Unfortunately, that is not something I have much of an idea on, however, forecast models can underdo precipitation north and west of the low tracks. As we saw with the 0z guidance, forecast models were pinging the greatest upward motion very close to the low centers. Typically with these bands of heavier precipitation induced by developing and strengthening low pressure centers, as the low pressure centers develop and mature, a band of heavy precipitation will develop well north and west of the low pressure centers, sometimes as far as 50-75 miles north and west. In situations where these lows become stronger, the banding will then collapse closer to the low pressure center. In this situation, the forecast models were bypassing the initial step here and just developing everything in close proximity. 

Ultimately, all forecasters should have held firm with original forecasts and not go into panic mode just because forecast models were cutting back on QPF and because model snowfall maps were indicating less snowfall. While this certainly should have been a flag, or a cause for concern, a more thorough and detailed analysis should have been conducted to determine why that was occurring and whether it made sense. It is also very likely the evolution of the northern stream energy, southern stream energy, and their interaction ended up being different than forecast models anticipated 12 hours out. These may have all ended up evolving in a way favoring a bigger hit. 

Below, was my forecast for this storm (top) versus what occurred. One could say all I had to do was just shift this down some miles and tighten the gradient. If I had more time/energy I would have made an updated map but l would have not made those changes. I would have went significantly less for Connecticut and maybe do 4-7 inches southern and 2-4'' northern so it would have been much worse if I did something so this map makes the forecast better than it would have been. I would have been JUST AS GUILTY for lowering totals and just taking the reduced precipitation totals and running away. I did also scream bust Monday evening. 


For anyone involved in weather forecasting, there is no doubt this event was a tremendous learning experience. We can't forget the basic fundamentals of forecasting and sometimes we need to rely on our knowledge and understanding of the atmosphere over computer forecast models. While computer forecast models are excellent and allow us to do our jobs, they are not perfect and we must remember, there is a recent they are called model guidance. 

This was also a great test in communicating weather and communicating uncertainty, especially as we get within hours of a weather event occurring. We live in a day and age where everyone expects perfection and everyone wants pinpoint details days out. Unfortunately, we must remember it doesn't always work out that way. 


Sunday, February 11, 2024

Tuesday, February 13, 2024 Winter Storm

 A significant winter storm is expected to impact southern New England overnight Monday through Tuesday afternoon. Southern areas, especially towards the coast will see rain to start, however, any rain is expected to transition to snow. Portions of the region will see a heavy, wet snow with strong winds along the coastal Plain. This will introduce the likelihood for scattered power outages. 

Synopsis

A potent piece of shortwave energy within the southern stream becomes an open wave as it rapidly ejects northeast from the southern Plains through the Tennessee Valley and towards the northeast. Meanwhile, shortwave energy within the northern stream undergoes slight amplification as it digs southeast across Canada. Several days ago, there were signs these two energies would phase. This would have resulted in an even more powerful storm, but could have raised concerns about track and how far north the rain/snow line would reside. The more likely scenario now is the northern stream acts as a bit of a kicker, this should help the system track a bit farther south and east, increasing the likelihood that most everyone sees accumulating snow:


Surface low pressure associated with the southern stream vorticity initially is located well inland, closely matched with the placement/track of the vorticity as the system is initially closed. However, as the wave opens up, the surface pressure will weaken with a secondary area of low pressure developing around Delaware/Maryland then tracking east-northeast, passing south of Long Island. The low-level and 700mb low pressure development track will be a bit northwest of the surface low. This will favor heavy precipitation traversing much of southern New England:


The 12z GFS suggests intense lift traversing across southern New England evident by strong frontogenesis moving across the region. This is very consistent with several other forecast models as well:


Discussion

The most noteworthy aspect of this storm will be the forward speed. This will be a factor in preventing greater accumulations from occurring. The worst of the storm will likely be confined to a 3-4 hour window where snowfall rates will range 1.5'' - 2.5'' per hour. Another factor influencing accumulations will be snowfall ratios. The greatest ratios will be across the far interior where a colder thermal profile will support greater snow growth and fluffier snow. Snowfall ratios across the far interior should range anywhere between 10:1 and as high as 13:1. Moving south across the region (through central and southern Connecticut into southeastern Massachusetts) snowfall ratios may struggle to near 10:1 and could be less. 

With a deepening surface low pressure, winds will increase towards the coastal Plains where winds could gusts 30-40 mph. This may induce blizzard conditions at times and lead to very low visibility. Inland winds may gust 20-30 mph yielding low visibility and some blowing/drifting snow where the snow is drier in nature. 

Power outages could become a concern towards southern areas where wind gusts will be stronger and snow will be more on the wet and heavier side. 

With this, let's get into the forecast:

  • Precipitation begins across southwestern Connecticut during the mid-to-late overnight Monday with precipitation overspreading areas to the north and east during the early morning. Many areas (especially along and south/east of I-84) may even briefly begins as rain or a wintry mix, however, precipitation should quickly change to snow. 
  • Snow intensity picks up extremely quickly, with the heaviest of the snow falling between about 9:00 AM through about 3:00 PM before rapidly tapering off. Snow tapers off quickly from the southwest to the northeast with the more intense snows occurring later and ending later farther northeast in the region. 
  • Snowfall rates during the height of the storm range between 1.5'' per hour and 2.5'' per hour. 
  • Winds along the coast gust 30-40 mph. Scattered outages are possible due to strong wind gusts combined with heavy, wet snow. 
  • Winds farther inland gusts 20-30 mph. Power outages will be more on the isolated side and occur where snow is on the wetter side. 
  • Major travel impacts are likely Tuesday. Travel is not recommended unless it is high priority.
Below is what I am thinking:


 

Saturday, January 13, 2024

Sunday, January 14, 2024 Snow Squalls

 A potent Arctic cold progresses across New England Sunday. While the brunt of the Arctic air remains confined to the central United States, upper-Midwest, Ohio Valley, and Tennessee Valley regions, it will still be very cold and wintry across southern New England behind the front. 

The leading edge of the Arctic air will be defined by elongated and potent vorticity. A vorticity maximum will round the base of an amplifying trough across our region and strengthen, resulting in a great deal of positive vorticity advection across New England Sunday:

This system will be associated with vigorous dynamics characterized by a 500mb mid-level jet streak which may approach or exceed 150 knots. This is about as strong of a mid-level jet you'll ever see and will help promote vigorous lift across the region:


Associated with the feature will be steep mid-level lapse rates ranging between 6.5 C/KM to as high as 7.7-7.8 C/KM. These lapse rates will help aid in weak instability within the region. This weak instability will aid in lift:

In addition, there will be a very steep lapse rate from the surface to about 15,000 feet above the ground. This bufkit sounding for Windsor Locks, CT illustrates this well. This look is suggestive of instability:


The response from the strengthening shortwave and positive vorticity advection will be a strengthening southerly flow in the low-levels of the atmosphere which will help transport sufficient moisture into the region to interact with the front. The steep low-level lapse rates will also help to promote the mixing down of stronger winds aloft:


What are we looking at? 

As moisture increases along and ahead of the Arctic front, scattered snow squalls are likely to develop ahead of the cold front with potential for a line of moderate-to-heavy snow to develop along the Arctic front. This can be thought of like summer downpours and thunderstorms where you get a 15-30 minute period of heavy precipitation, but in this case instead of rain you have snow occurring. 

These snow squalls could produce 2-3 inches of snow in a very short amount of time on a local level. Strong wind gusts would occur as well and this would contribute to extremely dangerous travel conditions for a brief period of time as visibility drops to almost zero and roads become icy very quickly. There is even potential for thunder and lightning given the steep lapse rates and instability. If you are caught in a snow squall the best thing you can do is pull over or off the road (in a safe spot) and let the squalls pass. Snow under squalls becomes intense very quickly and conditions can deteriorate in the blink of an eye.

The time frame for these snow squalls is between 1:00 - 5:00 PM. 




Friday, January 5, 2024

Sunday, January 7, 2024 southern New England Winter Storm

 Our first widespread, accumulating snowfall event of the season is upon us! Snow lovers were starved after pretty much a shutout (some places did get some snow) during the month of December. If you happened to read my winter outlook, I mentioned January and February were going to be active and we are off to those races! Anyways, let's get right into all the fun science and meteorological applications and thought process behind the forecast.

Synopsis

A frontal system moving across the Gulf Coast states Friday lifts northeast across the mid-Atlantic region during the day Saturday. An area of surface low pressure develops within the mid-Atlantic and strengthens as it lifts northeast off the mid-Atlantic coast and passing south and east of Long Island:


Discussion

As the low moves over the open waters, this is where the fun begins! Thanks to the temperature gradient which exists between the water and land this time of year, combined with the jet stream dynamics, the surface low will begin to strengthen. As the surface low strengthens and the circulation elongates we'll see a low pressure center develop about 5,000 feet above the ground. Thanks to a strong latitude horizontal gradient, this will help to strengthen this low pressure circulation as the system is passing northeast south of Long Island.:


This track of the 850mb low, combined with the fact that it will be strengthening places a good portion of southern New England in a very favorable spot for an intense band of snow to traverse. This is indicated very well assessing 850mb frontogenesis on both the GFS (left) and NAM (right) forecast models:

Several forecast soundings across the region all show good cross hair signature (when maximum lift bisects the snow growth zone). When/where this is occurring, snowfall rates will be heaviest, snow growth will be best, and snow ratios will be highest. We'll look at a few examples below:


This system will also have a copious amount of moisture to work with. Forecast model guidance develops 50+ knots of easterly inflow at 850mb, throwing a great deal of moisture into a cold low-level airmass. The result will be plenty of snowflakes developing:


While we're well within the 48-hour window of start time, there remain a quite a bit of uncertainties within our forecast model guidance. This has become an increasingly annoying theme the past few years. There are differences with the overall track and speed of development. Some forecast models are on the more amped side, resulting in a farther north and west track and development. This introduces warmer air into southern areas and would also result in the greatest frontogenesis and subsequent banding shifting farther north. This would also increase potential for subsidence in southern areas. 

Forecast 

  • The highest totals should be confined to interior east-central Massachusetts into southern New Hampshire, and southwestern Maine. This is where snowfall ratios should be the greatest throughout the duration of the storm and where the intense banding is likely to persist the longest. In fact, there may be a swath of snowfall of 12-16 inches within this zone!
  • Towards southern Connecticut. snow ratios will be held back due to some warmer air in the lower levels, just above the surface. Snowfall ratios improve briefly as the intense banding lifts north. 
  • Easterly surface winds off the warmer water will hold back accumulations greatly across extreme coastal Connecticut, southern Rhode Island, and far eastern Massachusetts. 
  • Snow will also be on the wet side across southern Connecticut into interior southeast Massachusetts. This could result in isolated downed tree limbs and power outages. 
  • Snow begins Saturday evening (between 7:00 PM-10:00 PM from southwest to northeast), with intensity quickly picking up through the evening. 
  • The worst of the storm is between 11:00 PM - 5:00 AM, however, for northeastern areas this will be extended through much of Sunday morning. 
  • Light snow lingers through much of Sunday with moderate to at times, heavy snow across interior northeast Massachusetts. 
Below is my snowfall forecast: