Tuesday, March 14, 2023 Significant Winter Storm

 Potential is increasing for a substantial mid-March winter storm to impact southern New England Monday night and Tuesday. This will be an extremely challenging forecast due to some large-scale and mesoscale factors which will play major roles in the storm's outcome. A scenario of rain transitioning to heavy, wet snow with strong winds is likely. Combination of heavy, wet snow and strong winds will also cause potential for tree damage and power outages. 

During the day Monday, shortwave energy within the northern branch of the jet stream digs southeast across portions of the Ohio Valley and Northeast with shortwave energy within the southern branch of the jet stream out ahead of it. The greatest questions we've faced the last few days is will the northern stream and southern energy phase, and if so, how does this process impact storm evolution? 

Over the past few days, forecast model guidance and their ensembles have been hinting a scenario in which the northern stream energy digs south of Long Island and closes off as it phases with this southern stream energy. If you're a snow lover in southern New England, this is a scenario that you certainly want to see. But with this still comes challenges. 

1. If the northern stream is too strong (or too amped) a scenario which pulls the developing surface low pressure too far northwest, hooking into perhaps Connecticut would result in a significantly warmer scenario for many, yielding mostly rain with the heaviest snow perhaps combined to the Berkshires of western Massachusetts or farther west into New York (such as the Catskills). 

2. If the northern stream energy is too fast, the southern stream won't have as much time to get out ahead of the northern stream energy. This would have a significant impact on phasing potential and could mean a weaker and much farther southeast low pressure. This could be good for snow, however, a more significant storm would become far less likely. 

Of course, perhaps the greatest challenge overall, is the antecedent airmass. We're lacking one major ingredient...cold temperatures. Some of our largest snowstorms and snowstorms which feature rain transitioning to snow, involve a large cold high pressure north of Maine, that is also lacking here. So, if there is no cold air, how are we going to get snow? Dynamics baby...we are going to be HEAVILY relying on dynamical process to help cool the tropospheric column which will help support rain transitioning to snow, especially under the heaviest banding. 

Ensembles from the European forecast model (shown below) and GFS forecast model (not shown) are in relatively strong agreement in a bit later of a phase. What this does is allow the surface low to be well southeast off the New England coast, however, as the phase occurs the surface low (not only rapidly deepens) but gets tugged northwest, towards the elbow of Cape Cod and tracks northeast. This is a nearly perfect track for heavy snowfall across a large portion of New England:

The details still need to be ironed out, however, at this stage, confidence has increased enough in the potential for a significant storm to create a first call forecast. Over the next 24-36 hours, the focus will shift towards pinning down and sniffing out the mesoscale details. Below are some factors which need to be closely assessed and taken into account:

1. Subsidence - In all likelihood, one or multiple bands of very heavy snow (CCB - cold conveyor belt) will likely develop. In between these bands will be subsidence (sinking air) which could significantly impact snow totals in this zone (as in much lower totals). Meanwhile. under the heaviest banding, totals could be significantly higher. 

2. Dry slot - Depending on storm track, and especially track of the 700mb low, a dry slot could traverse portions of the region. This would cut off precipitation, perhaps entirely.

3. Snowfall ratios - Ratios will be key, especially with the larger end of accumulation potential. Given the marginal airmass, snowfall ratios may be as low as 6:1 to 7:1. Under the heaviest banding where it is more likely to generate powerful upward vertical motion into the dendritic snow growth zone, ratios will likely exceed 10:1. 

4. Wind - Strong winds are likely, especially towards eastern Massachusetts. Wind gusts of 60-80 mph are a possibility and this would result in numerous power outages. Farther inland, gusts of 40-60 mph would be possible. Combined with heavy, wet snow this could yield numerous power outages and widespread tree damage. 

Below is my current thinking. Again, as mesoscale details become a bit more clear or confidence increases in this aspect, this map will likely be fine tuned a bit more. My next update is scheduled for late Sunday afternoon or early Sunday evening.

02/27/2023 - 02/28/2023 Accumulating Snowfall Event

 Well...it has certainly been a while since I fired up a blog post. This has been a horrific winter in the snowfall department across Connecticut. With this said, there is increasing confidence in a statewide accumulating snowfall event Monday evening into Tuesday morning. I don't have time (or energy) to go deep into the science stuff, so I'll just keep it a bit simple. 

A vigorous and potent piece of shortwave energy currently across southern California moves across the Southwest Sunday and then rapidly ejects northeast across the central Plains Sunday night, across the Missouri Valley Monday morning, then east-northeast across the Ohio Valley Monday afternoon. As the shortwave rapidly ejects northeast with a very strong jet stream it will start to become sheared apart and weaken.

At the surface, low pressure moves northeast across the Great Plains through the Missouri Valley and upper-Midwest. The track of this low will push a warm front towards Connecticut. At the same time, a secondary low pressure may develop south of Long Island. It is this push of warm air, warm air advection, which is the focal point of this system and a several hour period of moderate-to-heavy precipitation to traverse the state. This push of warmer air comes with some intense lift 

18z NAM for Windsor Locks, CT late Monday evening and early overnight shows some intense lift into the dendritic snow growth zone which would be suggestive of a several hour period of moderate-to-heavy snow. 

So what can we expect?

• Snow begins to overspread the state Monday evening with moderate-to-heavy snow falling across much of the state during the late evening through mid-to-late overnight.
• Snowfall rates of 1'' to 1.5'' per hour are likely during the most intense periods of snow. 
• Snow will also transition to rain across the central and southern part of the state moving into Tuesday morning as warmer air pushes in and the stronger lift exits. 

Below is what I'm currently thinking

Major Winter Storm Likely for Northwest Hills of Connecticut late Thursday night and Friday (12.16.2022)

We are very close to a significant winter storm for the majority of Connecticut outside of the coast, however, it appears the pieces just won't come together for that to happen with the exception of northwest Connecticut. 

The forecast weather pattern for Thursday evening features a very large closed upper-level low pressure system over the upper-Midwest region embedded within a longwave trough. Along the southern periphery of this upper-level low, embedded within the jet stream, will be several vorticity maxima's. Our area of focus (highlighted in purple box) is vorticity which is curling poleward towards our region:

As we now look at the 500mb jet stream and stick to the purple highlighted box, we see a very favorable region of surface convergence and upper-level divergence which will net the development and strengthening of a secondary low pressure right along the mid-Atlantic coast. As the system strengthens low pressures will also form aloft:

So why isn't the whole state getting snow? While we have cold air in place ahead of the storm and a supply of cold air to the north, unfortunately where the storm is strengthening and how the strengthening occurs will result in a surge of warmer air moving into much of the state. Due to the storm track colder air may remain locked in across the northwestern part of the state, keeping all precipitation in the form of snow.

As cyclogenesis occurs close the coast (and just inland) a strong easterly flow will develop within the lower-levels of the troposphere and at the surface. Unfortunately, since we are on the eastern side of this development, we'll see an easterly flow become established across the state which will yield an influx of warmer marine air:

The biggest question we still currently face is, where does this low pressure and resultant mid-level low pressure track? If the track was more northeast or even a bit more east-northeast, we would turn the wind direction across the state to more northeast to north-northeast and would lock colder air in place and that would give a predominately statewide snow event. However, guidance continues to suggest the low pressure will track more north-northeast. This will keep the majority of the state in a warmer east-northeast wind direction, with the exception being the northwest hills. 

Forecast models indicate that intense lift will be traversing the state Thursday night and the first half of Friday with the first wave of intense lift moving into the state during the early overnight. It is very possible that under the most intense lift, the northern half of Connecticut could see a period of heavy, wet snow with large flakes. Where the temperature profile is favorable in northwest Connecticut, this will yield very heavy and wet snow:

18z/14 NAM bufkit sounding from northwest Connecticut shows this intense lift described above with over 40 units of omega within the dendritic snowgrowth zone for a period overnight Thursday. This would result in a period of extremely heavy snowfall with rates upwards of 2-3'' per hour:

So what can we expect?

• Precipitation begins to move into the state Thursday evening. Across the northern part of the state, precipitation may be a rain/snow mix, however, late evening and early overnight when stronger lift traverses the state, this may yield a few hour period of heavy went snow. 
• Any areas with mixing will changeover to rain as warmer air filters in and the degree of lift weakens. 
• The northwest hills should remain all snow for this event, however, there could be some mixing during any lulls of precipitation when lift is weaker.
• Precipitation begins to wind down Friday evening and as colder air filters in on the backside the majority of the state (with the exception of the shoreline) could end as some snow with maybe an inch or two of accumulation.
• It will also be on the gusty side with sustained winds 10-20 mph with gusts 25-30 across the state.
• Isolated power outages possible across northwest Connecticut due to the wet nature of the snow.

Below is my current forecast:

Accumulating Snowfall Event Likely Sunday, December 11, 2022

 A potent piece of shortwave energy traversing the jet stream passes southeast across New England Sunday afternoon and undergoing minor amplification:

A weak wave of low pressure develops during the amplification process. The system will acquire moisture as the shortwave tracks across the Great Lakes and will draw in moisture from the Atlantic Ocean. With a cold enough airmass in place across the state precipitation should be all snow. The only exception may along the immediate shoreline where surface temperatures may be a bit too warm for snow, however, the airmass will be quite cold aloft and the warm layer will be relatively thin. 

Forecast models are in strong agreement that the low pressure will track over Connecticut Sunday afternoon into early Sunday overnight with forecast models indicating a several hour period of moderate lift traversing at least the western part of the state. The 18z/09 GFS bufkit for Windsor Locks, CT (BDL) indicates moderate lift within the dendritic snow growth zone early Sunday evening then briefly again overnight Monday. With sufficient moisture within the snow growth zone a period of moderate snow is likely:

Based on the forecast model agreement and likelihood for a period of moderate lift to traverse the state, a widespread 2-4'' is likely across the state with totals held down a bit along the immediate shoreline where surface temperatures may be a bit on the warm side. 

What we're looking at:

• Light snow arrives during the afternoon with snow coming down moderate at times during the evening and early overnight with snow tapering off during the overnight. 
• Roads will become slick as the afternoon progresses and expect a slick evening commute. 

Below is my initial forecast:

December 2022 Second Half Preview

 If you're a snow lover or an active weather pattern lover in the Northeast then there is a lot to be excited about as we move into the second half of December and for the final stretch of 2022. There has been strong consistency and agreement within long-range computer forecast models, global teleconnections, and evolution of the stratosphere in that the ensuing northern hemispheric pattern will be one that has historically favored an active storm pattern. Now this does NOT guarantee it will be snowy, but the potential will be there. I want to stress...while the potential is there and the look of the pattern is favorable there are some flags and we need to keep these flags in mind. So let's dive right into it. 

North Atlantic Oscillation (NAO):

One of the signals we will assess is the North Atlantic Oscillation (NAO). Computer forecast model ensembles are in strong agreement that the NAO will becoming extremely negative moving through the first week of December. The NAO measures the difference in sea-level pressure between the Azores high pressure and Icelandic low pressure. We typically get a visualization of the NAO by assessing 500mb height anomalies with a focus on the domain centered around Greenland. A negative NAO is typically characterized by above-average height anomalies around Greenland (higher pressure) with a positive NAO characterized by below-average height anomalies around Greenland (stronger low pressure). 

As we move towards mid-month, the NAO remains negative, but becomes less negative. Historically, during periods where the NAO is transitioning from extremely negative to less negative (or even positive) that transition period has been tied into East Coast storms. Why is this? Without getting too much into detail (otherwise it would take me until after this period to complete the post) the negative phase of the NAO correlates to a suppressed jet stream which can mean a storm track south of the Northeast. The negative NAO (also coined the term "block" can keep a storm track close to the coast. So if we visualize this, if a storm is coming up the East Coast (so associated with a trough- dip in the jet stream) the response upstream has to be a ridge. As the storm is evolving you'll start getting this dip in the jet stream to poke into the domain where the NAO is measured - this results in a weakening negative NAO - hence the rising index. Again, it's way more complex then that and this a very basic explanation. 

The NAO can also help drive Arctic cold into the eastern United States, however, this is not always the case as structure and placement of the core anomalies are critical and you must have a build-up of cold air in Canada. 

Arctic Oscillation (AO):

The Arctic Oscillation (AO) is a very close relative of the NAO, however, this is a measure of the polar vortex. you may have heard about the polar vortex as it has made it's way into the news the past several years. The polar vortex exists 27/7/365 and is a fixated low pressure over the Arctic. It's typically extremely weak during the summer and strengthens moving into winter. However, the strength of the polar vortex can fluctuate significantly during the winter season and there are some seasons where it can be very strong or very weak. The AO measures this. When the AO is negative, the polar vortex is in a weakened states and that can correlate to a suppressed jet stream with an increased likelihood for Arctic air to invade the United States. When combined with a negative NAO that can lock-in this cold. The caveat here, however, is you have to have a build-up of cold air in Canada to advect south and you need a cross-polar flow to direct this south.  

Like with the forecast of the NAO, computer forecast models are in strong agreement that the AO will follow suite and tank as we move through the first week of December then start to rise and become less negative towards mid-moth. This would help reinforce a blocking pattern which essentially would decrease the chance for any weather system to track off to our west (which typically results in warm-air advection meaning more ran and less snow) and can decrease the potential for a storm to track too far off the coast to miss:

East Pacific Oscillation (EPO)

The East Pacific Oscillation (EPO) is very similar to that of the NAO in the Atlantic. The EPO is a dipole of anomalies with one pressure anomaly in the Gulf of Alaska vicinity and one pressure anomaly of opposite sign south of this. When the EPO is negative this corresponds to above-average height anomalies within the Gulf of Alaska region (indicating a ridge of high pressure) with a trough south of this across the southern Pacific. It can said that the EPO (negative phase) can be a huge driver in delivering Arctic cold into the United States as the ridging into Alaska can efficiently displace colder air over the Arctic southwards. When in tandem with a negative NAO/AO cold Arctic outbreaks are nearly a given in the East. 

The forecast for the EPO is to become quite negative trough this first week of the month before becoming less negative moving towards the second week of the month. What really stands out here, however, is the extreme uncertainty moving towards mid-month as evident in the very large spread. This is important and will be discussed a bit later on:

Pacific-North American (PNA):

The Pacific-North American (PNA) is to the EPO as the AO is to the NAO (for the most part). The PNA measures the height anomalies in the vicinity of the eastern Pacific and western United States. When this index is positive it corresponds to above-average height anomalies within this region (indicating a ridge of high pressure). This enhances the likelihood for a trough to dig into the East. So, when in tandem with a negative EPO the potential for an Arctic outbreak of cold weather becomes enhanced across the East (especially when you have a negative NAO and AO. When the index is negative you have a reversal of this pattern (trough in west and ridge in east). 

Like with the forecast for the EPO, there is an extremely large spread in the state of the PNA moving towards mid-month after the negative PNA in place during the first week of the month. This will be important too and discussed below:

Assessing the Day 5-day averaged 500mb height anomalies for the day 12-16 period (December 12-17) we see a classic, textbook blocking scenario with the negative NAO/AO signal with heights well above-average across Greenland and the Arctic. As we look at the Pacific, we don't see a terrible look, however, the signal is much more mixed. How the Pacific evolves as we move into mid-month will be extremely critical:

So, what can we take from all this? There is a strong signal for storminess around mid-month. This storminess may also be a precursor to what could be a very active pattern for the second half of the month. for snow lovers, the mid-month period may be a bit difficult with the exception of interior Northeast/New England. Unfortunately, despite the blocking, we may be lacking a good solid Arctic airmass and may be dealing with an airmass that is more temperate and Atlantic nature (this is evident by the lack of cross polar flow and despite the fact we have negative height anomalies in the East during that period, they are retrograding from the Atlantic so the source region is that region as opposed to the Arctic.

HOWEVER, as we move into the second half of the month, there are signals that we could get enhanced support from the stratosphere and a Pacific which may become better configured. A better configured Pacific could be the key to dump Arctic air into the East, however, we first need to get Arctic air into Canada which the signals would indicate is a good chance.

All in all, if you're a snow lover in the East there is a lot to be excited about, HOWEVER, there are still some flags and details to iron out but at this stage, all you can ask for is potential and a favorable pattern and that's what we have. We'll see how this goes.

Tuesday, July 12, 2022 Thunderstorm Potential Update

Scattered showers and thunderstorms are expected to develop across New York and Pennsylvania Tuesday afternoon and progress east-southeast across the Northeast into New England ahead of an approaching cold front and developing pre-frontal trough. With the pre-frontal trough now in the picture, this may lead to earlier storm development. 

Severe Thunderstorm Potential 

The potential for widespread severe weather is exceedingly low, however, the potential exists for isolated-to-scattered pockets of severe weather across central/eastern New York down into Pennsylvania with severe weather potential more localized across New England. The greatest risk is for damaging wind gusts. 

In terms of the set-up there are actually some impressive ingredients. An unseasonably strong mid-level flow will overspread the Northeast with 50+ knots of flow at 500mb and 40+ knots of flow at 700mb. This will contribute to bulk shear values in excess of 40-50 knots over the region. This shear would be more than supportive of thunderstorm organization and with a predominately unidirectional flow this would support quick upscale growth into one or two line segments. Shortwave forcing is also pretty decent with much of the better forcing right along the international border:

Combination of surface temperatures climbing into the 80's with dewpoints surging through the 60's will contribute to about 500-1000 J/KG of MLCAPE into New England with upwards of 1000-1500 J/KG of MLCAPE farther west across New York and Pennsylvania. There are indications the pre-frontal trough may become established relatively early and well east. This would yield earlier cloud development and scale back temperatures and instability. Poor mid-level lapse rates and warm mid-level temperatures will yield a low ceiling on how much instability can develop.

How thunderstorms should evolve

With surface-based CAPE values approaching 2000 J/KG this will aid in the rapid acceleration of parcels to the LCL and subsequently the LFC as convective temperatures are reached. However, due to very poor mid-level lapse rates (5.5-6 C/KM), rather warm mid-level temperatures (500mb temps ~-8C), and mixed-layer CAPE only in the 500-1500 J/KG range, parcel acceleration through the mid-levels of the atmosphere will be reduced greatly. This will preclude more robust convection on a widespread basis. This is evident by tall skinny CAPE profiles which suggest a higher degree of water loading which will actually minimize the true CAPE.

Convection will begin to rapidly fire by early afternoon across central New York and Pennsylvania. The strong shear aloft will help aid in thunderstorm organization, however, the processes described above will hinder convection from becoming very deep and thunderstorms will struggle to intensify. With this said, combination of some steep low-level lapse rates (but this too could be impacted if full heating isn't materialized) and strong low-level winds will promote the risk for isolated-to-scattered damaging wind gusts within the strongest of thunderstorms. Given some signals for low-level winds to back a bit around the trough, a tornado can't be ruled out, especially if any thunderstorm can remain discrete. 

Tuesday, July 12, 2022 southern New England Thunderstorm Potential

A developing upper-level trough digs into the upper-Midwest region through the day Monday meanwhile at the surface an area of high pressure slides east off the Northeast coast. The result will be a developing low-level southwesterly flow across the Northeast and New England. The result will be strong theta-e advection associated with a hotter and much more humid airmass.

On Tuesday, the upper-level trough amplifies as it progresses east across the Great Lakes and southeastern Canada. At the surface a cold front slowly advances south and east across the Northeast. Out ahead of the cold front the airmass will be hot and humid with high temperatures away from the coast pushing into the mid-to-upper 80's. Dewpoint values will likely be pushing lower 60's by early Tuesday morning with mid-to-upper 60's by late afternoon to early evening. 

Instability

An area of steeper mid-level lapse rates (> 7 C/KM) will traverse portions of the Northeast Tuesday morning, however, move off the Northeast coast by early afternoon with mid-level lapse rates ranging between 5.5-6 C/KM. These poor mid-level lapse rates will hinder overall instability potential, however, combination of temperatures pushing well into the 80's with dewpoints climbing through the 60's should be enough to yield 1000-1500 J/KG of mixed-layer CAPE. While the highest dewpoint values may occur past peak-heating these higher dewpoint values will help offset the slight cooling that occurs towards evening. 

Wind Shear

An unseasonably strong wind max rounds the base of the upper-level trough characterized by 50+ knots of wind from the west-southwest at 500mb and 700mb which should yield bulk shear values of 40-50+ knots overspreading the region. This shear will be more than supportive for thunderstorm's to become organized. Forecast models are also suggesting the potential for some directional shear with winds in the lower-levels of the atmosphere and at the surface coming out of the southwest. 

Severe Potential

Thunderstorms are expected to rapidly fire within New York and Pennsylvania by early Tuesday afternoon thanks to an unstable and weakly capped airmass. Combination of mixed-layer CAPE values in the 1000-1500 J/KG range, strong bulk shear, and steep low-level lapse rates due to strong surface heating will result in the potential for the strongest thunderstorms to produce damaging winds. Given shear vectors storms should grow upscale into one or multiple line segments which will increase the potential for damaging winds in the strongest cores that can develop. While mid-level lapse rates will be weak along with warm mid-level temperatures (-8C to -9C) which will limit hail growth and potential, some hail can't be ruled out during the early evolution of storms, especially if any initial storms take on supercell characteristics. This could also come with the potential for a brief tornado. 

The greatest overall potential looks just west of southern New England across New York and Pennsylvania as the timing of the front closer to southern New England is more during the evening. Thunderstorms will likely begin to weaken as they move into southern New England due to the loss of daytime heating. However, given strong bulk shear and high dewpoints, there may be enough instability to keep the potential for an isolated severe thunderstorm into central Massachusetts and Connecticut through the evening. Given the weak lapse rates and warmer mid-level temperatures thunderstorms even across New York and Pennsylvania will really struggle to develop very deep cores even with the stronger shear. This will keep the wind damage potential more scattered. Cloud-to-ground lightning will also occur with the strongest thunderstorms but the degree of lightning will also be scaled back due to lower CAPE within the hail growth zone and warm mid-level temperatures. Noting the Storm Prediction Center (SPC) has the best chance for any severe thunderstorms just to the west of southern New England and given the above descriptions can't disagree. Thunderstorms will rapidly diminish as they move east of the Connecticut River Valley through Tuesday evening.