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. 

Record High's in the Hartford Area Likely 05.21.2022

 A warm front lifts northeast late Friday ushering in an anomalously warm low-level airmass characterized by 850mb temperatures running +2.5-3 standard deviations above-normal:

Bufkit sounding from the GFS forecast model for Windsor Locks, CT (BDL) indicates the potential for a very well and deeply mixed boundary later Saturday with mixing occurring as high as ~770mb (~7,000-7,500 feet) above the ground:

Forecast models also indicate a plume of extremely steep mid-level lapse rates in association with an elevated mixed-layer moving overhead. This is very important as this will promote a very strong capping inversion roughly around 10,000 feet above the ground. This will do a few things:

1) Likely suppress the development of any clouds though will have to watch for some high-level cirrus. 

2) Promote exceptional mixing below this inversion. This increases the potential for maximum heating and temperatures to be realized.

Forecast models are indicating much of interior Connecticut (with exception of the shoreline, northwest, and northeast hills though 80's are very likely) seeing temperatures climb between 90-95 Saturday. However, given the potential degree of mixing there is potential for temperatures to push into the mid-to-upper 90's in the Hartford area which would shatter the daily record high of 93! The all-time record high temperature in the Hartford area is 99. That won't be broken but there is a low probability that could be tied!

There are some caveats though which will determine whether we are in the 90-95 range or 95-99 range:

1) Dewpoints - If we don't mix as well and dewpoints maintain in the 60's this could limit the maximum temperature from being realized. 

2) Surface wind direction - If the surface winds have more of a westerly component this will increase the potential for maximum temperatures to be realized. If the surface wind has a more southerly component this will likely result in a bit of a marine influence, holding temperatures back a bit.

Saturday is certainly slated to be the hottest day of the year with record warmth likely!

Monday, May 16, 2022 Northeast Thunderstorm Potential

 A shortwave trough is expected to move across the Northeast Monday with cold front moving through at the surface. As the cold front moves across the Northeast, a line of showers and thunderstorms is expected to organize across western New York and western Pennsylvania and propagate east-northeast through the region. 

Combination of temperatures climbing well into the 70's (with some localized lower 80's) and dewpoints pushing into the 60's will contribute to about 1,000-1,500 J/KG of MLCAPE. This will help aid in the development of showers and thunderstorms along with strengthening of this activity. Strong jet stream dynamics will also be present with 40-50 knots at 500mb and 35-50 knots at 700mb. This will help contribute to about 35-45 knots of vertical shear - plenty sufficient for storms to become organized.

Given the combination of a modestly unstable airmass and strong winds aloft, the potential will exist for the strongest thunderstorms to produce damaging wind gusts and hail. Forecast models also indicate increasing helicity through the day as stronger low-level winds at 850mb overspread the region so a brief tornado is also possible.

Noting the Storm Prediction Center has placed has placed much of New York and Pennsylvania in an enhanced risk (level 3/5) for severe thunderstorms with the greatest potential being damaging winds with widespread damaging wind gusts possible. This line is expected to rapidly weakening as it moves across southern New England as instability quickly begins to wane, however, localized wind damage will still be possible within the region. 

When assessing the potential environment to distinguish a more typical Northeast severe weather event versus a more substantial threat there are several red flags to note;

1) Weak mid level lapse rate - 700-500mb lapse rates are forecast to be on order of 5.5 - 6 C/KM. Weak lapse rates will put cap onto how much instability can materialize during the day as well as limit overall updraft (and subsequent) downdraft strength. 

2) The strongest of the low-level jet winds are confined to the 850mb level with winds forecast to strengthen to around 30-40 knots with a general 25-35 knots at 925mb. While these winds are rather adequate for these levels, they are a bit shy of what you would like to see for a higher magnitude damaging wind event. 

3) Questions regarding low-level lapse rates - The BIGGEST discriminator between more mundane and higher magnitude damaging wind events in the Northeast is low-level lapse rates. Stronger low-level lapse rates (> 8-8.5 C/KM) correlate much higher to widespread damaging wind potential. While these values may be achieved locally, there are questions about this on a widespread or region-wide level. 

Based on everything stated above, showers and thunderstorms rapidly develop by mid-to-late Monday morning across western New York and Pennsylvania and propagate east-northeast across the region with this line strengthening through the morning and early afternoon. Given the modestly unstable airmass and strong wind shear aloft, some strong-to-severe thunderstorms are likely with scattered damaging wind gusts, some hail, and the risk for a brief tornado. Given some of the uncertainties a more widespread damaging wind event doesn't appear likely, but there may be localized areas of more concentrated damage. 

(Note: Defining a higher magnitude damaging wind event - Several-plus reports of 65+ mph winds). 

Wednesday, March 9, 2022 Connecticut Snow Event

An area of low pressure moving off the mid-Atlantic and passing just south of Long Island results in a cute little snowfall event for Connecticut Wednesday. Snowfall accumulations are expected to be on the minor side given marginal boundary layer and surface temperatures with the greatest accumulations occurring across the northwest and northeast hills. 

As low pressure moves northeast off the mid-Atlantic coast passing just south of Long Island, the combination of a strengthening system, favorable upper-level dynamics to support large-scale lift, and a large temperature gradient across the region will yield the rapid blossoming of precipitation and snow across the state moving through Wednesday morning. Temperatures at 850mb (~5,000 feet above the ground) will be as warm as +10C to +12C just off the mid-Atlantic coast with 850mb temperatures as cold as -5C to -10C across New England resulting in a large temperature gradient which favors enhanced upward vertical motion. This will be further enhanced by a strong 150+ knot upper-level jet streak north of New England:

Forecast models also indicate we'll see some very strong frontogenesis move across the state Wednesday afternoon (evident by the red and spots of black below). This is important as this will help achieve heavier snowfall rates which will make accumulations more possible especially within the hills and road surfaces within the hills:

However, in order to maximize the heavier snowfall rates we'll have to get this lift to occur within the dendritic snow growth zone. This should be around 15,000 -20,000 feet Wednesday (slightly higher than what is preferred). Below is the 18z/08 NAM bufkit for New Haven, CT indicating the snow growth zone (yellow/purple contours) and omega (negative values or red contours indicate upward vertical motion...lift). Here we see omega values as low as -10 to -15 and even higher later in the afternoon:

What we're looking at for Wednesday is due to boundary layer temperatures which will be above-freezing and surface temperatures climbing into the mid-30's this will make snow difficult to stick to paved surfaces, especially when snowfall rates are on the light side. Under areas of much stronger lift when precipitation rates will be heavier, accumulations become more likely, especially on colder surfaces and now untreated surfaces. The depth of the warm-layer, however, is on the shallower side this is why despite the surface temperatures being in the mid-30's precipitation will be snow. A shallower warm-layer gives less time for melting. Also, temperatures as low as 1,000 feet off the ground are quite cold (~ -3C to -4C). This cold layer so low will help with the greatest accumulations occurring across the hills (elevations above 500 feet and especially above 800 feet). 

Below is what to expect:

• Light snow breaks out across the state during the morning with areas and periods of moderate-to-heavy snow occurring at times.
• Snowfall rates between 0.5''-1'' per hour under the heaviest band of snow and where snow rates are highest.
• Timing for heaviest snow rates will be from late morning through late afternoon. 
• Greatest accumulations within the northwest and northeast hills. Across lower elevations and the valley, majority of snow accumulations will be on grassy and untreated surfaces.
• Significant travel disruptions aren't likely, but roads will be slick so give yourself extra time traveling. 

Friday, February 25, 2022 Connecticut Winter Storm

 A Southwest Flow Event (SWFE) or overrunning event is set to bring a wintry mess to the state of Connecticut. What is a SWFE or overrunning event? All this really means is the winds within the mid and upper levels of the atmosphere are coming from a southwesterly direction which typically transports warmer air which can overrun colder air locked in at the surface, hence, the term overrunning. 

This is an extremely challenging forecast for Connecticut as it is a big battle between temperatures warming above-freezing between 5,000 and 10,000 feet above the ground and temperatures remaining below-freezing throughout the troposphere. This is the difference between significant snowfall accumulations (>8'') and lesser snowfall accumulations with more in the way of sleet.

The NAM forecast model is the warmest of the guidance with the freezing line at 850mb (about 5,000 feet above the ground) already accelerating north through Connecticut by 7:00 AM EST Friday. The GFS and European models have this line in the vicinity of Long Island:

There are two key factors which will influence how much we warm between about 5,000 feet and 10,000 feet above the ground; in fact you can actually even say from the surface through about 10,000 feet:

1) There is a fairly stout, but small, high pressure system which will be positioned north of Maine and drifting off to the northeast. Clockwise flow around high pressure with high pressure in this position means a more north-northeasterly flow in the lowest few thousand feet of the atmosphere. This could inhibit warming within this layer, keeping temperatures below-freezing. Assuming temperatures warm above-freezing above this layer, this would have a significant impact on precipitation type with sleet vs. freezing rain and the precipitation type dependent on such factors as; the depth of the cold layer and how warm the warm layer is. 

2) Forecast models track the main surface low right across New England with some indications a weak secondary low may develop south of Long Island. This is something which would work to lock in colder air at all levels of the atmosphere and would favor predominately snow, especially for far northern Connecticut. 

It is very interesting to note, however, that forecast guidance; including the warmer NAM forecast model has continued to show colder evolutions throughout the day. This increases confidence in a much colder thermal profile across the state

Anyways, I don't really feel like going all crazy with the meteorology/science aspect (at least from a writing perspective...the work was done going into the forecast but I don't feel like translating it to a blog post). I'm actually not very thrilled with this threat...I hate sleet and I'm just ready for summer. So we're going to skip the science stuff and get into it.

What to Expect...

1) Precipitation begins moving into the state late Thursday evening (between 10:00 PM and 1:00 AM EST) and should begin as snow everywhere. 

2) Snow should rather quickly become moderate-to-heavy at times. There may be a several-hour duration of moderate-to-heavy snow as forecast models advertise high-levels of vertical velocity across the state for a several-hour period. 

3) The heaviest of the snow will occur between the hours of 4:00 AM and 10:00 AM Friday. During this window snowfall rates of 1-2'' per hour are likely. 

4) Despite the colder profile, enough warming should occur towards the shoreline which should result in snow transition to predominately sleet. This will cut down significantly on snowfall totals, however, will transpire to significant travel hazards. 

5) We will see very dangerous travel conditions for the Friday morning commute due to heavy snow falling inland and sleet along the shoreline. If you do not have to travel don't do so. 

Below is what I am currently expecting: