Thursday, July 27, 2023 New England Thunderstorm Potential

An intriguing setup overspreads New England Thursday yielding the potential for scattered-to-numerous showers and thunderstorms; including the potential for several strong-to-severe thunderstorms capable of producing damaging winds and/or hail. In addition, a few tornadoes are possible along with an increased likelihood for flash flooding given very saturated grounds thanks to a very wet month. 

General Synopsis

As of this writing, strong shortwave energy embedded within a seasonably strong jet stream was moving across Michigan with low pressure and associated warm front at the surface with the warm front having already moved across Michigan and headed towards the Northeast/New England. Forecast model guidance is in very good agreement in tracking this shortwave energy and associated surface low pressure and attendant warm front east across New England Thursday. The surface warm front is expected to move southwest to northeast across the region Thursday morning. This may be accompanied by isolated shower and thunderstorm activity. 

As the warm front passes through, a very rich low-level theta-e airmass is expected to overspread much of New England with surface dewpoints climbing into the lower 70's with dewpoints as high as 73-76 likely across southern portions of the region. While there may be widespread cloud cover during the morning (more likely towards northern sections of the region), forecast model soundings and guidance suggest there will be enough dry air aloft to result in thinning clouds which should help temperature soar quickly, especially with 925mb temps around +25C and 850mb temps around +20C. Temperatures should soar well into the 80's region-wide with the hottest locations ranging between the upper 80's and lower 90's. 

Instability

Combination of a very rich low-level theta-e airmass (theta-e between 360-370K) characterized by dewpoints into the 70's and surface temperatures well into the 80's to lower 90's should contribute to moderate levels of instability with mixed-layer CAPE values between 1,500 - 2,000+ J/KG. One inhibitor for stronger instability will be the presence of rather weak mid-level lapse rates (generally between 5.5-6 C/KM). While surfaced-based CAPE values will be high (potentially 2,500 - 3,500 J/KG) and mixed-layer CAPE values will be between 1,500-2,000 J/KG indicating moderate instability, the instability is largely a product of the rich low-level theta-e air - majority of instability is tied into the lower levels of the atmosphere and the CAPE profile is not exactly fat CAPE. This is evident on bufkit soundings within the region which have normalized CAPE values generally around 0.2. What all this means is with the rich low-level theta-e airmass, larger parcels accelerations will happen for surface parcels and will easily rise to the lifted condensation level (LCL) and then from the LCL to the level of free convection (LFC). Once parcels reach their LFC, they will continue to rise, however, the presence of weak mid-level lapse rates will result in slower parcel accelerations. This will limit more robust convection on a widespread scale. While weak mid-level lapse rates will hold back parcel acceleration above the LFC, it is possible dewpoints into the lower 70's (or even a bit higher) could help compensate for this a bit.

Wind Shear

A seasonably strong mid-level jet characterized by 500mb winds of westerly flow on order of 40-60 knots traverses the region as New England is situated towards the northern periphery of a strong upper-level ridge which has become established across the southern states. In the lower-levels of the atmosphere, an unseasonably strong low-level jet is expected as the 850mb jet strengthens in response to the approaching shortwave. 850mb winds are expected to increase to 35-45 knots across the region through Thursday morning from a west-southwest direction. At the surface, winds are expected to range anywhere from southwest to more south-southeast with proximity to the warm front a huge key in the direction. Closer to the warm front surface winds will be more south to south-southeast and farther away from the warm front wind will be more south-southwest to southwest. With winds increasing with height through the troposphere (speed shear) and changing direction with height through the troposphere (directional shear) there will be plenty of dynamical support (combined with the shortwave tracking across the region) for thunderstorms to become organized. Given the presence of directional wind shear, the more mature thunderstorms will take on supercell characteristics. 

Inhibiting Factors for a Severe Weather Outbreak 

While numerous strong-to-severe thunderstorms are possible, there are some inhibitors which may prevent a region-wide severe weather outbreak and higher-end severe weather (higher-end meaning wind gusts 74+ mph, hail 2''+ in diameter, or tornadoes EF2 or higher). As mentioned above, weak mid-level lapse rates may negate a more substantial threat for severe thunderstorms as this will yield slower parcel accelerations above the LFC. Steep mid-level lapse rates (> 7 C/KM) are a large discriminator between severe weather and higher-end severe weather, particularly in the Northeast. Larger parcel accelerations above the LFC can help thunderstorms become quite vigorous.

 Forecast model soundings also indicate there may be an abundance of drier air in the mid-levels of the atmosphere. While dry air is good in the mid-levels, too much dry air can be bad, especially without the presence of an elevated mixed-layer as this drier air can mix into the lower levels of the atmosphere, subsequently reducing instability and it can inhibit mid-level storm development. Given rich theta-e air, however, with strong low-level instability, parcel acceleration may be vigorous enough to transport moisture deeper into the troposphere. 

Forecast model soundings also indicate some pretty strong capping up around 500mb. This has some relation to the weak mid-level lapse rates which are a product of very warm 700mb temperatures (around +7C) and 500mb temperatures (around -8C) resulting in high freezing levels. The weak mid-level lapse rates and warmer mid-level temperatures significantly reduce parcel acceleration through these levels which negatively impacts storm strength potential. 

What to Expect

Based on everything above, the expectation is scattered-to-numerous thunderstorms develop across New York mid-to-late morning and push east. With the majority of forcing tied into the approaching shortwave, combined with the mentioned instability and wind shear parameters, the thinking is storms will be discrete. This will enhance the potential for supercell thunderstorms capable of producing damaging winds and large hail (despite the weak mid-level lapse rates, warm mid-level temperatures, and high freezing levels) and risk for a few tornadoes. These discrete supercell thunderstorms progresses across Vermont, New Hampshire, and western/central Massachusetts. As this activity pushes east, enhanced forcing from a likely sea-breeze boundary and wind shear becoming more unidirectional as the warm front continues to surge north should yield activity organizing into a squall line. This will increase the potential for damaging straight-line winds across southern New Hampshire, southern Maine, and eastern Massachusetts. 

One wild card is the state of Connecticut. Connecticut will be south of the strongest shortwave forcing. This combined with the capping mentioned above may mean much of Connecticut misses out on activity (especially western sections). However, the thinking is a sea-breeze front will develop through the afternoon and push north. This will help with thunderstorm initiation and thunderstorms may quickly develop along and east of I-84 during the late afternoon and quickly consolidate into a line with this moving across eastern Connecticut and Rhode Island producing some wind damage and hail (and of course a tornado can't be ruled out). 

Overall, this is a pretty intriguing setup for some strong-to-severe thunderstorms across the region, but several limiting factors outlined here will likely preclude a region-wide severe weather outbreak and preclude higher-end severe weather from occurring. However, a concentrated swath of wind damage is possible is a squall line materializes and this may be more likely towards eastern sections. 

Tuesday, March 14, 2023 Winter Storm Update

 As typical with phasing systems, computer forecast model guidance continues to struggle with the finer details and exact evolution of this system. The finer details and exact evolution will be fairly critical in local impacts, especially for those on the edge. The big question we're dealing with right now is the potential for a dual low pressure structure. Dual low's always throw a giant wrench into the forecast. The challenge with these is it can be impossible to determine whether that scenario is correct or not. While that can be challenging, this is where assessing the mid-levels of forecast guidance can perhaps provide a clue.

Below we will assess the 500mb vorticity and Sea-Level Pressure charts  from the 18z/12 NAM, 18z/12 GFS, and the 12z/12 European forecast model. We will focus on the northern stream shortwave, shortwave energy within the southern stream, and surface low pressure.

All three models continue to be in strong agreement that the northern stream energy will dig south of Long Island and close off. As stated in my Saturday forecast, this is a classic signature for significant southern New England snowstorms. It doesn't guarantee it, but this is a strong signal. While the graphic below does not show a closed off look at 500mb, in subsequent hours the vorticity tracks southeast into Long Island and closes off. The result at the surface is developing and strengthening low pressure right off the New Jersey coast.

Regarding the southern stream energy, we see shortwave energy pretty far off to the south and east. This shortwave energy is associated with convection (showers and thunderstorms developing off the southeast and mid-Atlantic coast and tracking northeast. One of the questions has always been, how the northern stream and southern stream interact, or phase. Forecast model guidance today has trended in a direction in which there is stronger emphasis on this southern stream shortwave energy. Forecast models largely want to make this the main low pressure which results in a significantly less impactful event for a large portion of the region. This is what we call, "chasing convection". Now, we've seen storm potential get destroyed because this scenario verified where the convective-induced low pressure became the main storm and much of the precipitation was tugged southeast with it. So the question is, how likely is that scenario to occur here? 

I believe the models are placing too much emphasis on the convective-induced low pressure. I come to this conclusion as the degree and strength of convection is not expected to be that intense or particularly deep. Deeper and stronger convection results in more intense vorticity. Also, given the structure and evolution of the mid-levels, this will favorable the low pressure development off the New Jersey coast to be the main low which will strengthen as it moves northeast. 

Even now, there is still some uncertainty with the forecast models, but most of this uncertainty involves surface features and evolution. A strong assessment of the mid-levels may help the forecaster eliminate some of this uncertainty. Based on the evolution of the mid-levels, especially at 700mb with the development of a closed 700mb low and track of this 700mb low, combined with the thinking the convective-induced low is overblown, I am thinking we'll see an intense band of heavy snow traverse a large portion of southern New England. 

I'm also becoming a bit more confident the system will occlude rather quickly as it undergoes rapid development off the coast. As occlusion occurs, this will shut off the inflow of warmer, moist air into the storms core which will result in the CCB (band of intense snow) to weaken rapidly and result in a weakening precipitation shield. Due to the increasing confidence in this aspect, I am reducing the totals of each range a bit. I am also becoming a bit concerned with subsidence within the Connecticut River Valley. This could result in significantly lower totals within the Valley, but should intense banding traverse the valley, this concern would be lessened. 

Outside of the Berkshires and far northwestern Connecticut, most should see precipitation start as rain Monday evening, however, as the storm evolves and dynamic cooling begins to take place, temperatures at the surface should drop close to freezing. As heavier lift begins to evolve, rain will rapidly change to snow. The changeover may be slower within the valley and along the coastal Plain. Snow will be wet and heavy with ratios well less than 10:1. Under the CCB, ratios may be as high as 13:1 to even 15:1 if the profile can become cold enough. Snowfall rates of 1-2'' per hour are likely under the most intense banding. Some forecast soundings also indicate a MAUL (Moist Absolutely Unstable Layer) indicating the potential for thundersnow which could yield locally higher rates. Winds are also expected to be quite strong with the potential for gusts 60-70 mph towards eastern Massachusetts and gusts 40-60 farther inland. Combination of strong winds and heavy, wet snow may result in widespread tree damage and power outages. 

Forecast Changes which reflect narrowing the ranges a bit as confidence in the finer details increases and to account for mesoscale features such as orographic support in Berkshire County, MA and subsidence into the Connecticut Valley (Note: I am a little concerned subsidence could be an issue farther north past Springfield):

• Added a 20-30'' contour within the Berkshires where orographic enhancement combined with the greatest ratios will occur. 
• 18-24'' contour was reduced to 16-20''.
• 12-18'' contour was adjusted to 12-16'' and also pushed farther east through Litchfield County, CT, far northwestern New Haven County, CT, and far western Hartford County, CT to account for the higher elevation and higher totals.
• 6-12'' contour was adjusted to 8-12''.
• 3-6'' contour was adjusted to 4-8'' and this contour brought higher into the Connecticut Valley to reflect subsidence concerns.
• 2-4'' contour for the Connecticut shoreline with a 1-3'' contour added in eastern Massachusetts.

 

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.