Major to Significant Winter Storm To Impact southern New England Thursday into early Friday

Over the past few days confidence has vastly increased that a storm system developing across the southeastern United States will work up and track close enough to the coast to impact southern New England.  With several sets of computer forecast guidance suggesting a rather strong system, these impacts will be rather significant as portions of the region will see major (8-12'') to significant (12''+) snowfall accumulations.  Given the expected track of the system, we will also have to contend with plain rain, sleet, and perhaps some freezing rain.  This system will also be accompanied by some strong winds as well.

While confidence is extremely high of a storm track close enough to the coast to impact the entire region, there is still a great deal of uncertainty with regards to the exact track of this storm system.  This is ultimately going to be absolutely huge when it comes to determining who remains all snow, who mixes or changes to rain, and where the axis of the highest snowfall totals will occur.  Unlike some of the past storms we have had this winter, we don't have a high pressure system to our north to keep colder air locked in place across the region and with the models strengthening this system very rapidly off the Carolina coast, this will allow for warmer air to try and work into portions of southern New England.  A track difference west/east of as little as 30-50 miles will be the difference between something like 3-6'' of snow and potentially as much as 12''!

What we also have to watch is the track of the 700mb low.  This is extremely important for two different reasons.  1) The highest snowfall totals along with the heaviest banding of snowfall occurs just to the northwest of the low pressure track.  This is where lift is typically the strongest along with very strong forcing.  2) Areas virtually in the direct path of the low track and just south and east typically end up being in the dry slot.  This is where drier air from aloft descends downward or when drier air gets wrapped into the system.  The drier air eats at the precipitation and the moisture and areas within this zone see very little to no precipitation and something like this can vastly affect snowfall totals.

Given how close to the coast this system will track and with warmer air that will be working into portions of the region, this will create a tight baroclinic zone (greater temperature difference over a small area).  This also works to vastly increase lift and forcing, just to the west of this zone and where this zone sets up is where the heaviest snows and higher totals will occur.

With a potent storm system which will continue to strengthen as it works up the coast this will also increase the pressure gradient across the region and a period of strong winds will be introduced.  Winds could gust as high as 35-45 mph region wide, and especially across the coast.  This will lead to isolated to scattered power outages along with perhaps blizzard conditions in some spots.

The first impacts from the storm will begin very early Thursday morning, right around dawn or just a bit after with everyone starting off as snow.  The timing of the storm will likely lead to a great deal of school cancellations throughout the region.  For those heading to work the travel commute will deteriorate very quickly and expect numerous delays.

The height of the storm will occur between about 10 AM and 10 PM.  During this time snowfall rates may approach 3''/HR in the areas where the heaviest banding occurs.  Winds will also increase as well as the afternoon goes on with gusts approaching 35-45 mph, especially towards the coast.  For areas south and east snow the snow will eventually transition over to sleet and then rain thus the lesser snow totals.

Given everything mentioned above, this is what I'm currently thinking as far as snowfall totals go:

Storm System to Bring Major Snowfall Accumulations to Much of southern New England Wednesday, February 5th, 2014

The active start to February continues on Wednesday as yet another storm system is expected to impact southern New England.  This system is forecasted to be a bit stronger than what we just dealt with as the system will have a bit more organized structure with much stronger lift and a further northward track will mean increased moisture as well.  All these ingredients will come together to bring the potential for major (8-12'') snowfall accumulations across a chunk of the region.  We can't even rule out some areas perhaps picking up over a foot of snow as well.

With a storm track which is expected to be virtually just south of CT moving across the extreme outer portion of the Cape this is going to allow for some warmer air to work northward across southern CT, southern RI, and far southeast MA (including the Cape) and this will reduce snowfall totals across these parts thanks to more in the way of mixing.  We also may see some sleet mixing in as far north as the CT/MA border for a period of time.  A storm track like this as well means an increased thermal (temperature gradient) in the mid/lower levels of the atmosphere which also means stronger forcing and lift over the region:

  When looking at how much moisture is associated with this system, computer forecast models are all in agreement there will be loads and loads of moisture associated with it.  In fact, the consensus between the NAM/GFS/Euro models all have about 0.50'' to 1'' of liquid falling in a 12-hour period!  Forecast soundings also show the snowgrowth zone will be located between about 15,000-20,000ft up which is what you want to see and the soundings do show some good lift within this zone at times.

While all of these are certainly positives for a pretty sizable snow event, there are a few negatives as well which may reduce the potential for a much higher snowfall accumulations.  Computer forecast models really begin to dry out the mid-levels of the atmosphere rather quickly during the late morning into the early afternoon hours as dry air races in from the west and southwest.  This dry air is not only a precipitation shield eater but it vastly reduces snowgrowth and ratios.  There are also some signs of some subsidence zones ( caused by "negative lift") which will mean there may be some spots where virtually no snow is falling (or the intensity is extremely light) while only a few miles away it's snowing quite heavily:

Given what we are presented with right now this is what I am currently thinking for snowfall accumulations.  The highest snowfall totals will be confined across far northern sections where dry air will be less of an issue and snowfall ratios will be a bit higher.  Interior northeastern MA back to Worcester will also be another jackpot area as dry air will be less of a factor here as well and guidance suggests the best banding may pivot over this area.

Snowfall Forecast Update: Forecast Still on Track

Reviewing data as it has become available since yesterday's forecast, at this time there will be no changes from the previous forecast and everything still appears to be on track.  Computer forecast guidance has remained quite consistent with how this system will evolve with the exception of a few minor changes but I don't think these changes will have substantial impacts on snowfall totals.  If anything, perhaps far western areas of the forecast might receive a bit less in the way of snow but at this time I see no need to make adjustments.  What I'm also watching out for is the possibility of an inverted trough developing late tomorrow morning which could prolonged periods of heavier snowfall across far eastern MA/Cape areas...if this is the case the potential would exist for more snowfall than currently forecasted for these areas.

Forecast soundings indicate we will be dealing with very good snowgrowth which will lead to beautiful dendrites...nice big fluffy snowflakes which will accumulate very quickly.  We will also be dealing with some incredible snowfall ratios which could perhaps exceed 15:1 or even 20:1.  This will lead to the higher totals we will see.

What to expect as we move through today:

• Snow will begin falling across the region from SW to NE as the afternoon progresses.  While snow will remain predominately light across the region, areas across the CT/RI coast may experience bursts of moderate to heavy snow as models have some pretty strong lift across these areas.  As the afternoon progresses, these bands of heavier snowfall will work northward. 

• Given very cold ground and cold temperatures, snow will begin sticking immediately, this will make more very slick roadways and hazardous travel. 

• The heaviest of the snowfall will occur between the hours of 7 PM-7AM tomorrow for much of the region with the exception of eastern MA/Cape where heavier snow may linger past 8-10 AM.  

• During the height of the storm winds will begin to increase as well with gusts potentially up to 35 mph across the region.  Across far eastern MA/Cape area, winds could gust to near 50 mph with sustained winds 20-30 mph.  This will not only lead to blizzard conditions but dangerous wind chill values as well.

As of now, no changes have been made from yesterday's snowfall map.  Any adjustments will be made as necessary as the storm is occurring and we start to see where the strongest forcing/lift will reside over and where exactly the heaviest banding sits over.  Typically you have to wait until the storm is occurring to determine this as these are what we call "mesoscale" factors.  This means they occur on a very small scale and our computer models have a very difficult time handling these features well.

Coastal Storm to Bring Accumulating Snowfall to southern New England Tuesday into Wednesday

Confidence in a coastal storm bringing accumulating snowfall continues to increase as computer model guidance over the past few days has come into further agreement on a further northward track.  The time of impact will be from late tomorrow morning/early afternoon lasting into mid to late morning Wednesday with the height of the storm occurring late evening through the overnight period.

Shortwave energy will be entering the United States from the northern Plains during the overnight period, rotating around a piece of the polar vortex which is situated north of Maine in Canada.  As this energy digs into the United States it is expected to strengthen and the response will be for surface low pressure development.

Throughout the day tomorrow this piece of energy is expected to strengthen as it continues digging into and through the Ohio Valley into the mid-Atlantic states and as it does show the energy/trough will become negatively tilted.  This will not only allow for further intensification (cyclogenesis) of the low pressure system but this will help to keep the track close enough to the coast for impacts.  We will also see the system slow down a bit as it passes just off the New England coast.

Late evening tomorrow and through the overnight period, as the system works northward just off the coast, it will undergo rapid cyclogenesis and we will see a rather intense and mature storm system develop.  This is evident as computer forecast models are developing closed off circulations at the 925/850/700mb levels.  When this occurs we will begin to see a very intense band of extremely heavy snowfall develop and where this band traverses is where the highest snowfall totals will occur.  At this time this appears to be across portions of SE CT/RI and into far SE MA which includes the Cape.

What's going to happen is light snows may begin to break out as early as late morning/early afternoon with periods of moderate bursts of snow during this time.  While a few inches may drop during the afternoon hours the heaviest of the snow is expected late evening/overnight.

While we have a rather cold airmass in place which is a positive for higher snowfall ratios, forecast model soundings are also highly suggestive of very favorable snowfall growth and snowfall rates.  This will lead to extremely fluffy snowfall composed of rather big snowflakes.  This will also be a positive for higher snowfall totals.

With an intensifying storm system just off the coast this will lead to a tightened pressure gradient which means increasing winds, especially across eastern MA.  This will lead to blizzard conditions across eastern MA and this can't even be ruled out for areas further west.  Given such an intense system with such strong lift and forcing, some thundersnow is also possible.

At the height of the storm, snowfall rates may approach as much as 2-3'' per hour, especially considering the snow will be so fluffy.  The highest snowfall totals should be confined to eastern southern New England as this is where the consensus is for the higher precipitation totals.

Below is what I'm expecting right now:

What is the Polar Vortex?

The recent blasts of very cold Arctic air which has invaded a large chunk of the United States, giving areas some of the coldest weather they've experienced in years has driven the media to discover the term "Polar Vortex" and run absolutely wild with the term.  With that,  I thought this would be a great time to dedicate a post about the polar vortex and explain exactly what the polar vortex is as well as discuss it's role in meteorology and in weather forecasting.  While this post will get a bit technical, everything will be explained as clear as possible and as simple as possible.  

What is the Polar Vortex?

As defined by the American Meteorological Society (AMS), the Polar Vortex is, The Planetary-scale cyclonic circulation, centered generally in the polar regions, extending from the middle troposphere to the stratosphere.  In other words, the polar vortex is an area of low pressure which is centered over the polar regions of the Northern Hemisphere and the polar vortex ALWAYS exists.  In the Northern Hemisphere, the polar vortex typically has two main centers of circulation with one typically located near Baffin Island and the other center of circulation typically located over northeast Siberia.  

The polar vortex (which remember is an area of low pressure with two circulation centers) varies in intensity/strength throughout the course of the year and the location of the two main centers also changes throughout the course of the year.  The polar vortex is typically more "active", meaning it's typically a more dominant signal, during the Northern Hemisphere winter as this is when the temperature gradients between the Equator and the North/South Poles is the greatest.  This leads to stronger wind gradients along with stronger pressure gradients.  

How is the Polar Vortex Measured?

We measure the strength of the polar vortex by the phase and strength of the Arctic Oscillation (AO).  The National Climatic Data Center (NCDC) defines the AO as, a large scale mode of climate variability, also referred to as the North Hemisphere annular mode.  The AO is a climate pattern characterized by winds circulating counterclockwise around the Arctic at around 55N latitude.  The Arctic Oscillation is measured in three different phases; negative, neutral, and positive.  Each of these three phases define the state of the AO and each of these phases influence the weather pattern not only across North America but other areas of the world as well.  Below we will take a brief look into each phase and what they mean in terms of weather.  

• Negative AO phase -  When atmospheric pressures up at 500mb (about 18,000ft AGL) associated with the Polar Vortex circulation are weaker than normal (meaning HIGHER atmospheric pressure values compared to the average) and zonal wind circulation around the vortex are weaker than normal, this indicates a negative phase of the AO.  During this phase, it becomes much easier for colder Arctic air to invade the northern tier of the United States.  Below are the monthly averaged 500mb heights/zonal winds for the Top 10 most negative AO January's along with the temperature anomalies for the month of January for each of the top most negative AO January's:

 

• Neutral AO Phase - The phase of the AO is termed neutral when there is no clear phase signal.  Typically, you're dealing with height anomalies in the Arctic regions which are much more neutral (not much above or below normal).  The graphic below displays 500mb height anomalies for the 6 January's with the three highest negative values and the three lowest positive values.  Looking at the height anomalies over the Arctic regions we see more neutral heights: 

• Positive AO Phase - The positive phase of the AO is a mere opposite of the negative phase.  500mb heights associated with the positive phase are lower than normal and the zonal wind circulation around the vortex is stronger than normal, meaning stronger westerlies.  These stronger westerly winds work to keep colder air bottled and locked up across far northern latitudes and increases the likelihood of above-average temperatures across the northern tier of the United States.  Below are the monthly averaged 500mb heights/zonal winds for the Top 10 most positive AO January's along with the temperature anomalies for the month of January for each of the top most positive AO January's:

Now that we have taken a brief look into what the polar vortex is and how the strength of the polar vortex is measured let's do a quick review.  

• Polar Vortex is an area of low pressure centered over the polar regions of the Northern Hemisphere and ALWAYS exists.  
• The strength of the polar vortex is measured by the Arctic Oscillation (AO).
• The Arctic Oscillation had three distinct phases; negative, neutral, and positive.  
• Each phase influences the weather pattern across North American and across other parts of the world as well.  

We've discussed what the polar vortex is and how the strength is measured but we will now take a look into other aspects of the polar vortex, such as movement.  While the polar vortex is normally situated over the polar regions of the Northern Hemisphere, certain atmospheric phenomena can actually cause the polar vortex to shift locations or even split into two separate areas of low pressure.  None of this is that uncommon either.  

In the case of this past Arctic Outbreak which plagued the United States east of the Rocky Mountains the first week of January, the weather pattern across the Northern Hemisphere allowed and favored for the polar vortex to actually sag southward into southern Canada and northern-tier of the United States, bringing down the brutally cold air we had.  Below are the average 500mb heights across the Northern Hemisphere, along with the average 500mb zonal winds from January 2nd, 2014 to January 6th, 2014.  The main areas of interest are all numbered.  Numbers 1-3 are showing the area of higher heights, indicating the ridging, while number 4 shows the very low heights, indicating troughing, associated with the polar vortex.  Number 5 shows the negative, or easterly zonal winds which allows the polar vortex to sink southward:

 What we all can take from this is, the polar vortex is nothing new and it's not some brand new creation caused by "Global Warming" or "Climate Change".  The polar vortex has always been in existence and was first discovered as early as the year 1853.  Despite alot of the talk about it within the media as well as social media, this isn't scary and it does not signal anything about the future.  It is not uncommon at all to have the polar vortex sag southward towards or even into the United States and this has happened many times before.  With all of this, hopefully there is now a better understanding of what the polar vortex is, how we measure it, what it does, and how it affects the weather pattern across North America.   

Winter Storm Set to Bring Major/Significant Snowfall Accumulations, Blizzard Conditions to southern New England

A rather potent coastal storm will bring major (widespread 8-12'') and significant (12''+) snowfall accumulations to southern New England beginning tomorrow and lasting into the first half of Friday.
A piece of shortwave energy is diving down from Canada into the central Plains as a trough develops and continues to amplify (strengthen).  This will allow the piece of shortwave energy to rapidly strengthen as well.  At the same time, a shortwave, albeit weaker, is moving out of Mexico into southern Texas and into the Gulf Coast states.  This shortwave will spawn an area of low pressure in the Gulf of Mexico and will begin to strengthen as it's influenced by the stronger shortwave.

As the trough continues digging into the Plains and begins advancing eastward, the trough will continue to amplify and dig, becoming quite strong which will allow for rapid cyclogenesis (deepening of the low pressure) just off the East coast.  The amplifying trough will also allow for the low pressure to track close enough to the region to allow for heavy snowfall.

What We Are Looking At:

• Periods of light snow are expected to develop early AM Thursday morning with pockets of moderate/heavier snowfall as well.  As the day wares on periods of moderate to heavier snow will become much more common. 
• The heaviest of the snowfall will be occurring during the overnight hours of Thursday into Friday morning.  During this time snowfall rates could potentially exceed 3'' per hour (especially with any thunder snow)
• With a strong area of high pressure off to our north and west and a rapidly deepening area of low pressure, this will create a several hour window of very strong winds across the region.  At times winds could gust as high as 35-45 mph.  This will lead to periods of blizzard conditions.  
• The heaviest snowfall accumulations are expected to occur across RI, eastern MA, and the Cape
• With very strong lift associated with this system along with some elevated instability, the potential will exist for thundersnow as well which will lead to locally higher snowfall rates.
• Temperatures will be quite cold during the duration of the storm as well with temperatures ranging from the single digits to the lower teens.  This coupled with favorable conditions aloft will lead to very high snow ratios, potentially as high as 15:1.  
• The snow will be very light and fluffy, allowing snow to accumulate very rapidly.  Given how light/fluffy the snow is along with very strong winds this will work to 1) Make snowfall measuring very difficult, if not impossible 2) Allow for drifting snow 3) Ratios could be negatively affected as strong winds may rip apart dendrites which could also have an impact on snowfall totals.  
• Based on what the computer forecast guidance is showing, this is what I'm currently expecting:

Clipper System to Bring Snow on Tuesday

One storm system has moved through and another is arriving as a strong clipper system is set to bring a period of light to moderate snowfall during the day on Tuesday to southern New England.  We aren't looking at a major snow event here, but we are looking at plowable snows.

  A trough will be working into the upper Mid-west region over the next few days and begin to amplify (sharpen) during the morning hours of Tuesday.  As this trough, with an associated Arctic cold front near the region, a piece of rather strong shortwave energy will rotate around the base of the trough.  With the trough amplifying, this will allow the shortwave energy to strengthen even further.  The trough amplifying will also allow for moisture to be drawn into the system from the Atlantic and this system will also have moisture associated with it.  Below is a 4-panel chart from today's 18z run of the GFS valid for 7:00 AM EST Tuesday morning.  The graphic on the top left displays the 500mb pattern (roughly 18,000ft AGL).  Circled is the piece of shortwave energy discussed above.  The graphic on the bottom left displays relative humidity values at 700mb (roughly 10,000ft AGL).  The darker the green shading, the more moist the airmass is at that level.  As you can see we are plenty moist.  The graphic on the bottom right displays 6-HR precipitation values and you can see the area of precipitation working towards the region.

  

Right now we are expecting light snows to break out very early Tuesday morning and lasting through late afternoon.  As stated above, we are not expecting a major snow producing event here given this is a clipper system which are typically very fast moving and they aren't usually associated with a great deal of moisture.
The airmass in place across the region will be cold enough to support snow for the entire region, with perhaps some mixing across the outer Cape.  Snowgrowth looks rather decent as well and snowfall ratios may be as high as 20:1!  This will help us in the snow accumulation department.  Mid-level lapse rates also look to steepen and this coupled with some strong lift which is suggested by stronger vertical velocities moving across the region suggest, there could be a very brief period of heavy snowfall somewhere across the region.  Given what we have presented to us now, this is what to expect right now.