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: