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.

Updated snowfall forecast for southern New England

Given the latest trends in the computer forecast models during the overnight period, snowfall totals have been increased slightly, although hesitantly.  A few negative factors for higher end totals is 1) The speed of the storm 2) The duration of the heaviest snowfall rates and 3) No real established mid-level circulation.  However, computer forecast models are continuing to indicate a tremendous amount of atmospheric lift coupled with a very saturated atmosphere and excellent snowgrowth, at least until some warmer air begins to work in aloft.  During the height of the storm, we could see snowfall rates exceed 2'' per hour at times, thus making some of the higher end totals quite possible.  Given the computer forecast models are projecting the strongest lift to occur across portions of extreme NE CT/ NW RI/portions of interior eastern MA, this is where the highest snowfall totals are expected.  

What to Expect

• Light snows to being breaking out, possibly as early as 2-3 PM
• Heaviest snowfall expected to occur between about 7-8 PM and 3-4 AM.  
• Snowfall rates at the height of the storm may approach 2'' per hour for a period
• Coastal areas will experience a mix/changeover to sleet keeping snowfall totals down across these areas
• As the storm winds down, many may see a period of mixing with sleet or changing over to sleet with the exception of western MA and extreme NW CT
• Storm is gone early AM Sunday with light snows lingering across NE MA until late morning

Accumulating snowfall Saturday into Sunday across southern New England

A winter storm is set to impact southern New England Saturday into Sunday as a quick hitting system is set to bring accumulating and plow-able snowfall to the entire region.  We have been closely monitoring this potential over the course of the past several days and as we draw closer to the weekend, the details being presented to us have become much clearer and we have a better idea of what to expect.

Meteorological Insight

Below are images of the 500mb pattern (images on the left) and sea-level pressure (images on the right) from today's 12z runs of the GFS and the Euro.  For the 500mb level pattern there are 2 highlighted areas.  The 1st highlighted area is the southern stream energy which will spawn the area of low pressure which will produce our storm.  The 2nd highlighted area is the northern piece of energy.  Initially we were watching as some model guidance did suggest that the possibility was there for these two systems to phase with each other, or in other words, combine.  In this type of solution we would be looking at a more power and potent storm system and a bit slower moving system.  However, it's now begging to appear that these two pieces of energy will not phase (at least over our area) so the southern stream energy will remain the dominant feature.

The image on the right, as mentioned above, is showing sea level pressure are there are two highlighted features here, with the exception of the Euro as the Euro has yet to develop our surface low yet.  The main feature here is the area of high pressure which is circled just to the north of New England.  This Arctic High pressure is expected to slightly strengthen this weekend and this will really help to keep colder air in the lower levels of the atmosphere and at the surface locked in place for a longer period of time and this colder air will also really help to make for fluffier snow, especially the further north you go.

Below we fast forward the GFS/Euro models to 72 hours out, with each projecting how the atmosphere will look come 7:00 AM EST Sunday.  What we can see is both models show a low pressure track very close to the coast and with that Arctic High Pressure eventually departing off to the northeast, this will eventually allow for some warmer air to work in towards the surface and aloft in the atmosphere.  This will be associated with a warm front which will work close to parts of the region.

As the area of low pressure develops and strengthens, this will allow for moisture to increase across the region and with a very cold thermal profile throughout the atmosphere, this will lead to some excellent snowgrowth and pretty high snowfall ratios, perhaps as high as 15:1 to 17:1.

What to Expect

This is not going to be a classic snowstorm by any means.  This isn't a storm forming from the phasing of two energies, as described above, and the mid-levels of the atmosphere aren't presenting us with that look which makes us go...WOW.  While light snows may being falling as early as early Saturday afternoon with snows lingering into perhaps late Sunday morning, the bulk of the heaviest snowfall is actually only expected to occur in a 4-6 hour window, likely from late Saturday evening into the overnight hours.  This is where much of the snowfall accumulations will occur.  The burst of the heaviest snowfall will be associated with the nose of the increasing warm-air advection aloft and a swath of very strong atmospheric lift.  During this 4-6 hour window, we could see snowfall rates potentially exceed 2''/HR.  As the Arctic high does begin to depart, allowing for some warmer air to work in, this will decrease snowfall ratios some and also may allow a changeover or a mix of sleet and perhaps some freezing rain, especially for portions of CT/RI/far SE MA.  Towards the end of the event though, all areas may experience some brief periods of sleet as well.  Below is current thinking of what to expect for snowfall totals.

Weekend Storm Potential for southern New England

By now I'm sure just about everyone has heard about the potential for a winter storm at some point this weekend with the time frame of impact somewhere between Saturday and Sunday.  At this stage, not only is trying to pin down the exact timing of the onset of any storm, but we are faced with many questions such as, will there be a storm and if so, how strong of a storm are we looking at?  When answering these questions we rely heavily on the many different computer forecast guidance models we have available and try to piece the puzzle together.  When there are differences among the computer forecast guidance models and not much consistency, it lowers the confidence of any forecast.  When looking at what potential this weekend may hold, we will look at the key pieces within the atmosphere and then see how different computer forecast guidance models are handling them.

Below we are looking at the 500mb level of the atmosphere (images on the left), or about 18,000ft AGL.  This level of the atmosphere is of major importance as the jet stream at this level, along with the upper-air pattern is the driver of our weather and we are also looking at 1000-500mb thicknesses along with sea-level pressure (images on the right)  The top image is from the European model (Euro) and the last two images are from the GFS model and the NAM model, two of the American models.  

With the images on the left, each model output graphic has two circled areas of interest.  These two features are what will eventually develop a storm off the East coast.  Each of these graphics are what each model is forecasting what the 500mb pattern will look like come 7:00 AM EST Saturday morning.  As you can see each model is handling the two features differently and that is one factor leading to a low confidence call right now, especially with regards to potential strength.

What each of these models do have in common is a rather strong northern branch of energy (the area near the US/Canadian border).  With each model in agreement on this, all 3 of the selected models will continue to lead to amplification of this feature (strengthening) and lead to a digging trough working into and through the central-tier of the US.  What we also notice is, the two American models, have another piece of very weak energy ahead of the more stronger energy.  The Euro has just the one piece of potent energy.

When looking into the southern stream energy (circled area across the southern-tier of the US), we can see various differences between each of these models.  The Euro and NAM models, however, keep the southern stream energies intact, with the NAM being stronger than the Euro with this energy.  The GFS, however, has the southern stream energy separate into two pieces.

The key for this weekend will be how each of these two streams interact with each other; when do they phase (combine), where do they phase, and how strong will each feature be?  As we go through the next few days, computer forecast guidance should begin to encounter a consensus of how the pattern will unfold and once this occurs, we will have a much better idea of exactly what to expect and when to expect it.

Going back to each model, we see what their projections are for 7:00 AM EST Sunday for the Euro/GFS models and for 1:00 AM EST Sunday from the NAM.  The NAM computer only runs out to 84 hours from it's initialized time.

From going by the latest data from this afternoon, what we see right now is the Euro/NAM computer forecast models are very close to the development of what would be a pretty strong storm system with the potential to produce plow-able snowfall across the entire region.  The GFS, however, wants to phase the two streams later rather than earlier, and that solution would reduce the risk for a more potent system.

Given how the GFS ensembles (a collection of different members associated with a particular model run) vary widely with it's solutions, this tells us that the GFS right now is really all over the place with this system and seeing this, certainly keeps the possibility in hand of a major coastal storm with the potential for plow-able snowfall throughout the region.

As new data continues to come in, over the course of the next 24-36 hours, the situation should become much more clear and details can start to be ironed out.