Significant Coastal Storm to Bring Snows to CT...But How Much?

While the airmass we have been locked into for some time now hasn't been really historic in terms of the degree of the cold but it is very impressive in terms of its duration.While the cold has not really come with much in the way of snow this will all change Thursday as an extremely strong coastal storm will be developing and will drop a significant amount of snows somewhere in southern New England, the question is just where? For us here in CT we will have to closely watch and monitor the exact track of the system (as well as other factors) as this will play a large role in what we are looking at for potential snowfall totals. Unfortunately, after this system passes it looks like things will become even COLDER for the first part of the weekend but there is a bit of a warm-up to begin next week! 

Currently, there are two pieces of shortwave energy we are watching. The first piece is located across the northern Plains and will continue to dive to the southeast. The second is located across the southern Plains and will move a bit southeast as well. These two will eventually phase (combine) across the southeastern United States coast:

As a result we will see the development of a surface low pressure just off the southeast coast and extreme cyclogenesis (strengthening of the surface low pressure) occur. This system is expected to develop and strengthen very rapidly:

 As the system tracks northeast off the east coast it will begin to undergo bombogenesis (a term used to describe EXTREME deepening of a surface low) off the New England coast.  Numerous computer forecast models strengthen the system well into the low 950mb range (and in some cases into the 940's). This would be really exceptional for our latitude and to give an idea of how strong of a system that is those pressure readings would rival that of a strong category 2 to a category 3 hurricane! With this system we will see substantial snowfall totals, very high snowfall rates within the heaviest banding which could exceed 5'' per hour, and very strong/damaging wind gusts of 50-60+ mph which would yield blizzard conditions and whiteout conditions. Again, the question is do we see any of this here in CT?

There are some large spreads within the computer forecast models with regards to how far west/east the system will track and again, this is critical to how much snow we see here in CT. We'll just look at the American forecast models such as the NAM and GFS (I can't post European model data unfortunately) to illustrate the differences. This afternoon's runs of the NAM and GFS show some big differences with the track and the result is big differences with how far west the heaviest snows get. The NAM brings moderate to heavy snows into CT while the GFS keep the heavier snows just east of the state:

However, this afternoon's GFS ensemble run suggests a bit higher likelihood for heavier snows to push into CT. Each of the individual images below represent a different "member" of the ensemble:

Not only does the surface low pass to our southeast but the 925mb lows, 850mb lows, and 700mb lows also pass to our southeast. These are prime tracks of these lows for moderate to heavy snowfall to occur somewhere in southern New England but again where that heavy snowfall occurs all depends on the exact track of these lows. the reason for the heavy snows to the northwest of these low tracks is due to the substantial upward vertical motion that occurs in this quadrant of a low pressure. In the case of this system we have substantial moisture being lifted well up into the dendritic snowgrowth zone, this is going to produce very heavy snows somewhere:

What is also rather impressive with this system is that computer forecast models develop a closed off circulation at 500mb which only happens in exceptionally strong systems:

This would work to not only enhance the bombogenesis process but would also yield to slow the storm's motion down a bit and also further increase the intensity of the upward vertical motion enhancing the snowfall rates in the bands and also increasing the strength of the subsidence and reducing snowfall rates and intensity in places.

Anyways I usually try to go more in depth and a little technical, especially in these types of storms but lack of time will prevent that. But for right now I will hold off on making a snowfall forecast until tomorrow evening. I would do it during the day but work calls. Below are some reasons for holding off on a forecast:

• The degree of spread and uncertainty with the storm track is just too much right now - We are looking at a situation where a track even 50-75 miles further west could mean potentially as much as 12-18'' of snow for some in CT where a track 50-75 miles east means maybe only a few to perhaps several inches for some (and this would mainly be eastern CT). This spread is just too much to even throw out a guess at this time. 
• Uncertainty with regards to where the heaviest banding sets up - This can again be tied to the track of the storm and also exactly where the 925mb, 850mb and 700mb lows track and how they develop. There is likely to be some extremely heavy banding with this system which could yield snowfall rates as high as 5'' per hour but it's not clear where this will occur. 
• Tying into the above, because we will have such extreme upward vertical motion we also must have extreme downward vertical motion which we call subsidence. This will create a zone where snowfall rates and intensity are vastly lessened and the result will be much lower snowfall totals for some compared to places around them. This is something that is also near impossible to pinpoint at this time. 
•  The forecasts for such a strong low pressure - This further complicates matters (at least in my eyes). When low pressures become this strong (and we see this happen with hurricanes) precipitation can often become more "band like" (bands f precipitation as opposed to an extensive precipitation shield). This means that you don't have a uniform precipitation field and this means you won't have uniform snowfall totals. Also, inbetween these bands you would have zones of subsidence which means lesser snows for these areas. For example, Windsor Locks could be right under a band and getting 2...3...4'' per hour while Hartford, CT is south of the band and getting barely 1'' per hour. That would result in a big snowfall gradient in just a small distance. Very, very difficult to forecast at this point. Such a strong system can also keep the precipitation field more compact and closer to the storm's center so if the system is further east the heaviest snows likely don't make it into CT. 

What we know...

• I do think that it is LIKELY we at least see moderate snowfall accumulations across much of CT (at least 6'' of snow). Once details become more clear we'll have a better idea of exactly how much.
• Snowfall begins to break out during the pre-dawn hours of Thursday with intensity increasing throughout the morning. How heavy the snow becomes, how long the heavy snow persists, and exactly how heavy the snow rates are will depend on all the factors described above. 
• We will see a period of very strong winds with gusts perhaps exceeding 60 mph. This would yield blizzard and whiteout conditions as well as tree damage and power outages. The extent of the gust potential will also depend on the track .
• As the system departs winds continue and even COLDER air works into the state for the weekend. 

I will have an updated blog and snowfall forecast up tomorrow evening...a bit later than I would like but work calls.