Major Storm Expected on Thursday

We are tracking yet another storm system to impact southern New England during the day on Thursday lasting well into the overnight hours but there continues to be some uncertainties within the computer forecast models and that does lead to a challenging forecast at this time.  A couple of our computer forecast models are in agreement that as the system works up just off the coast that it will undergo rapid cyclogenesis (strengthening) and become quite a powerful storm.  Meanwhile, some other computer forecast models aren't as impressive with the system and have a much weaker and faster system.  If this isn't tough enough there will be a bit of uncertainty as to exactly where the rain/snow transition line pivots and this is critical because it could mean the difference between only maybe 1-2'' of snow and perhaps as much as 6-7'' of snow.  These details should be ironed out over the next 24-36 hours.

Both the NAM and European computer forecast models are actually in fairly good agreement with each other on a rapidly intensifying storm system just off the coast.  Unfortunately I am not able to post Euro images, however, I can go ahead and post some graphics from the NAM.  The 12z run of the NAM from the 27th of December shows a very strong piece of shortwave energy just south of southern New England by 7:00 PM EST Thursday evening:

The forecast is for this piece of shortwave energy to continue pushing to the northeast and undergoing further and rapid strengthening.  The image below is a forecast for 1:00 AM Friday morning:

This will allow the entire system t rapidly strengthen so the response will be a rapidly deepening surface low pressure and closed off circulations at 925mb, 850mb, and 700mb:

In comparison, the GFS computer forecast model valid for the same forecast time shows a weaker system than the NAM and Euro and actually doesn't really begin to strengthen the system until it reaches the Gulf of Maine. Also note how much quicker the GFS is than both the other models:

Each of these forecasts presents a different set of solutions that we could potentially see.  The GFS forecast model also tracks the system pretty much over southern New England.  What this does is allow much warmer air to work into much of the region within the lowest 3,000-4,000' of the atmosphere.  This would eventually lead from a transition from snow to sleet/rain across many locations, particularly south and east of the MA Pike.  The NAM, on then other hand, also brings in some warmth ahead of the system.  This is due to the fact that it rapidly strengthens the system well to our south and since the track is tucked closer to the coast warm air floods in ahead of the system.  However, as the system rapidly intensifies and develops those closed off circulations (and the track is just to our south and east), this induces what is called dynamic cooling and will help to rapidly cool off the lower levels of the atmosphere.  This would mean MUCH more snow for a larger chunk of the region (with the exception of the south coast where the track would still drive in warmer air here).  This is what we need to watch over the next 24-36 hours.

One thing presented on all models is the possibility for a period of moderate to heavy precipitation (whether it be snow or rain).  The GFS has a much shorter window for this probability than the NAM or Euro.  Taking a look at some bufkit profile soundings from the 12z NAM for a few select locations across southern New England we see

1) Snnowgrowth zone positioned right between 12,000 and 18,000' (purple/yellow contours)
2) Strong upwards vertical motion present within this zone (the red contours with negative values)
3) -15C temperature contour crossing through this zone (thin yellow line running horizontally)
4) High relative humidity values within this zone (not shown below as it would look too "messy")

All four of these combined indicate that the potential exists for moderate snowfall accumulations to occur for any areas that remain all or mainly snow.  The NAM shows INCREDIBLE upwards vertical motion across eastern portions of southern New England including the Boston area:

If the NAM/European computer forecast models are correct we would see a very intense band of snow setup just to the northwest of the tracks of the 925/850/700mb lows and this is where the axis of the highest snowfall totals would occur.  Snowfall rates could approach 1.5'' per hour and we would also be looking at the potential for strong winds as well with gusts as high as 40-50 mph.  Given the degree of lift and even some weak instability thanks to presence of steep mid-level lapse rates, some thundersnow can't be ruled out wither which would locally enhance snowfall rates as well.  If the GFS is correct we would be looking at a much weaker system across southern New England with not as much impacts.  Northeastern MA into northern New England would now see the higher impact as the system strengthens as it moves into the Gulf of Maine.

As of right now, given the consistency of the Euro/NAM thus far and the GFS has seemed to trend a bit in the direction of the Euro/NAM, I am leaning more towards a solution depicted by the NAM/Euro.  This of course can easily change as if they are too quick with the strengthening of the system we see higher likelihood for warmer air to invade.  So this what I'm expecting:

• Snow begins in the mid-to late morning hours, especially across CT and parts of RI and early afternoon the further north into MA one goes.  
• Snow may transition over to rain briefly, especially the south coast, as warmer air initially moves in ahead of the strengthening system
• Rain will then begin to transition back over to snow as the system strengthens and dynamic cooling cools the atmosphere aloft.  The coast, however, will remain more rain
• There will be a zone through the system as well in which snow transitions between sleet and perhaps some freezing rain.  This will be the cutoff between the higher and lower snow totals
• During the height of the storm winds could increase to 40-50 mph causing isolated pockets of power outages
• Snowfall rates could approach 1.5'' per hour and perhaps as high as 2'' per hour if there is any embedded thundersnow 
• The precipitation begins to taper off during the overnight hours of Thursday into Friday.
• Thursday rush hour commute looks to be very bad so expect a very slow commute with numerous delays.  

Below is my initial snow map but there is not a high amount of confidence with this due to concerns mentioned above.  Typically I would be more shy than not in going with lower snowfall totals, however, when the NAM and Euro tend to be in agreement more times than not that is a very good indication of what will verify.  This doesn't always work but as of right now I would lean this way: