White Christmas On the Way

For those of you who dream for a white Christmas that dream will be coming true this year. An approaching amplifying (strengthening) shortwave trough will spawn the development of a surface low pressure which will go through fairly rapid cyclogenesis (strengthening/deepening). As this process occurs we will see a rapid increase in precipitation. For much of the state (pretty much along and north/west of I-84) this will be all snow, however, south and east of I-84 there will be some mixing issues and then a changeover to rsain and this will lead to reduced snowfall totals for these areas.  We will have to watch how the models handle some features such as the development and track of the 850mb and 925mb low pressure centers (this will be discussed in more detail below) because there is the possibility that if these develop further west, this could allow for a surge of warmer air to push in around from around 2000-5000' above the ground. This would reduce snowfall totals even along and northwest of I-84 and introduce the likelihood for mixed precipitation as well. 

An approaching shortwave trough will undergo strong amplification as a potent 500mb (~18,000' above the ground) jet stream rounds the base of the shortwave trough:

Within the jet streak will be some potent shortwave energy and this energy will only help to enhance and fuel the development of the surface low along with the rapid strengthening of the surface low:

As the system begins to rapidly strengthen and develop we will see low pressure centers develop at 925mb (~2500' above the ground), 850mb (~5000' above the ground), and 700mb (~10,000' above the ground). This will lead to a couple different things; 1) It will enhance upward vertical motion (particularly on the northwest side of these lows) which will increase the likelihood for the moderate to heavy precipitation. 2) Influence temperatures, especially between about 2000' and 5000'. While computer forecast models do agree that these lows will develop west of here but the main question is how far west? Typically a west track of these lows indicates warm air advection into these levels and computer forecast models do suggest that a strong enough surge of warm air could advect in which could change precipitation type from snow to perhaps sleet or even some freezing rain. However, this would also enhance an extremely band of heavy snowfall just west of this temperature gradient. Typically the strongest upward vertical motion is also on the northwest side of these low pressure centers, however, computer forecast models actually still yield a fairly impressive amount of upward vertical motion with the strongest occurring across the northwestern part of the state with a secondary area across northeastern CT. We should note, however, that in between these two areas of enhanced lift (of course this is assuming it verifies this way) there could be an area of enhanced downward vertical motion (what goes up must go down!). You have air rapidly rising upward in the area of enhanced rising motion so that air needs to eventually go somewhere and that somewhere is back downward. This is why in those big snowstorms where you see a narrow zone of really high snowfall totals (this is the area where the enhanced upward vertical motion occurred and remained the longest) but right near it you'll see much less snowfall totals compared to everyone else. This is the area where the subsidence occurred and. This is VERY difficult to forecast. Anyways, while temperatures will be marginal between about 2000' and 6000' (which is usually not a good thing) in this situation it will be (that's if you like alot of snow :) ). 

In place we will have very cold temperatures between about 700mb and 500mb (~10,000' to ~18,000') with temperatures between -10C (700mb) to as low as -20C (500mb). This is one key when talking about good snowgrowth production:

With great snowgrowth we will also see sufficient upward vertical motion along with sufficient relative humidity within the snowgrowth zone. This will yield to the generation of those really big and fluffy snowflakes which accumulate quickly (dendrites). For this we will look at a bufkit profile over Windsor Locks, CT:

Back to the marginal 850mb temperatures. In this situation since we will have excellent snowgrowth, due to the presence of sufficient relative humidity within the dendritic snowgrowth zone, and very strong upward vertical motion within this zone this would normally lead to rather high snowfall ratios. In this case, the marginal 850mb temperatures will work to reduce the snowfall ratios a bit but what they will also do is lead to the development of rather strong frontogenesis (strong vertical temperature gradient within the horizontal). This will yield to produce a very intense band of very heavy snow just west of this temperature gradient. The key is just figuring out where this sets up. This will only be further enhanced by the presence of very strong upward vertical velocity:

The surge of warmth, however, will yield a mix and changeover to even rain for some areas south and east of I-84

What should we expect?

• Flurries/light snows begin to break out between 10:00 PM and 1:00 AM. 
• Snow begins to increase in intensity after 2:00-3:00 AM and periods of moderate to heavy snows will occur through mid-to-late morning before quickly beginning to taper off from the southwest to the northeast. 
• Warmer air moves in south and east if I-84 and these locations will see snow transition to a mixture of sleet and then all rain. North and west of I-84 will remain snow but some mixing of sleet can't totally be ruled out.
• Under the heaviest banding snowfall rates could approach 2-3'' per hour. This would likely occur anywhere from about 5:00 AM to 9:00 AM. This is where the highest snowfall totals will occur. There could be two areas of heavy banding with the first area across northwestern CT and the second area across parts of northeastern CT.  If we do indeed see two bands, this could yield some subsidence in between these bands which could result in some lower totals for some. 
• Snow begins to taper off late morning and push out by early afternoon.