Saturday, January 18, 2020 CT Snowfall Forecast

I would like to begin by saying my notion of a "major" snowstorm for CT a few days ago was a bit misleading. Probably not the proper term to use for this type of setup, however, at the time I was not discounting the fact that there could potentiality be widespread 6''+ of snow.

Anyways, the forecast for accumulating snow Saturday is on track. This will be a very quick moving system and the majority of the state should even remain all snow throughout the duration of the event (with exception of the southern half of the state).

Normally, I wouldn't be as aggressive in terms of my snowfall forecast for such a setup, however, there are several things I like about this setup which I think will favor a bit more of snow then we would typically see. I will briefly go through these below:

1) Dynamic cooling - As low-level dynamics increase with this system this results in air rapidly rising upwards. This process results in cooling temperatures. When this process occurs rapidly there can be a rapid response in temperature. This will help with better snow ratios (see below) and preventing a changeover from snow to sleet (especially away from the shoreline). The GIF below illustrates how temperatures at 850mb (~5,000 feet above the ground) change during the day Saturday...notice how they cool quite a bit! Typically in this setup they would warm:

2) Favorable snow ratios - Typically in these setups our snowfall ratios are less than stellar...around 8:1 or so. However, given how cold the air mass will be (along with my expectation for some modest-to-decent lift into the dendritic snow growth zone...see below) I am thinking snow ratios may be as good as 15:1. This would indicate fluffy snow and possibly medium-to-large dendrites...the kind of snowflakes which accumulate quickly. Caution should always be advised when assessing snowfall ratios outside of 36-hours and with snow ratios on bufkit, however, the signal has been strong for solid snowfall ratios and given the environment I am inclined to believe they will be as good as 15:1 which is indicated below:

3) Forecast models are hinting at some decent lift penetrating the dendritic snow growth zone. This is also something which I think may trend a bit more impressive over the next 24-36 hours.

What can we expect?

• Snow begins to break out by late Saturday morning
• A 2-4 hour period of moderate to even heavy snow is possible mid-to-late Saturday afternoon
• During this period snowfall rates may approach 1.5'' per hour 
• Low-to-moderate travel impacts Saturday afternoon 
• Mixing with sleet southern half of the state...perhaps even to rain along the immediate shoreline
• Snow exits by late Saturday evening

Major Winter Storm on Tap for CT Saturday, January 18, 2020

We have been relatively quiet in the winter weather department across CT since ringing in the new year. In fact, we have seen temperatures quite a bit above-average. That is all about to change, however, as a potent weather system looks to deliver some snow...and potentially quite a bit this upcoming weekend.

A potent piece of shortwave energy forecast to enter the Pacific-Northwest Thursday evening will quickly traverse the fast flow aloft through the end of the week and enter the Ohio Valley region by Saturday morning:

A vigorous 110+ knot 500mb jet rounding the base of the trough will result in further strengthening of the system; including the surface low pressure:

There is pretty strong agreement within the computer model world the surface low pressure tracks through the upper-tier of the Ohio Valley, through southeastern Canada, and across northern New England. Typically this type of storm track results in warmer air flooding into the region...especially when the surface low continues to strengthen to our west. While there will be warmer air that works in which will likely result in snow changing to sleet/freezing rain or rain there are a few features which yield potential for major snow totals;

1) Strong surface high just north of Maine. This feature is departing to the northeast as Saturday progresses, however, it looks to have control long enough for the return flow (winds at the surface more north-northeast) to keep lower-level and surface cold air locked in:

2) Strong warm-air advection. Strong surge of warm-air advection results in very strong upward vertical motion resulting potential for moderate-to-heavy precipitation...in this case snow. This also helps provide the moisture needed for the generating of snow:

It is still a bit too far out to get into details and there are still some uncertainties, especially regarding the potential for mixed-precipitation and any changeover to rain and whether this will impact potential totals. Anyways, as it stands right now this could be a major snowstorm for a large portion of CT.