Sunday, December 1, 2019 - Monday, December 2, 2019 Winter Storm Update

First significant winter storm of the season is still on track for CT. While we continue to draw closer to the onset of the storm there are still some uncertainties to resolve, however, details are starting to become a bit clearer. This is going to be a long duration winter weather event with two different parts (or aspects to it). While I did go in-depth a bit the other day with the meteorology/science behind the storm, I will do so again and discuss some features in greater depth.

Part 1: Sunday, December 1, 2019:

Sunday starts cold and cold enough at all levels of the atmosphere for precipitation to begin as snow statewide. A vigorous piece of shortwave energy diving east-southeast through the Ohio Valley and associated surface low pressure weakens as it moves east through the Ohio Valley. The track of these features will yield warmer air to move towards CT. This warmer air (warm air advection) will be a focal point as it will help yield a several-hour period of moderate-to-heavy snow overspreading the state beginning late Sunday morning.

As the warmer air begins to work into the lower-levels of the atmosphere (and at the surface along the immediate shoreline) we'll see snow transition to sleet, freezing rain, and rain along the immediate shoreline. The biggest question here is going to be how much sleet vs. freezing rain?

Conducting a sounding analysis across the state of CT we see some differences come Sunday evening between the NAM forecast model (on the left) and the GFS forecast model (on the right). The green shaded color accounts for the spread in dewpoint temperature (green line) and temperature (red line) across the state. The NAM introduces a shallow-elevated warm layer...this would result in a changeover to sleet..eventually the depth the warm layer would increase with warmer air becoming more shallow (closer to the ground) which would indicate freezing rain potential. The GFS, however, keeps colder air in place longer and then quickly transitions much or the state to some rain:

The verdict: I think the GFS is a bit too aggressive with the degree of warmth. While the immediate shoreline (perhaps even 10-15 miles inland) may switch to rain, I don't think we'll see rain into central CT. This is where we'll see a mixture of sleet/freezing rain. Significant issues should not result from sleet or freezing rain as the intensity will be very light and more intermittent. This continues through the overnight into Monday morning.

Snowfall totals on Sunday should generally be 1-3'' along the immediate shoreline and 2-4'' the rest of the state. (I would lean towards the lower side of these ranges as opposed to the higher side).

Part 2 Monday, December 2, 2019: 

For my snow map to verify this will require part 2 panning out and there are still uncertainties with how this transpires. Going back a few paragraphs, I mentioned the surface low pressure moving east through the Ohio Valley and weakening. As this weakens a new surface low pressure begins to develop across PA which slides east...the question is how close to CT this tracks...if it's very close to CT warmer air wins out and the strongest atmospheric lift and heaviest precipitation occurs north of the state, if this is more over Long Island or just south, colder air quickly works back into the state, stronger atmospheric lift resides over the state, and we get heavier precipitation...snow.

The NAM indicates the stronger atmospheric lift develops north of CT through the day Monday while the GFS suggests some of the stronger atmospheric lift does indeed develop down into CT:

Verdict: After analyzing all model guidance, my thinking is we will see some stronger atmospheric lift work into the extreme northern part of the state during the day Monday. This will result in a second period of moderate-to-heavy snow developing through the morning and persisting into the late afternoon before tapering off. Periods of light snow remain possible through the overnight.

Snowfall totals with Monday's snow may add another inch or two across the shoreline (though it's very possible this is more of a mix or even rain) with another 2-4'' across interior CT...it's the extreme northern part of the state which sees the heaviest snow where additional totals of 4-8'' are possible.

Here is a breakdown of what to expect:

• Snow overspreads the state Sunday morning from west-to-east
• Quickly becoming moderate-to-heavy for a few to several hours
• Snow quickly transitions to sleet and then rain along the immediate shoreline and changes to a mixture of sleet/freezing rain inland
• Intermittent periods of sleet/freezing rain persist through the overnight into Monday morning
• Snow begins to re-develop across the state through Monday morning (though may be rain or a mix along the immediate shore)
• Heaviest snow along the CT/MA border where rates may approach or exceed 1'' per hour
• Thundersnow a small possibility 
• Significant travel impacts expected Sunday through Tuesday morning. 

Based on the latest guidance, I have made some adjustments to my snowfall map. I have shifted the 2-4'' zone farther north to incorporate a larger area of southern CT and I have shifted the 8-12'' zone slightly north (closer to the CT/MA border). 

This snowfall map accounts both Sunday and Monday:

Sunday, December 1, 2019 - Monday, December 2, 2019 Snow Potential

Are we looking at our first accumulating snowfall event for the majority of CT as we turn the calendar to December? There continue to be growing signals that this may be a possibility, however, there are still numerous details to iron out moving into the weekend. This is going to be a bit of a messy storm as there will be two different parts to it. This will also be drawn out over the span of a few days. (Note: I use GFS graphics for visual reference, however, analysis and my thoughts are derived from analyzing all forecast models).

They piece of energy which will be responsible for our potential storm Sunday and Monday is entering the southwestern U.S. This energy will also be a player in what will be a significant winter storm/blizzard across the Inter-mountain West, northern Plains, and Inter-mountain West moving into the weekend:

Sunday morning, the piece of energy is forecast to dive southeast through the Ohio Valley. At the surface an area of low pressure (weakening) will be moving towards the east-southeast. As this whole system progresses towards the east-southeast a warm front will lift towards southern New England. As the warm front approaches, warm-air advection will rapidly increase. This warm-air advection will be quite significant as it will result in a period (perhaps several hours) of moderate-to-heavy snow overspreading much of the state. As warmer air begins to arrive (first occurring a few thousand feet off the ground) snow will quickly transition over to some sleet (maybe some freezing rain) and even rain along the immediate shore. This will conclude part one of this system:

Moderate-to-heavy snow overspreads the state Monday morning before transitioning over to a wintry mix and rain along the shore by late afternoon. Several inches of accumulation will be possible:

As the energy dives south of CT Monday morning, a secondary area of low pressure is expected to develop...this is where things get a bit dicey as it's uncertain exactly where this happens and how quickly this happens. Prior to this development, warmer air will be in place, both aloft and at the surface, thanks to the warm-air advection. However, as the secondary low develops and strengthens dynamic cooling will take place and this will allow the lower-levels of the atmosphere to quickly cool with colder air working in at the surface.

My current thinking is this all happens a bit too far north. This will keep the column a bit too warm for snow here in CT (although a better chance would exist in the northwest hills and along the CT/MA border) and we'll see mostly rain or a mix with this second part. Although, this is still several days away and plenty can certainly change.

All in all this system will likely be a major snow producer (8-12''+) somewhere in New England, it's just uncertain as to where this area will be. My early estimate is along and north of the MA Pike.

Sunday, December 1, 2019 Forecast:

• Snow begins working into the state from west-to-east during the morning. 
• Snow quickly picks up in intensity; perhaps falling at rates of 1-1.5'' per hour.
• Snow begins to change over quickly to rain rain along the immediate shoreline first due to the influences of the warmer water. 
• Snow transitions over to a mixture of sleet...perhaps some freezing rain and rain inland as warmer air begins to work in aloft.
• Significant travel impacts expected beginning late Sunday morning into the evening.
• Winds do begin to strengthen overnight as the secondary low develops and strengthens. 
• Major uncertainties exist regarding Monday.

12z GFS bufkit for Windsor Locks, CT Sunday suggests ample upward vertical motion and more than sufficient relative humidity and ice (not shown) for the generation of moderate-to-heavy snow:

Below are my initial thoughts for Sunday ONLY. This DOES NOT INCLUDE WHAT MAY TRANSPIRE MONDAY:

I will have some additional updates late Friday afternoon.