Freezing Rain Tonight/Tomorrow Morning.

We have seen periods and pockets of mixed precipitation across the state throughout the afternoon and we are beginning to see freezing rain, and some accretions across portions of the state as well. This theme will continue into the overnight hours. We will continue to see these pockets of mixed precipitation and freezing rain move through the state through the rest of the evening and through the overnight hours, however, our attention turns to tomorrow morning when steadier precipitation will push into the state. There could be some concerns for prolonged duration of freezing rain within the CT River valley across interior CT. This is due to the possibility of colder air holding in place much longer than locations outside the Valley. (The meteorology behind this was discussed in the previous blog post).

A radar grab of the Northeastern United States from 6:42 PM EST highlights the two areas of precipitation currently. We see the pockets of mixed precipitation across CT and we see an extensive area of precipitation back off to the southwest. This area of precipitation is expected to begin pushing into the state as early as 4:00-5:00 AM tomorrow morning. (Remember we will continue to see periods and pockets of mixed precipitation through the overnight but it's the heavier and steadier stuff which begins to arrive by 4:00-5:00 AM:

Current surface observations from across the region as of 7:00 PM EST show much more mild air just to the south of CT with much colder temperatures just to the north of CT:

Also discussed in the previous blog post was an area of high pressure which would become positioned just to the north of ME. We currently have the high pressure positioned just to the north of ME and with this we have a generally northerly wind direction across the northern part of the state with a more easterly component to the winds across the southern portion of the state.  The positioning of this high (along with the strength of the high pressure) is allowing for much colder air up north to drain right down through the CT River Valley. As long as this high pressure remains in the position it is in this cold air will continue to drain down the Valley. Once the high pressure begins to move off to the north and east, the surface winds in the Valley will shift to a more easterly or southerly direction and this will allow for surface temperatures to warm above freezing:

For southern CT the high pressure isn't that much of an influence and with a more easterly component of the winds this will allow for surface temperatures to rise above freezing much more quickly and across some places they already are just above freezing. This will limit concern for any prolonged icing across southern CT.

Some of the computer forecast models do warm surface temperatures above freezing in the CT Valley as early as 10:00 AM tomorrow morning, however, there are some computer forecast models which keep surface temperatures below freezing much longer and this would pose an increased risk for prolonged icing which could lead to some downed trees, utility poles, and power lines.  This is going to be an extremely close call and we'll have to monitor this very closely. Some of what to watch include;

1) Onset of the heaviest precipitation: If that batch of precipitation off to the southwest moves in faster this gives the Valley prolonged exposure to steadier precipitation with surface temperatures below freezing. The result would be ice accretions which could approach or exceed 0.20'' which is when things could get dicey when talking about the possibility for downed limbs and power lines. If the timing is much slower than this likelihood is greatly reduced.

2) How long does the cold air remain locked in place? This right here in the million dollar question. Computer forecast models often struggle to handle these cold air damming situations very well and that is due to the resolution size of the models. They just don't have the power to analyze these very small scale phenomena. A difference of just two hours or even two degrees could mean the difference in only minor icing to accretions high enough to warrant the possibility for downed trees/limbs, power lines, and utility poles.

3) The area of high pressure to the north: How quickly does this high pressure depart off to the northeast? If it remains in place longer, this allows for a longer duration of the northerly component of the winds to remain in place across the Valley...the result: colder temperatures and prolonged icing. If it pushes out more quickly the result: Winds shifting to a more easterly or even southerly component and that will favor the advection of warmer temperatures from the east (Atlantic Ocean) and from the south.

This is going to come down to a nowcasting forecast and we'll have to watch it very closely. The map below highlights what areas have to be concerned the most from the scenario which would result in icing problems:

This map highlights the areas which could see icing issues if the worst case scenario verifies which is colder air remaining locked in longer. The rest of the state (with the exception of southern CT) will continue to see pockets of mixed precipitation and freezing rain through the evening:

Looking ahead the chances for a white Christmas are vastly increasing as plowable snows are looking more and more likely across the state but we'll look more into this tomorrow.