Ice Event Possible Across Interior CT Friday night into Saturday?

Just a day ago it looked as if we would see a major warming trend Friday night through Saturday as a storm system tracked well off to the west allowing for a surge of milder air to work into the state. However, substantial changes have unfolded within the computer forecast models and these changes suggest that we will keep the cold air locked in at the surface while the surge of warmth happens a few thousand feet above our heads. This type of scenario would favor the potential for freezing rain, especially across interior CT away from the shoreline. In my forecast video from Tuesday I went with a high of 57 Saturday along with a warming trend Friday night into Saturday...well this looks very wrong. So why the sudden change? This will be explained below.

One of the biggest key factors here will be an area of high pressure which slides through southern Canada and becomes positioned just to the north of Maine for several hours before slowly drifting off to the north and east. There is strong agreement on this scenario from the NAM, GFS, and European computer forecast models. The below image (using the GFS forecast model) is for 7:00 PM EST Friday evening showing the area of high pressure to the north of Maine. In the northern hemisphere, the flow around high pressure is clockwise (anti-cyclonic). Given the position of the high pressure, this would indicate surface winds coming from a more northeasterly direction:

This is extremely critical as the airmass to our north and east will be quite cold with surface temperatures into the teens and 20's. When we see high pressures lock in this position we have to watch for a situation known as cold air damming. When you have such a cold airmass to the north and east with the high locked in these cold air damming scenarios can be a major headache for forecasting purposes. What happens is the northeasterly winds transport this much colder (remember cold air is more dense than dry air and thus sinks and is much closer to the ground). air to the south and west. Here in New England we have the Connecticut River Valley which runs in a north-south fashion from VT/NH down through central MA and down through central CT. The CT River Valley (which has higher hills to the west and east) sits relatively low compared to the surroundings. The colder air being more dense sinks and sits in the valley and is continued to be funneled through the valley keeping the colder air locked in:

                      

While we have the surface high pressure locking in colder air at the surface much more mild air will be advecting in a few to several thousand feet off the ground Friday evening into Saturday. This will mean that temperatures a few thousand feet off the ground will warm to much higher than the freezing mark. As precipitation falls from the clouds, it will initially fall as snow as temperatures well above 10,000' certainly will be cold enough to support snow, however, once it descends into the milder air it will melt and transition into rain. Once this rain falls to within the lowest 1000' of the atmosphere it will once again encounter temperatures which will be below freezing. This will introduce the hazard for freezing rain away from the coastline:

A bufkit forecast sounding for Windsor Locks, CT from the GFS computer forecast model shows a nearly classic look for freezing rain Friday evening:

What should we expect with this?  During the day on Friday a warm front will slowly be approaching from the southwest as a storm system begins to strengthen off to our west. All indications right now are that this warm front will push into extreme southern CT but due to influences of the high pressure to the north will stall across extreme southern CT. This will lead to temperatures warming above freezing for southern CT, however, temperatures across interior CT will remain below freezing. Throughout the morning and afternoon hours periods of light to perhaps moderate mixed precipitation (snow and sleet) will fall across interior CT with rain across southern CT. While no major accumulations are expected, this will lead to slick roadways so extra time should be given when traveling.

By Friday evening, as the storm system off to the west not only strengthens but pushes closer, we will begin to see the warmth work in several thousand feet off the ground and we will see steadier precipitation push in. While southern CT will see rain this is when the potential will increase for freezing rain across the northern part of the state. The timeframe for the start of this will be between 5:00 PM and 7:00 PM. Periods of freezing rain will continue into and through the overnight hours and even into Saturday morning. While indications right now are the high pressure will depart off to the northeast, which would allow for warmer air to work in at the surface and transition the freezing rain over to a plain old cold rain, there are uncertainties with how quickly this happens so this will be heavily monitored over the next few days. As it stands right now ice accretion does appear to be a high possibility and this could lead to tree damage and power outages. Travel is NOT advised during freezing rain. Contrary to the belief, NO vehicle is safe to drive in during freezing rain and that includes Four wheel drive vehicles.

Further information will be known as well as a precise forecast as we continue to move closer to the event and the details continue to be ironed out.