A prolonged period of icing across parts of interior Connecticut is becoming more likely on Tuesday evening and Tuesday night. This has been in our forecast for a few days now and it appears as if parts of Litchfield County could see 1/4" of ice accretion.
The setup is not a classic ice storm look - with a relatively weak high pressure over Quebec retreating east into the Maritimes. But, it is a setup where stale cold air becomes easily trapped up against the east slopes of the Litchfield Hills. This happens quite often every winter. The reason why temperatures will stay cold enough for ice in the hills is that a weak area of low pressure will form south of us keeping winds light and out of the north. If that low to the south didn't form the wind would be out of the south resulting in a large jump in temperatures as milder air would flow north with no obstruction.
The reason we're expecting freezing rain is that temperatures in the clouds about 5,000 feet above our heads will be well above freezing - near 40 degrees! This will melt all the snowflakes falling toward the ground. However, a shallow layer of cold air near the surface will keep surface temperatures below 32 degrees resulting in the rain freezing on contact. This sounding from Norfolk, CT off the NAM shows the sub-freezing layer in the lowest levels of the atmosphere quite well.
While there may be some brief icing in the valley locations around Hartford requiring at least some treatment it appears areas above 400 feet are most at risk for a solid glaze of ice. Up to 1/4 inch of glaze appears likely with great agreement on all of our computer models. Generally this much ice is not enough to cause more than an isolated power outage but will make any untreated surface very slippery. It looks like the freezing rain and icing will begin in western Connectciut after 4 p.m. - with most areas not seeing it until after 6 p.m.