Saturday, December 18, 2021 Wintry Mix

 A fast moving low pressure will bring a wintry mix to much of Connecticut Saturday with the exception being near the shoreline where predominately rain is expected (though some mixing is certainly possible at times). 

While snow is possible at the onset of precipitation, this is looking like predominately a sleet or a light freezing rain event. This will not be a significant storm by any means, however, roads will become slick through the day Saturday, especially untreated surfaces. 

Low pressure moving northeast across the Ohio Valley begins to get shunted south as it moves into the Northeast. This is largely due to an area of high pressure across southeast Canada which will act as a "wall":

The flow around the high pressure system (clockwise flow) will help to feed some colder air from Canada into our region within the lowest 5,000 feet of the atmosphere, keeping temperatures through this portion of the atmosphere very close to freezing at the surface and just a few ticks below freezing as you begin ascending through the lower atmosphere. However, with the low pressure system ultimately tracking just to our north and west, this will promote a more west-southwesterly flow above 5,000 feet. This will bring about very strong warm-air advection. This push of warmer (and higher moisture content air) into colder (and drier air) results in a thump of heavy precipitation blossoming over the region:

So for precipitation type, why are we more likely to see a mixture of sleet and freezing rain as opposed to snow? In order to get dendrite (snowflake production) you need the following:

1. Cold enough temperatures (Colder than -4C) within the cloud for the formation of ice crystals and ice nucleation to occur.

2.  Sufficient upward vertical motion and moisture into the snow growth zone (with snow growth maximized between temperatures of -12C and -18C). 

In the case of Saturday, we are actually looking at relatively warm temperatures within the clouds. In fact, temperatures may be a bit too warm for the formation of sufficient ice crystals (first image below). In addition, the depth of the warmer air in the mid-levels of the atmosphere will also result in a relatively high snow growth zone (second image below). Initially, upward vertical motion may be strong enough to produce some snow (as evident by the green/light blue shadings inside of the black outline box in the second image below, however, as that strong upward vertical motion subsides the chances for snow vastly decreases and we are looking at sleet/freezing rain:

What to Expect:

• Precipitation begins to break out mid-to-late Saturday morning. There will be an initial burst of snow away from the shoreline, but this should quickly transition to some sleet and freezing due to the processes described above...very quickly. 
• The heaviest precipitation rates occur with the initial burst with rates more light when sleet and freezing rain is falling. This continues well into the evening. 
• Overnight, a second burst of heavier precipitation traverses the state, however, this will be predominately rain as warmer air has pushed in. 

Below are my thoughts: