NE Proverbial Mixed Bag of Winter Precip, and How

The proverbial "mixed bag" of winter precipitation will fall in the mid-Atlantic and Northeast from tonight into Tuesday night. Many areas will see snow, turning to rain, to sleet, then freezing rain. What will cause this and who will see which types of precipitation when? It's a complex question. Different layers of temperatures in the atmosphere will cause the precipitation to end up in various forms. Looking at the NMM computer model forecast prediction cross-section (a vertical "slice" of the atmosphere from the Maryland shore to Lake Erie) for Tuesday at 7 p.m., this is what we see (this map is from our Pro site, and I've annotated it):

I've darkened the top of the graphic because that's dry air. Precipitation is going to be in the lower levels of the atmosphere, specifically where you see relative humidity readings of about 80% or more. That precipitation will start out as snow in areas where the temperature ranges from about 0 C (blue areas) to -20 C (pink). As that snow falls into a layer of warmer temperatures, above freezing (green and yellow), it will melt into rain.

Now it depends where you are on the ground; if you're on the left side of the picture, which is the western part of the cross-section (you can see the mountains at the bottom in brown), that rain will freeze again on the way down and end up falling as sleet. If the layer of cold air at the ground was very short (and it will be later tomorrow night), the rain would splash to the ground but freeze on contact (freezing rain). If you're on the right hand, the eastern side of this picture, it's all rain in the lower atmosphere, and ground temperatures are above freezing, so you've got nothing to worry about. The official AccuWeather.com forecast for this storm system can be read through the link below.

The problem with the model graphic I showed, of course, is that this is only one of at least eight computer models' cross-sections, only valid for tomorrow at 7 p.m., and the models will continue to update between now and then. And of course, they could all be wrong. If we concentrate on one model, we can visualize this storm on a very simple map which shows the precipitation type on the ground from Tuesday morning through Tuesday night in the Pennsylvania area: Blue=Snow, Red=Freezing Rain, Orange=Sleet.

How does the atmosphere end up with layers like this? Typically, incoming warm air rides up and over cold air (because warm air is "less dense"). This process is shown in the oversimplified graphic below (which is backwards from the cross-section I showed above).