PHILADELPHIA (CBS) — Snow, sleet, and freezing rain…another mixed precipitation event is on the horizon this weekend. The winter season brings an assortment of weather to the Delaware Valley, and forecasting what will fall is quite complicated because a one degree temperature change can make all the difference.
The Delaware Valley is no stranger to snow, sleet and freezing rain. A myriad of factors come into play in a winter precipitation forecast. This includes the track of the storm system, intensity, moisture, and most critically the temperature profile, and how it evolves.
So what is a temperature profile?
You can think of it as layers of the atmosphere. Typically the higher up you go, the colder it gets, but not always. To determine whether snow, sleet, freezing rain or just rain will form, you must locate where that layer of freezing air will be.
Snow can be the most obvious precipitation type to determine. When all layers are at or below 32 degrees, a snowy scene will ensue.
Sleet is a bit trickier. Sleet occurs when a shallow layer of warmer air is introduced into the mid-levels of the atmosphere. Falling snow enters this warm layer and begins to melt, but only partially, before re-emerging into a cold, sub-freezing layer near the surface. There it refreezes and is observed as clear, small, frozen pellets.
If this warmer air is more expansive, falling snowflakes melt completely into rain, but if the surface temperature is 32 degrees or colder, it is considered ‘freezing’ rain. It looks like normal rain, but freezes as ice on contact with the ground or other objects creating hazardous, slippery conditions. If freezing rain accumulates, it has the potential to down trees and power lines.
Another less common cold season precipitation type is graupel. Graupel looks like small hail or cloudy-colored sleet. It forms when super-cooled water droplets freeze onto falling snowflakes.