When winter storms head toward the Carolinas, this is the most important question I have to answer and it's never easy. More often than not, we get all 4 types across our viewing area that spans nearly 100 miles from Laurinburg, Cheraw and Hartsville to Little River, Myrtle Beach and Georgetown. If you are a regular reader of this blog or my Facebook page, I often talk about the forecast models. High speed and powerful computers, using sophisticated physics and mathematical equations, simulate the future state of the atmosphere. Their results are not infallible, but that's a blog for another day. These forecast models give me the ability to look at the atmosphere vertically at a given point in time.
Take a look at image 1. This is a skew-T diagram with many lines. The solid and dotted white lines show the temperature and dew point from the surface (bottom) to 45,000ft (top). By knowing the temperature and moisture distribution, I can determine what kind of precipitation will likely hit the ground. In eastern South Carolina, most of our winter weather situations are characterized by cold, dense air at the surface and warm moist air streaming from the Gulf of Mexico and Atlantic Ocean. How thick the cold and warm layers are ultimately determine what precipitation type we get.
Image 2 - SNOW
Precipitation begins as snow and falls through an atmosphere that never goes above freezing (or 32F). Therefore, it reaches the ground as all snow.
Image 3 - SLEET
Precipitation begins as snow, but encounters a shallow layer of warm air that causes the flakes to partially or totally melt. It then falls into a sub-freezing layer thick enough to allow the melted flakes to refreeze as an ice pellet, or sleet, before hitting the ground.
Image 4 - FREEZING RAIN
Precipitation begins as snow, but encounters a deep layer of above freezing air. This results in complete melting of the snowflake. The liquid then falls into a very thin layer of below freezing air right at the ground causing it to freeze on contact on trees, power lines, roads and cars. While snow is the most glamorous winter precipitation type, Freezing rain is the most treacherous.
Image 5 - COLD RAIN
Once again, precipitation often begins as snow, but then encounters a warm layer of air that extends to the surface. Melting occurs as soon as the flakes fall into the warmer air and never has an opportunity to refreeze before reaching the ground.
Because of our proximity to the insulating Atlantic ocean, 95% of the time, we get rain or freezing rain as our winter weather. Christmas of 89' and Jan of 00' are proof that we do get big snows...even if they are rare!
