First off, thanks to all of you for extending a very warm welcome to me here on the blog earlier this week. It's very exciting to be a part of the WAVE 3 Storm Tracking Team! As a Louisville native I'm committed to continuing our tradition of leading the way in local weather. Plus, I'm just a huge weather geek like a lot of you who read the blog!
If you scroll down on the blog (or click here) you'll find Brian's detailed post on the possibility of a bit of freezing rain and minor ice accumulations across parts of Kentuckiana tomorrow evening. Since these events are tricky I thought I'd take a few moments to explain why we're seeing the potential for a little ice instead of a little snow.
Snow, whether you love it or hate it, is something that most of us have experience with and understand quite well. Small snow crystals form inside a cloud at cold temperatures and then fall to the ground and "stick" when it's at or below freezing. When a lot of them begin to accumulate we all know that it involves shoveling, school cancelations, and slushy or snowy roads. Pretty simple, right?
Here's where things get tricky and why wintry weather events like tomorrow's are difficult to forecast. When the atmosphere isn't below freezing at all the levels where snowflakes fall through they begin to melt. In situations like the warm front that's going to affect us tomorrow, warm air begins moving into the area aloft. Assuming that it's still at or below freezing at the surface, when enough of this warm air moves in a snow event can transition to an ice event with sleet and/or freezing rain.
Let's start with sleet. Snowflakes moving through a relatively small layer of air that's above freezing will only partially melt. These half-melted snowflakes then refreeze into ice pellets (sleet) once they move through a layer of the atmosphere that's well below freezing. This is cause for optimism among snow-lovers since you can say in most cases that "each ice pellet has a little bit of a snowflake in it" as you stare out the window in disgust at the falling sleet.
Freezing rain is more of a trouble-maker because we're dealing with liquid rain that freezes on contact with the ground. Much like sleet we have a layer of air aloft that's above freezing. This time though it's a larger layer and the snowflake spends more time falling through it. This causes the snowflake to melt all the way into a raindrop. The raindrop falls to the ground and creates a glaze of ice on everything it touches since the surface and a layer of air just above it are still below freezing. Quite often you'll see elevated objects like trees begin to pick up ice accumulation before roads since ground temperatures can take some time to respond to the below-freezing air temperatures.
(Diagrams from NWS JetStream)
You can see why we look over so much data when figuring out whether we'll get snow, sleet, freezing rain, or a mix of all three. A degree of difference either here at the surface or in the atmosphere aloft can mean a whole different ballgame. This is why telling us what's falling from the sky at your location via our Facebook page and Twitter is important!
Hopefully this will help you make sense of what's happening as we see a few instances of icy weather in parts of the area later tomorrow. By the way... BOTS!