Friday, February 15, 2013

Snow Saturday? Some questions, answers

Being a meteorologist or writing about the weather during the winter in the Carolinas is a no-win proposition.

That's due to the really "iffy" situations that are a part of forecasts that involve snow, sleet or ice. Saturday's forecast is one of those situations.

Basically, meteorologists are trying to predict whether or not it will snow, and it's based on a low pressure system that isn't forecast to become strong until it reaches the Carolinas (snow) or goes well off the Atlantic coast (no snow).  And it's based on a forecast over whether it will be cold enough in the atmosphere for snow, and whether it will be cold enough on the ground for anything to stick.

On rare occasions, it's a no-brainer.

Most of the time, it's a close call.  We've had two of those this season.  The first time, little or no snow fell. The second time, the forecast was for sleet and freezing rain, and enough fell to cause problems.

So here are some questions and answers about Saturday's forecast:

It's above 60 degrees today. How can it possibly snow Saturday?

Anyone who's been in the Carolinas for more than a few winters knows the answer to this.  Our winter events often come just a day or two after some very warm weather. It's part of what makes snow-ice forecasts such a "close call." In addition, there also are often 60-degree days a few days after snow or ice. That's especially true from the middle of February through March, when our weather is starting to trend toward spring.

Why do we think it will snow Saturday?

A low pressure system is forecast to move along the Gulf Coast and then cross the Southeast during the day. The latest runs of some computer models indicate the low will strengthen as it pushes across Georgia and the Carolinas. That will increase the amount of precipitation.

At the same time, much colder air is forecast to push into the region overnight and Saturday. Temperatures will be a bit too warm for accumulations, but if the storm system is strong enough, it can bring colder air down from the higher levels of the atmosphere.

Keep in mind, however, that the computer models are waffling all over the place.  One run, they show snow across the North Carolina, especially in the east. The next run, it looks like Georgia and South Carolina get it. We might not know until the last minute, because of the last-minute development of the system. 

Why would it not snow Saturday?

While the Global (GFS) model has been hinting at accumulating snow, the European model has not. It shows very light levels of precipitation, and temperatures that are cold, but above freezing. If the Euro model verifies, then we'll have a mostly cloudy and chilly -- but dry -- day.

When would it snow?

It seems almost certain that precipitation will start as rain in the Charlotte region early Saturday afternoon. As more cold air filters in, the rain would mix with snow, and possibly change to all snow. The mixing would start around mid-afternoon, with snow falling by late afternoon.

The storm system will move quickly, so the snow would end by mid-evening. Temperatures will plummet into the lower 20s Saturday night, so if the streets are wet, we could have black ice Sunday morning.

What is likely to happen?

I've seen this story before, back in January.  The cold air appears to be marginal.  Based on what I'm hearing from meteorologists, the best guess is that Charlotte gets a few snow showers Saturday evening -- enough for conversational purposes, but not enough to cause problems.

The better chance for accumulating snow seems to be east of here.

Stay tuned.