Still On Track For Another Winter Storm Friday – But Much Uncertainty Remains

Winter weather is still in the forecast for most of South Carolina later this week. However, much uncertainty remains in the forecast today because of vast differences in the computer model guidance. The current weather situation is complex and challenging to predict because the models need to keep up with several storm systems.

The different computer models, each with their own set of equations for crunching numbers generated by weather observations to produce a forecast, will create a different forecast. Sometimes the different models end up with big differences in their handling of the weather pattern. Today, this is the case for our potential winter storm later in the week.

Compounding the problem is that our storm will come from three of these features coming together over the Southeast. Models also sometimes struggle with the process of weather features merging.

Below is the output from some of the more commonly used computer models. You will see that there are a lot of differences.

Total precipitation by precipitation type from the most-used four of many computer forecast models available to weather forecasters. Top-left is rain, top-right is freezing rain, bottom-left is snow and bottom right is sleet. The scale was intentionally left off to limit hype and misinterpretation; darker colors are heavier precipitation. There are many differences with both potential severity of the upcoming storm as well as timing.
NOAA-Weather Prediction Center forecast weather map for early Friday EST.

As you can see, they all indicate potential snow and ice in South Carolina, but there are wide differences in their handling of the storm. Also, the above graphics show only placement of precipitation types. Look closer at the top-right at each set of maps; they are not all valid at the same time! In the case of the first one, the NAM (which stands for North American Mesoscale, run by the National Weather Service). This is the end of the run, it only goes out 84 hours, Friday evening in this case, at which point the model shows the storm is still ongoing in South Carolina.

So, we don’t get a complete picture of what the storm might do according to this model. The GFS (which stands for Global Forecast System, another model run by the National Weather Service) plot is for 120 hours out, Sunday morning, because it forecasts a slower progression of the storm, indicating a 1-2 punch of separate weaker storms, where the second one doesn’t leave South Carolina until Saturday night.

The other two, the ECMWF (which stands for European Center for Medium Range Forecasts, the model is commonly called the European or Euro) and GEM (which stands for Global Environmental Multiscale, run by Environment Canada, commonly just called the Canadian global model) show similar timing but very different impacts.

Our best guidance today is a general alliance found between the NAM model and the Euro. This also happens to be consistent with what these models were showing yesterday.

This means that our bottom line remains like what was said yesterday:

We still expect to see a winter storm affect South Carolina on Friday. The start time is in question, but it will come after some rain Thursday into Thursday night, turning to winter precipitation as colder air rushes in later Thursday night and Friday. The Upstate sees the transition first, which might be before daybreak on Friday.

More of the state sees impacts from this one than with Sunday’s storm since the air coming in as the storm arrives will be colder.

On the northwestern part of the storm, temperatures will fall below freezing aloft as well as near the surface, leading to the bulk of the precipitation falling as snow. The area where this occurs will likely include the entire Upstate. However, the transition period likely will likely consist of some freezing rain and sleet as well. Plan on difficult travel all day Friday, perhaps lasting through much of the weekend as below-average temperatures will continue behind the storm.

Hopefully, the models will harmonize by tomorrow and we can gain some confidence to forecast specific ice and snow amounts. For now, though, be sure to check South Carolina EMD’s Winter Weather Page for preparation tips.