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Jack’s

Social Media is Buzzing about Snow Chances

Posted on: 3:50 pm, December 11, 2012, by

We are in unique days in the business of weather forecasting. The Internet, social media, and the gift of access to tax-payer funded weather information and models often change the course of how meteorologists approach issuing forecasts.

Sometimes we have to answer rumors that are spreading, sometimes we have to answer bad science, and sometimes we just have to be a little more frank about long-term ideas than we would otherwise like to be. It’s not about “keeping” information from you; it’s more about only telling you the things on which we have enough confidence to put our name. That’s at least how I see it.

That being said, the horse is out of the barn on the potential for snow in December. I’ve been alluding to it for a few days, but confidence is low enough that I definitely would not broadcast a threat of snow on television. If you are reading this, that means you probably either have some understanding of how forecasts are made or want to learn more about it…so here goes:

Here’s the first image that you’re seeing spreading like wildfire on Facebook and Twitter:

Screen Shot 2012-12-11 at 2.52.18 PM

This is the 12Z (6 AM) GFS model forecast for next Tuesday evening. A passing storm system gives a “look” that some snow is possible; however, upon further investigation of the model by looking at the sounding below, you’ll see that there’s a very thick layer of the lower atmosphere where temperatures are above freezing:

Screen Shot 2012-12-11 at 3.00.27 PM

That means for most of us, we won’t see a flake if it pans out exactly like the model shows (which is pretty unlikely anyway). Some flakes could mix in with the rain as the precipitation ends in elevations above 1500′ in northeastern Alabama and East Tennessee, but snow is not likely in this scenario.

Remember this about models: they are guidance, not Gospel. The margin of error is large when you start looking at model data 6, 7, 8, 9 or 10+ days in advance. So what does that mean? Should we just throw it away? Nope! This is something we will watch and see if a trend develops toward a better chance of snow or toward just a chilly rain event. Trends are important; single model runs are much less important than what you can gather by watching and studying a set of them over a period of days.

The next image you’ll probably see will look like this for the weekend before Christmas:

Screen Shot 2012-12-11 at 2.52.29 PM

Now THAT is an impressive snow look. If that were less than 48 hours in the future, I’d have to say I’d be forecasting a good snow in parts of Alabama. Look at the time, though! It’s 276 hours in the future; that is Saturday night, December 22nd on the 12Z GFS, and it is a more classic look for Tennessee Valley snow.

Upon closer inspection of the atmospheric conditions around North Alabama, you find a problem:

Screen Shot 2012-12-11 at 3.10.00 PM

Almost all of the atmosphere is cold enough for snow or sleet, but the moisture is limited. Sometimes these kinds of storm systems just can’t push the moisture far enough north, and it just ends up being cold and dry.

In both cases, the storm systems’ paths could shift as much as 400 miles to the north over the next several model runs. I doubt that happens, but it’s a possibility.

So, what am I trying to tell you here? Seems like a long, drawn-out process just to get to the bottom line, but here it is:

The Bottom Line:

  1. WHNT is not forecasting any accumulating snow at this time. There is not enough confidence in the long-term modeling to support a definite “yes” or “no” answer.
  2. In the month of December, measurable snow has only fallen 20 times in Huntsville since 1907. Only EIGHT of those totaled up to an inch or more. I can remember multiple December snow threats in my lifetime that fizzled out at the last minute, so take what you see online with a grain of salt. If there is a legitimate winter weather threat out there, we will be talking about it. If it is just an unusual look on a forecast model, we will probably stay quiet about it just to reduce confusion.
  3. The probability of a White Christmas in any given year for the Tennessee Valley is less than 5 percent (statistic courtesy NWS Huntsville’s White Christmas Page). If you love snow, don’t lose hope yet. The chance is there! If you hate snow, don’t get too hot & bothered about the talk. At this point, it’s nothing but talk!

-Jason
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