WRAL WeatherCenter Blog

So goes the Snow...

Posted December 20, 2009

As you've no doubt seen over the past couple of days, a strong nor'easter that lifted out of the Gulf of Mexico and skirted across eastern NC brought a substantial mix of rain and snow to the state, along with wind gusts at the coast that reached as high as 40-60  mph in spots.

The storm tapped into just enough cold air to keep temperatures way down over a good part of the state at the surface, and to produce plenty of snow in the mountains and foothills, across the Triad and into the Roxboro area, and even a nice coating of an inch or two across northwestern parts of the Triangle and some of our Virginia border areas. The National Weather Service in Raleigh has released a nice map recapping the snowfall totals across the state, and they follow a classic pattern that is fairly common with well-defined low centers passing southeast of the region, with a sharp line running southwest to northeast separating the areas with significant snow from those that mainly got rain, with only minor amounts of sleet and freezing rain mixed in along that narrow transition zone at times.

I've included a couple of images here. The first is the snowfall totals map for our state, and the others are a visible satellite image from around 10:30 am on Sunday morning covering much of the southeast, followed by a similar image from about 11 am centered more closely on NC. At the time, there were a lot of low orographic standing wave clouds over the mountains, but only a few scattered clouds farther east, so you can clearly see the line dividing parts of the state with lingering snow cover from those areas that mainly got rain.  Incidentally, it was a substantial rain event in addition to the wintry conditions, as much of the region received in the range of one half inch to one inch of rainfall, including .90" out at RDU. That rain leaves us about 2.37" below normal there for the year 2009, and 2.23" below normal for the past full year, well within the typical year-to-year variation in rainfall amounts for the area.

Finally, the NWS made note of a useful web site for checking snow cover across the country in a nice, graphical manner. It is hosted by the National Operational Hydrologic Remote Sensing Center, and allows you to view zoomable and customizable maps of the U.S. showing things like snow water equivalent and estimated snow depth, based on a model that combines a number of surface observation networks, airborne snow cover surveys, and satellite-based snow detection. I've included a link to the site in case you'd like to look it over.

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  • Mike Moss Dec 24, 2009

    bbad238, When there is a single, main, well-defined surface low southeast of us and moving northeast, air that is cold enough for snow over a deep layer (not just near the surface) tends to be replaced by air that is a little too warm for that by east to southeast winds sweeping in off the western Atlantic a few hundred to a few thousand feet above the ground as the low is moving by, with that process often extending about 100-150 miles west from the track of the low center. Even when the surface temperatures show a fairly small spread, there may be a sharp warmup that occurs just above the colder surface layer. If the surface low tracks a little farther east, as you noted it is sometimes possible for the area of snow to shift farther east as well. Of course, the location and amount of snowfall depend on more than just the surface low track (character of upper level features, strength/position of surface high supplying or reinforcing cold air, temperature of the soil, etc).

  • schif777 Dec 22, 2009

    Great report, keep up the excellent work!

    http://www.weather-channel.org

  • bbad238 Dec 21, 2009

    Mike, very interesting and useful information. I'd just like to know what causes there to be such a quick transition zone, since there wasn't a big spread in temperatures at the surface. If the low had moved further off the coast would there have been more snow further south and east? Thanks!