Kevin Myatt is The Roanoke Times' weather columnist.

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Had you shown me in July how the weather map was developing for early next week, I would have called out "White Christmas."

But an important ingredient will not be arriving with Santa from the north pole: Arctic air.

A low-pressure system is expected to develop in the Gulf of Mexico late in the weekend and track northeastward. Meanwhile, high pressure to the northeast will bank cooler air against the mountains.

Typically, in the last week of December, this would be a perfect prescription for a winter storm ... snow, sleet, freezing rain or some mixture thereof. Almost certainly, if any Arctic air were available, or even decent Canadian air, there would be enough snow or sleet to make our Christmas white.

A couple of weeks of strong middle and upper atmospheric flow from the subtropical Pacific and the Gulf of Mexico have entirely scoured out the colder air over the eastern U.S. The high to the northeast will do its best to push what cold air is available southward, but it will be very shallow and not really that cold.

We can't entirely rule out some brief freezing rain on the front end of the precipitation, if the shallow dome of cold air is barely below freezing in a few locations. Also, snow can't entirely be ruled out on the back side of the precipitation Christmas night and Tuesday as colder air aloft moves in behind the storm.

But mostly, we'll be left with a chilly, dreary, damp Christmas.

Though Bing Crosby wished "may all your Christmases be white," a white Christmas is an uncommon event over most of the U.S.

According to a National Climatic Data Center study in the mid-1990s, Roanoke had at least 1 inch of snow on the ground on Christmas in 17 percent of the years between 1961 and 1990. Roanoke had at least 5 inches on Christmas 7 percent of the time, and never had at least 10 inches on Christmas.

Keep in mind that the 1961-90 period included most of our coldest and snowiest years on record, so a white Christmas may be even less likely now.

I came to Roanoke in late 1999 and have yet to see what would be classified as a white Christmas. We had light snow on Christmas Eve in 1999, a 3-inch snow on Dec. 20 that mostly melted by Christmas in 2000, and some leftover patches of sleet and snow in shaded areas last Christmas.

Meanwhile, Brownsville, Texas, which hadn't seen any snow in more than a century, had a white Christmas in 2004. Some south Texas locations that would have rated a zero percent chance of 1 inch based on the 1961-90 study had more than 10 inches on Christmas morning in 2004.

Even at Denver, which will have an overwhelmingly white Christmas this year thanks to this week's blizzard, only half its Christmases were white during the 30 years studied. Denver's chances of 5 inches or more of snow on Christmas are not a lot better than ours: 13 percent.

The only parts of the U.S. that can almost always count on a white Christmas are high mountainous areas above about 7,000 feet, some areas near the Canadian border, and locations in favored areas to receive snow squalls off the Great Lakes.

Besides the World War II-era song "White Christmas," so much of our Christmas lore dates to the 1700s and 1800s when the "Little Ice Age" was still ongoing. During that period of a much cooler climate that ended in the mid-1800s, it was much more common for locations in Europe and North America to have significant snow at Christmas.

But in the late 20th century and early 21st century in Southwest Virginia, it is commonplace, not unusual, for Christmas not to be white.

Have a merry Christmas, weather or not.