This winter weather pattern is a rattletrap.
Screws are loose in the atmospheric machine that would typically bring us snow in the winter, and storm systems easily get thrown off the track.
Thursday’s snow that wasn’t or wasn’t much, depending on where you are in Southwest Virginia, was only projected to be a couple of inches. But in a winter when there has been little snow, it felt about the same as missing a forecasted half-a-foot would have in a more typical winter.
Computer forecast models had a hard time zeroing in on this storm. Early in the week, the models depicted a strong low-pressure system moving up the spine of the Appalachians, bringing us a one-day warm-up and rain between outbreaks of very cold air.
Later, the track was modified for more of an East Coast storm, and it seemed like there was potential for our first real winter storm of the season. But a lot of things clearly weren’t in place that typically are for major winter storms, and a lot of meteorologists rightfully doubted the more extreme scenarios.
So the models again changed the track, making the storm weaker but with a flatter trajectory, straight west-to-east across the southern U.S. rather than turning it up the coast. It began to look like one of those quick-hitting snow episodes that happen three or four times for us in a typical winter, the kind that drop 2 to 4 inches in about four to six hours of steady snow.
Though the pattern was not even perfect for that, it seemed plausible, and a lot of forecasters bought into it.
But the real trouble began Wednesday night. One major American computer forecast model diverged on a new path, showing a much weaker system in two parts that would entirely go south of our area. The other major American computer model diverged the opposite direction, showing a light hit from the first piece of the storm while the second developed a stronger low-pressure system with widespread wintry precipitation.
Faced with conflicting evidence and a storm system with multiple parts only a few hours away, forecasters hedged more toward the middle, or more precisely, the lower end of the middle.
It turned out that the computer model showing the storm sneaking south of us was mostly right. Our quest for the least snowy winter in Roanoke’s recorded weather history remains intact.
The current weather pattern, largely unchanged since mid-January, is simply missing some important pieces of the snow-making machine.
This winter, we have not had a real “cold-air damming” episode yet, where a high-pressure system to the northeast keeps the cold air banked against the mountains.
We have not yet had a clearly defined subtropical jet stream, or juicy flow of air from the Pacific, undercutting the dome of cold air coming at us from the north.
We have not yet had the right kind of atmospheric pattern in the North Atlantic where high pressure blocks the jet stream, forcing it southward into the position where storms can form along the Gulf Coast.
We have not yet had a long-lasting low-pressure system off the coast of Newfoundland that acts like a traffic cop to guide systems on the right track and at the right speed for significant winter storms in our area.
Sometimes, a winter storm or at least a light snowfall can happen without those key ingredients. But the odds are against it unless at least a couple of those are in place.
The cold weather will reach its peak next week. The Monday through Thursday period next week will likely be the coldest four consecutive days of the 21st century to date. Lows could dip into the single digits on a couple of those nights, and I wouldn’t even be surprised to see a below-zero reading somewhere locally.
But it takes more than just being cold to make it snow, and at least through Thursday, this looks like another extremely dry period of Arctic chill.
Roanoke needs 2 inches of snow to keep this from being the least snowy winter on record. Having missed this week’s snow, and with no obvious new snow threats on the horizon, I think there’s at least a one in three chance that we won’t get those 2 inches.