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Dam the cold air.
That’s not a curse against cold air or a lack of cold air, but a description of something that often happens in our winters here in Southwest and Southside Virginia.
Cold-air damming refers to the phenomenon in which high pressure over southern Canada or the northeastern U.S. forces cold air southward and southwestward, trapping it against the Appalachian Mountains, directly over central and western Virginia (often, ironically, excluding the far southwest corner that is the coldest and snowiest in other atmospheric setups).
It is a common feature of our winters in Virginia, often present at least to some degree when we get significant snow or ice.
It is a little geographical trick our location in the western half of Virginia offers that sometimes makes it colder than it would otherwise get at our latitude with a similar atmospheric pattern were it just flat and the cold air dispersed and moderated over a larger area unchecked by the Appalachians.
Sometimes cold-air damming develops for a couple of days during otherwise milder periods and creates a window for wintry precipitation when none would otherwise occur. Other times, it can press in ahead of a wet system when it is already cold and turn what would be a marginal mixed precipitation or cold rain episode into a solidly cold winter storm.
Cold-air damming appears in summer too, bringing a drizzly day in the 60s to break us out of the sticky 80s and 90s for a day or two, to relief or annoyance, depending on one’s view of summer heat.
Much like the usually frequent northwest upslope flow over the mountains which only reappeared this past weekend for some light snow accumulations in our state’s southwest corner (see the last section of this column), cold-air damming has been largely missing-in-action this particular winter in Virginia.
Cold-air damming is not something that has gone missing repeatedly in recent winters. This matter isn’t a climate shift – I’m not immediately aware of any pertinent research on climate change and cold-air damming frequency or intensity – but, rather, it’s a peculiarity of this specific season, thus far.
Just a year ago, on Jan. 16, 2022, a complex storm system hurled Gulf of Mexico moisture into deeply dammed cold air, resulting in 3 to 9 inches of snow/sleet/ice layer cake in our region. Before that, cold-air damming patterns of varying strengths led to a 2020-21 winter full of seemingly weekly marginal mix, ice and wet snow events despite some largely unfavorable large-scale atmospheric patterns for wintry weather in our region. Winters like 2001-02 and 2007-08 got their only significant snowfall in parts of our region from a single episode of thick moisture ejected into cold-air damming.
The absence of cold-air damming might change in the latter part of this month.
Let’s be clear, there is no large-scale shift to a deeply wintry pattern afoot, at least yet. It remains very unclear if that might happen sometime in February, with warming high above the Arctic triggering a push of colder air to lower latitudes, funneled our way by atmospheric patterns.
But after a mild, showery period through the coming weekend, colder air from Canada starts leaking southward some for next week and the week after, and it may start complicating our precipitation forecasts at times as wet storm systems continue moving through, little ripples of that “atmospheric river” that you may have heard about swamping California and burying the Sierra Nevada mountains in piles of snow.
The strong westerly flow from the Pacific across the nation has, thus far in January, mostly warded off incursions of cold air from the Arctic, flooding most of the central and eastern U.S. with mild air. That flow may slow some in the weeks ahead, but likely continues carrying some wet storm systems across the nation.
There are some forecast model placements as we get farther out in the month which could allow high pressure over southern Canada or the northeast U.S. to press cold air southward east of the Appalachians while a moist system undercuts it across the south.
Depending on position and timing of various atmospheric features, moisture might overrun the cold air at the surface. If the dammed air is cold enough, that could bring snow or ice to our region at some point.
At this time, just take this as an educated guess on what might develop in our weather later this month, based on some evidence. Don’t get too lulled by this month’s repeating mild rainy systems to think at least one of them ahead might not be something other than rain.
Snow lovers have some hope, but it’s not a solid enough chance yet for winter haters to have great dread. It’s just way too early to declare: “Dam the cold air, full spring ahead.”
* * *
2022 AMONG WARMEST YEARS ON RECORD
Greenhouse gases from human industrial activities are widely considered the overwhelming factor in this planetary warming trend, according to a consensus of climatologists and other atmospheric scientists.
Regionally, 2022 was plenty warm, ranking highly at two sites with lengthy, mostly intact data going back over a century. But notable cold periods in January, October and December nudged the average down a bit from what it has been in recent years.
At Roanoke, it was the 15th warmest year on record, out of 105 since 1912 with fewer than 10 missing days of data, averaging 58.6 degrees. That was a step down from three previous years in a row averaging 59.5 degrees, tied for warmest on record with 2012.
Twelve of the 20 warmest years at Roanoke, among those with less than 10 missing days of data, have occurred since 2000. (1931 averaged 59.7 degrees but has 18 missing days of data for Roanoke.)
At Lynchburg, 2022 was tied with four other years for 12th warmest out of 125 years since 1897 with fewer than 10 days of missing data (only one year, 1938, was excluded, with 11 days of missing data). Lynchburg averaged 58.5 degrees in 2022.
While 2020 ranks second (59.4 average, right behind 59.5 of 1921), 2021 ranks seventh (59.1 degrees) and 2019 is among those tied with 2022 for 12th at 58.5, Lynchburg’s temperatures do not display the same clustering of recent years near the top as do Roanoke’s, although many recent years are among the warmest third.
There is a deep dive regarding thermometer placement to get into regarding Roanoke and Lynchburg, and relative urban warming comparison, that I once touched on in a Weather Journal column for The Roanoke Times, linked here.
(Records at Blacksburg and Danville, the National Weather Service’s other two major climate stations in Cardinal News’ region of coverage, have enormous stretches of missing data over many years, so weren’t included in this quick analysis.)
So much of the warming trend regionally being based on sticky overnight low temperatures, not spiking daytime highs, deserves a re-visit in this space when we get to a warmer part of the calendar.
* * *
SOUTHWEST VIRGINIA SNOW RECAP
What passes for a widespread snowfall in the 2022-23 winter to date occurred in the southwest corner of the state, mostly west of Interstate 77, on Friday and early Saturday.
Most reports ranged from less than an inch upwards to 3 inches. Burke’s Garden in Tazewell County lived up to its winter wonderland reputation, topping Virginia reports with 4.7 inches. (Billy Bowling captured some frozen-tundra video from Burke’s Garden, linked here, if that’s something you’d want to see.)
The snow was caused primarily by strong upslope flow over the mountains behind a departing low-pressure system. But, this being the 2022-23 winter that doesn’t want to do normal things, the winds turned to a bit more of a north-northwest trajectory, which didn’t allow as much upslope snow as we often see with more of a straight northwesterly trajectory extending narrow but sometimes heavy bands across the New River Valley to the Blue Ridge south of Roanoke. Accumulating snow was almost entirely contained to the area west of Interstate 77.
It did focus snow heavily on the higher elevations of the Great Smoky Mountains along the Tennessee-North Carolina line, where some 18-24-inch reports were received at 4,000-plus elevations. Virginia’s highest elevations near Mount Rogers and Whitetop Mountain in Grayson Couty were outside the core flow and got lighter amounts.
A mid-level trough and developing coastal low did cause some light snow to develop in central North Carolina northward into Southside and Central Virginia late Friday night and in the pre-dawn hours of Saturday morning, but surface temperatures were mostly above freezing and no significant accumulation occurred.
Truthfully, the setup on Friday and Saturday was only a couple clicks off a more widespread wintry episode for our region, if the coastal surface low and southward diving polar trough with colder air could have gotten in sync just 18-24 hours earlier.
Real winter may seem far from us, but it may only be a couple atmospheric wiggles away.
Journalist Kevin Myatt has been writing about weather for 19 years. His weekly column is sponsored by Oakey’s, a family-run, locally-owned funeral home with locations throughout the Roanoke Valley.
