A few days ago, it looked like this might be a 70-degree Wednesday afternoon.
Instead, it’s cloudy and damp with temperatures stuck in the 30s and 40s, and some patches north and west of Roanoke may even see some freezing rain overnight.
What happened?
Cold-air damming happened.
To be clear, this winter is still in a generally mild halftime period of sorts this week between our coldest January in many years and what is starting to look like a mid to late February with multiple shots of cold air and occasional flirtations with ice and snow. Next week will be colder and may have multiple chances of snow or wintry mix. We’ll get back to that.
But this week’s mild “halftime” has a halftime itself, this odd cold-air Wednesday wedge between mild temperatures earlier in the week and those returning for late week, possibly as early as Thursday afternoon, if the cold-air wedge breaks fast enough.
Cold-air damming
Cold-air damming occurs when a high-pressure system to the north pushes cooler air, at the surface and extending up to a mile high or so upward, southward and southwestward, trapping it against the Appalachians.
This phenomenon, sometimes referred to as a wedge, is not limited to winter. In the summer, we sometimes get a drizzly day in the 60s or 70s to break up a string of 90s highs when a cooler air mass is pressed southward and trapped against the mountains. Sometimes in warmer months from spring to fall, the boundary between cooler air and warm, sticky air can fuel thunderstorm development — the storm that spawned the April 2011 Pulaski tornado formed along such a boundary.
But in the winter, the trapped cold air mass can make critical differences in precipitation types, allowing snow, sleet or freezing rain to develop where it would only be a mild rain if the mountains weren’t where they are and westerly and southwesterly wind flow were better able to push colder air away.
Whether precipitation is snow, sleet, freezing rain or just a cold rain depends on the intensity and depth of the cold air over any given location.
As high pressure begins moving eastward away from the Northeast U.S., or if strong warm air advection from the south and southwest becomes too strong to resist, the wedge of colder erodes. But this can be a stubborn process that is hard to forecast, and temperature forecasts sometimes miss by 10 or 20 degrees if the timing of this is misjudged for a given location.
Thursday may provide just such a time of strong temperature differences across our region as the cold-air damming wedge holds longer in some locations to the north and east while it erodes and warmer air arrives to the south and west. But eventually we’ll all be back in milder air, probably Thursday afternoon and almost certainly on Friday. Just don’t get used to it.
Next week’s wintry wondering
The relative level of cold-air damming may become a critical factor next week in determining precipitation types across our region.
In the bigger picture, cold Arctic air begins to seep southward again over the weekend and early next week as a storm track becomes invigorated across the southern half of the U.S.
This will allow storm systems to lift moisture up and over the cold air for what may looks to be widespread precipitation across much of the eastern two-thirds of the U.S.
At this writing, it appears that Virginia will be far enough northward for wintry forms of precipitation to be in play, aided by potential cold-air damming, though there is still much up in the air (quite literally) about what types of precipitation, or how much of each type, will occur in various sections of our commonwealth.
The specific timing of what could be multiple waves of precipitation also isn’t settled yet. It appears, from this distance, that next Tuesday and Wednesday may be a period when wintry precipitation becomes likely over part or all of the Southwest and Southside coverage area of Cardinal News, but any day next week may have some chance of precipitation.
Hopefully some of these questions will become a little clearer over the weekend. If the risk of a serious winter storm is looming early next week, be it snow, ice or mix, we’ll post a new outlook here on Cardinal News.
Longer term, the bigger question is whether the polar vortex splits or becomes stretched to push a deeper, longer-lasting shot of Arctic air into the central and eastern U.S. in the second half of the month, perhaps extending into March. Again, this is not something that can be solved now, but hopefully will become clearer in days ahead.
The alternating pattern between milder and colder periods may harbor the potential for storm systems that could spread rain, storms, snow and ice to various parts of the central and eastern U.S. Where our corner of the map falls within that may vary storm to storm.
Considering we had some serious fires last week after several months that have tilted a little dry, the moisture is needed, whether or not its various forms are loved or disliked.
A January of shivering
Some early calculations suggest January may have been the coldest in the contiguous 48 states of the U.S. since 1988, but also it is likely to have been the warmest January on record globally going back to the late 1800s, topping several recent Januaries.
Specific to the Southwest and Southside coverage area of Cardinal News, it was a top-20 coldest January of the past century or so, based on mean daily temperature (average of highs and lows), and for most spots the coldest January since 2014.
Here is a list of several stations with 80 or more years of weather data in and close to the coverage area of Cardinal News and how this January ranked.
· Blacksburg, 26.3 average, tied for 10th coldest on record with 1985, coldest since 2014 (24.8). Coldest January on record: 19.0, 1977.
· Bluefield, W.Va., 24.6 average, fifth coldest on record, coldest since 1978 (23.5). Coldest January on record: 23.6, 1977.
· Burke’s Garden, 22.5 average, fifth coldest on record, coldest since 2014 (20.8). Coldest January on record: 17.2, 1977.
· Danville, 33.7 average, 13th coldest on record, coldest since 2014 (33.1). Coldest January on record: 27.0, 1977.
· Lynchburg, 31.9 average; tied for 15th coldest with 1979 and 2018, coldest since 2014 (29.7). Coldest January on record: 23.0, 1977.
· Martinsville, 30.7 average, tied for ninth coldest on record with 1981, 1988 and 1994, coldest since 2014 (28.9). Coldest January on record: 26.2, 1977.
· Roanoke, 32.6 average, 18th coldest on record, coldest since 2014 (30.2). Coldest January on record: 23.6, 1977.
· Tri-Cities Airport, Tenn., 30.2 average, 10th coldest on record, coldest since 2014 (28.5). Coldest January on record: 22.1, 1977.
Journalist Kevin Myatt has been writing about weather for 20 years. His weekly column, appearing on Wednesdays, is sponsored by Oakey’s, a family-run, locally-owned funeral home with locations throughout the Roanoke Valley. Sign up for his weekly newsletter:
