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The trains kept rolling through Lynchburg.
Not Amtrak or Norfolk Southern, in this case, but trains of thunderstorms, especially on Thursday and Saturday nights, arriving on tracks from different directions each night, sliding across the city one after the other dumping torrential rainfall amounts that quickly turned borderline drought and an annual deficit of rainfall into flooded streets and a rainfall surplus.
Meanwhile, spinning thunderstorm updrafts ominously helicoptered over Roanoke on Friday and Saturday, and a tree-breaking bowing line segment slammed into Bristol, among many other locations in Southwest and Southside affected by intermittent, and unevenly distributed, thunderstorms from Thursday afternoon through Sunday morning.
It’s largely the result of being between and betwixt the major players in the North American atmospheric pattern. A major heat dome is parked over the Southwest U.S. some eastern stretch of it extending close enough to us at times to bring in slightly above normal heat and sticky humidity. Meanwhile, a cool trough is spinning over the Upper Midwest and Great Lakes region, close enough to sling some upper-level disturbances and cold fronts our way at times.
Boundaries between the two air masses over our region were the breeding ground for thunderstorm development in afternoon heat and humidity Thursday through Saturday. Similar periods of scattered thunderstorms – though uncertain yet if they will match the strength of last week’s – are resuming for the latter half of this week. In between, a brief slightly less humid period on northwest wind flow ushered in another round of wildfire smoke from Canada on Monday.
On a more local level, almost imperceptible pre-frontal troughs and outflow boundaries from previous storms set up corridors where storms formed and moved more readily – and repeatedly – than they did other parts of the region. Lynchburg ended up under one of these angled northwest to southeast on Thursday evening, then another tilted southwest to northeast on Saturday evening.
Twice in three days, Lynchburg was the target for torrential rain, with 4-7 inches of rain common each time, and storm totals over the three-day period (there were shorter-lived strong to severe thunderstorms on Friday evening as well) of 8 to 12 inches common in and near the Hill City, including parts of Campbell, Appomattox, Amherst and Bedford counties.
Many streets were flooded, creeks rose out of their banks, and water entered some homes and businesses. The James River absorbed the downpours and rose a foot above flood stage downstream at Richmond.
It could have easily been much worse, but for two factors: (1) there was time between the rounds of storms for some draining and (2) Lynchburg was more than two inches below normal in rainfall and in “abnormally dry” conditions on the U.S. Drought Monitor going into the deluge. However, the Lynchburg area is now saturated, and any additional downpours this week that happen to occur right there could resume flooding.
While a single episode of narrowly focused downpours can’t be readily linked to large-scale climate, there has been considerable study pointing to increased frequency and intensity of localized, short-term rainfall events in a warming global climate. Intensely warm sea surface temperatures currently observed in the Gulf of Mexico and parts of the Atlantic Ocean are likely a factor in more water vapor being moved inland by some weather systems.
The Lynchburg area was the most severely affected by late-week storms, but not the only area, with other scattered pockets of heavy rain, gusty winds and hail, with lighter rain and lesser effects in between the pockets.
Rotating thunderstorms proved to be more visual curiosity than actual hazard for the Roanoke Valley, forming along some of those atmospheric boundaries both Friday and Saturday.
Defining when a storm is a “supercell” or merely exhibits “supercell structures” for a while can be blurry and perhaps even a bit esoteric. But there was little doubt to those of us who have traveled far and wide across many states pursuing rotating storm structures that what was being seen in the Roanoke area last Friday and Saturday were the result of rotating updrafts that lingered at least for several minutes to an hour or so.
There were lowered cloud masses called “wall clouds” and even some hints of funnels – but low-level shear, winds changing near the ground, was likely too weak to allow for a tornado to form.
Other clusters of storms took on a “bowing” effect at times, spread outward by outflow winds, as a cell did that crashed into Bristol downing trees on Friday.
There was plenty of lightning and rumbling and gusting and pouring to go around, though some places got much less than others did. While Lynchburg was swamped by a half-foot to foot of rain over three days, Danville, 70 miles down U.S. 29, got less than a half-inch total over those same three days.
Tracking exactly where the worst storms and heaviest rain will occur day to day is difficult, as the atmospheric chessboard changes with each move. The previous day’s storm can emit outflow winds that form boundaries that affect where the next day’s storms will form, sometimes hundreds of miles away from where prior forecasts from the day before indicated they were likely. Or, sinking air left behind a storm cluster can put a lid on convection over a wide area that would otherwise be favorable, turning seemingly likely thunderstorm chances into blue skies.
We’ll see more stickiness and scattered storminess through Friday, perhaps rewarded with a cooler, less humid break by the weekend, as a stronger push from the Great Lakes trough arrives – hopefully without a new round of smoke.
Smoke makes a return
Canadian smoke is going to be a hard habit to break.
The latest waft of it arrived late Sunday and became especially thick on Monday, briefly as thick at the surface as the more persistent late June wave of it. The wave of smoke arrived behind a pretty ineffectual cold front, that only managed to drop dew points about 5 degrees, so, as Twitter follower (and two-time Cardinal Weather newsletter Photo of the Week contributor) Matt Weddle noted, we got the “swamp air AND smoke” at once this time.
The silver lining is that the smoke has been moving across more like a wide squall line, rather than a continually renewing stream from Canada, so it will have mostly pushed past our region to the east by the time you read this on Wednesday evening or Thursday.
Most of the latest round of smoke swept in from western Canada, rather than eastern Canada as was the case earlier in the summer. Wetter weather has helped roll back, though not eliminate, the fires in parts of Quebec and Ontario. Meanwhile, Canada is sending troops to fight fires in British Columbia.
We do have a push of cooler air from the northwest potentially arriving this weekend. It will depend upon the steering flow around the north-central U.S. trough whether it is accompanied by another round of smoke. But it’s almost certain we will see more of it before colder weather finally squelches most of the fires this fall.
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.