A bow echo was a curve ball for Virginia weather on Monday.
Tracking farther south than what seemed likely several hours before, a windy squall line knocked power out to about 115,000 Virginia utility customers at its peak, causing damage to many trees. There were official reports of 60 mph or greater winds at Roanoke and Wintergreen Resort, with several personal weather instrument reports in multiple areas between those locations topping 70 mph.
There was at least one injury, as a tree was blown into an occupied camper near Gladys in Campbell County.
Southwest Virginia from the New River Valley westward was mostly spared Monday’s storm line, but got some rumbly rain with a few gusts the next day.
Tuesday brought one, two or three more storm clusters, depending on where you are in western, central or southern Virginia, with some sporadic reports of wind damage.
But we never recovered the atmospheric instability that existed Monday. That day’s storm line, another that fell apart entering Virginia early Tuesday, and a couple more later in the day used up what little unstable air was left underneath mostly cloudy skies and temperatures that didn’t make it to 80 for many.
As often seems the case with severe storms, the “day before the day” proved to be the most significant for much of our region — that is, the day prior to the one that had been circled for a serious severe weather threat a few days earlier.
So, was it a derecho?
Whether Monday’s line of storms — a squall line, mesoscale convective system (MCS) or “bow echo” for its curved signature on radar — could also be called a “derecho” is a bit murky.
The squall line that arrived Tuesday evening, weakening fast as it entered Virginia, was declared a derecho, preliminarily, by the Storm Prediction Center for its consistent severe winds from Iowa to northern Indiana.
No such d-word declaration has come out yet about Monday’s line of storms. It might fall a little short on the requirement to have 240 miles of nearly continuous severe-level winds (58 mph or higher). The core of almost-continuous damage with that line seemed to run from a little east of Charleston, W.Va., to a little west of Richmond — the air-mile distance between those cities is 241 miles.
Whether a storm line is a “derecho” or a merely a very windy squall line is a vastly overrated distinction.
It wasn’t on the same level as a regional weather event as the June 29, 2012, derecho event that defines the term in many people’s minds, but for those who lost power or had big trees blown down, it was bad enough.
Going with the northwest flow
Back on July 29 in this space, we talked about the northwest flow pattern that was setting up, as the hottest core of summer high pressure drifted westward and its counterclockwise circulation put us in that flow.
At different times, northwest flow has brought us puffs of wildfire smoke from Canada and/or the Pacific Northwest and heat-easing cold fronts.
Until Monday, the kind of storm clusters that form in the Upper Midwest or Ohio Valley and track southeastward across Virginia had largely been missing from this summer. We usually have a few rounds of those each summer.
We’ve had quite a few thunderstorms, but most have been homegrown, forming along stalled fronts or upper-level troughs or disturbances near us, or else induced by daytime heating and/or terrain.
Conditions finally lined up early this week with a hotter day on Monday, but not much in the way of locally developing storms to siphon off the moisture and instability, and a pattern of stronger winds aloft pushing storm clusters our way from where they were developing in the Ohio Valley.
Storm clusters like these are driven by a colder pool of air behind forming from their downdrafts. Sometimes the storm clusters seem to almost take on a mind of their own, moving in unexpected directions or odd angles, as the driving cold air pool proves a little pushier than modeled on computers.
That happened on Monday. By midafternoon, it became apparent the storm line would get farther south than expected.
Its western fringe dragged through the Roanoke and New River valleys, staying much more intact to the north and east through Lexington and Lynchburg. Southwest Virginia west of the New River valley was spared on Monday, but Tuesday’s storm clusters were a little more robust there than they were elsewhere in our region.
There may be more where this came from. Each day through the weekend has some potential for storm clusters to form to our west or northwest and move toward us, plus there could be some more local backyard storm development.
These northwest-flow storm clusters are the price for not having extreme heat like early July keep going all summer for us.
We still need the rain, but could do without the damaging winds.
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.
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