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Let’s get ready to rumble, spring yells in many ears like Michael Buffer starting a boxing match.
Generally speaking, severe weather — thunderstorms with 58 mph wind gusts, 1-inch hail or tornadoes — increases in intensity and frequency east of the Rockies in the months of March through June before becoming more scattered with more stable summer heat toward July and August. Very generally, we expect to see severe storms focus on the South early in the spring and gradually lift northward as we move through spring, but this is inexact, as we’re already off the grid with this in the current spring.
Virginia, not being a core state to either “Dixie Alley” in the South, the Tennessee/Ohio Valley region or the traditional “Tornado Alley” in the central U.S. Plains states, often falls through the cracks with regard to defining a “severe weather season,” which can be an amorphous concept anywhere.
Some of the state’s most notable tornado events have occurred in February (Appomattox County, 2016), August (Petersburg, 1993, Virginia’s only confirmed F-4 tornado on 0 to 5 Fujita Scale) and September (38 tornadoes spawned by remnants of Hurricane Ivan in 2004), outside the spring severe season. However, April, considering Saltville and Roanoke in 1974, Bedford in 2002, Pulaski and Glade Spring in 2011, Lynchburg in 2018 and Franklin County in 2019, does often seem to be a focal point for tornado risk in our region.
Rather than try to more precisely define a severe weather season for Cardinal News territory (Southwest and Southside Virginia) and the commonwealth as a whole, let’s blur it a little more by loosely defining five different but overlapping severe weather seasons that we often experience, covering the entire year.
(1) Low-instability, high-shear season.
This covers the cooler months from about November to early March, when despite it rarely being warm, there can be enough speedy and shifting winds aloft to give some spin to storm cells or squall lines within larger precipitation areas that move through with more vigorous upper-level and surface low-pressure systems. Sometimes these type storms don’t even have thunder or lightning, with instability of warm air rising into colder air aloft lacking, but can emit some gusty winds or the infrequent brief tornado.
We saw some of this on Jan. 9 — that odd day when Martinsville had a tornado warning at 38 degrees and there were several reports of wind damage across Southside and Central Virginia — and again on Feb. 10, with impressive shelf clouds and not-quite-severe gusty winds with a squall line. We are generally past this severe season on the calendar. Warmer days mean instability will increasingly become a factor in storm development.
(Tuesday afternoon brought on some thunder in some parts of our region with chilly surface temperatures in the 40s. This was due to rather potent instability aloft, above the surface cool layer, a different mechanism than the high-shear, low-instability storms discussed here.)
(2) Classic severe weather season.
This is simply the traditional spring severe weather season, generally March to June, over the central and eastern U.S. that sometimes affects Virginia. This often manifests for us in severe storm outbreaks affecting the Deep South and/or the Tennessee Valley that happen to extend eastward and northeastward into Virginia, or else, severe outbreaks focusing on the Carolinas that extend northward into Virginia. Our region is rarely the epicenter of severe weather in more classic spring outbreaks; we are usually a side-show for somebody else’s outbreak.
For some of the same reasons we didn’t have many winter storm threats, the main steering winds for the kind of storm systems that would cause such South-focused severe outbreaks have, so far this late winter and early spring, shifted north and west of our region, which could point to the season not being as extreme as it could be near us. The Upper Midwest and Ohio Valley have already become the focal point for tornadoes early in this spring, even dating back to unprecedented February tornadoes in Wisconsin. It’s almost as if the severe season skipped the “Dixie Alley” phase and jumped farther north to start.
However, recent cooler temperatures relative to normal in our region and an increase of snow cover in the northern tier of the U.S. signal how the stronger jet stream flow has slipped back to the south some, and there were severe storms in the lower Mississippi River Valley on Monday. With a cooler pattern apparently setting up for early April, the severe season’s gradual progression from south to north could reset itself in weeks ahead. We’ll just have to watch and see if more South-focused severe storm outbreaks become a habit and possibly reach us a time or two in the next few weeks.
(3) Squall line/derecho season.
Putting the “d-word” may get some extra attention here, but not every squall line or bow echo sliding in from the northwest across our region meets the longevity and/or intensity definition of a derecho.
June has seemed to be a focal point for some of the more serious of these type events — June 29, 2012, being the big one that comes to mind, of course, but 1993 and 2013 had some serious derecho episodes for our region, among other years. But last July 28 also brought a squall line/near-derecho event, and August and early September can see similar patterns.
The frequency of such squall lines varies a lot year to year, often spiking when strong heat-dome high-pressure systems set up over the central U.S. and we are on the downstream side with a northwest flow pushing occasional cold fronts across to collide with heat and humidity. If summer brings on more of a Bermuda high-pressure pattern, bulging northwest across us with hot, sticky weather, these cold fronts can be deflected well to our north and west and we don’t see many squall lines moving our way from the Ohio Valley.
It will be a while before we have much of a pulse on whether this year brings an active squall line season for us.
(4) Sporadic summer severe season.
Typical heat and humidity we often see from late May to mid-September or so are the main ingredients we need for rather frequent occurrences of scattered to isolated, usually slow-moving, thunderstorms that rumble, gust and pour on some while just being distant thunder on a sunny afternoon for others several miles away.
“Sporadic” does not necessarily imply “random” or “minor” here. There are often subtle atmospheric and geographic factors that act as triggers or enhancers for thunderstorms aside from just daily heat and humidity. Mountain ridges, the “lee trough” that often develops 20-50 miles east of the Blue Ridge, backdoor cold fronts sliding in from the northeast, almost imperceptible upper-level disturbances and outflow boundaries from previous thunderstorms are among the many factors that can play a role in triggering or intensifying summer thunderstorms.
And the most serious of these storms kick out wind gusts capable of quickly knocking out power to thousands or even structural damage to buildings, some pretty chunky hail, and, every now and then, a tornado. Sometimes summer storms, rather than being isolated or scattered, group into lines or clusters that affect larger areas.
These situations are always week-to-week, day-to-day, even hour-to-hour matters and not easy to predict as a season. Patterns that favor more subtropical moisture sweeping in from the Gulf of Mexico or western Atlantic usually enhance the chances we’ll see significant scattered summer thunderstorms. Which brings us to our last severe weather season on this list.
(5) Tropical-involved severe season.
The remnants of a tropical system that has come ashore along the Gulf of Mexico or Atlantic coast can significantly raise the potential for tornadoes or squally bands with gusty downdraft winds.
Even though tropical systems’ core updrafts decrease dramatically once removed from their warm ocean water food, there can continue to be a remnant and rather vigorous circulation well inland, sometimes enhanced by other weather features. This circulation can give spin to rain cells rotating around the old storm center, spawning tornadoes or localized strong downdraft winds, in addition sometimes to more widespread gusty winds.
Though possible any time through the summer (even May at times), August through early November is when our region has seen the most significant involvement of remnant tropical systems. Torrential rain causing flash flooding is usually the chief concern with an inland tropical system, but often there are a few tornadoes spawned, and sometimes many, as with the aforementioned Ivan in 2004.
It is often the case that the tornadoes and squally winds of inland tropical systems occur without thunder and lightning — which brings us full circle to the high-shear, low-instability season.
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.
