Tuesday, June 25, 2019

Charleston tornado deep dive (for non-meteorologists)

A tornado hit parts of Charleston, WV on Monday evening (June 24).  This is not an area that's familiar with tornadoes.  So...

What happened?

A marginally favorable environment for tornadoes shifted from eastern Kentucky through central West Virginia.  A leftover circulation from an area of storms off to the southwest the night before probably enhanced the change in wind speed and direction with height needed for tornadoes.  A small line of storms approached Charleston from the southwest during the evening.  One part surged out in front, a sort of hybrid between a line of storms and a supercell (a single storm with a persistent rotating updraft).  It then produced an EF1 tornado along an intermittent 11-mile path from the Lincoln County line to the Kanawha River.

At 6:49 p.m., this was the Charleston radar's view.  The hole at the center is the radar on top of the hill at Southridge.  Warmer colors indicate heavy precipitation and the black triangle marks the ongoing tornado near Corridor G.


These were the winds blowing toward and away from the radar at the same time, with cooler colors toward the radar and warmer colors away.  Arrows show the sense of rotation.  The area of bright red on the south side of the tornado is where the radar software probably misinterpreted extremely strong winds toward the radar. 




Drivers on Corridor G had close encounters with the tornado.


The tornado passed very close to the radar itself.


And at this stage, despite rain wrapping around from the south side, the tornado was surprisingly visible in the Southridge area.


At 6:59 p.m., the tornado was in or near the South Hills area of Charleston, increasingly wrapped in rain. 


The tight couplet between light winds toward the radar (light green) and intense winds away from the radar (deep red to magenta) was still obvious.


At this time the storm was visually intimidating from downtown Charleston.  This video focuses on the rain and cloud bearing down on downtown, but briefly pans northward to the area of the tornado.



The precipitation and outflow wind surging over downtown is the "rear-flank downdraft."  This mass of storm-cooled air forces warm inflow air up over it into the eerie, smooth-looking shelf cloud stretching across the sky at the edge of the storm.  While the RFD looks scary in this case and did quite a bit of wind damage all over Charleston, it's not the tornado-producing part of the storm.  The tornado at this time is north of the RFD, where the gust front (the leading edge of the outflow) visibly bends backward in a little notch over South Hills.


The most widely seen and spectacular photos from Charleston show the RFD and attendant shelf cloud.




The NWS survey found that the tornado dissipated close to the Kanawha River in Charleston.  At 7:04 p.m., the remaining circulation had crossed I-64/77/79 and was approaching Yeager Airport.


Most of Charleston experienced damaging straight-line winds on the south and west sides of the remaining rotation even as it continued to weaken.  Although areas across the Kanawha were not impacted by the tornado, there were numerous downed trees and power lines from the RFD winds.


How rare?

Pretty rare.  Long-term averages from the Storm Prediction Center show West Virginia gets about two tornadoes a year.


The timing is typical.  WV averages one tornado in the month of June, and less than one in all other months.  (The first weather event I can remember?  A tornadic supercell in the Kanawha Valley on June 2, 1998.)


On average, an EF1+ tornado passes within 25 miles of Charleston about once every 5 years.


Because the state is almost entirely rural, hilly, and wooded, it's particularly rare that the tornado and its parent storm were so widely seen.

Why so rare?

Start by ruling out the predominant myth...rugged terrain doesn't stop or destroy tornadoes.  Exactly how tornado-producing storms interact with terrain is an area of active research with a lot of unanswered questions.  But here's a violent tornado in Alabama showing exactly how much it cares about some hills and valleys.




The Charleston tornado came 75 years and one day after the June 23, 1944 outbreak.  In that event, an F4 tornado tracked over 40 miles across north-central West Virginia.  That tornado alone killed 103 people, flattened a large part of Shinnston in Harrison County, and climbed a good part of the way up Cheat Mountain before finally dissipating.  It remains the strongest tornado in state history.  The West Virginia hills aren't enough to prevent tornadoes--even violent ones--if the right ingredients are there.

That implies that it's rare to get the right ingredients.  There are several likely reasons for that.

- A typical tornado environment in the central or southeastern U.S. involves winds that turn with height:  out of the south or southeast near the ground, and out of the west or southwest higher up.  But in most of West Virginia, a south or southeast wind is downslope.  Air coming downslope is under higher and higher pressure, and warms accordingly.  Air that warms (without adding any moisture) becomes drier, as my sinuses know when it's cool outside and the heater starts to run.  Dry low-level air is unfavorable for tornadoes.

- If a tropical airmass works its way in around the mountains from the west instead of over and down, then the low-level wind is from at least a little bit west of south.  Then there's less turning with height to the prevailing westerlies aloft.  In short, it's tough to get both rich moisture and favorable wind shear into the WV foothills.

- A common ingredient for severe weather, especially in the Plains, is a plume of midlevel air from the high terrain of the southern Rockies and the Mexican highlands (the elevated mixed layer).  West Virginia is a long way from the source of this air.  When it does get that far northeast, it's usually been diluted by thunderstorms over the central U.S.

Was the forecast good?

Yes.  The Storm Prediction Center upgraded from a slight (2 out of 5) to an enhanced (3 out of 5) risk for severe weather at 12:30 p.m. Monday.  The red dots mark the tornado reports.  Local TV meteorologists and the Charleston NWS office passed along this outlook.


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