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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Wednesday, November 29, 2017

May 2017: McLean, Elk City, upward lightning

16 May

My first return trip to Oklahoma after starting Ph.D. work at North Carolina State was in the middle of May.  It wasn't really a chasing trip.  There were people and places to visit.  But the timing was no coincidence.  Climatology obviously favors May and I aimed about a week earlier than I otherwise might have, for fear of the death ridge setting in early in a warm spring.  So on the 16th, I was in the convoy departing westward from Norman.  Even the night before we had been uncertain about the moisture quality and storm mode/spacing.  But the environment in the Texas Panhandle and western Oklahoma had materialized about as favorably as it could have by late morning and everyone latched onto the trend.  SPC stepped up to a MDT.  While we were still en route, a PDS box came out and the dryline erupted.  We arrived in the Panhandle just in time.  A messy supercell crossing I-40 between Groom and McLean was neither approachable on the road network nor very promising, but a new cell on its southern flank was organizing.  We dropped south from McLean on Hwy 273.  The base barely came into view over the rolling terrain of the eastern Panhandle.  There were suspicious features moving around underneath but we let it come to us on 273.





I tend to be transfixed by a storm to the point of letting it move too far away while I stand gawking by the road.  We realized the base was getting to our north and adjusted a couple of times up 273, back toward I-40.  The storm produced a large and persistent cone funnel without much of a wall cloud.





The storm toyed with tornadogenesis for several minutes before the funnel narrowed and a dust cloud appeared beneath it, followed quickly by full condensation.




The tornado took on a thicker elephant trunk shape and receded into the precipitation to our north.  We followed.  It took a moment to recognize the tornado reappearing as we came out of a draw.  We were among the relatively few chasers with a view of the dramatic rope-out over a field of blooming yucca and primroses.







The remnants of 2"+ hail, the largest I've ever seen in person, lay strewn in the grass.  Many chasers positioned for a closer and higher-contrast intercept when the tornado began were blasted.




The tornado dissipated and we edged northward to I-40 as the precipitation core cleared our path.  The McLean storm was still healthy, but a true monster of a supercell was all on its own a county or two south of us.  We decided to race it east and intercept near the Oklahoma line.  Considering the environment and the spectacular 17-minute-long tornado we had just seen, we expected quite a show when we set up on Hwy 30 south of Erick.  That was not the case.  The storm was going HP, to the naked eye appearing to have more precip falling in the RFD than in the forward flank (though there were baseballs and softballs flying through that deceptively clear area).




It would turn out that McLean was the highlight of the day.  We made a long trip on the sparse road network to close on the storm again.  Somewhere in the rain, the storm produced a deadly EF3 tornado on the south side of Elk City.  We were fairly close but saw nothing, except a few ribbons of dust under the base that some reported as a tornado.  There was never a chance to stop and watch again after the Erick intercept.






18 May

We played a succession of supercells in what should have been a ripe environment south of SPC's HIGH, from Hobart to northwest of OKC.  Storm interactions seemed to keep them slightly elevated most of the time (there was some damage from a rainy mess of a storm near Corn and Colony).  Bust.  But there were some otherworldly skies over central Oklahoma after we called it a day and stopped at Pops.






20 May

Not far east of my parents' home in the DFW metro is a town called Cedar Hill, where there's a hilltop tower farm visible for many miles.  I had wanted to shoot upward lightning there for years. Well after midnight, I got my chance as an MCS with extensive trailing stratiform passed over.  I caught only two of what must have been 10-12 flashes.





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Monday, May 30, 2016

Another week in May

Sunday, May 22

Mini-MPEX headed out I-40 into the Texas Panhandle, expecting initiation on a confluence axis ahead of the dryline, which lagged near the New Mexico border.  Storms erupted quickly as we launched on the north side of Pampa, becoming tightly spaced with several left splits.  A cell to the south produced a tornado near Howardwick, but the most discrete cell was to the north, near Spearman.  We set up on a gravel road in a remote, wildflower-dotted part of the Canadian River valley and released a sonde in strong inflow with a wall cloud and brief funnel to our north.



The storm failed to produce a tornado there, and we had to race out of the river valley to avoid a left-mover colliding with our storm from the south.  But later in the evening, we collected a series of inflow soundings on the same storm as it produced a couple of large tornadoes somewhere between Canadian and Spearman.  Howling inflow winds whisked the last sounding up from the ruins of the Gageby Store on Route 83 with mammatus hanging overhead.



Monday, May 23

Mini-MPEX had a head start after waking up in Shamrock and downing Texas-shaped waffles at the Sleep Inn.  Both vans and the CLAMPS trailer met up in Hammon, north of I-40 back in Oklahoma.  The vans started south through Granite, the most aptly named town in the state, and the unusually green Wichitas to Route 62 in southwestern Oklahoma; the near-field van continued into the Texas Panhandle to launch on a young storm.



Despite an apparently favorable environment with strong instability and deep-layer shear, the storm evaporated, hampered by extremely dry midlevels.  We seemed snakebitten all day.  The other van encountered a flooded road and had to turn around.  A jackrabbit ran under our wheel.  One of the thermometers on our van started reading over 100 degrees Celsius.  The spare tire cover under our van came loose and started bouncing on the road, hanging by a cable.  Other new supercells in the eastern Panhandle formed, but quickly split and disappeared one after another.  And as we headed back to Norman, two supercells northeast and south of our target area produced tornadoes, one near Woodward and one near Turkey, TX.

Tuesday, May 24

This was mini-MPEX's down day with CLAMPS needing maintenance.  We'd operated on every Southern Plains severe weather day of note since late April and expected to do the same the rest of the week, so I left after lunch on a personal chase in a caravan of OU met students.  We arrived in Buffalo, OK, where familiar faces and research vehicles were around every corner, all looking for the same thing.  As we baked in a gas station parking lot on the north side of town, I was beginning to think the optimal environment, an area of enhanced low-level shear near the subtle remnants of an outflow boundary, would go without a robust storm.  Suddenly a thick tower was exploding over the rooftops and trees across the street to our northwest.  We were on the road flying toward it before it even broke 30 dBZ.  It was just a tiny blip--but the fine line marking the boundary was curving out into the blip as the updraft sucked in vorticity.

In that environment I thought it was possible that we'd come up on the storm in Minneola, KS, to find a tornado already in progress.  But the updraft base was high, and we navigated a stream of chasers to find a relatively quiet dirt road intersection where we could watch the new supercell evolve.  The base lowered rapidly into a wide, ragged wall cloud.  Then the storm toyed with tornadogenesis for 10-15 minutes in a couple different locations before finally extending a condensation funnel into a whirl of dust.








The tornado grew into a fat cone.  We realized a little too late that while we had been mesmerized, the storm had been moving to the north, and we never quite got back to a good viewing range and angle.  We took off in pursuit.  The tornado slid back into the rain.


Another tornado appeared after the first roped out and it wasn't long before the ground-scraping wall cloud to its east produced simultaneously.  We stopped with the twins for a minute or two before giving chase again.





The storm was headed toward Dodge City just north of us.  There were some tense minutes on the way east to Route 283 and northward toward the town.  We had already seen at least one tornado that produced a horizontal vortex as it faded into the rain, an exclusive hallmark of a strong to violent tornado, and the updraft base was almost to the ground in some places.  The storm looked poised to do something nobody wanted to see.  With a parade of chasers filing into town on 283, we chose not to risk being in the way of first responders should Dodge City take a direct hit and instead detoured eastward.

Dodge City was spared by a few miles.  We were far out of position but our vantage point offered a rare perspective:  the full sculpted updraft of a classic supercell with either several short-lived tornadoes or one long-lived tornado visible underneath, in an assortment of shapes and sizes.  The structure was something out of a storm spotting guide.  There were very few other chasers around us.





We noted a left-moving supercell closing in from our south and repositioned northeastward to get away from close CG lightning.  In the distance we saw one tornado vanishing into the precip in a prolonged rope-out while another formed.


Ordinarily it's a mistake to walk away from a supercell before twilight, as I've learned the hard way, but we had seen the feature presentation.  Storms were merging and getting messy, road options were becoming limited, and it was a very long drive back to Norman.  We left the storm early, got back across the Oklahoma line by sunset, and ended up missing virtually nothing.

Wednesday, May 25

Mini-MPEX was back in full swing and targeted south-central Kansas.  Field work isn't just enjoyable because of the storms; the Plains are worth exploring in their own right, and I have a soft spot for Kansas vistas in particular.  These launch sites were just outside Wellington and Leon.



Storms in south-central Kansas were LPs that struggled along for a couple hours, then evaporated.  The evolution was almost identical to Monday's in the Texas Panhandle. 


One storm did thrive, a supercell that produced a long-lived EF4 up on I-70, but we couldn't get there.  We were glumly driving home through north-central Oklahoma when a disorganized cluster of storms near Enid congealed and, presumably with the help of the incipient low-level jet, became an intense supercell.  In-cloud and cloud-to-air lightning flickered continuously on the edges of a perfect anvil in the dying light.  We sped west from I-35 and launched in front of the almost stationary storm at its peak, close to the time it produced a tornado that derailed a train.


Only after we had maneuvered through Enid and onto back roads south of town for another couple launches was there time for photos, and by then clouds atop a surge of outflow were obscuring most of the lightning.


The Carrier supercell was mostly a lucky break.  While we did know better than to shut everything down and ignore ongoing storms with LCLs lowering and the low-level jet kicking in, nobody on the crew expected what we got.  If we'd tried to make it to the I-70 storm, we probably would have missed both.  We had been unlucky with our target area Monday and missed a prolific tornado producer on our down day; things swung back our way Wednesday night.

Thursday, May 26

It was the most befuddling setup of the week, maybe of the Plains season, and for the first time I had the job of nowcasting and putting both vans in position to sample storms.  Junk convection and diurnal heating played havoc with a relatively shallow moist layer, creating unpredictable pockets of dry air throughout the warm sector from Kansas to Texas.  We intercepted a weak supercell in northwestern Oklahoma near Freedom; it promptly died.  Discouraged, we headed into Woodward to see if anything could come off the dryline in the Texas Panhandle and survive the mediocre moisture.  Finally, a struggling updraft came up through the Oklahoma Panhandle and approached two or three outflow boundaries; we committed to working that storm and were rewarded as it grew into a healthy supercell that dropped enormous hail in southwestern Kansas.  We passed through Coldwater between launches with a stout updraft looming overhead.


The Coldwater storm succumbed to the flawed environment and storm interactions and lined out after a couple pairs of soundings.  We tried to work transient supercells in the western half of Oklahoma after darkness fell, but we struggled to get in position on tightly spaced and erratically moving storms.

Friday, May 27

We knew this could be the final day for mini-MPEX, so we started southwestward out of Norman despite a marginal environment for supercells.  After a couple false starts trying to get on a tail-end cell out of a cluster of convection that began near Lawton, we settled in on a storm that tracked across the south-central section of the state.  It rarely had strong rotation and only once or twice looked like it could come close to producing a tornado, but maintained a little spin at some level well into the evening.  Soundings showed ample deep-layer shear, but small SRH helped explain the absence of an intense low-level mesocyclone.  Our storm finally gusted out after we followed it into the jungles of southeastern Oklahoma.



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