Thursday, May 30, 2019

A violent tornado close to home: The May 28, 2019 EF4 Lawrence-Linwood, Kansas tornado

The third violent (EF4 intensity) tornado in the U.S. so far this year occurred on Tuesday May 28 near Lawrence, Kansas (KS) and west of Kansas City, Missouri (MO), not far from my home.  It was the first violent tornado to affect the Kansas City metro area since 2003, and sent my wife Shawna and our cats to the basement for the first time ever.

We were on a 3-day Memorial weekend trip to the High Plains of Colorado (our first multiple day chase trip in over 3 years), but awoke Tuesday morning in North Platte, Nebraska to find Kansas CIty under the gun.  So, instead of chasing north-central Kansas, we rushed back to Kansas City to check on family and cats, and found ourselves in our basement taking shelter not long after we got home.

The track map below shows how the tornado would have come close to our house had it not dissipated near Bonner Springs, KS.  We're extremely grateful that the tornado did not directly affect us or family members, but are sad for those near Lawrence and Linwood, KS (see photos of the tornado near those locations above) who lost property and homes.  Good warnings and media coverage undoubtedly saved lives, but 18 people were injured.

I've had several people ask me why the tornado was violent and long-lived (nearly 32 miles and just short of an hour on the ground).  The surface map at 4:00 pm CDT showed a stationary front (reinforced by earlier storms) draped acoss northeast KS and northwest MO, with the Lawrence-Linwood supercell taking shape near Emporia, KS (circled "S"  on map below), in the warm sector well south of the front:

Computer model forecasts from the morning suggested that low-level winds and the low-level jet would strengthen over eastern KS during the day, nearly doubling in speed between noon and 7:00 pm CDT on NAM forecast graphics for 850 mb (roughly 5000 ft MSL):

When we arrived home at late afternoon after passing through the front from north to south, we noticed that winds were strong from the southeast, which tended to confirm the model forecasts.  This in turn increased low-level shear, which combined with instability resulted in increasing energy-helicity index (EHI) values during the afternoon, supportive of significant tornadoes (compare the RAP model EHI forecasts for noon and 5:00 pm CDT below):

Typically, the low-level jet is at a diurnal minimum in speed at late afternoon in late spring. But on May 28, a strong upper system (see the NAM 500 mb upper air forecast in midlevels for 4:00 pm CDT below) was lifting northeastward through the Plains.  The increasing low-level jet and 850 mb flow was in response to this strong dynamic system:

Going back to the surface map earlier, notice the backed southeast winds at Topeka (TOP) and Olathe, KS (IXD).  That further increased the low-level wind shear ahead of the soon-to-be tornadic supercell, and likely helped it to produce the large tornado long before it interacted with the front farther north.

The HRRR model forecast of the fixed-layer significant tornado parameter (STP) also suggested strong support for tornadoes at late afternoon over northeast KS:

A dangerous aspect of this violent tornadic supercell was that it was very HP (high-precipitation) in nature, wrapping the large tornado in rain, making it difficult if not impossible to see.  The views of the tornado at the top of this blog post are from the north or northwest looking toward the south or southeast, where the back edge of the tornado was sometimes visible.  However, from the east, northeast, or southeast, a large rain-wrapping shield hid the tornado, as in this view toward the west or southwest from Highway K-10 east of Lawrence:

The experience Shawna and I had with the El Reno tornadic supercell back in 2013 has forever made me very leary of HP supercells, so much so that I now always put lots of distance between me and such storms.  In fact, I tend to avoid chasing them.  That's because HP supercells are so difficult to view and assess both storm movement and structure, not to mention any tornado, which is truly frightening.

A related dangerous factor of HP storms is the rain-filled rear flank downdraft (RFD) that often surges and wraps around the tornado from the south and southeast.  On the radar reflectivity images below just after 6:00 pm CDT, notice how the wet RFD surges eastward on the south and east side of the tornadic circulation.  If trying to stay with the storm by stair-stepping eastward and northward, it is not unusual to have this surge overtake you, as can be seen along north-south Highway 59 south of Lawrence in the images below:

My experience in the field is that rain-filled RFD surges are often strong wind producers (apart from any nearby tornado), as with the 2013 El Reno tornadic storm where we nearly got blown off the road retreating south of the intensifying and rapidly expanding tornado.

I've seen a couple videos of chasers following the May 28 Lawrence-Linwood tornadic storm way too close, hooping and hollering while surrounded in blinding rain and seemingly oblivious to the danger near them.  I used to do stupid stuff when I was younger 25-30 years ago, but experience and maturity in recent years has taught me to give these storms a very wide berth, even if it sacrifices the chance to view a large tornado.  I fear that more chasers will get killed in future years on HP-type storms of this nature.

It is truly miraculous and awesome that no one got killed with the May 28, 2019 Lawrence-Linwood tornado.  That is something to celebrate.

- Jon Davies 5/30/19

Monday, May 13, 2019

Boundaries aiding or inhibiting tornado potential: Examples on May 5, May 6, and May 7, 2019 in the Plains

A week ago, May 5 - May 7, 2019 saw several supercells and tornadoes over the central Plains (for example, see images above by Tony Laubach on May 6 after dark in central Kansas ).  Thankfully, there were no serious injuries from any of the storms.  But boundaries on these days certainly had some influence on whether particular storms did or did not produce tornadoes.

This post will look at boundary details that likely contributed to certain storms being tornadic or non-tornadic on these three days, using surface, satellite, and radar data.

First, on Sunday May 5, an outflow boundary was leftover from morning convective showers over central and southern Kansas, seen on this noontime visible satellite image:
By mid-afternoon, this boundary was still visible over central Kansas, along with a dryline wind shift boundary over southwest Kansas that was marked by newly-developing cumulus (also see the 4:00 pm CDT surface map below):
By 5:00 to 6:00 pm CDT, storms had developed from Kansas to west Texas:
It's notable that the storm/storm cluster in central Kansas near the old outflow boundary on May 5 produced tornadoes off and on for at least a couple hours as the cluster moved southeast near this boundary, including this EF1 tornado northwest of Hutchinson, Kansas:
The SPC mesoanalysis depiction of the significant tornado parameter (STP) at 5:00 pm CDT suggested that factors supporting supercell tornadoes (such as wind shear and CAPE) were maximized somewhat in central Kansas, probably due in part to this boundary:
It's interesting that the morning HRRR model forecasts (not shown) did not show any storms developing along this boundary.  The tornadic storms on May 5 in central Kansas near Great Bend and Hutchinson probably would not have occurred without the remnants of the morning outflow boundary enhancing convergence and local wind shear.

A landspout also occurred on May 5 northwest of Dodge City between 5:00 and 6:00 pm CDT, as storms developed along the sharp wind shift boundary in southwest Kansas seen on the satellite photos earlier, where surface heating and CAPE intensified along the vorticity-rich boundary.
A large dusty tornado (EF2) also occurred with a supercell in west Texas south of Lubbock (image below), within the warm sector and not associated directly with a boundary.  This was the strongest and longest-lived tornado of the day, and formed ahead of  a dryline bulge within an environment of enhanced STP values in that area (see earlier STP graphic).

The next day (Monday, May 6), the 7:00 pm CDT surface map showed a slow-moving east-west front over central Kansas that initiated storms during the late afternoon and evening:
Between 6:30 and 7:00 pm CDT on May 6, storms on radar were ongoing in central Kansas, including a cell that was tornado-warned near McPherson (MPR, south of Salina/SLN).  Note the blue "fine line" on the radar-image below, located just _south_ of McPherson and indicated by the small white arrows:
This strongly suggests that the tornado-warned storm was undercut by cold air north of the front, which is probably why the storm did not generate a tornado, although it did produce large hail (2"+).  A photo of the McPherson storm also visually suggests it was occurring atop cold surface air, with lines of scud clouds visible near the ground and a laminar shelf forming:
Another tornado-warned storm was located farther west, northeast of Dodge City (DDC), also just north of the surface front (dashed line below) with cold air likely undercutting it, as suggested by the frontal "fine line" position (not shown) from the Dodge City radar:
As the evening went on, the boundary slid southward through Dodge City (still visible as a "fine line"), but then slowed down as storms consolidated into a significant supercell just northeast of Kinsley, Kansas after 9:00 pm CDT:
Notice that the boundary east of Dodge City (as marked by the dashed line above) had gotten "pulled" back north into the strong supercell northeast of Kinsley so that the storm could access the unstable warm sector air mass, instead of being undercut like earlier with cool post-frontal air.  That likely had something to do with the supercell producing a tornado after dark east-northeast of Kinsley shortly before 10:00 pm CDT (see images at top of this post):

The SPC mesoanalysis depiction of STP at 9:00 pm CDT showed an environment supportive of tornadoes feeding into this supercell from south of the boundary:
It's also interesting that, farther west,  a tornado occurred around 9:20 pm CDT at the location where the aforementioned boundary hooked into the approaching squall line west of Dodge City (see the 9:16 pm CDT radar image up above).  This boundary intersection would be a favored location for a tornado from an embedded circulation at the intersection point.

So, it seems that the boundary positioning on May 6, depending on whether a particular storm had access to warm and unstable surface air or was located over the top of cold post-frontal surface air, had much to do with tornado potential.

I don't have much room here to go into Tuesday, May 7 in the Texas panhandle.  But it is worth noting that a stationary front oriented southwest to northeast appeared to have much to do with supercells' ability to produce or not produce tornadoes that Tuesday afternoon.  Storms immediately north of Amarillo at mid-afternoon appeared unable to produce tornadoes when they moved north of a fine line (not shown) marking the frontal position and colder surface air.  However, some storms located more to the northeast of Amarillo were able to access warm and unstable surface air due to their location just south and east of this same boundary:
SPC's own Roger Edwards was chasing near Amarillo that afternoon, and noted on his Twitter post to #txwx the cold air coming out of the storm he was chasing just north of the boundary, indicative of the coldness of the surface air associated with the boundary:
Here's the SPC depiction of STP at mid to late afternoon, suggesting a supportive environment for supercell tornadoes over much of the Texas panhandle, but not doing much to indicate where the surface frontal boundary and cold surface air were truly located: 
This is why it is so important to keep track as much as possible where relevant surface boundaries are located when considering which storms are more likely to produce tornadoes.

Finally, an aside on a different subject... images (below) of the EF2 tornado on the west and southwest edge of Lincoln, Nebraska on Sunday, May 5 show how difficult to classify some tornado events are.  Although radar images depicted a rotating supercell (below), images of the tornado (also below) look more like a gustnado along a storm gust front:
This high-based tornadic storm formed just ahead of a slow-moving east-west front where SPC mesoanalysis graphics indicated steep low-level lapse rates (below).  This suggests the tornado might have been some kind of a "hybrid event" where a front flank mesocyclone and the storm's gust front got together in some way to produce the tornado.

In summary, May 5, 6, and 7 had some very interesting settings for tornadoes in the Plains, and provided some excellent examples of how boundaries can sometimes help and other times hurt tornado production.

- Jon Davies  5/12/19

Friday, May 3, 2019

First large Plains tornado outbreak of 2019 on April 30 results in two deaths

Tuesday's tornado outbreak on April 30 in Oklahoma (OK), Missouri (MO), Arkansas (AR) and Texas (TX) saw many tornadoes that were widely-photographed, and unfortunately, one death from a large tornado after dark in southeast OK (see 1st image above).

*** Update 5/4/19  -  Sadly, a 2nd woman has died from the large tornado near Blue in southeast OK just after dark on April 30 -- that makes 2 tornado deaths on April 30, instead of only one. ***

For so many tornadoes (some large, some rain-wrapped), it is surprising that there were only a few injuries and just the one death (two other deaths occurred due to flash flooding in OK and MO).  That may speak to the effectiveness of warnings on April 30, and the fact that no large towns were hit directly.

The 2nd and 3rd images above show a large and dangerous EF2 tornado at mid-afternoon north of Tulsa, OK that was difficult to see at times due to rain wrapping; it is fortunate that this tornado did not strike any towns.  The 4th image above is a large EF1 tornado in northwest AR near Bergman.  And the last image above is a "landspout-type" tornado that developed in west Texas northwest of Snyder with a supercell over an east-west stationary boundary.

The outbreak was generated by a strong shortwave trough of energy at 500 mb (roughly 18,000 ft MSL) moving out of a large western trough and into the Plains (see thick red dashed line on NAM forecast for midday below).  Midlevel winds spread out widely ahead of this shortwave, inducing large-scale ascent over the area where tornadoes occurred on April 30:

The surface map at 2:00 pm CDT (1900 UTC) showed a stationary front draped from MO to west TX, with several localized areas or "waves"of low pressure:
In particular, cool air and easterly winds from overnight and morning storms over eastern KS and western MO had reinforced the front over northeast OK just ahead of the surface low there.  This increased low-level wind shear in that area, reflected by increased values of the effective-layer significant tornado parameter (STP) on the SPC mesoanalysis at 2:00 pm over northeast OK into southwest MO:

That's the area where the most tornadoes occurred during the afternoon, seen on the visible satellite image below from 3:00 pm CDT (2000 UTC), with some tornadic storms labelled:

Other storms in northwest TX near Wichita Falls, and west TX near Snyder, produced tornadoes after 4:00 pm CDT (2100 UTC) as noted on the above satellite photo.  (The last tornado image at the top of this post, northwest of Snyder TX, may have involved some landspout-type processes with strong heating along the stationary boundary and near the storm there, labelled on the satellite image.)

Moving into evening on April 30, the 8:00 pm CDT (0100 UTC) surface map and composite radar image below showed storms "lining out" across OK along the front, with some outflow boundaries hinted at over northern TX and southern OK, and also southwest MO and northwest AR:

It's possible these subtle outflow boundaries may have helped with tornado production with storms ahead of the line in the warm sector over southern OK around 9:00 pm CDT where the large EF3 tornado killed a woman between Bokchito and Blue, and also the EF2 tornado south of Springfield MO around 8:00 pm CDT where 3 people where injured.

Also notice how the effective-layer STP from the SPC mesoanalysis highlighted southern OK and southwest MO at 8:00 pm CDT, largely due to an increasing low-level jet (southerly winds at about 5000 ft MSL, not shown) within the warm sector around sunset:

This outbreak had a little of everything regarding tornadoes.  It involved tornadoes occurring well inside the open warm sector (the infamous drone-viewed tornado near Sulphur OK around noon), tornadoes near convectively-enhanced and shear-enhanced boundaries (the afternoon tornadoes in northeast OK, southwest MO, and northwest AR), the deadly evening tornado in southeast OK within the warm sector (possibly helped by a subtle outflow boundary), and the aforementioned afternoon tornado in west TX that may have involved landspout processes. 

- Jon Davies  5/3/19