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 

Friday, April 26, 2019

More tornadoes and more deaths in Texas and Louisiana, April 24 and 25, 2019

Yet another tornado episode occurred in the South on Wednesday, April 24 and Thursday, April 25 in Texas (TX) and Louisiana (LA), adding 2 deaths to 2019's tornado toll during the early morning hours of April 25 .

The tornado pictured above near College Station, TX at mid-afternoon on Wednesday didn't kill anyone, but was rated EF2 and was widely photographed.  This tornado is interesting because it formed rapidly from a small cell that was not very impressive on radar (see images below).  As a result of being small and relatively distant from radar sites, there was no tornado warning at the time.

Here's the setting at 4:00 pm CDT... A surface low was just west of College Station (CLL), with a stationary east-west outflow boundary (from earlier storms to the north) also located near CLL.  The circled "S" shows the location of the small supercell that produced the tornado as it crossed this boundary going from south to north:
Why did the storm produce a tornado so quickly?  It probably had something to do with the boundary; remember from other posts I've made this year that tornadoes definitely seem to favor boundaries.  This is suggested by the SPC mesoanalysis images below at 4:00 pm CDT:
Notice in the 2nd graphic above (SRH, or storm-relative helicity) how there was a band of enhanced low-level wind shear along and north of the outflow boundary, likely promoting low-level storm rotation in that area.  This same area overlapped low-level CAPE (surface-based instability that can promote rapid vertical stretching) along and south of the boundary, likely making a very favorable environment for a tornado as the cell crossed the boundary in a narrow corridor before it encountered cooler air farther north.

There's not room to show it here, but the RAP model 1-hour forecast sounding at 4:00 pm for CLL had 1200-1300 J/kg of total MLCAPE, over 300 m2/s2 of 0-1 km SRH (a nicely curved low-level wind profile), and over 100 J/kg of 0-3 km MLCAPE in low-levels.  Along with over 60 kt of deep-layer wind shear, and lifting condensation level heights (LCLs, or estimated cloud bases) well below 1000 m to reduce cool outflow, this was a very favorable environment for supporting tornadoes along and just north of the outflow boundary.  Farther north of the boundary, storms became increasingly "elevated" (less surface-based regarding the instability feeding them), which would be less supportive of tornadoes.

Later that evening and night, a long-lived supercell producing multiple tornadoes emerged from the southern segment of a line of storms over far eastern TX and moved to north-central LA (see radar images below), eventually killing 2 people at Ruston, LA just before 2:00 am CDT on April 25.
The EF3 damage from this tornado at Ruston included this motel (photo below), which lost most of it's 2nd floor.  Although no one died at this location, this is a pointed reminder that shelter below ground (or at least in a small interior 1st floor room) is crucial in surviving tornadoes!

The surface map at 1:00 am CDT early morning on April 25 showed an outflow boundary that was now oriented north-northeast to south-southwest over east TX to northwest LA, with the long-lived supercell (circled "S") moving northeastward roughly along this boundary for 2-1/2 hours:
A subtle and weak warm front progressing northward over central and northern LA (thick red dashed line on surface map above), suggested by slightly backed surface winds and larger dew points by 3-4 degrees F, may have also helped in sustaining the tornadic storm.

SPC mesoanalysis images at 1:00 am CDT (below) also showed a favorable environment for supporting supercell tornadoes over west and northwest LA along and just ahead of the outflow boundary and subtle warm front:
MLCAPE (not shown) was around 1200 J/kg, 0-1 km SRH > 300 m2/s2, and low-level MLCAPE > 100 J/kg over a sizable area through which the supercell was moving.  The orientation of the boundaries apparently allowed the northeastward-moving supercell to take advantage of this supportive environment for tornadoes over a long period of time, while utilizing increased shear and convergence along/near the same boundaries.

Here's a quick look at the larger scale upper air pattern via a 500 mb forecast map from the NAM model, valid at 1:00 am CDT:

A large, intense midlevel trough was moving from TX eastward, forcing ascent over LA with the "spreading jet" pattern ahead of it inducing general upward motion and strong wind dynamics across the area where tornadoes occurred overnight.

Tragically, the tornado outbreaks in the southern U.S. on Feb 23, March 3, April 13, and now April 25 have claimed a total of 29 lives so far in 2019.  Other deaths have resulted from thunderstorm winds blowing trees down on houses and vehicles, and flash-flooding.

As severe weather season continues in 2019, know how to stay safe... watch Surviving the Storm: What Chasers Want You to Know, free on YouTube:

- Jon Davies 4/26/19

Wednesday, April 24, 2019

Landspout-type setting produces first tornadoes of 2019 for Oklahoma & Kansas on April 17

It has been a slow start to tornado season in Oklahoma (OK) and Kansas (KS), very much like last year.  Last year's first Kansas tornadoes were on May 1, and Oklahoma's were on May 2.   But this year's first tornadoes in both states (see images above) were a week ago on April 17 near Shattuck, OK (top image) and Wellington, KS (bottom image).  These appeared to be due to mainly "landspout" processes (storm updraft stretching directly over a pre-existing wind shift boundary), rather than last year's supercell tornadoes on May 1 and May 2 when plentiful low-level wind shear was present over large areas.

The surface map setting at 3:00 pm CDT on April 17 (below) showed a stationary front present as a sharp wind shift over northwest OK into the northeast Texas (TX) panhandle:

This sharp boundary could be seen on high resolution visible satellite images at early to mid afternoon:

Radar reflectivity images from Amarillo showed a new storm developing on this boundary at 3:00 pm CDT in the northeast corner of the TX panhandle (1st image below), north of other storms farther to the south:

A radar image less than an hour later (middle image above) showed the same cell exploding with a red core.  At this time (3:52 pm CDT), a rope-like tornado (not pictured) was on the ground northeast of Canadian TX, while new cells began to develop northeastward into OK along the boundary.  The last radar image above at 4:49 pm CDT showed a supercell storm had developed in northwest OK on the boundary near Shattuck, where two tornadoes were occurring simultaneously (see the photo at the top of this post).

Although at least a couple of the storms in the northeast TX panhandle and northwest OK were supercells (storm rotation indicated on radar, but not shown here), my opinion is that the actual tornadoes were due mainly to "non-mesocyclone" or "landspout" processes, or at least that the tornadoes were a "hybrid" event (combining non-supercell and supercell processes).  This is suggested by the SPC mesoanalysis images below at 3:00 pm CDT before the tornadoes:

These graphics clearly show the stationary front wind shift boundary in the surface vorticity field (blue analysis lines and thick black dashed line on 1st image), steep low-level lapse rates from surface heating (axis of red dots, 2nd image), and no low-level wind shear (storm-relative helicity or SRH, 3rd image) over the northwest OK and the TX panhandle  where the tornadoes occurred during the following couple hours.  These are all factors that relate to landspout tornado formation.

It's also interesting that a supercell farther south that moved into west-central OK (visible on the last two panels of the radar images shown earlier) did not produce any tornadoes, although it was later tornado-warned based on radar.  This storm was within the warm sector and away from the stationary front.  But, without the sharp boundary wind shift, it had no source for low-level vorticity ("spin") as a result of the lack of SRH (low-level wind shear) over western OK during the afternoon.  (Low-level wind shear helps generate low-level rotation and mesocyclones that can give birth to supercell tornadoes.)

Here are a couple more images of one of the two tornadoes near Shattuck, OK between 4:30 and 5:00 pm CDT along the boundary (probably the tornado at right in the earlier photo):

Another tornado (a landspout) occurred in the central TX panhandle just after 5:00 pm CDT east of Amarillo, along the same boundary that extended some distance southwestward:

And, essentially the same boundary also extended into south-central KS as a pre-frontal trough, helping to generate the landspout tornado pictured earlier near Wellington around 6:40 pm CDT.

Again, this case seems to be a good example of how landspout processes can produce multiple tornadoes via strong low-level stretching directly over a sharp wind shift boundary with little immediate temperature contrast (see here for another more prolific case).

Jon Davies - 4/24/19