Sunday, August 2, 2020

A 'surprise' tornado on July 29, 2020 north of Kansas City - another subtle, marginal summer tornado setting

I've had several people ask me what was going on with the unexpected but well-photographed tornado (above, EF0) last Wednesday 7/29 at early afternoon near Smithville, MO, north of Kansas City, only a few mile north of where I live.  I was quite surprised when I heard about the warning associated with it, and wondered myself how there could be a tornado on this particular day.

Although not something one could really anticipate, a careful look back at the setting revealed that it was not 'random' or 'from out of the blue'.

There was a well-defined boundary (dashed red-blue line below; a weak stationary front?) on the surface map at noon on 7/29/20 across the Kansas City metro area, delineating south winds to the south of the boundary from light easterly winds on the north side of it:
As many forecasters and researchers have pointed out in recent years, tornadoes like boundaries, often because favorable wind shear (a change in wind direction and speed as winds veer with height) that might help generate a 'spin up' can be present along and across such boundaries.  The black-circled 'S' on the map above shows that the location of the small supercell that later spawned the weak tornado was indeed very close to the boundary.

Here's satellite and zoomed-in radar reflectivity near the time of the tornado (1:45 pm CDT, or 1845 UTC), with a white circle showing the location of the supercell mesocyclone and rotation in the storm near Smithville:
It is also interesting that a broad, weak low was present at 700 mb (roughly 10,000 ft above sea level) nearby over southeast Nebraska at midday on the SPC mesoanalysis:
This weak low aloft and a short wave trough (heavy black dashed line) moving around it helped provide lift for generating thunderstorms over northeast Kansas and northwest Missouri.  In a way, this looked like a weak 'cold-core' tornado setting, and coupled with the east-west surface boundary, was another factor that may have helped set the stage for a tornado.

The storm environment, though subtle, also offered some ingredients that were marginally supportive of supercell tornadoes.  The first panel below from the SPC mesoanalysis at 1800 UTC (noon CDT) showed that low-level wind shear, though not large, was enhanced some just north of the east-west boundary (0-1 km storm-relative helicity / SRH > 50 m2/s2), as one might expect with easterly surface winds just north of Kansas City on the surface map earlier:

The 2nd panel above showed that instability in low-levels close to the ground was large (0-3 km mlCAPE > 150 J/kg) near the boundary across the Kansas City area.  Although subtle, combinations of both low-level shear and instability together were largest in that area (see black oval in 1st and 2nd panels above), just south of Smithville.

One last ingredient was deep-layer wind shear (surface to 6 km above ground, 3rd panel above) that was near the lower limit (around 25 kt) of what is considered supportive for supercell storms.  But on this day it was enough to do the trick.

Here's a RAP model analysis sounding estimate of the environment near Smithville at midday along the east-west boundary:

Although not impressive, the instability (which I've manually shaded in red above) and vertical wind shear were apparently just enough to help generate and support a tornado as the supercell crossed the surface boundary moving north-northeast.  

In particular, the instability in lowest levels close to the ground probably helped with stretching of air in the small storm's updraft as it interacted with the local boundary.  The photo below shows a cloud ridge flowing into the storm with 'vapory-looking' scud close to the ground, a typical visual 'look' when large amounts of low-level CAPE are present not far above ground, and low-level stretching in updrafts may be enhanced.

This weak event wasn't of much importance (brief tornado, no damage), but is another illustration of how subtle ingredients can come together to produce a mesoscale 'accident'.  Given how marginal the setup was, an experienced nowcaster and meteorologist would not expect anything more than a weak tornado or two from such a setting.

- Jon Davies  8/2/20

Thursday, July 16, 2020

EF4 tornado in Minnesota on July 8, 2020 - a rather subtle environment for a violent tornado

After four EF4 tornadoes in April (3 in Mississippi, 1 in South Carolina), there were none in the U.S. in May and June.  But on July 8 in west-central Minnesota (MN), an EF4 tornado (images above) near Dalton, MN shortly after 5:00 pm CDT (2200 UTC) killed one person.  This was the first violent tornado in July in the U.S. since 2004!

The environment at first glance did not appear likely to generate a violent tornado, as low-level wind shear (storm-relative helicity, or SRH) wasn't notably large.  But there was a boundary in the area and large instability (CAPE), along with enough deep-layer wind shear to support tornadic supercells.  Here's a look at the setting...

SPC mesoanalysis graphics at 2200 UTC (5:00 pm CDT) showed large total CAPE (mlCAPE > 4000 J/kg, 1st panel below) near the soon-to-be tornadic storm that had developed during the previous hour in west-central MN:

Deep-layer shear (2nd panel above) was enough (30-40 kt) to help organize the isolated cell into a supercell.  Just as important, the cell developed near an east-west boundary (warm front /stationary front, also shown on the graphics above) that extended eastward from a surface low, a favorable setting for tornadoes when the surrounding environment offers additional support.

Also at 2200 UTC, low-level instability (0-3 km mlCAPE) was large over west-central MN (> 125 J/kg, 1st panel below), although 0-1 km SRH was nothing that would raise eyebrows much (0-1 km SRH 75-100 m2/s2, 2nd panel below):

But together, the combination of just enough low-level shear/SRH with strong low-level and total CAPE in the vicinity of the east-west boundary east of the surface low was more favorable for tornado potential than a look at wind shear parameters alone might suggest.

In fact, supercell and tornado development was rather fast and 'explosive', as seen in the two satellite images below:

In just less than an hour from initiation, the storm became a supercell and planted a strong/violent tornado on the ground.  That's quite fast compared to many supercells that take two hours or more to produce tornadoes.

A contributing factor to this rapid development may have been low-level lapse rates (below), not shown in the SPC graphics above:
The axis of red dots in the graphic above shows strong surface heating extending from the southwest into the supercell's environment near the boundary over west-central MN.  Such steep lapse rates (> 7.0 degrees C per km, a rapid change in temperature above ground) can enhance the stretching of low-level air entering storm updrafts, adding a 'kick' to an already unstable situation and helping to make the most of available low-level wind shear (SRH) as it tilts and stretches into an updraft.

Back around 2005, I came up with a parameter called 'enhanced stretching potential' (ESP) that combined low-level lapse rates with low-level CAPE to help locate areas that might have this extra 'kick'.  It was designed mainly for diagnosing short-term potential for landspout tornadoes along sharp boundaries (see this paper).  But on 7/8/20, ESP overlapped the area we discussed above that already had potential for tornadoes suggested by more typical supercell tornado parameters:
It may have added some 'gasoline' to the fire in this case, speeding up supercell and tornado development.  But that's only speculative.  I will mention that the prolific tornadic supercell in southern Saskatchewan (Canada) on July 4 that I touched on in my previous post had some of the same ingredients (SRH < 100 m2/s2, large CAPE and large ESP, as well as an east-west boundary; not shown).

As ESP was originally intended for highlighting short-term possibility for landspouts, it is worth noting on 7/8/20 that the ESP graphic above showed an ESP maximum over north-central Nebraska, where landspouts did occur later that evening along a boundary southwest of a storm near Thedford, Nebraska:

And other landspouts occurred on 7/8/20 along a boundary and boundary intersection over northeast Colorado where ESP was also large (not shown).

Finally, here's the large-scale NAM model 500 mb forecast for mid-afternoon on 7/8/20 showing the strong shortwave disturbance (red dashed line) in mid-levels moving through the northern plains that helped intiate the tornadic supercell in MN:

It is interesing that the early half of July 2020 has been more active for tornadoes in the central and northern Plains of the U.S. than in the last half of June.

- Jon Davies  7/16/20

Monday, July 6, 2020

Unusual EF2 tornado in South Dakota on July 4, 2020 is 1st strong tornado in U.S. since June 10

July 4th was a rather active day in the northern Plains of the U.S. and southern Canada.  In particular, a photogenic supercell with a sequence of tornadoes occurred in southern Saskatchewan on July 4 (see photo above).  This blog post isn't about that supercell, but I will discuss it briefly at the end here.

What I do want to focus on is a tornado (rated EF2) northwest of Aberdeen in South Dakota near the town of Wetonka, around the time that the tornado pictured above was happening in Canada.  The South Dakota tornado was the first significant tornado in the U.S. since June 10th, a surprising 25-day streak during a late spring / early summer period that has seen below normal tornado activity in 2020. 

**** UPDATE Note:  NWS Cheyenne WY was slow in rating an EF2 tornado northwest of Hemingford, Nebraska on July 2, 2020.  Now with that new information, I make the correction that the Nebraska tornado on 7/2/20 was the first significant tornado in the U.S. since June 10. ****

The Wetonka, South Dakota tornado came from this supercell, shown around an hour before the tornado:

I wanted to document that the evolution of the supercell and its associated outflow boundary before the tornado was something you don't see that often.

Below is a 3-panel base reflectivity sequence during the 30 minutes prior to the tornado.  Notice the outflow boundary (fine blue line) ahead of it:

This outflow surged out southeast in front of the storm in the hour before the tornado, suggesting that the supercell was "outflow-dominant" (rain-cooled air moving outward from the storm), usually signaling to meteorologists and storm spotters that tornado development is unlikely.  In other words, the cool outflow air would undercut the storm updraft beneath which a tornado might form.

However, in this case, the outflow _slowed_ and the rotating supercell storm caught up to it, pulling and wrapping the outflow boundary back in under the storm and updraft.  At the white arrow in the radar images above, see the fine blue line slow and then curl back northwest into the supercell as a 'hook-shaped' echo forms (see last panel above at 7:27 pm CDT), allowing the supercell updraft to access warm moist inflow air at the surface.  This was likely an important factor in the storm's ability to produce a tornado.

The zoomed-in radar image below is during the tornado moving southeastward near Wetonka.  The circle indicates the supercell mesocyclone where the tornado was located, and I've also indicated with a dashed white line the boundary and inflow wrapping way back into the mesocyclone from the east during the tornado.

As seen in the earlier radar sequence, outflow initially surging out ahead of the storm was probably a reason why the storm was not tornado-warned, only severe-warned.  Typically, once a supercell becomes outflow-dominant, it remains that way, unlike this case.   It is somewhat unusual to see an outflow boundary out ahead of a storm get pulled back in underneath it, suggesting that the supercell mesocyclone on July 4 was rather strong, while at the same time the initial outflow air in advance of it was modifying and weakening. 

From the radar images above, it appears the supercell mesocyclone was largely wrapped in rain, a probable reason why there are no photos of the tornado from local residents near Wetonka.

The storm environment, while not remarkable, was certainly supportive of supercell tornadoes.  The SPC mesoanlysis graphic below shows that the effective-layer significant tornado parameter was maximized over north-central South Dakota in the vicinity of the supercell:

Also, here is a RAP model sounding (below) from a point just south-southwest of the supercell at 7:00 pm CDT, about a half hour before the tornado:

Low-level wind shear was just enough (storm-relative helicity or SRH near 100 m2/s2) with a looping low-level wind profile, and over 30 knots of deep-layer wind shear was also present.  Total mixed-layer CAPE was large (> 3500 J/kg), and low-level mlCAPE was also significant (0-3 km mlCAPE around 140 J/kg).  This latter ingredient, along with the available low-level wind shear, would likely facilitate low-level stretching and 'spin' to help generate a tornado, _if_ a rotating storm could access the unstable inflow air ahead of it's own outflow.

Again, this was something of an unusual case in that the storm was able to pull its own outflow boundary (initially surging southeast) back in underneath its updraft to access the unstable warm sector inflow air necessary to support the formation of a tornado.

This suggests that meteorologists and storm chasers should remain alert to the possibility of slowing outflow that might allow a supercell to catch up to its leading edge and reconfigure the storm's outflow boundary underneath it. 

Now, let's go back to the Saskatchewan supercell I mentioned at the top.  It moved east-southeast along a wind shift boundary marked by the dashed red-blue line in the satellite image below at 0016 UTC:

This supercell produced three tornadoes as it moved along this boundary, but thankfully all were in open country.  

What's interesting about this supercell is that it appeared to produce tornadoes in an environment with rather high lifting condensation level (LCL) heights (1750-2000 m above ground), as seen below in the 00 UTC 7/5/20 SPC mesoanalysis:

It also appeared to occur in a setting with little low-level wind shear (SRH < 50 m2/s2, not shown). These factors are generally negatives for supercell tornado potential. 

However, the tornadic Canadian cell did occur in an area of enhanced low-level CAPE (see 2nd SPC panel above), which could enhance low-level stretching, as well as enhanced surface vortcity (light blue lines in 2nd panel, a source for 'spin') near the aforementioned boundary.  Low-level lapse rates were also steep along this boundary (near 9.0 deg C, not shown), which could help with low-level stretching.  

These ingredients together suggest that there may have been some non-supercell processes contributing to this Canadian tornado event, in addition to supercell processes (see my prior blog post here).  But that's speculative, because the SPC data is at the edge of the SPC mesoanalysis and RAP model domain, which could affect the accuracy of soundings and parameters.  Still, this Canadian case deserves some further study.
So, even in a slow tornado year, there are definitely interesting cases to examine!

- Jon Davies  7/6/20

Thursday, June 25, 2020

A look at landspout tornado formation in northwest Kansas on June 21, 2020

I've heard quite a few chasers "complaining" :-)  about the lack of tornadoes in May & June 2020 over the central Plains.  That dearth of significant tornadoes is actually a _good_ thing, because they obviously can kill and injure people and turn lives upside down.  But I do sympathize with chasers who have a desire to see tornadic storms out in open country where damage is minimized. 

I haven't done a blog post since mid May (life has been busy on several fronts).  So, with the relative lack of Plains tornadoes in 2020, I thought I'd write something about last Sunday's (6/21/20) landspout tornado in northwest Kansas west of Hill City (see images above).  It's also interesting to look at how a day that appeared somewhat promising to many chasers for supercells and possible tornadoes in west-central Kansas didn't pan out very well.

First, the landspout tornado.  These are not really forecast-able, as they are the result of mesoscale "accidents" where ingredients (a sharp slow-moving boundary with instability and steep low-level lapse rates in the area and a storm intensifying right over that boundary) have to come together just right.  But in the one to three hours leading up to such an event, it may on occasion be possible to see some of those ingredients coming together.

Here's the surface map over Kansas at 2:00 pm CDT:
Note the wind shift boundary (a weak stationary front) over northwest Kansas west and northwest of Hill City.  This boundary was close enough to Goodland's radar that it could be seen as a fine line (see white arrows on lowest elevation base reflectivity images below) as storms built rapidly in the Oakley area ahead of the boundary at early to mid afternoon:

Notice on the radar images at 1944 and 1948 UTC (2:44 pm and 2:48 pm CDT) how the boundary began to bow northwestward in response to outflow from the rapidly developing storms to the southeast.  However, the same boundary on its northern end stayed largely stationary in place near Hoxie (HOX) and west of Hill CIty (HLC), and a cell formed quickly right over this segment of the boundary in the 1944-1922 UTC time frame, indicated on the last two panels of the radar above. 

This is the cell that produced the landspout tornado roughly 1950-2005 UTC (2:50 pm to 3:05 pm CDT) northeast of Hoxie and west of Hill City.   Contributing ingredients were the storm forming right over the boundary in an environment with large CAPE and steep low-level lapse rates (> 9.0 deg C in the lowest 3 km above ground, see RAP model 1-hour forecast sounding for Hill City at 1900 UTC / 2:00 pm CDT below) to enhance stretching of vertical vorticity on the boundary:

It also helped that this segment of the boundary was somewhat removed from the spreading outflow from the storms farther to the south and southwest (see radar graphic earlier).

I've always found landspouts fascinating as an alternative way to generate tornadoes without the horizontal wind shear (storm-relative helicity or SRH) needed for most supercell tornadoes.  Indeed, there was very little low-level wind shear early Sunday afternoon 6/21/20 in northwest Kansas (see the sounding above).  In this case, the vorticity (source of "spin") for the non-supercell tornado (landspout) came directly from the boundary.

Regarding the potential for long-lived supercells and supercell tornadoes on 6/21/20, it turns out the models from that morning were not very encouraging over west-central Kansas.  We'll look at that next.

Below is the NAM 9-hour 500 mb forecast for mid-afternoon showing a short wave disturance approaching from the northwest providing upward motion to fire up storms rapidly in an unstable environment over western Kansas.  Note that winds in mid-levels weren't that strong (only 20-25 kt):
This flow aloft was enough to marginally support organized storms and supercells, but if storms developed rapidly over a large area, a pool of outflow air would probably undercut any supercells and make them short-lived as a squall line formed.

Also below are the 9-hour NAM forecasts of mixed-layer CAPE, 0-1 km SRH, and surface-based lifting condensation level (LCL) heights valid at mid afternoon:

Notice that, although forecast CAPE was large, 0-1 km SRH was very much lacking (as noted earlier), and in particular,  LCL heights were all quite high (> 1500-2000 m above ground).  Such high-based storms (LCL is an estimate of cloud base height) with rain passing through a relatively deep layer of unsaturated air below cloud base would result in evaporational cooling and enhanced cool outflow.  Indeed, although there was a short-lived supercell north of Dodge City around 4:00 pm CDT, cold outflow from the expanding cluster of storms over west-central Kansas at mid-afternoon began to race south, destroying the potential for discrete supercells and short-lived supercell tornadoes (see the photo below as storms were gusting out into a squall line near Dodge City):

Some strong wind gusts were recorded in the Dodge City area and other parts of southwest and south-central Kansas as the storm cluster lined out and moved southward.  

Key ingredients suggesting that this would be more of  a "gust-out" event instead of supercells and tornadoes over west-central Kansas were:
    1)  High LCL heights to encourage evaporative cooling and strong outflow from storm clusters
    2)  Lack of low-level wind shear, reducing potential for low-level rotation in early discrete cells
    3)  Relatively marginal wind fields in mid-levels such that early supercells could not become well-organized and established 

The landspout tornado in northwest Kansas came from non-supercell / non-mesocyclone processes before outflow began to dominate from expanding storm clusters.

All photos in this blog post are from various posts on Twitter made by local people and chasers last Sunday as I watched this event unfold from home in Kansas City.

Shawna and I have been able to chase a couple low-end but interesting cold-core tornado events this spring.  I'll try to post something about those in July as I get time.

- Jon Davies  6/25/20

Sunday, May 17, 2020

May 14, 2020 tornadoes in the Flint Hills: A late start to Kansas' 2020 tornado season.

Systems coming through the central Plains the first half of May have been generally unimpressive, but last Thursday's setting suggested at least a chance of supercells and maybe a brief tornado in east-central Kansas (KS).   This potential was relatively close to Kansas City, requiring only one tank of gas and little exposure to Covid-19.   So Shawna and I  headed toward Emporia, KS at late afternoon for our first storm chase of 2020.

To our surprise, we ended up seeing several tornadoes in open country northwest of Emporia (see images above).   Because some didn't seem to get reported, this blog post will document what we saw,  and I'll also discuss the setting for Kansas' first tornadoes of 2020.

After watching two marginal supercells dissipate near Emporia around 6:00 pm CDT, we nearly turned around and headed back to Kansas City.  But model forecasts from the morning (more about that later) had consistently indicated that wind shear over eastern KS would pick up at early evening, so we re-focused on a new cell west of Emporia, approaching it heading west from Osage City around 6:30 pm CDT:

Notice the inflow cloud streak pointing into the storm at lower left in the photo above.  That often means a storm is beginning to really organize and become a supercell.

We found a hill a couple miles west of the KS turnpike east of Admire, and watched as the storm base to our west lowered and took on some structure just before 8:00 pm CDT:

Around 8:05 pm CDT, some scud developed just under the lowering, and Shawna said, "I think that's about to drop a tornado."  A moment later, several miles to our west-northwest, it did:

This tornado north of Bushong lasted a couple minutes.

Here's a wide view of the Topeka NWS radar lowest-elevation base reflectivity at the time of the tornado, with the meso location and our viewing location indicated (click on images for a larger view):

A few minutes later, another "needle" funnel (possibly a tornado) developed from the same mesocyclone as it moved slowly northeast:

In the image above, notice the 2nd rain-free base farther west, lower to the horizon.  We  soon became aware that this was a separate storm some distance to our northwest, and around 8:19 pm CDT, some dust appeared to rise below this base, along with a small "nub" that formed to the right:

Here's the radar lowest-elevation base reflectivity view at this time:

This storm near Alta Vista, KS was 20+ miles to our northwest, but it began to capture our attention while the previously tornadic storm closer to us began to look less organized and moved away.  Rather than driving closer through rugged Flint Hills and risk losing visibility behind hills, we decided to stay on the same hill and observe the storm's evolution by zooming in with the video camera.

We soon saw rain bands develop along the distant base, and at 8:20 pm a thin funnel extended nearly all the way to the ground.  Here's the funnel/tornado at full zoom backlit by lightning, located somewhere southeast of Alta Vista:

This lasted a minute or two while a lowering farther right became visible (see right edge of the image above). 

After the thin funnel dissipated, I zoomed back out to shoot lightning for a couple minutes, but at 8:27 pm CDT I suddenly became aware of what looked like a "cone" tornado on the horizon where the new lowering had been.  Here are grainy zoomed-in video images of that feature (apparently a tornado) both in regular contrast at dusk, and also with contrast strongly enhanced:

This feature lasted several minutes before a curtain of rain (the "hook"?) wrapped around from the left and hid it from view.  Here's a radar lowest-elevation reflectivity view at about the same time:

This sizable-looking "tornado" was located in southwest Wabaunsee County somewhere east-southeast of Alta Vista and west-southwest of Eskridge, KS.  I haven't seen any images of it from other chasers.  But it sure looked like a tornado, and was co-located with a mesocyclone circulation (marked with black circle on the image above) from radar velocity images (not shown).

After that, we watched lightning for a few minutes, then decided to retreat eastward as two other cells approached from the southwest at dark.  With my aging eyesight, I tend to avoid chasing after dark, so we headed back toward Kansas City.

One of these new cells from the southwest produced yet another tornado, observed by Reed Timmer around 8:45 - 8:55 pm CDT near Council Grove, KS.

Looking back at the May 14th setting, it wasn't particularly impressive for supercell tornadoes.  The morning NAM model 500 mb forecast for that evening showed a weak shortwave trough (red dashed line) approaching Kansas in west to east midlevel flow aloft, but only 20-25 kts of wind over east-central KS, not exactly great support for supercells:

Yet the new HRRR version 4 model (HRRRv4) on the College of DuPage site (with a new panel showing 0-3 km CAPE) showed a decent overlap between low-level CAPE and low-level wind shear (storm-relative helicity or SRH) from the morning forecast valid at early evening:

The same model forecasted a storm that looked like a supercell in the Emporia-Topeka area at the same time (see right-most panel in image above). So, this low-level CAPE/shear overlap suggested a chance of supercell tornadoes via enhanced tilting and stretching of horizontal vorticity within storm updrafts developing in this low-level CAPE/SRH environment, _if_ there would be enough midlevel wind flow and deep-layer shear to support decent supercell storms.

Moving ahead to the evening, here's the NWS-analyzed surface map at 7:00 pm CDT, with a black oval highlighting the area where storms were developing in east-central KS near and just south of a stationary front:

With surface winds from the morning forecasts expected to become more southeasterly at evening over east-central KS, low-level shear would likely increase, along with the deep-layer shear important for supporting supercell storms.  That was indeed the case, as seen in this graphic from the SPC mesoanalysis comparing surface to 6 km wind shear at 5:00 pm CDT (left panel) and at 8:00 pm CDT (right panel):

Notice how the deep-layer shear increased to greater than 30 kt from afternoon to evening, enough to help organize storms into persistent supercells in the Emporia-Topeka area.

The SPC mesoanalysis at 8:00 pm CDT also showed low-level CAPE and low-level wind shear (SRH) overlapping nicely in the Emporia-Topeka area as south-southeast low-level winds picked up near dark:

This confirmed the HRRRv4 forecast from the morning model run (shown earlier).  I've discussed overlapping low-level CAPE and SRH areas in other 2020 case blog posts, and it seems there is increasing evidence that these two parameters _together_ can be useful in assessing environments with supercell tornado potential when deep-layer shear is also supportive.

Thankfully, Thursday's tornadoes hit in open country, and lasted only a few minutes each.  So there was no damage reported.

As storms organized in the increasing wind shear at dark, thunder became constant and lightning prolific as we watched the supercells to our west and north:

Here's an image Shawna took looking south at the edge of the anvil from our cluster of storms, with mammatus visible and mostly clear skies to our south beneath the "cap" at dusk:

You just don't see or experience the wide-openness of the Plains living in Kansas City, so it was invigorating to watch storms in open Flint Hills country on May 14!

- Jon Davies 5/17/20