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. 

The 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

Thursday, April 23, 2020

April 22, 2020 tornadoes in southern Oklahoma - A "high-end" cold-core event?


***** Update 4/24/20:  The strongest-rated of the southern Oklahoma tornadoes so far was the one that hit Madill (2nd image above), killing two people (there was some confusion in online reporting the day after the tornadoes, with many news outlets reporting only one death).  Also, the strongest-rated tornado of the day was the one that struck Onalaska in southeast Texas, killing 3 people.  This massive tornadic supercell moved on across central Louisiana during the evening, killing one other person south of Alexandria as it went on to produce several tornadoes.  There were 6 total tornado deaths on April 22, 2020.  ***** 

Yesterday's tornadic supercells in southern Oklahoma (OK) were photogenic, from the photos above, and also deadly.  The tornado in the 2nd image above (by Lane Chapman) killed one person when it hit the south side of Madill, OK shortly before 5:00 pm CDT.

The top photo above is a large tornado that hit in open country northeast of Ardmore and Springer, in south-central OK, and the 3rd image above is a tornado near Wapanucka, OK, west of Atoka in southeast OK.

Much farther southeast, a large tornado struck Onalaska and Seven Oaks in southeast Texas (TX) around 6:00 pm CDT (bottom image above).  Sadly, this large "wedge" tornado killed 3 people near Onalaska.   It was far removed form the setting in Oklahoma and spawned by a monster supercell that continued through southeast TX and on into central Louisiana during the evening.

My focus in this discussion will be on the tornadoes in southern OK, where forecasters expected tornado development, and did a great job anticipating this event.  But it also was a bit unusual for April in that it involved a "cold-core" low at 500 mb (roughly 18,000 ft MSL) near the OK-Kansas border moving east-southeastward as a "positive" tilt wave disturbance across Oklahoma and Texas (see NAM 500 mb model forecast for mid-afternoon below, with "spreading" flow indicated ahead of the wave):




























My experience has been that 500 mb "cold-core" lows (see this paper) moving east-southeast early in the season aren't that effective at producing significant tornadoes, but this system was an exception.  This was probably due to the large amounts of total CAPE available (2500-3500 J/kg, not shown) because of surface dewpoints in the upper 60's and low 70's F into southern OK at mid-afternoon (see 4:00 pm CDT / 2100 UTC surface map below):
Most early spring cold core events in the Plains involve dewpoints only in the 50's F, usually resulting in comparatively weak tornadoes.  But there are "high-end" cold-core events that have much more moisture, such as 7/19/18 in central Iowa and 10/4/13 in northeast Nebraska.  With access to larger moisture and CAPE, and these can produce stronger tornadoes.  

Notice how narrow the moisture axis was over south-central OK on the surface map above, southeast of the surface low.  This is typical of cold-core tornado events.  Also notice how the 500 mb and surface pattern matched this composite "cold-core" pattern below, _if_ you rotate the composite pattern clockwise 60-90 degrees:

In the April 22nd case, the warm front was moving eastward instead of northeastward, due to orientation of the upper flow, and the tornadoes occurred along the warm front just southeast of the surface low.

Here's a visible satellite image at 4:26 pm CDT showing the "arc" of discrete tornadic supercells in southern OK associated with the cold-core setting, as well as the soon-to-be tornadic supercell in southeast Texas that was far-removed from the cold-core pattern farther north:
































And here is the SPC mesoanalysis depiction of 0-3 km MLCAPE (low-level CAPE) and 0-1 km storm-relative helicity (SRH; low-level wind shear) at 4:00 pm CDT / 2100 UTC, shortly before the tornadoes in southern OK:























In the low-level CAPE field, you can see how narrow the moisture axis was over southern OK.  Often, narrow axes of moisture work against tornadoes because the storms "outrun" the surface-based moisture before relevant processes can work together to generate tornadoes.  But in the case of tornadoes associated with cold-core tornadoes, the low-level CAPE is so large that, if sufficient low-level wind shear or SRH is also present, low-level stretching within storms and rotating updrafts can occur rapidly, so that the narrow moisture axis is not a problem.  The fact that all this is occurring along an advancing warm front probably helps, too, because warm frontal boundaries are often good at generating tornadoes due to increased wind shear and warming/moistening air along them.

Another clue that the April 22nd case had some "cold-core" factors involved is some of the photos I've seen.  This one (by Maddi Frizzell on Twitter) shows the ending phase of the tornado northeast of Springer, OK:

This image shows the full updraft visible, with the tornado at the very back edge of the storm, so typical of many cold-core tornadic storms.  Although the April 22nd supercells in southern OK weren't "mini-supercells" as in so many early spring cold-core events, they weren't especially tall, as this image shows.

A final note: Another tornado from the same tornadic supercell that killed 3 people near Onalaska, TX also killed one person in Louisiana (LA) south of Alexandria during the evening.  Yet another person died in Louisiana after being swept into a drainage ditch filled with rushing water. 

A very interesting case to study, although I'm sad to hear of the deaths in OK, TX, and LA.  We'll see in the coming days what intensity ratings the National Weather Service gives the various tornadoes on April 22nd. 

- Jon Davies  4/22/20  

Tuesday, April 14, 2020

Easter Sunday tornado outbreak in the South: Big shear & CAPE combinations on April 12-13 2020 !


Easter's big outbreak of tornadoes (see the scary photos above in southern Mississippi)  caused 29 deaths due to tornadoes across the southern and southeastern U.S. that I've been able to confirm online (updated as of 4/15/20), and several others from falling trees and flooding.  Sadly, here's a list of tornado deaths so far by state:

southern/southeastern Mississippi (MS), p.m.: 10 dead, 2 tornadoes, both EF4 from same supercell

northwest Georgia (GA), evening: 7 dead, EF2 tornado

southeast Tennessee (TN), late evening:          3 dead, EF3 tornado

northwest South Carolina (SC), early a.m.: 1 dead, EF3 tornado

southern South Carolina (SC) pre-dawn: 8 dead, 2 tornadoes from separate supercells, both EF3

The outbreak started on Easter at mid to late morning in northern Louisiana (LA), including an EF3 tornado that struck Monroe LA.  It progressed through MS during the afternoon, and tornadoes then developed into Alabama and northern Georgia during the evening, and across South Carolina in the early morning hours before dawn.  Many tornadic supercells were embedded within lines, and the number of warnings (> 140 within a 24-hour period) came close to the April 27, 2011 super-outbreak.  But thankfully, there weren't as many tornadoes and deaths as with that historic outbreak, nor were most of the tornadoes as strong.  Yet 29 deaths from tornadoes is unfortunate.

Another characteristic in common with the April 27,2011 outbreak was the degree of low-level wind shear and instability, which were quite large over a big area.  Going back to my work with Bob Johns in the 1990's, here are low-level shear (SRH, or storm-relative helicity) and instability (CAPE) combinations on Easter Sunday (red dots) from RAP model soundings representative of areas where tornado deaths occurred.  I've plotted these on top of a scatterdiagram from our study of 1980's tornadoes that was used to develop the energy-helicity index (EHI), still used today in forecasting:
I've also plotted in yellow above from 2011 values of SRH and CAPE representative of the April 27, 2011 super-outbreak, and the Joplin tornado environment the same year.  Notice the broad range of SRH and CAPE combinations, some almost getting up into the range (middle of the diagram) of what was seen during the April 2011 super-outbreak.

The biggest and strongest tornadoes on Sunday (see tornado pics up at top) were in southern MS deep within the warm sector at mid to late afternoon with two large semi-discrete supercells moving parallel to each other from northeast of McComb MS, to northeast of Laurel MS.  Two tornadoes from the southernmost supercell were rated EF4, one near Salem to near Bassfield MS, and another from southeast of Bassfield to areas past Soso and Heidelberg MS (this tornado was 2 miles wide at one point!).  These two tornadoes killed 10 people.  A 2nd supercell paralleled that supercell to its northwest, producing a very long-track EF3 tornado (path > 90 miles).

Here's the RAP model 1-hour forecast sounding near Laurel MS while tornadoes from these two supercells were in progress to the northwest and northeast:



The SRH and CAPE combinations (350+ m2/s2 and 2000 J/kg) were typical of strong or violent long-track tornadoes in the Plains, so it isn't a surprise that two of the tornadoes were violent in intensity.

At the same time, here's the SPC mesoanalysis depiction of the effective-layer significant tornado parameter (STP) and low-level wind shear (0-1 km SRH):

As on the RAP sounding above, the STP values were large (> 6.0) over southeast MS, and SRH was also large (300-400 m2/s2), all supportive of strong or violent tornadoes with the supercells indicated.

Here's a different RAP sounding, this one in northwest Georgia near Dalton during the evening near the warm front where two separate tornadoes killed 10 people total (Murray County GA, EF2, and just east of Chattanooga over the border in TN, EF3) :


Notice how different this one is compared to the southeast MS sounding!  The SRH was huge,  > 850 m2/s2 (!), but the CAPE only 700-800 J/kg.  This is a reminder of how SRH and CAPE can work together in many different combinations to help generate deadly tornadoes.

Now a look at the bigger picture... below are 500 mb forecasts (midlevels of the atmosphere) from the NAM model run on Easter morning showing features at 4 pm CDT (1st graphic, click on it to see full size) and 4 am CDT the next early morning (2nd graphic).  Inset on both graphics in the upper right hand corner are 0-1 km energy-helicity index (EHI) forecasts from the NAM at the same times (see the scatterdiagram earlier to see how EHI is computed):











Notice the strong shortwave (thick black dashed line) that was moving through the large longwave
trough over the central U.S., providing strong lift as jet stream winds "spread" (thick white arrows) ahead of it moving east and northeastward across the South.  Also notice how much of the southern and southeast U.S. was "overrun" by sizable SRH-CAPE combinations ahead of this shortwave trough, as indicated by the inset EHI forecasts, setting the stage for a potentially deadly tornado outbreak:

Compare this 500 mb pattern to the one associated with the Nashville tornado in early March, which was a more localized tornado setting.  There's quite a difference in breath and orientation of the two different systems!

It is also interesting to note that tornadoes on Sunday and early Monday occurred largely between the surface warm front (see NWS surface maps below at 6-hour intervals), and the southern "branch" of the fanning jet stream at midlevels (thick white arrow) superimposed from the 500 mb graphics above:
 

In a broad sense, this is typical in most outbreaks, with that southern branch of the spreading jet pattern defining the southern extent of tornado activity.

Most tornadoes on Easter weren't very visible due to supercells embedded in lines, or the tornadoes occurring at night.  But here's a photo of a tornado that happened around noon in west-central MS northwest of Yazoo City... this is how the Monroe, LA tornado might have appeared had it been somewhat visible within the rain:

A few final comments:  SPC forecasters did a great job forecasting this outbreak and getting information out to the media several days in advance!   That probably saved some lives.

Also, I have to shake my head a bit at the number of storm chasers traveling long distances to see tornadoes in the middle of a historic coronavirus outbreak.  I know travel isolated in a car is probably relatively safe, but contact at convenience stores, gas stations, and hotels seems risky and a little questionable.

Chaser behavior was also an issue again on Sunday, with video showing two chasers driving the wrong way on one side of an interstate in MS to get around traffic slowed/blocked by debris!  And in another video in east-central MS, two chasers appeared to be following a large and difficult-to-see rain-wrapped tornado, driving into what appeared to be the back edge of the tornado circulation where tree tops were bending almost horizontal at road side!  That seems too dangerous, and can leave a bad and misleading impression on viewers.

Anyway, I'm grateful that the death toll on Easter and early Monday wasn't greater, and for the job that forecasters and media did this past weekend.  Stay safe and healthy, everyone!

- Jon Davies  4/14/20   (tornado death counts were updated 4/15/20)