Tuesday, December 14, 2021

Tornadoes, Warehouses, and Factories - Can these businesses do better to keep workers safe?

(This is an essay/opinion piece my wife Shawna wrote and wanted to share here on my blog after last Friday's catastrophic and stunning tornado event in Kentucky, Illinois, Arkansas, Tennessee and Missouri. - Jon Davies)     

(You can help Western Kentucky tornado victims by donating here.) 

I was heart-broken watching the devastating tornado outbreak unfold last Friday night (December 10-11) in the central Mississippi River Valley area .  It was the most catastrophic U.S. tornado episode since the 2011 Joplin tornado and the 2011 Dixie Super Outbreak, and the deadliest tornado outbreak ever recorded in the U.S. during the month of December!

Kentucky bore the brunt of it, with at least 74 deaths in the state confirmed so far (the death toll may rise as there are still people unaccounted for), and additional tornado deaths in Illinois, Arkansas, Tennessee, and Missouri.  This outbreak included a tornado that would have had the longest track ever recorded (> 220 miles across parts of four states) except for a short break at one point in its path.  This tornado killed at least 57 people, and while officially rated EF4 intensity at this time, there were areas of damage that could conceivably suggest EF5 wind speeds.  Regardless, last Friday was a truly historic yet dark and sad day.

From a safety viewpoint in the future, is there anything we can learn from this event?

I want to focus on a couple locations:  First, the Amazon facility that was struck by a narrow but strong tornado (EF3) near Edwardsville, Illinois, killing six people (see photos below):  

I'll also discuss the candle factory in Mayfield, Kentucky where eight people died when it was completely destroyed by the large long-track violent tornado (at least EF4 intensity) that tore through western Kentucky (see photos below):

Both facilities had ample warning via NWS warnings, around 20 minutes lead time in each case.  The Amazon facility in Illinois, which is now being investigated by the federal government via OSHA regarding structural issues (see here), had no below-ground area or solid concrete floor-level safe rooms for the 40+ employees working on Friday evening.  Similarly, the Mayfield Consumer Products candle factory apparently had no basement or above-ground concrete shelters when the half-mile wide violent tornado struck, although employees were told to go to hallways and bathrooms.  Given the typical low-end construction quality of many such warehouse structures (most large industrial buildings are not designed to withstand extreme winds, with roofs often blowing off and adjacent walls collapsing), the lack of true shelters and safe rooms in these cases probably contributed greatly to the deaths.  That said, it was very fortunate that, of 140 total employees in these two cases, only 14 lost their lives. 

For reference, here is a regional radar image (Edwardsville and Mayfield are marked) at about the time the Amazon facility was struck, and roughly an hour before the factory in Mayfield was hit:

That two large industrial buildings populated with many employees were hit directly by strong and violent tornadoes on December 10 highlights sheltering and other problems in such situations.  It seems clear that large warehouse and factory buildings should be built or modified to have some sort of below-ground shelter or ground-level rebar-reinforced concrete safe rooms sizable enough for workers to evacuate to when tornadoes or strong winds approach.  Mega-profit companies like Amazon can surely afford to provide such shelters for their employees.

Other issues have surfaced in news stories about these two facilities (for example, see here).  One revolves around policies that don't allow employees at work to have cell phones (a good source for weather warnings and simple-to-view radar apps).   Such rules could be relaxed on days when severe weather is likely.  It would also seem very prudent for companies with many employees to have a designated manager to keep abreast of days when severe weather is possible, and to monitor warnings and radar for storms that might directly affect their location.  

==>To illustrate, on December 10, Mayfield came under a late afternoon tornado warning that sent candle factory workers to halls and bathrooms for shelter.  That small storm (not shown) passed well south of Mayfield and produced no tornadoes.  However, three hours later when the mega-supercell with a confirmed large long-track tornado on the ground directly approached  Mayfield, there seemed to be confusion at the factory.  Some workers in interviews claimed that at least one supervisor threatened to fire employees who wanted to head home for shelter with 20 minutes of lead time before the tornado hit (see here).  A designated manager with some basic weather training and online or cell phone access to warnings and radar could probably distinguish between the two vastly different situations, and help make urgent and intelligent decisions to keep workers safe.

The last couple years have not been particularly active regarding tornado activity in the U.S.  But this violent and deadly December event is a strong reminder that severe weather and tornadoes can happen any time of year, and that we all need to stay aware.  Have a safe place to go in severe weather, stay alert, and keep your cell phone on and handy (particularly at night) when thunderstorms are in the forecast and severe weather may be possible.  

On a positive note, I also want to say how impressed I am with the many people in media interviews who were well-informed and took the right action to protect themselves and their families as tornadoes approached last Friday.  Sheltering in interior bathrooms or closets with mattresses when there was no basement was a common action among survivors interviewed.  

Unfortunately, when a violent (EF4 or EF5) tornado strikes, particularly when there is no below-ground shelter or safe room, even the smartest actions may not guarantee safety.  I'm paraphrasing here, but what I've heard legendary severe weather researcher Dr. Chuck Doswell say is very true:  "Tornadoes are going to occur, people just happen to be in the way." 

Here is a 30-minute video that a crew of storm chasers with production experience and I produced several years ago about staying safe in tornadic and severe weather. I check this video periodically, and it still stands with accurate information (including workplace safety):  


A little time spent watching this video and planning ahead may help save you and your loved ones' lives.  Let's all stay responsible, alert, and safe as possible!

Shawna Davies  12/14/21  (with the assistance of Jon Davies) - updated 12/16/21 and 12/17/21

(Update 12/16/21: After writing this article, our unusual and record-breaking December severe weather hasn't let up, with the first December tornado ever reported in Minnesota occurring last night as a strong storm system roared through the central U.S. on Wednesday 12/15/21)

(Update 12/17/21:  A class-action lawsuit has been filed on behalf of candle factory workers in Mayfield alleging they were told they would be fired - see discussion earlier - if they left work ahead of last Friday evening's deadly long-track tornado in western Kentucky - see here.) 

(Update 12/17/21:  As of this morning, the tornado death toll in five states from the December 10-11, 2021 tornadoes is at least 88 dead.  Western Kentucky is in particularly bad shape, and it will take some towns and citizens years to recover.  You can help by donating to the Team Western Kentucky Tornado Relief Fund established directly to help the tornado victims there.)

Friday, October 29, 2021

Cold-core tornado chase in northwest Missouri on October 24, 2021

The past few months have been been a difficult period for my wife Shawna and me.  Her mother died, a friend died, our loving cat Sadie died, and there have been both health and family issues going on.  That's why I haven't posted in a while, and with  some other changes for us in progress over the coming months (such as readying our house for sale and deciding where to move), blog posts here will likely  continue quite few and far between.

However, we both were able to do a rare October storm chase last Sunday (October 24, 2021) close to home north of Kansas City (see image above), our first storm chase in months due to all that's been going on with us personally.  This blog post presents some photos and description documenting our October 24 chase.

The satellite and surface map image below at early to mid afternoon (satellite at 2:51 pm CDT or 1951 UTC; surface data at 2:00 pm CDT or 1900 UTC) shows that the setting over northwest Missouri (MO) was a "cold-core" situation (see here) with a surface low just west of St Joseph, and a mid-level low over southwest Iowa, with a warm front/Pacific cool front intersection near St Joseph:

With a strong shortwave disturbance moving across the area (not shown, connected with the 700mb/500mb low) and associated cold air aloft located above surface temperatures in the 70's F and dew points in the 60'sF making for significant instability, we figured tornadoes were likely along the warm front just east of the surface low.  Backed easterly surface winds along and north of the warm front also made for sizable low-level wind shear in the area near St Joseph and eastward, upping the potential for tornadoes.

We headed for St Joseph around 1:15 pm CDT, but were delayed helping a family with car and other issues along I-29 near St Joseph.  As a result, with storms erupting west of St Joseph, we missed the first tornado just west of St Joseph near Troy, Kansas at roughly 2:12 pm CDT.  But we could see a mesocyclone and lowering associated with the same storm looking southwest from north of St Joseph around 2:25 pm CDT:

We moved east from Savannah, MO to follow this mesocyclone as it moved east-northeastward along the warm frontal boundary, and with cloud bases lowering in the warm frontal mixing zone, we watched what appeared to be a brief tornado that lasted 30-45 seconds, looking to our south-southwest from north of Avenue City (northeast of St Joseph):

On the satellite photo shown earlier, the black arrow indicates the cell cluster that produced this probable brief tornado east of the surface low.

We intercepted this same mesocyclone near Rochester, MO, watching leaves swirling in the air and Shawna fascinated by wispy cloud filaments accelerating rapidly upward into the updraft as it passed just in front of us.  Here's the mesocyclone viewed moving away from us on Hwy 169, with both the flanking line and short updraft tower visible looking to our northeast:

We had to drop south to Hwy 6 to head east and catch up with the mesocyclone along I-35, which took some time.  Meanwhile, our storm had combined with another nearby cell to form a supercell that produced a weak tornado southwest of Pattonsburg MO around 3:35 pm CDT that we couldn't see.

When we headed north on I-35 toward Pattonsburg, we could see the storm's mesocyclone again, with rapidly-moving cloud tags and brief funnels visible looking toward the northeast located in an area just east of Pattonsburg around 3:50 pm CDT:

After that, with the storms moving away from us at close to 40 mph, and hilly, tree-covered terrain to our east and northeast, we decided to head back to Kansas City.  

Out of reach to our southeast, we could see what had coalesced into a separate supercell between Cameron and Chillicothe, MO, and this storm produced several tornadoes, including the large tornado that later hit Purdin, MO shortly before 5:00 pm CDT doing EF2 damage (see image of that tornado below by Stephen Jones):

The 3-panel radar image below shows the evolution of storms between 2:12 pm CDT (1912 UTC) and 3:55 pm CDT (2055 UTC), with the small white arrow indicating the northern cell(s) we were following and documenting near the warm front:

The more southern cell west of Chillicothe (CDJ) on the last panel above is the one that produced the most tornadoes, including the EF2 Purdin tornado.  

Most cold-core tornado settings associated with mid-level lows play out fairly quickly with brief, weak tornadoes.  With the EF2 Purdin tornado, this case was an exception that featured a broad (rather than narrow) warm sector, and instability (mlCAPE > 1500 J/kg, not shown) and low-level wind shear (0-1 storm-relative helicity or SRH > 200 m2/s2, not shown) increasing during the afternoon near and just south of the warm front into north-central Missouri.  

This is highlighted in the panels below from the SPC mesoanalysis showing the Tornadic Energy-Helicity Index (TEHI, combining CAPE, SRH, and deep-layer shear with other parameters) at 1:00 pm CDT (1800 UTC) and 4:00 pm CDT (2100 UTC):

Notice the TEHI "max" of 3.0+ over north-central Missouri at 2100 UTC (2nd panel above) before the Purdin tornado.  

Later that evening, the environment over east-central and southeast MO away from the cold-core mid-level low over Iowa became quite favorable for strong tornadoes (hinted at in the last TEHI panel above) when storms there became supercells that produced EF3 tornadoes after dark at St Mary, MO and Fredericktown MO.

It may be a while before I post again, so I just want to express my appreciation to all who follow and give me feedback about my work and severe weather posts.  Thanks a bunch, everyone! 

- Jon Davies  10/29/21

Wednesday, June 23, 2021

EF3 tornado in Chicago suburbs on June 20, 2021: NOT a QLCS tornado

Sunday night's tornado after dark (photo above) that struck the western Chicago suburbs of Naperville, Woodridge, and Darien in Illinois (IL) shortly after 11:00 pm CDT injured 11 people.  Fortunately, no one was killed, with good watches and warnings in advance.  The tornado was EF3 in intensity, and left a damage path of around 16 miles in length.

I noticed on social media early on that several people called this a "QLCS tornado" (QLCS stands for 'quasi-linear-convective-system'), a type of non-supercell tornado that can occur within bowing squall lines.  It is true that the tornadic storm was embedded within a line of storms.  However, to be clear and accurate, this was definitely _not_ a QLCS tornado -- it was a supercell tornado with a well-detected mesocyclone (rotating updraft) on radar, seen on the inset in the tornado photo above showing storm-relative velocity.  Because it was a supercell tornado with a distinct mesocyclone on radar before the tornado, a radar-based tornado warning could be issued for parts of DuPage County (the county where Naperville is located) around 20 minutes before the tornado developed.  

QLCS tornadoes tend to be brief, relatively weak, and more difficult to see on radar, making them more difficult to warn.  A QLCS tornado that struck the southeast corner of El Reno, Oklahoma after dark on the evening of May 25, 2019 (not shown) lasted only 4 minutes and traveled only around 2 miles.  It was briefly detectable on radar due to the close proximity of El Reno to the NWS radar near Norman, but could only be warned with 2 minutes lead time, not even enough time for sirens to sound in El Reno before the tornado struck.  It killed 2 people and injured 29, even though deaths and injuries are rare with QLCS tornadoes.

Back to June 20, 2021, the 3-panel composite reflectivity radar image below shows the broad evolution of the line of storms and the embedded supercell and mesocyclone (small white circle) from 10:45 pm to 11:15 pm CDT:

It is important to remember that it is not unusual for supercells to be embedded within a line of storms, but that does not mean that tornadoes associated with such storms are QLCS tornadoes.  A primary difference is the mid-level mesocyclone visible on radar, often in advance of a tornado.  

A brief look at the local setting and environment over northern IL on Sunday evening June 20th showed a quasi-stationary outflow boundary associated with the east-northeast to west-southwest line of storms across the Naperville-Chicago area:

This boundary may have helped to generate the tornado as the embedded parent supercell moved east along it, taking advantage of increased low-level wind shear and convergence on and near the boundary, as well as the temperature/moisture contrast across it.

SPC mesoanalysis graphics near the time of the tornado also showed the environment near the boundary over northeast IL to be supportive of supercell tornadoes:  

In particular, a version of the energy-helicity index (a parameter combining low-level wind shear and CAPE/instability) augmented by enhancements/limitations related  to deep-layer wind shear, 0-3 km CAPE, and estimated cloud-base heights (see 1st panel above) showed a supportive environment for supercell tornadoes in the Napervlle-Chicago area.  Also, a new version of the significant tornado parameter (2nd panel above) using low-level wind shear (storm-relative helicity or SRH) in the lowest 500 m above ground indicated similar support for supercell tornadoes.

A short-term HRRR model forecast of the basic energy-helicity index from early evening also showed maximized combinations of SRH and CAPE near and just south of the local boundary across the Naperville-Chicago area:

So, ingredients supportive of significant supercell tornadoes were definitely in place over northeast IL late on the evening of June 20, 2021.

Again, the fact that the tornado was associated with a supercell and detectable mesocyclone on radar made warning in advance more possible, an important difference in contrast to QLCS tornadoes.  QLCS tornadoes are shallower circulations that are more transient and harder to detect on radar, and therefore more difficult to warn.

It is important to understand this distinction.

- Jon Davies  6/23/21


Thursday, May 13, 2021

The May 8, 2021 HP supercell in central Kansas & comparison to the May 1, 2018 Tescott tornado setting

Last Saturday's HP supercell at early evening west of Salina (see Shawna's dynamic photo above) was our first chase in nearly a year, as Kansas (KS) has been rather quiet concerning severe weather recently.  The setting did have some similarities to the May 1, 2018 tornadic supercell set up northwest of Salina three years ago in that storms formed in west-central KS at the north edge of a "cap" (warm layer of air aloft near 10000 ft MSL), and in each case at least one storm moving east-northeast became an evening supercell near Ellsworth, KS.  

But Saturday's supercell was non-tornadic and high-precipitation ("HP") in character, while the May 1, 2018 supercell produced an EF3 tornado on the ground for around 14 miles.  Sometimes it is informative to compare cases in the same geographic area that appear to have similar settings at first glance, but end up with very different results, which I'll do briefly here, as well as summarize our storm chase.  

The NWS-analyzed surface map at 7:00 pm CDT (0000 UTC 5/9/21) in the first panel below showed a double-barrel surface low near a triple point over west-central KS, northeast of which storms had formed at mid to late afternoon... a classic severe weather setting  sometimes associated with tornadoes:

The 2nd panel above also shows the NAM model 12-hr forecast of the 0-1 km energy helicity index (EHI, combining instability and low-level wind shear) valid at the same time as the surface map in the first panel.  EHI values approaching 2.0 into central KS suggested that tornadoes might be a possibility, along with the morning SPC outlooks (not shown) that showed a 5% probability of tornadoes in central and north-central KS... not high but not insignificant, either.

Shawna and I followed a couple cells that had formed west and southwest of La Crosse, and had became a loose cluster of storms as they moved north of Great Bend.  Out of this "cluster", a high-based supercell formed after 7:00 pm CDT west of Ellsworth, KS:

This storm moved across Ellsworth producing wind gusts, heavy rain, and some nickel-size hail, and we were able to get in front of it again on Hwy 140 heading east toward Brookville and Salina.  Here's Shawna's shot of this high-based cell near Carneiro looking west-southwest as I snapped some pics in the foreground:

This HP storm really took on some impressive structure west of Brookville, with a dark roiling high-based shelf (the storm was outflow-dominant at this point, with winds already shifting to the west at our location in front of the shelf).  Staccato cloud-to-cloud and cloud-to-ground lightning bolts were visible along the outflow shelf, and I took the picture below of this "angry" supercell looking north (similar to Shawna's photo at the top of this post, but without the cool lightning) from west of Brookville as the storm approached us:

The high outflow base and outflow-ish nature of the storm eliminated thoughts of tornadoes at this point, but it was a beautiful storm in a pretty setting (the Smoky Hills).  We reached Salina with darkness upon us, and after watching lightning for a while, we headed back to Kansas City.

One problem working against tornado potential on this day appeared to be high lifting condensation level (LCL) heights (up near 2000 m above ground) that allowed evaporative cooling as rain fell through the deep sub-cloud layer, generating cool outflow air that undercut the storm's rain-wrapped mesocyclone.  Another issue was somewhat weak winds aloft in mid-levels that generated just enough deep-layer shear (25-30 kt) to support a supercell, but one that was HP in character, adding to the outflow-ish nature of the storm.  The morning HRRR model forecast these limitations rather well:

But the morning HRRR model also did a good job forecasting the supercell at early evening, with a mid-level rotation track indicated west of Salina:

As mentioned earlier, the track and timing of this supercell were similar to the May 1 Tescott tornadic storm back in 2018.  However, there were some key differences in the environments, as indicated by the red highlight boxes in the table of parameter comparisons from SPC mesoanalysis graphics (not shown) between the two events listed below:

Apart from smaller low-level wind shear (0-1 km storm-relative helicity or SRH) on May 8, the main differences were much higher LCL heights compared to May 1, 2018, and smaller deep-layer shear (0-6 km bulk wind difference or 6BWD), confirming the same issues raised by the HRRR model forecast discussed earlier.

Here's a RAP model analysis sounding (similar to what is used in generating SPC mesoanalysis graphics) at Ellsworth, KS valid at 7:00 pm CDT on May 8, 2021, highlighting in yellow and red these same problematic issues in the environment regarding tornado potential at the time the supercell was ramping up west of Ellsworth:

This sounding confirms that LCL heights and deep-layer shear were two negative factors in the environment that probably worked against tornadoes on May 8.

As the evening went on, low-level shear increased (SRH, not shown) and LCL heights lowered quite a lot (not shown) as surface temperatures cooled.  But, low-level stability (convective inhibition or CIN) also increased (not shown), suggesting that storms were not strongly surface-based near and after dark, so that much of the instability was coming from well above the ground, working against the formation of tornadoes.

Before closing, it is interesting to look at a later RAP model analysis sounding for Salina at 10:00 pm CDT (0300 UTC 5/9/21).  This sounding was estimating the environment in front of a 2nd supercell that formed behind the one we chased earlier, and was tornado-warned based on radar-detected mid-level rotation around 10:11 pm CDT:

Notice how large the CIN (blue area on the thermodynamic diagram) had become after dark, with more than 225 J/kg of inhibition.  This was the result of outflow air behind the earlier supercell, now bowing out well east of Salina (SLN) on the radar image below, shortly before 10:00 pm CDT:

I've indicated on the radar image the new tornado-warned cell west of Salina and the "fine line" showing the trailing outflow boundary from the earlier supercell (white arrows).  Also note the temperature and dew point at Salina (63 over 59 deg F) within this outflow just before 10:00 pm CDT.  All this information suggests that this new supercell was ingesting cool outflow air that would undercut and eliminate any true tornado potential with this new supercell.

I was a little surprised that NWS issued the tornado warning, based on radar and environmental data above.  Although ping-pong size hail fell with this 2nd supercell, no tornado occurred and the warning was soon dropped.  This re-affirms how outflow near the ground can really interfere with tornado production.  

The May 8, 2021 central KS case is an interesting case to study regarding how factors like LCL height and relatively weak deep-layer shear can limit supercell tornado potential.  These are two easy-to-overlook ingredients that require careful consideration in tornado forecasting.

- Jon Davies  5/13/21

Thursday, April 1, 2021

March 25, 2021 deadly tornado outbreak in Alabama and Georgia

One week ago today, a deadly tornado outbreak in Alabama (AL) and Georgia (GA) caused 6 deaths and a number of injuries, making it the deadliest tornado episode of 2021 so far.

Similar to March 17, 2021 (see toward the end of my post here), the outbreak was well-forecast and well-warned, and as a result many lives were likely saved, even considering the deaths that did occur.  As a result, some people on Twitter have mentioned the famous April 27, 2011 tornado "super outbreak" ten years ago, when over 200 tornadoes occurred on one day in the Dixie states and more than 300 people were killed.  

The graphic at the top of this page offers a quick comparison of the tornado environments between that day back in 2011 and the recent outbreaks in AL on March 25 and March 17, 2021 using a scatter-diagram showing storm-relative helicity (SRH, a measure of low-level wind shear) and mixed-layer CAPE (a measure of instability).  Sizable low-level wind shear can generate low-level storm rotation leading to supercell tornadoes, and CAPE (convective available potential energy) helps support and sustain those rotating storm updrafts.

The big pink dot in the middle of the diagram up at the top of this page is representative of the environment on April 27, 2011, showing an unusually large combination of SRH and CAPE leading to the massively deadly tornado outbreak that day nearly ten years ago.   It's pretty clear that 4/27/11 had much larger support for strong and violent tornadoes than either 3/25/21 or 3/17/21.

Getting back to 2021, I've plotted on the same diagram four of the strongest tornadoes from March 25 last week (small yellow-filled dots) and four of the strongest tornadoes from March 17 a couple weeks ago (small red-filled dots), and enclosed those areas on the diagram in black ovals.   Comparing 3/25/21 with 3/17/21, it appears that March 25 had somewhat larger low-level wind shear, one difference between the 3/17/21 setting that supported shorter-tracked EF2 tornadoes, and the 3/25/21 setting where tornadoes were stronger (EF3 and EF4) and longer-tracked.

Another difference between March 25 and March 17 (not shown here) was deep-layer wind shear, where 0-6 km bulk wind difference (BWD) on 3/25/21 in central AL averaged near 70 knots or more, whereas on 3/17/21 the 0-6 km BWD averaged only near 50 knots,  In other words, the general wind fields and resulting wind shear at multiple levels appeared stronger on March 25, 2021, helping to generate stronger and longer-tracked tornadoes like this one near Greensboro AL and west of Centreville AL at mid-afternoon:

To very briefly review the setting on March 25, here's the tracks of EF2+ tornadoes last Thursday plotted on top of a surface map at 2100 UTC (4:00 pm CDT) on 3/25/21:

And here's an SPC mesoanalysis graphic of composite tornado parameters at 1800 UTC (1:00 pm CDT):

The warm sector over central Alabama south of the warm front was where wind shear and instability parameters were maximized and the long-track afternoon tornadoes occurred.

Here's a composite radar image at 1930 UTC (2:30 pm CDT) just before the town of Ohatchee in east-central/northeast AL was hit by a tornado that killed 5 people.  

And here's a very interesting photo as that killer tornado approached Ohatchee around 1940 UTC (2:40 pm CDT) -- it appears to show two large vortices, reminiscent of the multi-vortex Elkhart Indiana tornado photographed on Palm Sunday in April of 1965:

This tornado was from the same supercell that produced a 50-mile track EF3 tornado that struck earlier south of Birmingham with damage and injuries.  

And, on the radar image earlier, a new supercell is indicated entering west-central AL that would later generate a strong 80-mile track EF3 tornado starting west of Centreville, AL that at times was more than a mile wide (see the tornado photo earlier by Max Olson):

Here's a RAP model mid-afternoon analysis sounding at Centreville, AL showing the environment supporting this longest-track tornado of the day that could have possibly been rated EF4 had it directly hit a town:

With around 300 m2/s2 of 0-1 km SRH, 1400 J/kg of mixed-layer CAPE, over 80 kt of deep-layer shear, and sizable low-level CAPE, this was a fairly potent environment for supporting supercell tornadoes.

As the outbreak extended into evening on March 25, a new storm became tornadic in central AL around 9:00 pm CDT (0200 UTC 3/26/21, not shown).  On the graphic below, the wind and instability environment for this storm at 0300 UTC 3/26/21 on SPC mesoanalysis composite tornado parameters was still quite good for supporting strong or violent supercell tornadoes:

And that's in fact what happened, with a low-end EF4 tornado striking Newnan GA over the border from AL around 0400 UTC (11:00 pm EDT), killing one person.  After that, storms tended to outrun the warm sector instability axis as they moved eastward, and tornadoes ceased occurring.

Finally, to back up in scale and time a bit, here's the NAM model 9-hr forecast of 500 mb features and energy-helicity index (EHI, combining SRH and CAPE) at mid-afternoon on March 25:

As on March 17, it was again a classic setting with plenty of SRH and CAPE in an area ahead of a strong trough at 500 mb (this time, a strong mid-level shortwave) where lift and forcing resulted from a "spreading" jet branches pattern ahead of this mid-level disturbance.

Last Saturday March 29, 2021 saw yet another deadly tornado day within an active sequence of days, with tornadoes in Texas, Louisiana, Arkansas, Tennessee, and Mississippi; one person was killed in northeast Texas (see photo of EF2 tornado near Carthage, Texas below):

As I write this, it looks like conditions supportive of tornadoes will take a break and hold off until the middle of next week (around April 7th or 8th?) in the central and southern U.S.

-  Jon Davies 4/1/21  (no April Fooling)

Wednesday, March 24, 2021

Active March severe weather pattern continues across the central and southern U.S.!

After last week's St. Patrick's Day outbreak in the South on March 17 (more about that farther down), yesterday saw yet another cold-core type setting with tornadoes in west-central Illinois (photo above) and southeast Iowa, on top of last week's cold-core activity (see my last blog post here).

Tomorrow (March 25, 2021) will likely see another tornado outbreak in the southern Mississippi River Valley into parts of Mississippi, Alabama, Tennessee, and Kentucky.  This will probably be similar in scope to the March 17 outbreak with several large long-track tornadoes, so people living in those areas should monitor weather information carefully!

Yesterday's cold-core event was weak (2 EF0 tornadoes), but fairly classic pattern-wise, occurring in the vicinity of a Pacific cold front intersecting a warm front in the northeast Missouri (MO), west-central Illinois (IL), and southeast Iowa (IA) area east of a closed mid-level low.  That was unlike last week's cold-core activity in Kansas, Nebraska, and Missouri (see blog post here) when the tornadoes occurred along occluded sections of a frontal boundary near the mid-level low, instead of the more common Pacific front/warm front intersection area. 

Here's the 0000 UTC 3/24/21 surface map (7:00 pm CDT 3/23/21) shortly before the tornadoes yesterday:

The tornadoes occurred near Rockport, IL (photo at top of this page), and Ft. Madison, IA, marked on the surface map above, close to the Pacific cold fornt / warm front intersection as mentioned earlier, roughly matching the Fig. 16 composite in Davies (2006).

The visible satellite photo at the same time (tornadic cell locations at arrows) showed the cloud swirl associated with the mid-level closed low at 700 mb (roughly 10,000 ft MSL) and 500 mb (roughly 18,000 ft MSL):

The 12-hr 700 mb forecast from the NAM model morning run on 3/23/21 showed the mid-level low over southwest IA, and a broad area of cold air aloft well below freezing (the blue colors) spreading over the warm sector into western IL:  

With surface dew points in the mid-50's (deg F) near the warm front, that cold air aloft generated plenty of instability to support storms and some brief tornadoes.

The 9-hr HRRR model forecasts from mid-morning suggested the location of the Pacific front/warm front intersection fairly well from surface temperatures and winds (1st panel below).  Plentiful low-level CAPE (0-3 km, 2nd panel) was also forecast to surround the boundary intersection area and extend northward into southeast IA:

The low-level CAPE north of the warm front into IA may have been a contributor to the brief tornado with minor damage near Ft. Madison, IA that occurred some distance north of the warm front within a linear feature on radar (not shown), unlike the tornadic supercell near Rockport, IL.

Back on March 10 (2021), there was a cold-core setting that affected south-central Minnesota that was _not_ tornadic, although a photogenic supercell (not shown) occurred near the warm front in that case.  Several people have asked me why that setting did not produce tornadoes, and while I don't have definitive answers, comparing that evening's surface map with yesterday's surface map suggests some clues.

Notice on the 3/10/21 surface map at 2300 UTC (5:00 pm CST, shown below; location of tornado-warned supercell marked by circled "S") that the warm front was "bulging" much farther northward relative to the surface low:

As a result, comparing this map with the surface map earlier from yesterday, there was not much of a "spiraling" wind flow pattern into the surface low.  Instead, winds went from a south or south-southeast direction across southeast Minnesota and most of Wisconsin abruptly to a north or north-northeast direction in the area northeast of the surface low.  This was different than yesterday's surface setting (see the surface map shown earlier) where winds from the warm sector over Illinois and then flowing into the Iowa surface low (ignoring a few "outflow" locations) backed more gradually across the warm front and farther northwestward, going from southerly to southeasterly to easterly.  This more "spiraled" backing wind flow pattern into the surface low may have helped set up a better low-level shear environment for low-level rotation in yesterday's storms near and just north of the warm front.

I'll make one other observation comparing the two surface maps.  In the non-tornadic March 10 case, the temperature contrast across the Minnesota warm front was > 10 deg F in a short distance (say 10-15 miles), which would likely have a detrimental effect on low-levels for tornado potential with the supercell that was crossing the warm front that day.  Yesterday, the temperature contrast across the warm front and for miles north of it was rather gradual, only around 5 deg F between west-central IL and southeast IA, and would not have as abrupt an impact on storms crossing the warm frontal area.

Before I close, I wanted to talk briefly about last week's St. Patrick's Day tornado outbreak in the South.  It was well-forecast days in advance by SPC, announced aggressively in media, and there were  no deaths and only a few injuries in spite of 30+ tornadoes occurring!  The excellent forecasts and media coverage appeared to really contribute to the low injury toll and the lack of deaths.  I truly hope the same will be true with tomorrow's expected outbreak in some of the same area.

Briefly, regarding the 3/17/21 outbreak, here's the morning NAM model forecast at 500 mb valid at mid-afternoon, along with an inset of the NAM 0-1 km energy-helicity index (CAPE and low-level shear combined) for the same time:

The setting was fairly classic, with a strong trough moving eastward across the southern U.S., and "spreading" jet branches providing forcing for generating storms out ahead of the trough where CAPE/shear combinations were strong and very supportive of tornadoes across the broad warm sector in the Dixie states.   

Notice too, that there was yet another "cold-core" setting associated with the same system, this time over southwest MO, where some weak tornadoes occurred.

And getting back to the Dixie states, below are a couple images of a supercell and long-track tornado (note the horizontal vortex on the second image) that moved across southwest Alabama (see tornado track on inset in image above) at mid-afternoon.

It is a little unusual to be able to see such visible supercell structure in the South.

I hope people in the Dixie states and the southern Mississippi River Valley/Ohio River Valley areas pay careful attention to weather information tomorrow, March 25, 2021.  Stay safe!

- Jon Davies  3/24/21