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)

Wednesday, April 1, 2020

The Jonesboro, Arkansas tornado on March 28, 2020: A subtle and difficult forecast setting.

As Covid-19 keeps expanding throughout the U.S. (we all need to keep following social distancing guidelines!), thoughts and well wishes go to people like fellow storm chaser Dr. Bill Hark in Virginia who has been sick with the virus.  We hope you recover soon, Bill.

With this going on and more than 4500 Covid-19 deaths now in the U.S., tornadoes may seem like "small potatoes".  But spring is upon us, along with the threat of tornadoes, no fooling intended on this April Fools Day.

Saturday's EF3 tornado that struck Jonesboro, Arkansas (AR) around 5:00 pm CDT (2200 UTC) is a reminder of that (see photos above).  With a tornado watch out a couple hours in advance and good warnings, no one died with 22 injuries reported, which is good news.

Most meteorologists on March 28 (including me) were focused on the potential for tornadoes over Iowa and Illinois, but the local environment really ramped up quickly in northeast AR during the afternoon, and was more subtle than one might expect prior to a large tornado.  So, I spent some time taking a closer look at this event.

Morning model forecasts did not suggest much low-level shear over AR compared to farther north near a warm front (see 0-1 storm-relative helicity / SRH forecast for mid-afternoon below):

But as the southern end of a large upper system approached (not shown), a low-level jet at around 5000 ft MSL (not shown) increased from 30 kt to 45 kt over northeast AR during the afternoon, helping to generate enough low-level shear to support tornadoes, which we'll discuss in a bit.  What might also be easy to miss is that a subtle boundary also appeared to play a role.

Below is a 3-panel composite radar image from late evening on March 27 through midday on March 28 showing this northeast-southwest boundary (see white arrows) drifting eastward across Arkansas during the 18 hours before the tornado:

Although it's not clear what originated this boundary (NWS-analyzed surface maps showed this as a cold front, which I don't think it was), it appeared to stop near Jonesboro and even back up a bit to the west during the afternoon.  Here's the surface map I analyzed at 2100 UTC (4:00 pm CDT) about 45 minutes before the tornado struck Jonesboro (notice the backed southeasterly surface wind at Jonesboro). This boundary is shown as a thick red-blue dashed line over AR:
And here's a composite radar image at 2125 UTC (4:25 pm CDT) with the boundary from the 4:00 pm CDT surface map above superimposed as a white dashed line:

Notice that the Jonesboro tornadic supercell (white arrow, producing its first tornado near Amagon AR at this time, southwest of Jonesboro) appeared to be moving northeast right along this boundary.  It's possible this boundary provided increased low-level wind shear and convergence to help support tornadoes.

Regarding storm environment parameters over northeast AR, the SPC mesoanalysis showed an area of enhanced low-level MLCAPE (0-3 km above ground, 1st panel below) and, although not particularly impressive, an area of somewhat enhanced 0-1 km SRH (around 150 m2/s2, 2nd panel below) near Jonesboro:

This "overlap" may have set the stage for enhanced tilting and stretching of low-level SRH with the supercell storm moving along the aforementioned boundary.  The 2100 UTC SRH over northeast AR (larger than forecast from the morning model runs) appeared to be the result of the low-level jet increase during the afternoon that I mentioned earlier.

An additional graphic I put together (1st panel below) shows areas of overlap of 0-3 km MLCAPE > 75 J/kg and 0-1 km SRH > 150 m2/s2 in purple from the 2100 UTC SPC graphics above:

Notice that the genesis region of the Jonesboro tornadic supercell was in the purple area over northeast AR, again suggesting localized potential for enhanced low-level tilting and stretching of SRH along the boundary discussed above.  The 2100 UTC effective-layer significant tornado parameter (STP, 2nd panel above) also suggested support for supercell tornadoes over northeast AR.

Here's the 1-hour forecast sounding from the RAP model for Jonesboro valid at 5:00 pm CDT (2200 UTC) at the time of the tornado, suggesting the localized support for supercell tornadoes near the boundary:

Although both STP and the energy-helicity index (EHI) weren't especially impressive (both parameters around 2.0) for a large EF3 tornado, they do indicate an environment supportive of tornadoes, one that may have been given a "boost" by the presence of the boundary in addition to the sounding environment shown above.

Here's a larger view of the forecast afternoon setting from the morning NAM model run, showing the large 500 mb trough moving through the central U.S. with the typical spreading jet stream pattern (thick white curved arrows) ahead of it, providing dynamic forcing and lift:

The insets on the graphic above show EHI and fixed-layer STP forecasts for mid-afternoon (again, not very impressive for northeast AR compared to areas farther north), and SPC tornado reports during the afternoon.

This case is a good illustration of how important it is to monitor features and parameters in real time, as the mid-afternoon SPC mesoanalysis and surface map indicated increasing low-level shear and support for supercell tornadoes near the boundary over AR, which was not forecast well by the morning model runs. 

Away from Arkansas, notice that over Iowa this was a "cold-core" tornado event  with a closed 500 mb low nearby and a boundary intersection (warm front and Pacific cold front) over central Iowa (see the surface map earlier above).  As is typical with cold-core settings, several tornadoes occurred near this boundary intersection over Iowa as it evolved northeastward during the afternoon, including this tornado northeast of Des Moines near Rhodes, Iowa after 4:00 pm CDT:

Back to Arkansas, the Jonesboro tornado setting and environment was more subtle than those accompanying recent large tornadoes such as the nighttime Nashville tornado back on March 3 and yesterday morning's tornado near Eufaula, Alabama (March 31).  In both those cases, the tornadic supercells were near a warm front or stationary front, and low-level shear was larger and more evident (0-1 km SRH 300-450 m2/s2) than near Jonesboro on March 28 (0-1 km SRH 150-190 m2/s2).

I hope everyone reading this navigates the Covid-19 outbreak carefully and in reasonable health over the next couple months!

- Jon Davies  4/1/20