Monday, February 22, 2021

A difficult tornado to forecast - 2021's 2nd deadly tornado on February 15 kills 3 people in North Carolina

Last Monday's EF3 tornado near the coast of southeast North Carolina (north of Sunset Beach NC) killed 3 and injured 10.  Because it occurred at night (between 11:30 pm and midnight EST), there were no photos of the tornado, but a storm chaser managed an image of the tornadic supercell (top photo above) off the coast east of Myrtle Beach, South Carolina (SC) around 30 minutes before it struck on land just north of the NC-SC border.   The 2nd photo above shows some of the impressive damage (see this page on the tornado from NWS Wilmington NC).

There was no tornado watch in NC ahead of the storm, and a tornado warning was not issued for Brunswick County where the tornado occurred until 5 minutes after it developed.  That's because it was far from an obvious forecast setting, which is worth posting about here.  The environment was also evolving rapidly, and the tornado developed fast on radar (not shown) when the supercell moved on land.

A forecast and nowcast problem in this case was that surface-based (sb) instability at 11:00 pm EST 2/16/21 (0400 UTC) appeared to be well off shore from the Carolinas (see sbCAPE on the first panel below from the SPC mesoanalysis):

When there is no surface-based instability within a severe weather threat area, storms that do occur are considered to be "elevated", meaning that the instability (CAPE) supporting the storms comes from lifted air located somewhere above the ground.  Because tornadoes by definition occur at the ground (different from a "funnel" aloft), tornadoes are generally not expected in such elevated settings because air at the ground is typically too stable to support tornadoes.

This "elevated" factor affected composite tornado forecast parameters such as the effective-layer significant tornado parameter (STP), shown in the 3rd panel of the SPC graphic above, where effective STP was near 0 along the coast of the Carolinas, a result of this apparently "elevated" environment. 

However, looking at mixed-layer CAPE (mlCAPE, 2nd panel in the graphic above), which uses an average of the temperature and moisture properties within the lowest 1 km above ground, notice that some CAPE (around 500 J/kg) was present right along the northeast SC and southeast NC coast at 0400 UTC near Sunset Beach (black square dot).  This is because there was significant moisture and increased temperature flowing northward above a shallow stable layer at the ground.  A RAP model sounding profile at Southport, NC (roughly 25 miles east of Sunset Beach), modified using a 0430 UTC temperature and dew point surface observation of 64 deg F and 64 deg F, showed this:

Notice  that the yellow dashed lifting curve at left on the graphic above (representing a surface-based lifted parcel of air) generated _no_ CAPE, suggesting an "elevated" environment.  But in contrast, the yellow dashed lifting curve at right (representing a lifted parcel of air from around 0.3 km above ground), did indicate notable CAPE, as seen by the red mlCAPE area on the sounding.  

In other words, significantly unstable air was present less than 1000 ft above the ground, just above the very shallow stable layer.  It was this instability, combined with large low-level wind shear (storm-relative helicity or SRH of 350-400 m2/s2) near a warm front that helped support low-level rotation and a tornado in the supercell that moved on shore near Sunset Beach NC.  Apart from the shallow stable layer, the environment was a small instability and large wind shear setting often typical of southeast U.S. tornadoes in the cool season.

Warm fronts with significant combinations of CAPE and SRH along and near them are recognized in forecasting as potentially supportive of tornadoes with supercells that move along with or across them.  The surface map below at 11:00 pm EST (0400 UTC) shows that, indeed, a warm front was present moving northward near the NC-SC border along the coast:

Also, the SPC graphic below at 0400 UTC showed that some low-level instability (mlCAPE in the lowest 3 km above ground) and large low-level wind shear (SRH in the lowest 1 km above ground) were both present in a small corridor along the NC-SC coast, near the warm front seen on the surface map above:

Although these ingredients were present only in a narrow strip along the coast, they apparently were enough to support the deadly supercell tornado near the warm front and the coast.  But the shallow stable layer near the ground discussed earlier masked this potential by suggesting an elevated environment along the same warm front that was actually not as significantly "elevated" as it looked at first glance.  This was due to the shallowness of the stable layer along the coast and the warmth/moisture flowing northward just above the ground.

Radar below shows the supercell just off shore at 10:00 pm EST (0300 UTC) that eventually moved on shore in NC near Sunset Beach around 11:30 pm EST (0430 UTC), interacting with the surface warm front and CAPE/wind shear just above the ground to produce the deadly tornado that lasted until midnight EST:

As the supercell moved farther inland away from the coast, the low-level stable layer apparently became too deep, eliminating near-surface CAPE, and there were no other tornadoes over eastern NC.

Forecast maps from the morning of February 15 did hint that there might be ingredients for a marginal possibility of tornadoes near the SC-NC coast, but that potential was certainly not very evident.  Below are the NAM model 500 mb forecast for mid to late evening, and a 2-panel NAM model forecast of mlCAPE and 0-1 km SRH:

At 500 mb (roughly 18,000 ft above sea level), a large trough was moving through the mid-section of the U.S., bringing with it last week's news-making frigid arctic cold to the central Plains and particularly Texas.  Ahead of this trough, a spreading jet branch pattern was over the southeastern US providing lift through diffluence, and at mid-evening some mlCAPE was forecast just touching the NC-SC coast, barely overlapping large 0-1 km SRH near the afforementioned warm front.  This narrow  juxtaposition of ingredients was a hint of sorts, but only a small one and nothing that appeared very threatening or alarming for the evening forecast environment. 

Earlier on 2/15/21 (not shown), an EF2 tornado struck southwest Georgia at mid-afternoon, also located near the same warm front that traveled rapidly during the day from the Florida Panhandle all the way to the Carolinas. The warm front in essence outran the instability axis as it moved across eastern Georgia and into South Carolina during the late afternoon and early evening, but then encountered a new fetch of instability again by late evening coming off the Atlantic Ocean along the coast of the Carolinas when the deadly tornado occurred.

Warm fronts are a feature to watch carefully in tornado forecasting, particularly fast-moving ones as in this case.  Often the environment in the immediate vicinity of the warm front may appear "elevated" with a layer of cool surface air north of the front.  But if the front is moving rapidly, the environment may be changing rapidly as well with warm/moist near-surface air flowing quickly northward, producing CAPE where there was none an hour or two earlier.  

This case also emphasizes that when a cool near-surface layer is very shallow along a warm front, sufficient instability may be present to support tornadoes even if the setting appears "elevated" at first glance.  Rapidly-moving warm fronts can really modify and change a local environment quickly.

- Jon Davies  2/22/21 

Friday, January 29, 2021

First strong tornado of 2021 kills 1 and injures 30 near Birmingham, Alabama on January 25th

It's been awhile since I've done a write up about a weather event.  Frankly, there has been so much going on in our nation that it's been very hard to focus on weather.   

The first tornado death of 2021 occurred this week on Monday, January 25 in Alabama (AL) where an EF3 tornado struck Fultondale, a northern suburb of Birmingham, around 10:43 pm CST (0443 UTC 1/26/21).  There were also 30 injuries.  The first image from video below shows one of the huge power flashes in northern Birmingham as the tornado moved through, and the second image shows some of the damage:

Here's a quick look at the meteorological setting:

It took a long time for the cell that produced the Fultondale tornado to get its act together.  As you can see on the radar images below, the storm originated in Louisiana at mid-afternoon on the 25th, taking roughly 7 hours to get around to producing the deadly tornado in AL that evening:

It is noteworthy that, apart from the death and injuries near Birmingham AL, the same supercell also  killed one person and injured two others from strong non-tornadic winds in central Mississippi (MS) around 6:40 pm CST (0040 UTC 1/26/21 - see middle panel above).  It's easy to forget that, in addition to tornadoes, strong thunderstorm winds can kill people, too.

Also worth noting is the thermal boundary from central MS into central AL (red-blue dashed line in middle panel above) that was in place due to storms that had already tracked east-northeast across this area (this boundary was also evident on surface maps, not shown).  Plenty of research over the years indicates that tornadoes like such boundaries because of the focus of convergence and the energy from contrasting temperatures and wind shear that can help supercell storms spin up tornadoes.  This particular boundary may have helped intensify the deadly supercell that eventually produced the tornado near Birmingham.

The NAM model forecast from the morning of 1/25/21 suggested that tornadoes might be possible during the evening.  Below, the 300 mb forecast for mid-evening (300 mb is the best level in this case for showing the jet stream pattern and branches; see thick white lines and arrows) depicts the spreading wind pattern and associated broad lift across the southeastern U.S.  Also shown (inset) are combined instability and low-level wind shear via the 0-1 km energy-helicity index (EHI), indicating a potentially favorable area for tornadoes with storms just north of the southern jet stream branch across MS and AL:

The EHI values above from southwest MS to central AL at mid-evening suggests that the supercell  moving across MS into AL during the evening (see earlier radar images) had a long residence time moving through a potentially favorable environment for tornadoes.  This being the cool season, the supercell may have needed this broad warm sector environment to eventually produce the tornado near Birmingham.  And, as noted earlier, the storm was moving along and parallel to a thermal boundary.

A closer look at the environment using SPC mesoanalysis graphics (below) suggests that the environment along the boundary as the evening progressed was more favorable for tornadoes over AL.  The 0-1 km storm-relative helicity (SRH, a measure of low-level wind shear that can help with generating tornadoes) was larger over Alabama (middle panel).  Some low-level mixed-layer instability (0-3 km mlCAPE, 3rd panel) was also co-located with the increased SRH over central AL, suggesting potential for low-level storm updraft stretching with the supercell moving into the Birmingham area:

Finally, here is a 1-hr HRRR model forecast sounding for 0400 UTC (10:00 pm CST) at Birmingham, roughly 40 minutes before the tornado (important parameters are highlighted in yellow):

The total mlCAPE was only 900 J/kg, but 0-1 km SRH was 350 m2/s2 (quite significant), and some low-level CAPE was also present (40-50 J/kg, not large but adequate).  Deep-layer shear (surface to 6 km above ground) was also quite large (> 65 kt).  As is typical in cool season cases with not a lot of CAPE but large wind shear, this environment was supportive of tornadoes, particularly with the aforementioned boundary, the broad warm sector through which the supercell was able to travel and organize during the evening, and the increased SRH over AL.

The Storm Prediction Center (SPC) did a great job issuing a tornado watch shortly before 6:00 pm CST that included western, central and northern AL.  Local NWS and TV coverage in the Birmingham area was also on top of the situation during the 10:00-11:00 pm hour, which probably saved lives.

Let's hope, on top of everything else going on in our country right now, that 2021 is not a year with many tornadoes hitting populated areas.  Over the past year, we've been through a lot (ongoing Covid-19, a national election, attempts to deny and overthrow documented election results, a violent assault on our Capitol).  And specific to weather, going back to 2019, there was "SharpieGate" and the attempt to deny and alter facts about the path of Hurricane Dorian.  

Hopefully, with new leadership emphasizing science and facts, we'll get this deadly coronavirus under control in the coming months, and life ahead will begin to look better again.   

- Jon Davies 1/29/21 

Saturday, October 31, 2020

Robert (Bob) Johns - dynamic SPC forecaster and researcher - passes away at 78.


This blog post is about Bob Johns, long-time Storm Prediction Center severe weather forecaster/researcher, who passed away at age 78 this past week after a long illness.

He was such an incredible influence in my life.  

In the late 1980's, I had worked in television and consulting firms for several years without finding much stability or direction in my weather career.  I had moved back to my home area to help take care of my ailing mom and assist my dad with his expanding home medical business in rural Kansas.  I thought my career in weather was over. 

However, I still had an interest in severe weather.

In 1989, I drove to Kansas City and the National Severe Storms Forecast Center (NSSFC, now the Storm Prediction Center) to find meteorological maps for some 1988 tornado events (there was no internet with online weather information yet).  As I was shuffling through paper in their map archive room, Bob Johns walked by and introduced himself.  We started talking, and that conversation changed the course of my life.

Bob sensed the untapped passion I had for tornado forecasting and severe weather research, something that hadn't been encouraged by anyone I had worked with so far.  To my surprise, he enlisted me in a research project he wanted to do using weather balloon soundings archived from the 1980's to examine environments of significant tornadoes using newer measures of wind shear.  Something like that wasn't easy to do back then without the sophisticated computer software for analyzing soundings that we have now.  

The result was several research papers we authored together in the early 1990's that laid some groundwork for using wind shear and instability combinations in tornado forecasting. This included the Energy-Helicity Index (along with SPC forecaster and programming wiz John Hart) as a composite parameter in tornado forecasting. 

I was thrilled that he asked me to work with him on this project.  In his quiet but enthusiastic way, one of Bob's gifts was recognizing people's abilities and inspiring them to use and develop them.   I am very blessed that he took an interest in me, and his mentorship was essential in nudging me in a different direction at a key time in my life.

Here'a photo of NSSFC staff back in 1977: along with Bob Johns, it includes forecasters Steve Weiss, Larry Wilson, and Jack Hales.  Bob introduced me to these NSSFC 'stalwarts', who were all very kind and encouraging to me.  I also met severe weather icons Chuck Doswell, Don Burgess, Robert Davies-Jones, and others through him, which was quite amazing yet humbling for someone so new to weather research and writing. 

Bob started at NSSFC in Kansas City in 1971, learned much about severe weather forecasting and issuing watches from forecaster Larry Wilson, and became a lead forecaster at NSSFC by 1979.  He worked in that capacity until 1994 when he became Scientific Operations Officer at the time SPC moved to Norman, Oklahoma.  He retired in 2001, after which he worked part-time.  It is noteworthy that Bob introduced the enhanced "Particularly Dangerous Situation" (PDS) wording to tornado watches during the 2 April 1982 tornado outbreak in Texas, Arkansas, and Oklahoma.

Chicago weather legend Tom Skilling wrote the following on Facebook after Bob John's passing this week:

"Anyone who has worked in the meteorological profession has read and studied Johns' remarkable work on severe weather--work produced over an amazing decades-long career...

I first met Bob at one of our Fermilab Tornado and Severe Weather seminars back in the 1990s... For someone who had so profoundly impacted the meteorological profession over his decades long career, it was such a joy to discover a soft-spoken man with a warm smile. Bob was so kind and so easy to approach... He had a passion toward meteorology and, in particular, severe weather and the manner in which nature put it together.  He spent his professional life cataloging what he had learned and worked tirelessly passing this information on to his colleagues and all interested in the subject."

I couldn't describe Bob better myself.  As Tom also mentioned in his FB post:

"His work on the prediction of "derechoes", the fast moving and destructive squall lines like the one which raced across our area at highway speed this past August, has been described as the "bible" on the subject for forecasters. His forecast techniques are still in use today. Johns published his study on the phenomenon with his National Weather Service colleague Bill Hirt in 1987."

Here's that paper:  Johns, R. H., and W. D. Hirt, 1987: Derechos: widespread convectively induced windstorms. Wea. Forecasting, 2, 32-49.

Bob's work on derechos (see my blog post on last August's derecho) helped make meteorologists and the public more aware of these large, destructive, and potentially deadly windstorms that can blast across several states.  His research has saved lives as a result.

Here's references for a little more of Bob's work:

As the last paper above reflects, Bob spent years studying and reconstructing the damage path of the famous 1925 Tri-State tornado in Missouri, Illnois, and Indiana.  It was a serious and fascinating subject for him.  That classic 2013 paper Bob wrote with Chuck Doswell, Don Burgess, and John Hart serves as the authoritative documentation and reference for that remarkable tornado.

Bob even wrote a book on the Tri-State Tornado, which you can buy on Amazon.

A more comprehensive list of Bob's published papers can be downloaded here:

And for those who'd like more background on Bob John's growing up in Indiana and the development of his passion for severe weather, here's John M. Lewis' article about him in 2007:

Back in 2001, I was honored to be part of an evening seminar and review of the 1991 Andover, Kansas tornado where Bob talked about his handling of the event as a forecaster at NSSFC/SPC:

He taught me so much about research, staying disciplined, being practical, and staying clear in my focus and writing.  Bob helped give me confidence that I was missing in my young life, and he also taught me about being a decent human being,  I owe him a great deal.  

A final thought... Bob Johns was a wonderful reminder that we should treat those newer people finding their way in our chosen profession with respect and a helpful spirit.  He exemplified the importance of 'paying it forward' when we've had the good fortune to do what we love.

Rest in Peace, my friend.  You will not be forgotten.

- Jon Davies  11/30/20

Tuesday, September 1, 2020

Landspouts and the meteorological ingredients that can produce them

I've had some requests to update some material about landspout tornadoes that used to be on an old web site of mine, which is the reason for this post. 

A landspout tornado is a type of non-supercell tornado that can occur with a thunderstorm that doesn't have a rotating updraft detectible on radar (a mesocyclone).  Because of this, warnings can't be issued for landspout tornadoes based on radar - they have to be reported by spotters and storm chasers, and there is no warning lead time.  Thankfully, most landspout tornadoes are relatively weak, but on occasion they can become strong in the right setting.

How do landspout tornadoes form?

First, a sharp wind shift boundary must be present.  That's what provides the vorticity (or 'spin') needed for the tornado, when the low-level wind shear that helps to generate supercell tornadoes is absent:

Second, instability (or CAPE - convective available potential energy) must be present along the boundary for a thunderstorm updraft to develop.  In particular, CAPE in the lowest 3 km above ground helps because it can facilitate low-level stretching in updrafts along the boundary due to the instability being located closer to the ground.  

Third, steep low-level lapse rates (a rapid drop off in temperature in the lowest few kilometers above ground due to strong daytime surface heating) can also help accelerate low-level air upward in updrafts, increasing low-level stretching.

Here's one example of these ingredients coming together in a setting that might generate and support landspout tornadoes, using SPC mesoanalysis page graphics:  

If a thunderstorm updraft forms on a sharp boundary in such an environment with everything coming together just right (for example, the storm updraft must develop directly over the boundary and align properly with a small 'wiggle' or pocket of 'spin' on the boundary), the thunderstorm may then generate a landspout tornado through upward stretching of that 'spin':

Here's an example of a setting for landspout tornadoes that occurred on May 25, 2018 in both southern Minnesota (MN) and eastern Nebraska (NE).  The surface map at early afternoon on 5/25/18 showed a relatively stationary wind shift boundary (thick brown dashed line) stretching from a low in southern MN into east-central NE:

SPC mesoanalysis graphics at early afternoon on 5/25/18 showed plentiful low-level instability (0-3 km CAPE, in red, 1st panel below) over southern MN near the surface low where a storm was developing on the boundary (light blue lines also show surface vorticity or 'spin' along this boundary).  The 2nd panel below showed low-level lapse rates (0-3 km) to be steep over a large area, including all along the boundary, with lapse rates > 8.0 deg C shaded in orange:

Below is the developing storm on visible satellite (yellow arrow) at about 2 pm CDT.  Less than an hour later, a landspout tornado developed on the southwest edge of this new storm (photos also below):

From the 5/25/18 surface and SPC graphics up above, notice that all of the ingredients listed earlier for supporting landspout tornadoes (boundary, instability, steep low-level lapse rates) were present in this case, increasing the possibility of a landspout tornado _if_ the storm aligned properly with the boundary and the 'spin' along it.

Later in the afternoon, other storms developed southwestward on the wind shift boundary over Iowa and Nebraska.  After 5 pm CDT, a small high-based storm on the boundary over northeast/east-central NE (yellow arrow in satellite photo below) apparently "phased' with the boundary enough to produce yet another landspout tornado (see photos below):

As can be seen from the satellite photo, there were _many_ storms that developed along the boundary by late afternoon, but only _one_ of the Nebraska storms produced a tornado.  This certainly highlights that landspout tornadoes are not really forecast-able in advance.  A meteorologist can only note that ingredients possibly supportive of landspout tornadoes (the boundary, instability, low-level lapse rates) appear to be coming together in an area, so that reports of such tornadoes are not a surprise and can be warned immediately when spotted. 

Here are a few landspout tornado cases I've posted about on my blog:

    June 21, 2020 landspout tornado in northwest Kansas

    April 17, 2019 landspout tornadoes in Oklahoma, Kansas, and Texas

    May 19, 2012 non-supercell tornado outbreak in southern Kansas

And, for more technical reference, here are some papers that discuss landspout tornadoes:

    Non-supercell tornadoes (Wakimoto and Wilson 1989)

    Tornadoes in non-mesocyclone environments with pre-existing vertical vorticity along convergence boundaries (Caruso and Davies 2005)

    Tornadoes in environments with small helicity and/or high LCL heights (Davies 2006) 

It is also important to recognize that there are some cases where both non-supercell and supercell processes appear to be at work, particularly when winds aloft are strong enough to support and organize storms into supercells while ingredients related to landspout tornadoes are also present.  This is particularly true in the High Plains of the U.S., and such "hybrid" events are difficult to categorize.  Here are a couple examples:

    June 29, 2019 long-lived tornado in southwest South Dakota

    June 6, 2018 large and long-lived tornado near Laramie, Wyoming

Hopefully, this short discussion will help those interested in understanding some of the important factors and ingredients that contribute to landspout tornadoes.

- Jon Davies 9/1/20

Thursday, August 13, 2020

The August 10, 2020 Midwest Derecho - how did it develop?

Winds from the derecho on Monday, August 10 (pronounced 'day-ray-cho', a widespread convective system with high winds over a large area that can affect several states) killed two people, one in Iowa and one in Indiana, while leaving over a million people without electricity.  The images above show the derecho moving into Chicago at mid-afternoon, the derecho's shelf cloud approaching Sioux City, Iowa on Monday morning, and major damage in Marshalltown, Iowa at mid to late morning.   

The derecho capped off a very active (for August) 7-day period of severe weather.  August 3-4 saw numerous tornadoes associated with the remnants of Hurricane Isaias from North Carolina (where 2 people died in a nighttime EF3 tornado) to Virginia, Maryland, Delaware, Pennsylvania, and New Jersey.  Then an EF2 tornado on the evening of August 7 killed two people in southwest Manitoba, Canada.

Monday's derecho started in the early morning hours over South Dakota, crossed Iowa during the morning, and hit Chicago and northern Indiana in the afternoon.  Here's an hour by hour composite radar image from NWS Chicago that shows the derecho's rapid progress:

And here's the associated storm reports showing the wide axis of damage across several states, including some embedded "bow echo" or QLCS-type EF0-EF1 tornadoes in the Chicago area:

How does a derecho like this form?  Large convective systems are pretty common across the Midwest in summertime.  So, what causes one of these systems to become a dangerous and deadly squall line or cluster of storms?  Answers have to do with the origin area of the derecho (south-central/southeast South Dakota in this case), as well as the synoptic setting out ahead of it.

Below is the surface map at 5:00 am CDT on 8/10/20, along with a radar inset focused on south-central South Dakota (SD).  I've indicated the genesis region of the derecho, where an area of thunderstorms had formed during the early a.m. hours within the thick black oval on the surface map, north of a frontal boundary over Nebraska:

A RAP model forecast sounding at Lake Andes, SD, just east of these thunderstorms, is shown below:

Notice that there was a significant layer of dry air in the lowest 3 km, indicated by the broad distance between the red temperature and blue dew point curves in lower levels.  This was _below_ a relatively moist layer beginning at roughly 3 km above ground where significant CAPE was present from a lifted parcel at that elevated level.  With little or no convective inhibition (CIN) at this level, elevated storms could initiate well north of the surface front over Nebraska (see 5:00 am surface map earlier), and rain into this dry layer below.

This would produce strong evaporative cooling, creating dense cool air accelerating downward and outward and generating strong surface winds beneath and ahead of the expanding elevated storms over southern SD.  Here's a diagram illustrating that process for many summertime derechos that initiate in the northern Plains of the U.S.:

This may look similar to diagrams of downbursts and microbursts, which are much more localized.  But what is different and important in Monday's case is that the evaporative cooling and downward air acceleration was taking place over a larger area with the _cluster_ of developing storms, rather than a single thunderstorm.  This then spread out into the broad squall line and convective system seen in the composite progressive radar image earlier.

I should emphasize that soundings like the one at Lake Andes up above, with a moist layer of sizable elevated CAPE above a significant layer of dry air (a type of  'overrunning' situation, north of the Nebraska frontal boundary seen on the earlier surface map) are not that common.  With a developing thunderstorm cluster, it's a little unusual to see such an elevated moist layer located that far above a depth of much drier air beneath.

Below is the 500 mb NAM model forecast for 4:00 am CDT in the mid-levels of the atmosphere on 8/10/20, showing a significant shortwave disturbance (thick dashed black line) moving across the northern Plains. This disturbance provided the upward forcing that helped fire up the elevated thunderstorms over southern SD that evolved into the derecho:

The inset on the graphic above is a forecast of mlCAPE valid at 7:00 am CDT, showing a long axis of instability extending eastward to Chicago, a corridor that would feed and maintain the convective complex and derecho as it evolved and moved eastward across several states during the morning and afternoon.

A similar situation accompanied a developing derecho that my wife Shawna and I experienced at Pierre SD in the early morning hours of 6/22/15, five years ago.  Below is the surface map with the genesis region of the derecho indicated around Pierre at 1:00 AM CDT on 6/22/15:

Here is the RAP model sounding at Pierre SD at about the same time:

Notice that the environment was similar to the 8/10/20 "genesis region" sounding shown earlier, with an elevated moist layer and sizable CAPE located _above_ a dry layer in the lowest 3 km.  As new storms formed west of Pierre around midnight, evaporative cooling of rain through this dry layer resulted in the generation of very strong winds (120+ mph measured at Hayes SD!) that caused widespread damage in Pierre (including our motel) and locations eastward on 8/22/15 as the storms morphed into a derecho event.

Many derechos that initiate over the northern Plains and move eastward through the Midwest (the most common area for derechos in the U.S.) probably have this type of atmospheric setting in their genesis region.  The drier air in the lowest 2-3 km below an elevated moist layer can result in rapidly-developing strong convective surface winds that spread eastward as a derecho along a west to east corridor of instability.  

An important factor is the presence of a stationary or quasi-stationary west-east front (check out the surface maps earlier) with this axis of unstable air along it, helping to provide a corridor along which the recently-intiated derecho can intensify, spread, and maintain itself moving eastward.  This was a key feature of Robert John's seminal work on derechos at SPC in the 1980's (see this paper).  Johns deserves much credit for making forecasters and meteorologists more aware of these dangerous convective systems.

Derechos are more complicated than this brief analysis would suggest.  But I've touched on a few factors that are important for many warm season derechos across the Midwest.

- Jon Davies 8/13/20 

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