Thursday, December 19, 2019

Southern tornado outbreak leaves 3 dead on December 16, 2019

A strong storm system moving across Louisiana (LA), Mississippi (MS), and Alabama (AL) on Monday, December 16, 2019  caused the first tornado deaths in the U.S. since May 2019.  A large long-track EF3 tornado above (at top) in west-central and central LA left one person dead near DeRidder, and was on the ground for over 60 miles, striking Alexandria around noon on Monday.

Another tornado (EF2) at late afternoon from a supercell embedded within a squall line killed a married couple in Lawrence County in northwest AL.  The bottom photo above shows an EF2 tornado north of Tupelo MS earlier from the same embedded supercell.  In all, around 30 tornadoes were reported in LA, MS, AL, and Georgia on December 16 through December 17.

The 2-panel base reflectivity radar composite image below shows the deadly tornadic supercell at midday over west-central LA (1st panel), and the deadly tornadic supercell within a bowing line segment at late afternoon over northwest AL (2nd panel).  Prolific tornadic supercells over southern MS are also indicated:

This storm system was a good example of a large "positive" tilt trough in mid-levels (see thick red dashed line on 500 mb forecast graphic from the NAM model below) causing a significant amount of severe weather.  As noted on the NWS "Jet Stream" educational site, positive tilt upper troughs (where the southern end of the trough is much farther west than the northern portion) tend to produce the least amount of severe weather, but that certainly wasn't the case on December 16:

A broad pattern of spreading jet branches (thick white lines and arrows on graphic above) ahead of this trough was causing upward forcing over the southern states.  With plenty of wind shear and enough instability present at midday over LA and MS to support supercell tornadoes (see inset showing energy-helicity index or EHI above, combining instability and low-level shear), the outbreak was in progress from late morning on.  Note that the southern jet branch above tended to define the southern end of the severe outbreak area, while the northern extent of instability defined the northern end.

Here's a U.S. surface map at midday showing the cold front and surface low pressure associated with the midlevel trough moving east across the western Southern states:

A RAP model sounding at midday at Alexandria LA shows how primed the atmosphere was for tornadoes over this area:

There was close to 2000 J/kg of MLCAPE, around 250 m2/s2 of 0-1 km storm-relative helicity (SRH), and over 50 kt of deep-layer wind shear.  Add to that a lack of convective inhibition (MLCIN) and plenty of CAPE in low levels below 3 km above ground (100-150 J/kg) to promote low-level stretching of updraft parcels, and this was an environment very supportive of supercell tornadoes with discrete or semi-discrete cells.

In recent years, the introduction and use of updraft helicity in computer model forecasts can help forecasters predict where rotating supercell storms (and, by association, possible tornadoes) may develop and track.  A HRRR model forecast from the morning of December 16 (below) suggested that long-track supercells would develop from central LA into southwest MS between 1500 UTC (9 a.m. CST) and 2100 UTC (3 p.m.CST), which was a good model forecast:

By mid to late afternoon at 2200 UTC (4 p.m. CST), the focus for supercells and tornadoes had shifted east to Mississippi and northwest Alabama, as seen on the SPC mesoanalysis graphic below:

The best combinations of low-level wind shear (0-1 km SRH) and low-level instability (0-3 km MLCAPE, see black oval-enclosed area above) were over Mississippi into northwest Alabama, where low-level stretching and tilting of wind shear within stronger storms would be enhanced, supporting low-level rotation and tornadoes.  The cells marked "A", "B", and "C" in the graphic above were consistent long-track tornado producers.

The supercells marked "B" and "C" on the SPC graphic above were particularly effective tornado producers.   From these cells, images in sequence below show a large mid-afternoon tornado northwest of McComb MS, a large tornado shortly before 5 p.m. CST near Columbia MS, and the EF3 tornado that struck Laurel MS at dark.  Power flashes are quite visible in the latter two images:

As is so often the case these days, excellent NWS warnings and media coverage probably saved many lives in this outbreak.

- Jon Davies  12/19/19

Tuesday, October 22, 2019

October 20, 2019 Dallas, Texas tornadoes after dark - no deaths or injuries!

Wow... it has been an active several days for tornadoes this past weekend in October (see my recent post about the EF2 tornado in Florida from T.S. Nestor).

As anyone watching news the last couple days knows, Sunday evening (10/20/19) saw several tornadoes in the Dallas, Texas area after dark, including one EF3 that struck the North Dallas-Richardson corridor around 9 pm CDT (top image above).  Another of EF1 intensity hit the Rowlett, Texas area a little later from the same supercell storm (middle and bottom images above, note the very impressive power flash!) .

**** Update 10/24/19:  Additional tornadoes have been surveyed by NWS Dallas/Ft. Worth in recent days, including a short-track EF2 tornado in Garland just before the Rowlett tornado mentioned above, and an EF1 in Rockwall after the Rowlett tornado. These tornadoes were from the same supercell that produced the North Dallas and Rowlett tornadoes. ****

No one wants to see damaging tornadoes in a metro area, especially after dark, but it is great news that there were no injuries or deaths reported in the Dallas area, largely due to a tornado watch and good warnings by NWS.

I've had several people ask me, "Isn't that odd for this time of year?"  Not really.  Based on  statistics over the past 25 years, Texas sees an average of around 9 tornadoes in October each year, and 58 tornadoes occur on average nationally in October.  So it does happen with the right meteorological settings.

Sunday evening's surface map (below, 7:00 pm CDT) showed a dryline west of Dallas with moist air (dew points upper 60's to near 70 deg F) that had moved back into north Texas on south winds during the day, with the deepest moisture from the Red River southward:

Tornado parameters from the SPC mesoanalysis at 7:00 and 8:00 pm CDT (below, enhanced energy-helicity index or EEHI, and the effective-layer significant tornado parameter or STP) suggested that combinations of instability and wind shear were quite supportive of tornadoes over the Dallas area as storms were developing rapidly and becoming supercells to the west:

The northernmost supercell on the 7:00 and 8:00 pm SPC images above produced the damaging EF3 tornado in north Dallas around 9:00 pm CDT, followed by the Rowlett tornado just after 9:30 pm CDT.  In fact, the warm sector setting east of the dryline over north Texas was so supportive of tornadoes that no boundaries were needed to help produce tornadoes, unlike last Friday evening in Florida.

A forecast of instability and wind shear from the RAP model sounding at Dallas a couple hours before the tornado shows excellent combinations of CAPE (instability), low-level wind shear (0-1 km storm-relative helicity or SRH), and deep-layer wind shear (0-6 km shear) were in place, with not much convective inhibition (CIN) in the environment.  These factors were all supportive of significant supercell tornadoes if discrete storms developed:

It is worth noting that the High-Resolution Rapid Refresh (HRRR) model from earlier on 10/20/19 (see below) was not successful in forecasting convective storms and supercells over the Dallas area that evening, although it did forecast storms in Oklahoma, and over west central Texas:

So, even with our often impressive automated model guidance of convective storms these days, the human forecast element is still greatly needed!

In my post this past weekend about the Lakeland, Florida tornado, I pointed out from radar images how that supercell behaved somewhat like a typical Plains tornadic supercell with one tornadic mesocyclone occluding and dissipating, while a new one formed to its east-southeast.  That same evolution was seen Sunday evening with the North Dallas and Rowlett tornadic mesocyclones, as is evident on the reflectivity and storm-relative velocity images below:

Looking at the larger synoptic picture, the NAM model 500 mb forecast for that evening showed a very large and strong midlevel trough (dashed red line below) moving through the Central Plains, with a typical "branching" jet pattern ahead of the trough.  This area of dynamic forcing overspreading the returning low-level moisture through the Plains was where the bulk of severe weather occurred Sunday evening and Sunday night:

One final note... The north Dallas tornado touched down 15-20 miles east-northeast of AT&T Stadium where the Dallas Cowboys were playing at the time of the tornado.  Although the soon-to-be tornadic North Dallas supercell stayed well north of the stadium, it is nevertheless very fortunate that the EF3 tornado did not directly impact the thousands of people at the game!

- Jon Davies  10/22/19

Saturday, October 19, 2019

October 18, 2019: A strong Florida supercell tornado with Tropical Storm Nestor

It's been awhile since I posted about a recent tornado case, so I pulled together some graphics about the meteorological setting with last evening's EF2 tornado after dark (see images above) near Lakeland in west-central Florida east of Tampa.  This tornado was associated with a tropical storm (T.S. Nestor, centered well out in the Gulf of Mexico at the time).  But instead of the supercell being embedded within an outer band of storms as with many tropical systems, it was discrete and occurred near an east-west stationary front, behaving in some ways more like a Plains supercell storm.

The stationary front is shown on the surface map below at about 0300 UTC (11:00 pm EDT), about the time of tornado development near Lakeland:

There was not a tornado watch in effect at the time, probably because the environment most supportive of significant tornadoes appeared to be out in the Gulf of Mexico closer to the center of Nestor, as indicated on the 0300 UTC (11:00 pm EDT) SPC mesoanalysis graphic below using the effective-layer significant tornado parameter (STP):

However, low-level wind shear (0-1 km storm-relative helicity or SRH) and low-level CAPE (0-3 km MLCAPE) on the SPC mesoanalysis at 11:00 pm EDT (below) were notably co-located together over west-central Florida near the stationary front:

This was in the same area where a supercell storm (shown in later graphics down below) was moving north-northeast near and across the stationary front.  The combination of these two low-level parameters near the ground probably facilitated low-level stretching and tilting of environmental vorticity into the storm's updraft to produce strong low-level rotation, even though total instability and numerical shear/instbility combinations did not appear especially large.  The stationary front likely provided additional low-level shear to add to the background environment as the supercall moved across the front.

A 3-hr forecast sounding from the RAP model at Lakeland valid at 0300 UTC (11:00 pm EDT) also suggests that the environment was supportive of supercell tornadoes:

Notice above that the red CAPE area extended only up to around 30,000 ft MSL (300 mb), with the "fattest" area of CAPE located near 10,000 ft MSL (700 mb), rather low in the sounding relative to the ground.  This is similar to many "cold-core" low-topped supercell  environments that support tornadoes in the Plains, suggesting significant upward air acceleration in low-levels, resulting in strong vertical stretching of low-level vorticity near the ground, even though total CAPE does not appear unusually large.  

So, the vertical distribution of CAPE near the stationary front was probably important in this case.  Also, deep-layer shear (0-6 km shear) near 30 kts on the sounding above, while not overly impressive, was just enough to support supercells and tornadoes.

This discrete supercell associated with a tropical system also behaved more like a Plains supercell on radar.  The Tampa radar reflectivity and storm-relative velocity images zoomed in below show the original tornadic mesocyclone northwest of Lakeland occluding and dissipating (drifting toward the back side of the storm while wrapping in rain-cooled air), while a new mesocyclone formed to its east and southeast, similar to the evolution of many Plains supercells:

Thankfully, even though the EF2 tornado was on the ground after dark for 9 miles and was nearly a third of a mile wide at times, there were no injuries due to timely warnings from NWS Tampa.

- Jon Davies  10/19/19

Saturday, August 17, 2019

Tornadoes in Kansas in mid-August! 8/15/19 in northeast Kansas

Supercell tornadoes undoubtedly can occur in Kansas in early to mid-August, but they don't happen that often, and even as a native Kansan, I've never seen one.  That changed on Thursday, August 15 when a well-established supercell (see Shawna's photo above) in northwest flow produced tornadoes southeast of Manhattan and southwest of Topeka.

Here's a map showing the approximate location of three tornadoes that occurred with the supercell pictured above in the 7:00-9:00 pm CDT time frame:
(A fourth brief tornado occurred with a separate trailing supercell after 9:00 pm CDT.)

I was chasing with James Skivers and my wife, following the southernmost supercell that developed northwest of Manhattan and moved sharply southeast across Wabaunsee County, Kansas (KS).  We missed the first brief tornado (EF0) from this supercell near the western Wabaunsee County line south of I-70 around 8:12 pm CDT when we were driving south to stay ahead of the storm through a low area with trees.  Here's Brian Miner's cool photo of the full supercell base and this first tornado, all in the same shot:

The second tornado (rated EF1 by Topeka NWS) was longer-tracked near dark, southwest of Alma, KS and northeast of Alta Vista.  The image of this tornado below is from Shawna's video, backlit by lightning looking east-northeast, with the rear-flank downdraft (RFD) and gust front visible:

A third brief tornado (EF0, below, looking north and also backlit by lightning) occurred near the time the second tornado was ending, back to its northwest under an occluded mesocyclone behind the one that generated the EF1 tornado:

A separate supercell back farther to the northwest produced a brief EF0 tornado after 9:00 pm CDT east of Alta Vista (not shown).

The satellite image below, with relevant features superimposed, shows the setting just before 7:00 pm CDT (0000 UTC 8/16/19):

Note the stationary front and outflow boundary (from earlier storms that moved into northern Missouri) close together over northeast Kansas near Manhattan and Topeka. Winds from the low-level jet (LLJ, around 850 mb or 5000 ft MSL) were impinging on and overrunning these boundaries, helping to initiate storms northeast of a "cap" that was inhibiting convection to the southwest.

Three different supercells are visible in the image above (black arrows show their southeastward motion), but only the southernmost cell near Manhattan was able to access the most unstable air along and south of the boundaries by virtue of its location and motion, probably a big factor in its ability to produce tornadoes about 90 minutes after the time of the satellite image.  The low-level jet was also increasing near dark, as is typical, enhancing low-level shear supportive of tornadoes.

The SPC mesoanalysis depiction of the effective-layer significant tornado parameter (STP) at 8:00 pm CDT (shortly before the tornadoes) showed large values supportive of tornadoes along and near the aforementioned boundaries in northeast Kansas (the enhanced energy-helicity index, an experimental parameter, is also shown):

Notice that, even though these tornado forecasting parameters appeared "favorable" over a rather large area extending north of the boundaries, the tornadoes were limited to the air mass south of and near the boundaries.  This suggests the importance of following the location and evolution of relevant boundaries in forecasting/nowcasting tornado potential, rather than just relying on parameter "bulls-eyes".

Also, here's the NAM model 500 mb forecast (winds at roughly 18,000 ft MSL) showing the northwest flow driving into Kansas, a little unusual that far south in mid-August, and the reason the supercells moved southeastward to the right of the midlevel flow:

Two shortwave troughs are marked on the forecast above (thick dashed red lines) showing a lead shortwave associated with the morning and early afternoon storms that moved from northeast Kansas into northern Missouri, and a fairly strong trailing shortwave not far behind crossing Nebraska.  Often, morning and early afternoon storms leave subsidence (sinking motion) and a "worked-over" air mass in their wake, reducing the chances of severe storms behind them.  But that was not the case here, with another shortwave immediately behind generating upward motion and intensifying low-level winds converging on and overrunning the surface boundaries.

All in all, an extremely interesting case for mid-August in Kansas!

- Jon Davies  8-17-19

Thursday, August 8, 2019

Updated map and images of May 28, 2019 EF4 tornado in northeast Kansas

It's been over 2 months since the long-track EF4 tornado in northeast Kansas (KS) on May 28 that just missed the main population of towns like Lawrence, Linwood, and Bonner Springs KS.

Some interesting video has surfaced since May 28.  I thought it would be useful to correlate some images of the tornado with a newer map I put together (above) that shows the relative width of the tornado path (> 1 mile near Eudora & Linwood) at various points from my own survey.  The smaller tornado that occurred earlier and merged with the larger one as it formed southwest of Lawrence is also shown.

The map above shows image locations (in red) by image number as discussed below, along with direction of view.

Image 1 below (from Quincy Vagell's video) shows the wet portion of the storm in southwest  Douglas County looking west around 5:57 pm CDT where a small EF1/EF2 tornado was embedded that struck Silver Lining Tours (SLT).   The new mesocyclone from which the large EF3-EF4 tornado developed about 10 minutes later is also visible north of the rain-wrapped area concealing the smaller tornado:

Image 2 below (from video by Robert Reynolds) shows this smaller EF1/EF2 tornado emerging out of the rain near Lone Star Lake around 6:05 pm CDT, looking east about 1/2 mile away.  This is the same tornado that struck SLT 2 to 3 minutes earlier:

Image 3 below (from video by Dalton Coody) shows the large tornado shortly after forming southeast of the town of Lone Star (in Coody's video, it is only visible briefly before rain wraps around it).  This was at roughly 6:10 or 6:11 pm CDT, after the merger of the smaller tornado with this larger one as it was developing:

Image 4 below (from video by Jack Miller) shows the HP supercell at about 6:12 pm CDT looking southwest from the south side of Lawrence.  The tornado, visible in the Coody video, is hidden by rain wrapping in from the southeast (a wet rear-flank downdraft or RFD):
Image 5 below (from video by Matt Grantham) jumps ahead to near Linwood KS around 6:35-6:40 pm CDT, looking west along Highway 32 west of Linwood.  The tornado was about a mile wide (!) at this point, but somewhat visible even though still wrapped up in rain:
Image 6 (from video by Cybil Walters) is from a housing development north or northeast of Linwood, and shows suction vortices (smaller intense "swirls" within a larger tornado) clearly visible.  This was around the time of EF4 damage near Linwood, and suggests why some houses north of Linwood were totally demolished, while others were still standing with mainly roof and outer wall damage:

Image 7 (from video by Matt Grantham again), looking southwest from northeast of Linwood around 6:40-6:45 pm CDT, also shows the tornado at about the time of EF4 intensity, although it is still somewhat difficult to see from some rain-wrapping:
The more I look at the path of this large tornado, the more I'm struck by how very fortunate it was that it did not directly hit any population centers, including the Kansas City metro area!

The images in this post also show how difficult it was to see this tornado (and the smaller one before it), as they were both associated with a large high-precipitation (HP) supercell.

Spotters and chasers BEWARE such storms, especially when they are moving faster than 30 mph... give them a very wide berth!

Thanks to Robert Reynolds, Quincy Vagell, Dalton Coody, Jack Miller, Matt Grantham, and Cybil Walters for their video images, and thanks to Rick Schmidt and Eric Lawson for pointing me to additional documentation and information.

- Jon Davies  8-8-19

Monday, July 1, 2019

A surprise photogenic tornado in southwest South Dakota on June 29, 2019

Similar to the unexpected and stunning Laramie tornado last year (see here), June 29 saw a striking and widely-photogrpahed tornado (photos above) that lasted around 40 minutes west of Martin, South Dakota.  It occurred mainly over open fields, and so was rated only EF1, although it may have been more intense.

The Storm Prediction Center (SPC) had outlooked a 5-10 % chance of tornadoes over parts of North Dakota on Saturday, but nothing over southwest South Dakota (SD).  Why?  The reason is that this tornadic storm was not a typical tornadic supercell storm, and like the Laramie tornado, was difficult if not impossible to forecast.

Looking back at Saturday afternoon June 29, there were certainly doubts about whether storms would even develop over SD - see the model radar simulation forecasts for mid-afternoon below:

Given these model radar forecasts and very high cloud bases expected (around 2500-3000 meters above ground, not shown) along with little if any low-level wind shear, no forecaster would anticipate a long-lived tornado hours in advance within such a setting.  Tornadoes from purely supercell processes typically occur with cloud bases under 1500 m and at least some low-level wind shear; it is unusual for tornadoes to occur when temperatures are near 100 deg F, which guarantees high cloud bases.

How could such an environment support a long-lived tornado?  Some important ingredients can be seen coming together just before the tornado occurred. 

Here's the surface map at 3:00 pm CDT, roughly 30 minutes before the tornado.  Notice the northeast-southwest boundary/trough over southwest South Dakota:

A storm or two did begin to form on this boundary shortly before 3:00 pm CDT (see satellite image below) with strong surface forcing and convergence on the boundary.  The very hot temperatures (around 100 deg F, see heat axis on map above) also helped to break through a stout layer of warm air aloft (700 mb temperatures > 14 deg C, not shown) to initate a storm.

SPC mesoanalysis graphics at 3:00 pm CDT also showed heating and steep low-level lapse rates poking into southwest SD along the boundary, along with large surface vorticity (light blues lines) along the same boundary:

This combination of strong low-level lapse rates and vorticity (slowly "spinning" air) along the boundary apparently set the stage for rapid vertical stretching beneath any storm updraft that could form and establish itself directly over the sharp boundary.

Typical supercell tornadoes require significant low-level wind shear (a rapid change in wind speed and direction with height) in the lowest 1 or 2 km.  But this was definitely not a typical case, as there was very little low-level wind shear (storm-relative helicity, or SRH) over southwest SD on SPC mesoanalysis graphics at 3:00 pm CDT:

However, the storm updraft essentially "locked on" to the sharp wind shift boundary to provide the necessary vorticity ("spin") to stretch vertically into a tornado, in lieu of significant SRH/low-level wind shear (again, note the large surface vorticity along the boundary on the earlier SPC graphic).

In the 2nd panel of the graphic immediately above, also notice that there was just enough deep-layer vertical wind shear (25-30 kt) to support a supercell storm.  So, instead of only brief updraft stretching from steep lapse rates along a boundary, the storm became more organized and long-lived through interacting with this deep-layer shear.  Indeed, it took on supercell characteristics with a hook-shaped echo and clear rotation detectible from a distance on radar (see radar inset below):

Thus, the tornado was able to keep going for quite awhile.  With the contribution of non-supercell processes (stretching of boundary vorticity in a steep lapse rate setting) and not a lot of rain falling from the storm (see photos at top), the high cloud bases and potential evaporative cooling did not seem to be a negative factor.  The tornado was also quite visible from a distance due to these same high cloud bases.

Because this setting combined both non-supercell ("landspout") processes with supercell characteristics, I would call this a "hybrid" tornado event.  WIthout the boundary and the storm directly linked to it, the tornado probably would not have occurred in the absence of notable SRH.  But with enough deep-layer shear to support a supercell, that was also part of the "recipe".

Farther north (not shown), tornadoes were expected over North Dakota on June 29, bur did not occur.   This was probably due in part to the slight cooling influence of an outflow boundary from morning storms.   Without as much surface heating, a strong layer of warm air aloft (temperatures around 14 deg C at roughly 10,000 ft MSL) helped to "snuff out" evening storms that managed to form over south-central North Dakota before they could really mature and take advantage of increasing low-level shear/SRH as nighttime approached.

Believe it or not, another tornado from processes similar to those on Saturday formed again on Sunday afternoon (June 30) over south-central South Dakota near the town of Burke, along yet another sharp boundary with strong heating and ingredients lining up enough to support a tornado:

But to reiterate, such tornadoes are almost impossible to forecast hours in advance because everything has to come together just right, and typical supercell tornado forecasting factors (such as SRH and lower LCL heights) don't work with these type of "hybrid" tornadoes.  Yet when the proper ingredients do come together, these "hybrid" tornadoes can be quite spectcular visually!

- Jon Davies  7/1/19