Friday, March 23, 2018

Alabama tornado outbreak March 19, 2018 -- A look at the meteorological setting

My wife Shawna has been bugging me to start up my severe weather blog again 😊, so I thought I'd do a post about the setting for last Monday's episode of tornadoes in northern Alabama (March 19, 2018). With the exception of the February 24 outbreak from northeast Arkansas to south-central Tennessee, this was the first tornado outbreak heading into spring after a rather quiet winter.

Here is my preliminary mapping of the tornadoes in northern and eastern Alabama on 3/19/18, from online NWS surveys:

The tornadoes were produced mainly by three different supercells moving across the northern third of Alabama, as is suggested by the way the tornado damage paths line up from west to east or east-southeast along three different paths. Later in this post, I'll pay particular attention to the evening tornadic supercell that struck Jacksonville, Alabama with an EF3 tornado.

The Storm Prediction Center (SPC) did a great job of forecasting this outbreak. Also, local awareness and timely NWS warnings resulted in no deaths and only a few injuries. That's always great to see!

Forecast-wise, it wasn't too difficult to pick out the potential for tornadoes. The model forecast below showed strong west to east winds at roughly 18,000 ft MSL (a "southern-stream" jet, see thick white line and arrow) into the southeastern U.S., with an embedded short-wave disturbance approaching Alabama at 7 pm CDT on 3/19/18. The model forecast of 0-1 km energy-helicity index (EHI; see inset), which combines instability and wind shear important for supporting tornadoes, was also maximized over northern Alabama ahead of the short wave where thunderstorms were expected:

A side note about west to east southern-stream jet wind axes: Tornadoes seem to be more common along and just north of the southern-stream jet, rather than further south. This has to do with jet stream dynamics and related issues I won't get into here. But it's worth noting that the day before (3/18/18, not shown), tornado potential that was forecast for eastern Texas but did not verify was in an area south of this southern-stream jet axis. Yet on 3/19/18, when supportive tornado environment parameters were maximized just north of this same southern jet stream core, a number of tornadoes occurred.

The 3-panel radar graphic below, from NWS BMX radar, is after 7 pm CDT (00 UTC), and shows the three main tornado-producing supercells (marked 'A', 'B', and 'C'):

Supercell A near the AL/TN border had been producing tornadoes during the prior hour and was weakening, at the end of its tornado-production phase. Supercell B was the largest, beginning in Mississippi before 5 pm CDT, and produced a number of weak short-track tornadoes across Alabama. However, the tornadic supercell of the day was the last one to form (supercell C), which developed rapidly after 7 pm CDT, and produced the long-track EF3 tornado that hit Jacksonville; this tornadic supercell was responsible for at least 7 injuries.

Why was cell C the strongest tornado producer? It's hard to say, but there are some possible clues. First, supercell B moving across Alabama during the late afternoon was large and long-lived, generating a traveling meso-low over northern Alabama (see the southernmost red "L" on the surface analysis below) that helped to focus and converge the low-level wind environment:

This wind stream out of the south and southeast can be seen on the streamline map of surface winds below at 7 pm CDT (00 UTC), flowing and converging into the area where supercell C formed rapidly between 00 UTC and 01 UTC (see the earlier 3-panel radar graphic):

This area of increased low-level wind flow (also helped by the tendency for low-level winds ahead of storm systems to increase near and after dark) would certainly increase low-level wind shear (storm-relative helicity, SRH) that supports supercell tornadoes. The significant tornado parameter (STP) incorporates low-level wind shear/SRH as one of its key components, and, in the 8 pm CDT (01 UTC) graphic below from the SPC mesoanalysis page, STP was quite large and very focused just east of Birmingham:

Another factor may have been the location of the stationary front (refer back to the surface map shown earlier) over east central Alabama. There's been plenty of research to indicate that tornadoes favor surface boundaries, such as warm fronts and stationary fronts, which increase wind shear through backing of surface winds. Supercell C from our earlier radar graphic would have interacted with this stationary front as it moved east and east-southeastward, improving its chances of producing tornadoes. However, as this supercell moved over into Georgia, it encountered increasingly cooler surface air northeast of the stationary front, and only produced an additional brief tornado or two.

So, with unobstructed wind flow and SRH to the south of newly formed supercell C northeast of Birmingham, it only took about 45 minutes from the storm's initiation for it to begin generating tornadoes, which is an impressively short time. The increasing southerly low-level flow, due both to supercell B's inflow and the diurnal increase of SRH near dark, in addition to the cell's interaction with the northwest-southeast stationary front, may have all contributed to this storm's efficient tornado production. The third tornado from supercell C (see photo below) hit Jacksonville shortly after 8:30 pm CDT (0130 UTC):

This tornado continued onward to the Georgia border for a track of over 30 miles, and was over a mile wide at one point. It is truly fortunate that this specific tornado only caused 4 injuries.

I'll try to do more posts as severe weather warrants and time permits this spring. Thanks for reading!

- Jon Davies 3/22/18

PS:  For anyone interested, my detailed peer-reviewed study of the meteorological setting for the devastating Joplin tornado back in 2011, published last December, is online here.


Monday, May 30, 2016

Counting Tornadoes: Dodge City 24 May 2016 - How Many?

**** Note  -- I added an important update at the bottom of this post on 6/1/16 ****

Most storm chasers count the number of tornadoes they see on any given day.  The bigger the number, the more "impressive" a chaser you are, I guess.  But many times, from a useful operational meteorological view, such "counts" are inflated.

Last Tuesday (May 24) near Dodge City is a good example.   I've heard several storm chasers tell me they saw anywhere from 10 to 17 tornadoes on the prolific tornadic supercell that moved from near Minneola across Dodge City, thankfully missing any direct hits on towns.

My wife Shawna and I followed the storm starting from a distance (near Bucklin, Kansas) up to fairly close just south of Dodge City.  I'm conservative regarding tornado counts, and we saw at least 4 "primary" tornadoes, each from its own mesocyclone cycle as new parent circulations developed north or east of each tornado, all fascinating to watch.  

Any other tornadoes seemed to be brief and transitional, including a rope tornado in the inflow to the 2nd primary tornado.  One of the mesocyclones just SW of Dodge City produced several visual "spin ups" with occasional multiple vortices before putting a fully condensed tornado on the ground.  But from a practical viewpoint, I would count that as one intermittent tornado circulation (again, same mesocyclone) instead of several individual tornadoes.

Following is documentation of what we saw using video captures (some of these contrast enhanced to make it easier to see features).   All views are toward the west-northwest or northwest.

Not counting any brief early touchdowns we couldn't see, here's the first primary tornado touchdown northwest of Minneola, viewed from near Bucklin (25 miles away!) around 6 pm CDT, along with a wide view of the storm (Bill and Anna Stromberg were in the vehicle ahead of us):

This first tornado was on the ground around 25 minutes, and here are several images at various zooms.  Rain wrapping to the west of the tornado soon made it harder to see, and a new lowering (at right) signaled new mesocyclone development northeast of the tornado:

Here's the "rope-out" phase of this first tornado, followed by an image that shows where the 2nd primary tornado soon would form:

When the new tornado formed, a thin rope tornado was visible in its inflow stream (see 2nd photo below). Meanwhile, the primary tornado became large but was in poor contrast when viewed from the southeast:

Here's the "rope-out" phase of this 2nd primary tornado, which was on the ground around 15 minutes.  Notice a new mesocyclone again to its north or northeast, with a new tornado forming.  This new tornado was hard to see (again, poor contrast), did not appear to last that long, and seemed somewhat transitional in nature.  So I don't know whether it would be considered a "primary" tornado:

During this phase after the "transitional" tornado dissipated, it was hard to tell if the associated mesocyclone was splitting or just elongating eastward closer to Highway 283.  Here's a photo Shawna took:

As we drove closer, the lighting improved, and rapid rotation from this mesocyclone was evident, soon resulting in dust whirls and multiple vortices under a low ragged cloud base. 

After a couple rounds of "spinning and relaxing", a fully condensed new primary tornado developed under this mesocyclone, and lasted at least 10 minutes:

After a while. this new tornado began to narrow and elongate, signaling another "roping" phase as yet another new mesocyclone formed over Dodge City:

Fortunately, the elongating tornado moved west of Dodge, while the new mesocyclone dropped a new tornado (the last primary one) north of the town (notice Dodge City and the storm chaser/truck traffic in the foreground):

This last tornado appeared to be on the ground for 5-8 minutes or so, after which we lost sight of it as heavy rain from another storm overtook us from the south around 7:15 pm CDT:

So, based on  what we could see with this supercell, there were 4 primary tornadoes on the ground for more than 5-10 minutes each, which seems to agree generally with the map NWS Dodge City put on their web site:

We also saw one peripheral tornado in an inflow band, and one transitional tornado during a mesocyclone evolution phase that was difficult to assess, making 6 total for my conservative count.  I don't doubt that there were other brief peripheral or transitional tornadoes that one could count differently, but the 4 primary ones we saw appeared most significant.  Sadly, the tornado that passed just west of Dodge City damaged several homes and resulted in 2 injuries.   

This event was also notable for the circus of chasers, storm tours, and local traffic jamming up Highway 283 south of Dodge City.  One tour van nearly ran over my wife when they abruptly pulled off the highway, even though she was standing well off the road.  I'm pretty upset about that, and clearly the traffic issue continues to increase the danger of storm chasing.

If I get time in a few days, I'll post something about the meteorology of this severe event.

Jon Davies - 5/30/16

Update on 6/1/16:

After viewing responses and photos from several other chasers, I clearly need to update my observations/opinions above.  

I'll use Dave Lewison's excellent images from Facebook to highlight the changes... here's his Facebook photo link that you should check out.  He was farther north on Hwy 283 and in better lighting conditions than Shawna and I were.  On a couple of his images re-posted and labeled here, I've inserted/referenced small images from my original post to roughly indicate the corresponding features and phases.

First, when the 2nd primary tornado was on the ground west of Hwy 283, the rope tornado I originally called an "inflow band" tornado was actually a tornado from a new mesocyclone:

Second, shortly after this image, 2 rope tornadoes or vortices were visible under this new mesocyclone northeast of the 2nd primary tornado (I missed these), suggesting that there might have been 3 tornadoes on the ground for a short time:

Third, these appeared to morph into a somewhat larger rope tornado northeast of the 2nd primary tornado that we could not see from farther south in poorer lighting conditions:

And last, when the 2nd primary tornado "roped out", this larger rope tornado became a sizable and probable primary tornado (not a "transitional" tornado as I originally characterized it) while yet another new mesocyclone was forming to its east or northeast:

I don't know how long this new primary tornado lasted because we were fighting traffic and dealing with Shawna's tour bus encounter.  But it seems clear from Dave's photos that this was a separate mesocyclone/tornado cycle of its own starting with the earlier rope tornado(s) northeast of the 2nd primary tornado, and not a short "transitional" phase as I characterized it in my original post.

Given this new information, I'd probably update the number of primary tornadoes on the Dodge City NWS map to 5 as follows, changing the "?" to number 3, and moving what I'd shown as numbers "3" and "4" to numbers 4 and 5:

With the new information above, I would probably up my conservative tornado "count" (not that important, and only an opinion!) to 7 tornadoes, and maybe 8 depending on how one might view/interpret things.  But certainly, I don't intend that to be definitive, and my original post wasn't meant to ruffle feathers.  I only intended to get some discussion going about a fascinating event.

Thanks to Dave Lewison for his images, and for the responses and additional helpful information from several chasers on my wife's Facebook page!

Jon Davies - 6/1/16

Sunday, May 1, 2016

Why no tornadoes? April 26, 2016 outflow boundary in northeast Kansas

I haven't done a blog post in a long long time.  But several people asked me why last Tuesday (April 26, 2016) wasn't a big tornado day in north & northeast Kansas as had been forecast.  So... it seemed a useful topic to start up posting again. 

With our many computer models, it's not hard to see potentially significant systems coming several days in advance, but whether they produce tornadoes or not often has much to do with features such as outflow boundaries and their orientation, which models certainly can't forecast very well.

Tuesday morning April 26 saw a strong area of thunderstorms move east-southeast across northeast Kansas and over into Missouri, producing a strong trailing boundary of cool outflow air (see satellite photo and surface map below).  

Strong surface-based instability (large CAPE) was present south of this boundary, but southeast winds producing wind shear supportive of potential tornadic supercells were largely limited to the area north of this boundary, separate from the warm sector instability (see SPC analyses of sfc-based CAPE & storm-relative helicity/SRH below at 20 UTC / 3 pm CDT). 

By 20 UTC, storms were forming along this outflow boundary, but because of the south-southwest to north-northeast mid-level flow aloft (see SPC 500 mb chart below), these storms moved immediately into the cooler surface air north of the outflow boundary.  

I adjusted the 21 UTC (4 pm CDT) Topeka RAP analysis profile (below), which was forecast too warm at the ground, by substituting the 21 UTC observed TOP temperature (69-70 F). The large green area (convective inhibition) on this profile confirmed the cool near-surface air north of the outflow boundary and the stable low-levels, which work against tornadoes, even though rotating storms (supercells) can still form in the overrunning instability and wind shear above the cool surface air.

Indeed, several supercells did develop north of the outflow boundary, although none could produce tornadoes in the cool surface air.  The radar panels below show one of these supercells (white arrow) that developed southwest of Topeka and moved north-northeast, prompting a radar-based tornado warning just northwest of Topeka shortly after 22 UTC (5 pm CDT).

The photos below show the cool air clouds north of the stalled outflow boundary, and the non-tornadic supercell as it passed west and northwest of Topeka within the cool surface air.

A somewhat similar situation (not shown) involving outflow and a stationary front over northeast Texas three days later on April 29 did generate a significant tornado at Lindale Texas, northwest of Tyler.  In that case, mid-level flow aloft was more west to east so that supercell storm motion was mostly parallel to the boundary, instead of perpendicular and up into the cool air, as in the Topeka April 26 case.

Outflow boundaries can help produce tornadoes, or they can hinder tornado production... it all depends on their orientation and strength relative to how storms move near them.

- Jon Davies 5/1/16