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

Tuesday, May 12, 2015

May 9, 2015 cold core tornado setting over eastern Colorado

I haven't done a weather analysis blog post in nearly 3 years (wow, long break!).  But Saturday's picturesque cold core tornado event (5/9/15) in eastern Colorado prompts me to do a short write up.

I've heard from several people who didn't realize this was a cold core setup, so I'll present a few maps to show that it was.  Just click on each image below to see it larger.

First, the surface pattern (3 surface maps below at noon, 3 pm, and 6 pm MDT) was a fairly classic cold core surface low pattern with several boundaries converging into the low... the dryline/pacific front (dashed brown), the warm front (red solid), and the cold front/occluded front (blue and purple solid):

Second, what made it a true cold core setting was the close proximity of the strong 500mb closed low (NAM 9 hr forecast for 21 UTC/3 pm MDT) over Colorado:

Notice the 500mb cold air aloft (dark blue) coming out over the warm sector in eastern Colorado and western Kansas.  The graphic above also shows the enhanced storm-relative helicity (SRH, a source of low-level storm rotation) forecast over eastern Colorado north of the surface warm front (circled).

Given the cold air aloft over the relatively warm surface air with dew points in the low 50's F and easterly surface winds, the stage was set for rapid upward stretching and tilting of horizonal vorticity ("spin") associated with the 0-1 km SRH, for possible tornadoes.

The graphic below shows the location (arrow) on satellite of the storm of interest north of Lamar (see also the circled "S" on 2107 UTC surface map earlier) that encountered the increased SRH as it moved north with the 500 mb winds across the warm front seen in our earlier surface maps:

With subsidence (descending stabilized air) over western Kansas in the wake of the midday storm cluster there, this cell was the easternmost and primary cell to take advantage of the warm frontal zone in a favored location just east of the surface low. 

The accompanying radar image above is a couple hours later as the supercell produced a large tornado (see photos below), before being undercut by the cold front/occluded front (radar "fine line", see arrows above) around 6 pm CDT as it moved north into the cold air.  

Another clue that this was a cold core setting is the smallness/compactness of the supercell, allowing for getting the whole storm and tornado into the first photo image above (by my wife, Shawna).  Many (if not most) tornadoes associated with cold-core 500 mb lows are small and weak, but this day saw at least one large long-lived tornado due in part to the strong dynamics from the big upper low nearby within a full longwave trough in the jet stream.

I should mention that the midday storm cluster and outflow over Kansas on Saturday would have killed most settings regarding tornado potential.  But with cold core patterns like this, the atmosphere can recover quickly with the very cold air aloft if there is sun's heating and dew points in the low 50's F, particularly over elevated terrain like eastern Colorado. Cold core settings are also just about the only pattern where you can get tornadoes with a supercell only 60-80 miles from accumulating snow, as was happening near Limon/Denver northwest of the surface low.

A truly fascinating day, with the best part being that the tornadoes occurred over wide open country without damage or injury.

Jon Davies - 5/12/15