After-dark EF4 tornado in NW Iowa 6/25/10 - a look at the environment

Violent (EF4-EF5) tornadoes are rather rare after dark in the Plains. Last Friday's tornadoes during 03-05 UTC 6/26/10 after dark in northwest Iowa (see video image above) included an EF4 intensity tornado near Sibley, Iowa, which certainly makes the environment worth examining.

The tornadoes occurred just ahead of a cool front moving slowly southeastward (not shown) and an approaching upper trough (not shown), associated with the west end cell of a line sagging southward over northern Iowa (see SPC mosaic radar overlays above). The 03 UTC SPC mesoanalysis of 0-1 km energy-helicity index (EHI; see 2nd image above), which combines MLCAPE and 0-1 km SRH, showed very large values (near 10!) ahead of this west-end supercell over northwest Iowa, suggesting strong potential for low-level mesocyclones and possible tornadoes with discrete or tail-end supercells. The SPC mesoanalysis of low-level CAPE (0-3 km MLCAPE, see 3rd graphic above) at the same time also showed a notably surface-based setting over northwest Iowa, with low-level MLCAPE values near 100 J/kg. From a 2009 NWA electronic journal paper by Davies and Fischer, environments with large CAPE and SRH that are also relatively surface-based offer important support for tornadoes after dark, but aren't that common in the Plains due to nighttime cooling beneath typical warm layers advected eastward from the desert southwest. This latter issue did not appear to be a problem last Friday evening over northwest Iowa.

The 03 UTC RUC analysis sounding at Sheldon, Iowa (just south of the tornadic tail-end supercell; see 4th graphic above) confirmed large MLCAPE and SRH (near 4000 J/kg and 500 m2s2) and sizable deep layer shear (> 40 kts), along with a relatively surface-based setting (0-3 km MLCAPE > 100 J/kg, and MLCIN around -50 J/kg), all excellent supporting ingredients for tornadoes with supercells in that environment. In contrast, 110 miles to the east-southeast with another tornado-warned embedded supercell near Clarion, Iowa, the RUC sounding (see last graphic above) showed _no_ low-level CAPE, and MLCIN was quite large (-250 to -300 J/kg) with not nearly as much total MLCAPE above the CIN layer (only around 1600 J/kg),. This suggested somewhat "elevated" nighttime storms with considerably less tornado potential, and no tornadoes were reported in this area farther east.

NWS Sioux Falls has a page detailing their survey of the EF4 tornado near Sibley, Iowa, north of Sheldon.

- Jon Davies 6/30/10

Matt Hughes

This has been a sad couple weeks with Wichita chaser Matt Hughes (of Discovery Channel's Storm Chasers show) hospitalized in serious condition, and then passing away this past Wednesday. I am so very sorry for his family's loss.

I haven't known Matt for very long, but it is clear he was a bright and enthusiastic person, with much passion, and also very sensitive.

With the deadly Yazoo City MS and Oklahoma City OK tornadoes this year, and now Matt's death, I hope these events will serve as reminders to weather enthusiasts that there are more important things in life besides storm chasing, and to keep all that in its proper perspective.

Matt, we will remember you fondly...

- Jon Davies 5-28-10

Brief "chaser convergence" thoughts, and Missouri cold core tornado setting 5/20/10

Wednesday's massive chaser convergence and careless driving in central Oklahoma has been a huge topic of conversation the past couple days. Shawna and I actually decided not to chase that day because of the expected chaser hordes and narrow severe focus in central Oklahoma, and we both had work to do. Sometimes one asks, "If there's already hundreds of people out there shooting the same storm, what's to learn and what's the point of being out there adding to the dangerous traffic jams?" Wednesday was just such a day. Things don't look to improve in similar future situations, unless someone gets killed in traffic and it is well documented. Shawna and I are getting increasingly picky about going out on "big" days with a small focus drawing hundreds of chasers and weather yahoos to the same spot.

Anyway, stepping away from the situation in Oklahoma, Wednesday 5/19 brought some "cold core" action to the Garden City/Dodge City area of southwest Kansas in the form of "landspouts" along a surface boundary close to the 500 mb low, photographed nicely by Mike Umscheid. And Thursday 5/20 brought a few "cold core" supercell tornado reports to the Sedalia area in Missouri east-southeast of Kansas City (see photo above). I had family visiting in town, so no storm chasing for me or Shawna. But with my interest in such settings, the Thursday setup is worth documenting briefly. Thankfully, as with most "cold core" setups, none of the tornadoes were strong.

The 2nd graphic above shows visible satellite with overlaid surface and 500 mb features at 2145 UTC. Notice the boundary intersection in west-central MO, similar to the "cold core" tornado composite shown in this paper. This can be a favored area of severe focus (enhanced low-level shear and forcing) for tornado development when a midlevel low (strong cold air aloft) is nearby to the west (northeast KS on 5/20) and there is some instability. In this case, the N-S boundary wasn't a dryline, but a subtle wind shift intersecting the warm front southwest of Sedalia at late afternoon, notable on satellite along with some clearing for sun's heating. The 3rd graphic shows selected images from the SPC mesoanalysis at 2200 UTC , including the midlevel low, total CAPE (near 1000 J/kg over west-central MO, but certainly not as impressive as down in Texas), low-level CAPE (a significant maximum near the boundary intersection), and SRH (enhanced northeast of the warm front, but probably under-represented closer to the front in west-central MO). The presence of the 500 mb low in northeast KS and the low-level CAPE maximum over west-central MO near the boundary intersection were probably the biggest "heads-up" flags seen here for tornado potential in what otherwise looked like a fairly benign environment.

The 4th graphic above is a radar image at about 2340 UTC when the tornado in the photo above (likely EF0) was occuring near Sedalia (see arrow for cell location). The RUC analysis at 2300 UTC (last graphic above), as with many "cold core" type settings, showed most of the CAPE below 500 mb, suggesting sizable low-level stretching. The model-derived estimated hodograph (same graphic) suggested good clockwise curvature to the wind profile and more low-level shear than shown on the SPC mesoanalysis during the afternoon. So, while not an easily forecast event, the presence of these ingredients suggested taking rotational signatures on the nearby Pleasant Hill MO radar and spotter reports very seriously. Indeed, the Kansas City area NWS office did a good job jumping on the situation with tornado warnings and statements as the situation developed and evolved prior to the Sedalia tornado, which is what situational awareness is all about.

- Jon Davies 5/21/10

Monster Medford OK meso & tornadoes 5/10/10

My wife Shawna, her son Zach, and I intercepted the large supercell near Medford OK last Monday (5/10/10). As a rule, I don't chase storms going 50+ mph, but we decided to try it, given the high-end potential tornado environment (see significant tornado probability mesoanalysis graphic for 20z, 2nd image above, an experimental product that Bill Togstad in Minneapolis and I have been working on). We were prepared for very fast storm motions, probably giving us only a few minutes of intercept time.

There were many chasers on the Wakita-Medford storm, but we managed to avoid most of the crowds (and all the road "drama" south of Wakita) by pulling off on a county road 5 miles north of Medford and letting the dangerous storm come toward us. As it emerged from the haze and broken low clouds near Medford around 3:50 pm, the mesocyclone at first appeared to be rain-wrapped, but then we could begin to see features behind the rain curtains, including a narrow tornado on the north side of the meso (see 3rd image above, please excuse the poor video quality). We were using WxWorx due to some internet problems, and Threat Net was indicating that the meso would pass just to our northwest. Not so! It quickly became apparent that the storm was moving more rightward/eastward, and that we needed to move south FAST, out of the path.

It was unnerving to see a meso that large coming toward us at nearly 60 mph (a mile a minute!), and the pressure drop with the fast movement was causing our ears to "pop", the first time I remember experiencing this with an approaching mesocyclone. As we blasted south, the first tornado dissipated, and a larger one formed near the center of the meso, with rain and dust curtains moving around it. In fact, thoughout much of the meso, what appeared to be wispy "spin ups" and brief condensation columns were also occurring, causing us to wonder if much of the mesocyclone might actually be a very wide lower-end "tornado" a mile or more across, with more intense vortices moving around inside it (see 4th image above). There's certainly no clear-cut answer to that. Regardless, we needed to get out of the way.

As the larger cone tornado within the meso sped toward us (see 1st image above), we cleared the path of the meso and RFD winds to the south, and pulled over just north of Medford. Looking back to our north, the tornado(es) were no longer visible, as rain and hazy sunlight obscured our view (chasers farther east reported seeing only a rain-wrapped meso at this point). Looking to our west, we noticed an anti-cyclonic wall cloud and circulation on the trailing gust front (a feature not that uncommon), whch produced a very brief spin up beneath a small condensation funnel aloft (not shown). With the incredibly fast storm motion, our chase was over in only minutes, and Shawna's son Zach had videotaped his first tornado(es).

The last graphic above is a schematic and approximated path of this large fast-moving mesocyclone (several miles across), one of the largest and fastest I've ever seen. Despite the damage path from near Wakita OK to east of Arkansas City KS, it was great to hear that there were no deaths or serious injuries from this dangerous supercell.

For those on Facebook, additional video captures are under Shawna Davies' photo folder titled "5-10-10 Medford OK meso /tornadoes".

- Jon Davies 5/15/10

Dick McGowan's photos of 4/6/10 Wakeeney KS tornadoes

Dick McGowan (www.tornadolive.com) was kind enough to send me a few images from his Wakeeney KS chase yesterday, to compliment my earlier post about the meteorological setting for these unusual tornadoes (surface temperature mid 60s F, dew points mid-upper 40s F).

The first image shows the first tornado north of the interstate, with a condensation funnel well aloft and dust circulation at the ground. Dick says this tornado formed rapidly from a bowl-like high-based mesocyclone circulation and lasted maybe 2-3 minutes. The last image shows this tornado dissipating (right) with a 2nd tornado at left in what appears to be an occluded area. Dick says this 2nd tornado lasted about 3 minutes, and even hit a small farm structure, lifting pieces of debris into the air.

Looking back at the Hays RUC profile in my previous post, the high cloud bases and steep low-level lapse rate suggest to me that these tornadoes may have been a "hybrid" type of event, combining supercell and non-supercell processes, with significant heating at the surface (possibly upper 60s F) and very cold temperatures not far aloft (0 deg C around 700 mb). Dick says that he had the visual impression of updrafts that were shrinking horizontally and stretching very rapidly in the vertical as the tornadoes occurred. At any rate, these weak tornadoes occurred in an odd location relative to the surface pattern, away from the deeper moisture, and were the only ones I know of that were photographed over the past 4 days of severe weather in the Plains.

Dick, thanks for sharing these images.

- Jon Davies 4/7/10

Update 4/8/10 AM - I looked back at the surface analysis I did in the previous post, comparing it to Dodge City radar base reflectivity just before 2100 UTC on 4/6/10, and found I missed a boundary (Dick McGowan also mentioned this). I've drawn this in on the updated surface map (with radar) above, suggesting that the Wakeeney tornadoes occurred at or near a pretty decent boundary intersection, which may have added some nice vorticity ("spin").

Surprise tornadoes near Wakeeney KS 4/6/10?

Although not in yesterday's NWS storm reports for 4/6/10, storm chaser Dick McGowan (on his way to Oklahoma from Denver) happened to be in the right place at the right time, apparently photographing a couple tornadoes near Wakeeney, Kansas, just north of I-70. I haven't seen any pictures posted yet. but this was well northwest of the true warm sector, and in the vicinity of a secondary front moving across northwest Kansas. This wasn't a forecastable event, but it is worth looking at because it was certainly unusual.

The first graphic above is a SPC mesoanalysis estimation of 0-3 km MLCAPE and surface vorticity at 2100 UTC (4 pm CDT), with radar reflectivity overlaid. Notice the pronounced low-level CAPE maximum in the Wakeeney/Hays KS area. By itself, this doesn't mean much, but given its location far away from the warm sector over eastern Kansas, it suggests an area of cold air aloft combined with surface heating. The surface analysis just before 2100 UTC (2nd graphic above) shows an east-west frontal boundary in the area, with surface temperatures in the mid 60s F along this front (70s F further south) and dew points in the mid-upper 40s F. The RUC midlevel map analyses at 2100 UTC (3rd graphic above) definitely show cold air coming in aloft at 700 mb (0 deg C over northwest Kansas) and a closed 500 mb low over northern Colorado. So there was warmth at the surface, cold air aloft at 700 mb, and a frontal zone with just enough moisture in the local area. One probably wouldn't call this a real "cold-core" setting given the odd surface pattern and the distance from the midlevel low, but the cold air aloft with this system certainly played a role.

Visible satellitie and radar (4th graphic above) show the location of a small cell (possibly a mini-supercell) near Wakeeney, moving east, with plenty of clear skies (heating) to its south and east. This is near the time the tornadoes were reported. A RUC analysis sounding at Hays Kansas at 2100 UTC (east of Wakeeney, last graphic above), modified for more heating than the raw RUC file indicated, yields 200-300 J/kg of CAPE, all bunched below 500 mb, with decent wind shear, fairly steep low-level lapse rates, and small CIN. This is another one of those small CAPE soundings I've discussed recently, and the type of environment that would likely generate rapid low-level stretching within local updrafts. Given all the above ingredients, it's not too much of a "stretch" to see how the setting might support some weak tornadoes with the isolated cell over northwest Kansas. But it was certainly not an event that could really be anticipated.

What an oddball case! And as I write this, more cold air aloft from the same system is now generating a small late pm severe cell over north Kansas City, just behind a secondary surface cold front.

- Jon Davies 4/7/10

Another small CAPE/large shear tornado setting: 3/28/10 in North Carolina

Last Sunday afternoon and evening (3/28/10) in central North Carolina was a great example of a small CAPE setting (300-500 J/kg) that had very large low-level and deep layer shear and was able to support a strong tornadic supercell (see photos above), including an EF3 tornado at High Point, just southwest of Greensboro.

Lowest elevation base reflectivity from radar and a surface map just before 2300 UTC (7 p.m. EDT, see 2nd graphic above) showed this supercell moving northeast near a southwest-northeast stationary front with a well-defined moisture axis (dew points low to mid 50s F) impinging on it from the south. The SPC mesoanalysis at 2200 UTC (3rd graphic above) showed very large storm-relative helicity (SRH) over central NC but only weak/marginal CAPE, with larger CAPE indicated back to the southwest. But as suggested by the moisture axis on the surface map, strong moist advection was definitely taking place across North Carolina.

This case highlights how important it is to update and modify model soundings with actual surface data when estimating environments. The raw RUC sounding at Asheboro (HBI, about 20 miles southeast of the tornadic supercell track, see 4th graphic above) showed only 61/55 F as temperature and dew point at 2300 UTC, but observed surface data indicated at least 63/57 F, which can make a considerable difference in CAPE computation. Modifying this sounding with the observed data (see last graphic above) and using a surface-based lifted parcel (the low-levels were nearly saturated) changed the CAPE from barely over 100 J/kg to well over 400 J/kg, a nearly four-fold increase! Combined with the very large 0-1 km SRH (> 500 m2/s2) and deep layer shear (near 70 kts), this was more than enough to support a significant tornado that caused injuries.

When using model soundings to estimate a severe weather environment, it's vitally important to watch how well the model is performing by checking actual observations, and to make adjustments when necessary... no April fooling!

- Jon Davies 4/1/10