Wednesday, April 7, 2010
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").
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
Thursday, April 1, 2010
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