Monday afternoon, an energetic upper trough--with most of its jet energy on the back side--was digging through the wrn CONUS with 543-dm 5H closed low centered over swrn UT. Associated sfc pressures were anomalously low from the 4 corners states swwd thru the lwr CO river valley; a 988 mb sfc low was centered near PUB, and sfc pressures as low as 991 mb reached the srn half of AZ (which is in excess of 5 standard deviations below climatological norms for that area; i.e., very, very rare for early April). Forcing for upward vertical motion likely ejected across the central High Plains by late afternoon in support of convective development. A diffuse dryline would be the focus for initiation, with the segment from some distance NW of AKO (where in intersected a sagging cold front; see rough sfc analyses below) to 50SW GLD looking to have the highest probability of initiation based at least partly on background modulation on the synoptic scale.
A rather veered nocturnal LLJ acted to shallow the return-flow moist layer somewhat over the srn Plains by Monday morning, with boundary layers only 1 km deep at DFW/OUN capped by deep/seasonably warm EML layer immediately above. Throughout the day a modest sely LLJ re-developed over KS/srn NE, as the primary LLJ TX-OK-MO gradually backed... contributing to moisture advection E of dvlpg/effective dryline. By 00Z a 1250 m moist layer was observed at DDC (although this was co-located with the dryline); while 1500 meter depths were observed at LBF (just south of aforementioned cdfnt) and LMN; and a nice 1900 m moist layer depth was observed at TOP.
The 00Z LBF RAOB's T/Td profile looked gorgeous, with backed mid-level trajectories during the day having maintained cool capping aloft (e.g. 1 C at 7H) above the boundary layer, and 68/55 F at sfc contributing to ~2000 J/kg MLCAPE and ~0 J/kg MLCIN; and moreover, a profile that would support a weak CIN environment even with diurnal cooling of the near-surface layer. However, the DDC RAOB showed 7 C at 7H, as did the DEN RAOB with its deeply mixed sfc-based layer. Model initializations/mesoanalyses also indicated the tightest gradient in the capping aloft (e.g 3-6 C at 7H) was hugging the NE-KS border region late aftn-evening, with trajectories there resulting in WAA having developed in the capping layer. In fact, mesoanalysis time series showed 5-6 C at 7H consistently in the near-storm environment of the small storm cluster that moved from 40ESE LIC to 20N MCK and ultimately produced the damaging nighttime tornado.
In Davies and Fischer 2009 (http://www.nwas.org/ej/2009-EJ3/), hereafter DF09, 1705 RUC soundings associated with non-tornadic, weak tornadic, and significantly tornadic supercells were examined from a thermodynamic and vertical shear standpoint to see which parameters might best discriminate among the categories both during the day and at night. Ultimately, MLCIN proved to be the strongest discriminator among nighttime non-tornadic supercells and nighttime significant tornadic supercells... with almost no overlap in the middle 50% of each category, a strong overall difference from the MLCIN distribution within the daytime subset. MLCIN of less than 50-75 J/kg was very common among nighttime weak and nighttime significant tornadoes. In fact, 90% of the nighttime significant tornadoes occurred with less than 75 J/kg MLCIN. (These overall findings are not surprising when viewed in the context of Davies' groundbreaking paper on CIN & LFC heights associated with tornadic and non-tornadic supercells: http://www.jondavies.net/waf796/waf796.htm). 0-1 km SRH also showed a pretty dramatic increase when moving from nighttime nontornadic supercells to nighttime significant tornadoes, especially compared to the daytime subset, but a lot of overlap occurred among the nighttime categories.
In a companion paper (Fischer and Davies 2009 at http://www.nwas.org/ej/2009-EJ4/), hereafter FD09, we looked carefully at two long-lived supercells in rather strong CIN environments which produced 2 and 4 nighttime sig tors respectively: the 5/29/08 Jewell and Belleville KS area tornadoes; and the 06/11/08 Salina, Champan, Manhattan, and Soldier KS area tornadoes. Additionally... the 04/24/08 Beloit KS sig tor and 4/09/11 Storm Lake IA area sig tors had a lot of similar storm-scale and environmental characteristics to those storms in FD09. All of these events involved well-established long-lived supercells which started out generally in rather strongly/deeply mixed environments, and toward darkfall progressed fully into the moist axis as the boundary layer diurnally cooled. All 4 events' near storm environments possessed 1500-3000 J/kg MLCAPE, 80-140 J/kg MLCIN, 500-800 m2/s2 0-1 km SRH, 6.5-11.5 0-1 km EHI, and 60-80 kts 0-6 km bulk shear. In all cases the CIN resulted primarily from diurnal cooling beneath a warm EML base (i.e., moisture was seasonably favorable and reasonably deep at sig tornado time). Very largely-looped and long hodographs developing in the near-storm environment of the well-established supercells likely contributed to intense vertical pressure gradients such that the stable layer was able to be overcome to allow for production of significant tornadoes in spite of larger MLCIN than is typical of the majority of sig tor near storm environments. While the 0-1 km SRH and 0-1 km EHI distributions in DF09 were each by far the largest in the nighttime sig tor category compared to the other 5 categories, the 0-1 km SRH and 0-1 km EHI within the two FD09 events/other two aforementioned events easily exceeded the 90th percentile of each for nighttime sig tors in DF09; i.e., off the charts, so to speak, and climatologically rather rare.
Given the type of set-up on Monday, a fairly quick transition from high-ish LCL/weak CIN environment to low LCL-strong CIN environment was expected--not strongly different from the environment evolutions in FD09 as described above. Though backing a moist boundary layer up onto the High Plains is typically a big--and underappreciated--positive for retaining weak-CIN after dark environments in the Great Plains (given that elevation minimizes the impact of the EML)... the lack of richer/deeper moisture, and the lack of cool capping aloft in and upstream of the area, far overwhelmed the elevation impacts. Sfc temps fell pretty quickly in the "moist sector" too, despite the strong low-level gradient...with 65-70 F by 23Z invof IML-MCK-CBK-GLD.
The 02Z RAP initialized sounding (see below) was favored over that at 03Z as the near-sfc T/Td/wind details were far more accurate on the 02Z sndg. (Given I was away from work, I used Earl Barker internet data and thus soundings & hodographs are not modified). With a 1 km deep moist layer with mean (ML) dew point of 56F, topped by a sharp (i.e., dry adiabatic nearly all the way down) EML base warm enough such that 7H temps were around 6 C... MLCIN of 145 J/kg was accumulated via the sounding. The similarities of this sounding's shape with the two events' proximity soundings from FD09 are underlined by the fact that all three had a ML LFC between 2900 and 3000 meters. The striking differences between this case and the FD09 cases, however, aside from the simple fact that this case's damaging tornado received only an EF-1 rating, are: 1) modest instability, with under 1000 J/kg MLCAPE; 2) a rather young storm, being less than 90 minutes old at the time of the tornado; and 3) somewhat weaker low-level shear, with 0-1 km SRH near 400 m2/s2 (the impact of a dramatically weaker observed motion vs. Bunkers overwhelmed the impact of a more rightward observed motion vs. Bunkers--result in this case being weaker-than-shown SRH, for what it's worth). The fact that the tornado was brief and the storm was not cyclic would tend to make sense. The storm was also about to cross the sagging cold front, though with 1) slow storm motions and 2) (primarily non-NWS) sfc obs immediately N of the sfc front indicating (away from rain-cooled areas) quite similar sfc theta-e as just S... a further increase in CIN due to crossing the front may not have occurred particularly quickly.
Images:
-poorly analyzed 00Z and 03Z cntrl High Plains sfc analyses (obs via Al P.); red dots = rough positions of 01-02Z tornadoes on 00Z map... and of ~03Z EF-1 tornado E of Benkelman, damaging wind/false tornado report SSE of Max, and supposed "snownado" W of Akron on 03Z map
-low res radar evolution 02-03Z; recall westernmost supercell (which passed just nw outside of the tor warning polygon), whose appearance at least via base reflectivity was the less impressive of the two, was the one that produced the tornado
-02Z RAP 00-hr fcst sounding at McCook NE. Note the lifted parcel path shown on the sndg is via a SB parcel, though a lowest 100 mb ML parcel would be more appropriate. (Given the shallow drier near-sfc layer, and the lowest 100 mb temp profile being less than dry adiabatic... the potential temperature and the mixing ratio of the SB parcel are smaller than that of the ML parcel; and thus the SB parcel path rises to the left of--and accordingly has weaker CAPE and stronger CIN than--the ML parcel.) Parameters from the sndg are as follows...
Unmodified model sfc T/Td: 62/55 F (thermodynamic parameters below are based on unmodified sndg)
Observed sfc T/Td: 63/55 F
SBCAPE: 803 J/kg
SBCIN: 178 J/kg
Unmodified model ML T/Td: 64/56 F
MLCAPE: 893 J/kg
MLCIN: 145 J/kg
0-3 km MLCAPE: 0 J/kg
MLLCL: 614 m
MLLFC: 2945 m
Bunkers storm motion: 209@ 27 kts (storm-relative parameters below are based on this motion and the unmodified model hodo)
Observed storm motion: 235@15 kts (0-1 km SRH "area" using this motion is swept out for visual purposes on the hodo)
0-1 km SRH: 436 m2/s2
0-3 km SRH: 557 m2/s2
0-6 km bulk shear: 57 kts
5 km sr wind: 18 kts
10 km sr wind: 41 kts
0-1 km MLEHI: 2.4
