05/08/09 KS-MO derecho environment




Preliminary summary:


A long-lived intense bow echo system (derecho) originated over western/central KS early in the morning, and then moved through southeastern KS, southern MO/extreme northern AR, and into the OH River Valley.  The system caused 1 death in KS, 2 deaths in MO, and 1 death in southern IL.  In what was an otherwise rather quiescent year for severe storms, this derecho certainly was one of the bigger weathermakers of 2009.


The system initiated within an absolutely classic pattern for derecho systems: 1) downstream of a low-amplitude shortwave trough, in an area of nocturnally-enhanced, intense low-level moisture advection/convergence near a west-to-east oriented surface boundary; and 2) on the sharp northeastern edge of an expanding/strengthening EML cap, within an environment characterized by very strong potential instability and pronounced dry air in the middle and upper levels.  A very preliminary meteorological summary follows.  I may add more detail in the future.




00Z RAOBs at DFW, SHV, LZK, OUN, LMN, DDC, SGF: note lowest 100 mb ML dewpoints around 72F over southern plains the evening prior to the event... which is about as favorable as low-level moisture quality gets in early May in these locations!  This moisture amidst favorably steep lapse rates aloft contributed to 3200-4000 J/kg MLCAPE, modestly capped by 10-12C 700mb temperatures.  Boundary layer depth was quite impressive as one moved east and north toward SHV/LZK, with a depth of near 3 km.  Meanwhile, a hot EML was progressing eastward off the southern Rockies (not shown), with a deeply mixed/hot boundary layer present at MAF and AMA amidst 13-15C at 700mb.  Despite CIN being locally reduced along the effective dryline (e.g. CDS-ABI corridor), increasingly warm capping above the boundary layer coupled with relatively modest convergence and a lack of background support probably contributed to the suppression of thunderstorms prior to sunset.  (This convective failure is probably something I'll examine more in the future, as I haven't looked at it closely as of yet). 


All RAOBs also showed markedly dry air in the middle and upper levels of the atmosphere, which combined with explosive instability would certainly be favorable for intense evaporative cooling/damaging winds.  Also note the comparatively dry/stable boundary layer along the DDC-SGF corridor... suggestive of the intense west-to-east oriented boundary layer moisture gradient near the KS/OK to MO/AR borders.  The 00Z to 06Z evolution at Lamont, OK (LMN) sampled the intense moisture advection particularly well, but I didn't save the supplementary 06Z LMN sounding in NSHARP and thus won't post a version of it here.




12Z RAOBs at DFW, OUN, SGF (each overlaid on respective 00Z RAOB): the evolution on these RAOBs shows the classical nature of the derecho environment very well.  At DFW and OUN, note "shallowing" of boundary layer from 00Z to 12Z beneath the strong, east- and northeastward-advecting EML off the southern Rockies, with very strong MLCIN (300-400 J/kg).  Meanwhile, intense moisture advection and boundary layer deepening occurred from 00Z to 12Z at SGF (although a very shallow near-sfc inversion had also developed), just downstream of the bow echo system--reflective of the retreating moist layer on the poleward edge of the expanding EML cap.  MLCAPE is near 4000 J/kg, with essentially no inhibition to ML parcels.  And in association with the deep boundary layer, lowest 100 mb ML dewpoints near 70F, and cool capping aloft away from the expanding EML... low-level instability is extreme with 0-3 km MLCAPE of over 200 J/kg, at the diurnal minimum no less!  Even without the unbelievably ideal low-level thermodynamics, the derecho would likely have had no problem producing damaging wind gusts at the surface.  As it was, the strongly sfc-based environment only served to enhance the intensity of the system.




06/09/12/15Z radar mosaic: initial development around 03Z over the central high Plains (not shown) appeared to be relatively high-based and tied primarily to large scale ascent downstream of the short-wave trough.  Storms became progressively more intense as they moved ESE and approached the western edge of the retreating low-level moisture and explosive CAPE... where the derecho was then born.  In the last image, hints of the developing comma head can also be seen.




09/12Z subjective surface analyses: note intense surface pressure falls spreading eastward in immediate advance of the evolving derecho--generally right around 6 mb per 3 hours.  This is a very impressive surface reflection to be seen in the overnight hours given the low amplitude of the background middle/upper level pattern and the lack of a more powerful surface cyclone.  This is accordingly suggestive of significant QG forcing that evolved downstream of the shortwave trough.  In the second analysis note the strong cold pool the system has developed, resulting in a substantial pressure rise/fall couplet.




00/03/06/09/12Z wind profiler/VWP plot at 5kft, 12kft, and 18kft MSL: nocturnal acceleration of the LLJ to 60 kt focused extreme mass convergence and sharp moisture advection into and downstream of the derecho.  "Hints" of the shortwave trough can be seen in the 12kft and 18kft plots... with the derecho's impressive (70-kt) rear inflow jet also being sampled directly by 09Z.