This Valley, Nebraska, 00Z sounding was modified thermodynamically for estimated conditions through the troposphere near the Spencer supercell, based on interpolations of mandatory-level raob analyses and surface data. [Winds are not modified due to software limitations; upper level flow was weaker than this over southeast SD while low level storm inflow was probably much stronger.] This is a "best guess" of the near-storm environment; but given the distance to nearest actual soundings, there is still considerable margin for error. Here are some derived thermodynamic parameters:

The sounding is practically uncapped with a convective inhibition (CINH) of only 5 J/kg, which can be overcome with weak convergence and/or slightly stronger heating. Such convergence may have occurred near the remains of the outflow boundary, or along some undetected meso-beta scale feature. The relatively steep low-mid level lapse rates are associated with a combination of elevated mixed-layer (EML) air advected off the higher terrain of the central high plains, and diabatic near-surface heating. The capping/heating process is evident in a daylong loop of observed soundings at OAX, 155 nautical miles SSE of Spencer.

During the middle-end part of the Spencer tornado's life cycle, considerable rain-wrapping was observed, with the updraft region trending from classic to high-precipitation (HP) in character. This sounding supports abundant precipitation production by the storm with mean mixing ratio near 15 g/kg, precipitable water near 1-1/3 inches. Also, despite storm-relative anvil level (near EL) flow about 75-80 kt, HP transition was also supported by the deep layer of relatively high RH off the surface, and possible seeding of the Spencer storm's updraft from other anvils [Extensive anvil interaction was indicated by satellite imagery and observed by storm chasers]. For more info on the anvil-level SR flow and other processes relevant to storm type, see this reference:

Rasmussen, E. N., and J. M. Straka, 1998: Supercell morphology variations. Part I. Observations of upper-level storm-relative flow. Mon. Wea. Rev., 126, 2406-2421

Observers drove through relatively warm rain, with some non-severe hail, N thru NW thru W of the Spencer tornado, in a part of the precipitation hook characterized by very large/damaging hail in many other tornadic supercells. Note the high wet-bulb 0 (WBZ) and freezing (FGZ) levels. Two contributors to lack of large hail may have been the great depth of superfreezing air through which it would fall, and the lack of low level drying which would provide evaporative cooling in falling precipitation.