Based on the 18z RUC analysis (shown below), the cap is beginning to weaken slightly in north-central South Dakota, as can be seen in the white region of the CAPE/CIN analysis below (blue coloring represents substantial CIN).  As expected, dewpoints have mixed out slightly into the mid 60s over this region, resulting in CAPE values of around 3000 J/kg.  This mixing out of the dewpoints has also resulted in ~30 degree temperature/dewpoint spreads over this region, which is a huge negative factor for tornadoes today, since the storms will have high bases and a tendency to become outflow dominant.  Any storms that initiate in this region must move eastward into a more favorable thermodynamic environment to have a good chance of producing tornadoes, but the cap is much stronger further east.   Right now, it appears that our decision of not chasing this event was the correct move, but several more hours of good heating still remain!