The purpose of this study is to evaluate the large-scale forcing data diagnosed from the ECMWF model for driving Single-Column Models (SCMs). Using data from a strong convective period during the ARM summer 1997 Intensive Operational Period (IOP) and a non-convective period during the winter 2000 IOP at Southern Great Plains, we demonstrate that the ECMWF diagnosed forcing fields are reasonable overall during the non-convective period while they show considerably large differences from those derived from the ARM observations using the objective variational analysis during the convective period. We show that errors in the ECMWF predicted precipitation and other thermodynamic constraints in the column-integrated budgets of energy and moisture have large impact on these diagnosed forcing fields. Results from an SCM driven by these two types of forcing data sets are comparable over the non-convective period while the SCM with the ECMWF diagnosed forcing exhibits larger simulation errors during the convective period.