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NCEP Service Center Evaluations SPC Steve Weiss
The largest differences were associated with low level moisture, as the parallel run consistently exhibited a more rapid northward progression of moisture inland from the Gulf of Mexico during the return flow phase over the plains in the wake of retreating surface ridges across the southeastern states. As a result, the NAM parallel BL dewpoint values were often 3-8 degrees F higher than the operational values, and this effect typically extended well inland on the leading edge of the moisture return. Comparison of the NAM parallel forecasts with verifying 2m dewpoints from METAR sites usually indicated the predicted dewpoints were too high. Within the zones of increased low level moisture in the parallel runs, larger values of MUCAPE and MLCAPE were often found when compared with CAPE forecasts from the operational run. The differences were typically on the order of 500 J/kg. General precipitation areas in the two runs were often rather similar, although when differences were observed they usually showed earlier and somewhat heavier convective precip in the parallel runs. We did note that the parallel run occasionally exhibited more structure and organization in the 3-hour accumulated precipitation field compared to the operational run, although we did not focus on specific QPF issues.