Verification of Convective Environment Forecasts from the NAM Parent and CONUS Nest within SPC Outlook Areas

High resolution convection-allowing models (CAMs), which have the ability to generate explicit storms, are primarily used as for identifying specific storm attributes via diagnostic output, such as simulated radar and updraft helicity. While CAMs generate explicit storm structures, knowledge of the near-storm environment remains an important aspect of understanding the potential severe weather hazard type. Storm Prediction Center (SPC) forecasters have a long history of examining aspects of the pre-convective and near-storm environment from relatively coarse numerical weather prediction (NWP) models which utilize convective parameterization schemes. However, as the usage of CAMs becomes more ubiquitous, it is important to consider their skill in depicting the pre-convective and near-storm environments as well. In CAMs, instability and shear fields can be highly detailed and strongly modulated in-and-around explicitly modeled storms, potentially making a diagnostic assessment of the near-storm environment more difficult.

This study examines forecasts of environmental variables important for deep convection and severe weather (e.g., CAPE, dewpoint, 0-1 km SRH) during the 2017 convective season from the operational NAM Parent (12-km grid spacing) and CONUS Nest (3-km grid spacing). Forecasts are verified using the SPC surface objective analysis (sfcOA; 40-km grid), requiring that the NAM forecasts be regridded to the sfcOA 40-km grid. Verification statistics (e.g., root-mean-squared error and bias) are calculated for 0000 UTC forecasts on a CONUS scale, as well as using an innovative technique in which fields are masked to the Slight Risk areas in the SPC 0600 UTC Day 1 Convective Outlook. This technique ensures that verification statistics are meaningful to a convective forecaster by only considering areas with severe weather potential. Overall model performance will be discussed, as well as an in-depth investigation into performance differences between the NAM Parent and Nest at the peak of the convective diurnal cycle.