P38 Extended range severe weather forecasts using WRF and MPAS.
Sobash, Ryan, and Craig Schwartz, NCAR/MMM
Extended range
forecasts of severe weather events were generated with WRF and MPAS
convection-allowing ensembles. The 10-member, 132-hr, ensemble forecasts were
initialized daily at 00 UTC between 23 April 2017 and 27 May 2017 with GFS
ensemble (GEFS) members. Both WRF and MPAS had 3-km grid spacing over the
CONUS, with MPAS having a 15-km mesh elsewhere on the globe, while the WRF
forecasts used GEFS boundary conditions. Severe weather forecasts were produced
using updraft helicity (UH), with evaluation focusing on daily (12 UTC – 12
UTC) and hourly probabilistic guidance for Days 1 – 5, verified using NWS storm
reports.
Differences in UH computations in the MPAS and WRF ensembles resulted in
differences in the UH model climatology, necessitating a bias-correction approach
to produce equivalent guidance. The bias-corrected guidance from both WRF and
MPAS exhibited positive skill over climatology for days 1 – 5, with skill
decreasing with lead-time. The MPAS forecasts were more skillful than the WRF
forecasts for Day 1, while the WRF forecasts were more skillful for days 2 – 5,
with the skill gap increasing with lead-time. The diurnal cycle of severe
weather reports was better captured in the WRF forecasts, especially beyond Day
2 and during the overnight periods (00 UTC - 12 UTC). Preliminary results
suggest this may be due to deficiencies in the representation of large-scale
overnight convective systems, such as MCSs, in the MPAS forecasts.