Final Upgrade to the NAM

One final upgrade is planned for the current version of the North American Mesoscale (NAM) Forecast System

The North American Mesoscale (NAM) Forecast System is scheduled for its final upgrade on February 1st.  Just in time for Groundhog Day, this upgrade to NAM "version 4" includes a number of major changes including:

• Changes in the grid spacing of the CONUS NAM nest from 4 to 3 km, Alaska nest from 6 to 3 km, and CONUS fire-weather nest from 1.333 to 1.5 km.  Yes, the latter is a decrease in resolution, but probably not significant.
• More frequent calls of some model physics packages to every 2nd time step and more frequent radiation calls for the NAM 12-km domain
• Specific humidity advection now done every time step (shockingly, this wasn't already being done)
• Changes to the model convective parameterization in the 12-km domain
• Updated cloud microphysics
• Land-surface model improvements
• Completely updated data assimilation system
• Use of a new climatology of fresh water lake temperature for inland water bodies not resolved by the current 1/12th degree analysis
• Reduced terrain smoothing in NAM nests

Gory details available from Technical Implementation Notice 16-41, available here, and the poster below.  

I think it is likely that these changes will result in some improvements in the skill of the 12-km NAM domain.  That domain, however, has been a pretty reliable performer for event water-equivalent forecasts over the central Wasatch (despite its low resolution), so I'm hoping that the biases don't change much.  My conversations with the NAM developers this past week suggest they think that the changes will also help some of the overforecast problems that plague the NAM nest, but they really haven't done a careful analysis over the west.  Instead, their inferences are based on examining non-orographic precipitation events east of the Rockies.  Time will tell.

It is anticipated that this will be the final upgrade to the NAM and that eventually a new cloud-allowing modeling system will be developed using the Geophysical Fluid Dynamics Laboratory (GFDL) Finite-Volume Cubed-Sphere Dynamical Core (FV3), which was recently chosen for NOAA's next-generation global environmental modeling system and replacement for the GFS.