Use of the NAM Model 1.33 km Movable Fire Weather Nest for Severe Weather Applications during the Spring of 2012

The North American Mesoscale (NAM) modeling system consists of the 12 km grid spacing Non-hydrostatic Multiscale Model run on an Arakawa-B grid (NMM-B) four times daily, and multiple higher horizontal resolution concurrent nested runs over the contiguous United States (CONUS), Alaska, Puerto Rico, and Hawaii. These include a 4 km CONUS nest and a movable, limited domain 1.33 km Fire Weather nest (hereafter “Fire Nest”). The various nested runs were new additions to the NAM model during the fall 2011 implementation. The NAM is run within the NOAA Environmental Modeling System (NEMS) framework which features the unique capability to run embedded higher resolution nests concurrently during the integration of the parent model. This provides an initial look at very timely model guidance across multiple resolutions designed to serve a variety of general forecasting and high impact weather prediction needs.

The geographic domain of the Fire Nest is determined by a cooperative effort involving primarily NCEP Service Centers, the National Interagency Fire Center (NIFC), and NCEP Central Operations. The Storm Prediction Center (SPC) selects the domain during the spring and fall months when severe convective and/or fire weather concerns prevail, and the Hydrometeorological Prediction Center selects the domain in winter. NIFC selects the domain in the summer during the peak fire season, but they also have priority at other times of the year when major wildfire threats may occur.

Starting in the fall of 2011, SPC forecasters have on occasion centered the Fire Nest over areas with severe weather potential. This can consist of placing the domain over a stationary location for consecutive runs, or moving the domain on successive runs to accompany a traveling cyclone and warm sector associated with a progressive severe weather episode. During the spring of 2012, the Fire Nest was utilized by SPC forecasters during the major tornado outbreaks on March 2 and April 14. Hourly maximum fields (HMFs) of specialized convective storm parameters from the Fire Nest, such as simulated reflectivity, updraft speed, updraft helicity, and surface winds, are compared with similar fields from several 4 km convection-allowing models (CAMs), including the CONUS nest and NSSL WRF-ARW, to assess the impacts of higher spatial resolution in the Fire Nest for providing guidance on severe storms. Initial findings indicate that that long-track updraft helicity (UH) paths in the Fire Nest output, associated with model-generated supercell storms, tend to exhibit considerably larger UH magnitudes compared to lower resolution CAMs. However, details in stormscale structure as seen in simulated reflectivity images do not necessarily appear to benefit fully from the smaller grid spacing in the Fire Nest.