Verification of URMA and National Blend of Models Grids in Complex Terrain

The National Weather Service is developing the National Blend of Model (NBM) grids, which will eventually be used as initial forecast grids at all National Weather Service Forecast Offices (WFOs). The NBM utilizes a gridded representation of the surface weather conditions, called the Unrestricted Mesoscale Analysis (URMA). Input from WFOs is being solicited to identify shortcomings in the NBM and URMA, so the grids can be improved.

A quantitative analysis of the temperature and dew point in these grids is performed using hourly data from reliable ASOS and AWOS sites in Colorado, including many sites in mountainous terrain. The URMA has 2.5 km by 2.5 km grid boxes, and hourly ASOS/AWOS values will be compared to the value of the grid box containing the site. The values in the URMA gird box should be close to the ASOS/AWOS values. Nearby mesonet sites can affect the value of the grid box containing the ASOS/AWOS site, but the mesonet values have a lower weight in the URMA than ASOS/AWOS, and mesonet observations are sparse in rural Colorado. In addition, ASOS and AWOS sites are sparse in Colorado in locations away from urban areas, and surrounding ASOS/AWOS sites have little influence. Analysis will also be done for sites on the flatter eastern plains of Colorado to see if there are any differences between the mountainous and non-mountainous sites.

A similar analysis will be performed for the NBM to identify any potential systematic biases in the NBM grids.

Data from the cold season of 2015-2016 will be analyzed. One specific goal of this study is to quantify how well the URMA and NBM perform for cold pools in mountain valleys as WFO Pueblo has observed instances of the URMA and NBM being too warm for cold pools in mountain valleys.