On 20 July 2005, several eastwest-oriented bands of clouds and light rain formed over eastern Montana and the Dakotas. The cloud bands were spaced about 150 km apart, and the most intense band was about 20 km wide and 300 km long, featuring areas of maximum radar reflectivity factor about 50 dBZ. The cloud bands formed poleward of an area of lower tropospheric frontogenesis, where air of modest convective available potential energy was being lifted. During initiation and maintenance of the bands, mesoscale regions of dry symmetric and inertial instability were present in the region of the bands, suggesting a possible mechanism for the banding. The release of these instabilities produced circulations with enough vertical motion to lift parcels to their lifting condensation level, resulting in the observed cloud bands.
On 16 February 2007, the Front Range of the Rockies experienced strong winds (45 m/s gusts), heavy snow, hail, graupel, and lightning. Some of the high-impact weather associated with this event occurred from nearly stationary convective snowbands formed under strong northwesterly flow aloft, one of which produced up to 3 inches of snow in Fort Collins. These bands occurred on the anticyclonic shear side of a midlevel jet in a region with negative potential vorticity and absolute vorticity, indicating dry symmetric and inertial instabilities. Both human and operational NWP model forecasts failed to predict the convective bands in this case.
In these and other similar cases, moist convection initiates and quickly organizes into bands that are much narrower than those typically observed in regions of frontogenesis. We will examine observations and high-resolution WRF simulations of these two cases to illustrate how the release of low- and mid-level inertial instability contributes to the growth and development of narrow, slow-moving bands of precipitation that are challenging to predict.