Improvements in quantitative precipitation forecasting over the western United States require advances in several areas including the understanding of key dynamic and microphysical processes in orographic precipitation. In response to this need, the Intermountain Precipitation Experiment (IPEX) was conducted in northern Utah from 31 January to 25 February 2000, with an emphasis on sampling orographic precipitation along the Wasatch Mountains. One of the key observing platforms for IPEX was the NOAA P3 aircraft, which was used to examine precipitation dynamics and microphysics upstream, over, and to the lee of the Wasatch Mountains. The P3 was equipped with an X-band Doppler radar to map precipitation structure and three-dimensional air motions. It was also instrumented with several in situ sensors to measure air, cloud, and precipitation characteristics. Among these, this study primarily makes use of the microphysical probes (King, FSSP, 2DC, 2DP) to document the sizes, concentrations, habits, and masses of cloud and precipitation particles.

Six Intensive Operation Periods (IOP's) involving the P3 were executed during IPEX. This study focuses on the microphysical aspects of an orographic precipitation event that occurred on 12-13 February during IOP 3. This storm produced more than two feet of snow at some locations along the Wasatch. Over a six-hour period, the P3 executed four separate microphysics stacks across the Wasatch Front near Ogden. Each microphysics stack was composed of four level flight legs oriented WSW-ENE at altitudes corresponding to critical air temperatures for microphysical processes (-5 to -20 C). Analysis to date indicates that supercooled cloud liquid water was only available in limited amounts during this event. The largest values observed were on the order of 0.15-0.20 g/m^3. This may be an underestimate, however, since the aircraft was not able to sample the lower parts of the orographic feeder cloud. In any event, accretional growth was probably minimal, a hypothesis that is currently being tested by analysis of particle image data. The results of this analysis will be presented at the conference.