This paper investigates the kinematic flow and precipitation evolution of a winter storm over and upstream of the Wasatch Mountains [Intermountain Precipitation Experiment third intensive observing period (IPEX IOP3)] using a multiply nested version of the fifth-generation Pennsylvania State University (PSU)––National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5). Validation using in situ aircraft data, radiosondes, ground-based radar, and surface observations showed that the MM5, which featured four domains with 36-, 12-, 4-, and 1.33-km grid spacing, realistically simulated the observed partial blocking of the 8–12 m s−1 ambient southwesterly flow and development of a convergence zone and enhanced lowland precipitation region upwind of the initial Wasatch slope. The MM5 also properly simulated the advance of this convergence zone toward the base of the Wasatch during the passage of a midlevel trough, despite not fully capturing the westerly wind shift accompanying the trough. Accurate simulation of the observed precipitation over the central Wasatch Mountains (within 25% of observed at all stations) required a horizontal grid spacing of 1.33 km. Despite close agreement with the observed surface precipitation, the Reisner2 bulk microphysical scheme produced too much supercooled cloud water and too little snow aloft. A model microphysical budget revealed that the Reisner2 generated over half of the surface precipitation through riming and accretion, rather than snow deposition and aggregation as implied by the observations. Using an intercept for the snow size distribution that allows for greater snow concentrations aloft improved the snow predictions and reduced the cloud water overprediction. Sensitivity studies illustrate that the reduced surface drag of the Great Salt Lake (GSL) enhanced the convergence zone and associated lowland precipitation enhancement upstream of the Wasatch Mountains. The presence of mountain ranges south of the Great Salt Lake appears to have weakened the along-barrier flow and windward convergence, resulting in a slight decrease in windward precipitation enhancement. Diabatic cooling from falling precipitation was also important for maintaining the blocked flow.

The influence of orographic circulations on the precipitation structure of a Wasatch Mountain winter storm is examined using observations collected during the third intensive observing period (IOP3) of the Intermountain Precipitation Experiment (IPEX). The event featured the passage of a midlevel (700–550 hPa) trough followed 3 h later by a surface trough. Prior to and during the midlevel trough passage, large-scale southwesterly flow impinged on the Wasatch Mountains. Low-level confluence was observed between this southwesterly flow and along-barrier southerly flow within 20–40 km of the Wasatch Mountains. This confluence zone, which moved toward the Wasatch Mountains during and following the passage of the midlevel trough, was accompanied by low-level convergence and precipitation enhancement over the upstream lowlands. Dual-Doppler analysis revealed the presence of a shallow along-barrier jet near the base of the Wasatch Mountains that was surmounted by southwesterly cross-barrier flow at mid- and upper-mountain levels. This cross-barrier flow produced strong (1–2 m s−1) ascent as it interacted with the steep windward slopes of the Wasatch Mountains, where precipitation was roughly double that observed in the lowlands upstream. Flow deflection and splitting were also observed near the highest terrain features. A narrow region of strong subsidence, which at times exceeded 2 m s−1, was found to the lee of the Wasatch and, based on radar imagery, appeared to modulate hydrometeor spillover aloft. Processes contributing to the evolution of the near-barrier flow field, including topographic blocking, diabatic effects, and surface friction contrasts, are discussed.

IPEX-a wintertime research program in the Salt Lake area-aims to improve models, predictions, and understanding of precipitation in a region that is difficult to observe and forecast E ighteen million residents are impacted by winter storms each year throughout the Intermountain West (Wilkinson 1997, p. 30)-the geographic region east of the crests of the Sierra Nevada and Cascade Mountains, and west of the Continental Divide (Fig. la). Although historically characterized by low population density, the region has experienced rapid population growth. Between the 1990 and 2000 national censuses, the five fastest growing states belonged to the Intermountain West (Ne-vada, Arizona, Colorado, Utah, and Idaho). For example, Utah experienced a 29.3% increase in population during this 10-yr period. As a result, intermountain winter storms impact a growing population and regional economy. The socioeconomic impacts of winter storms over the Intermountain West include public costs of road maintenance, private costs of property damage, disruption to daily commuter traffic and interstate commerce, and threats to public safety arising from snow-AFFILIATIONS: SCHULTZ

The influence of topography on the evolution of a winter storm over the western United States and distribution of precipitation over northern Utah are examined using data collected during the third intensive observing period (IOP3) of the Intermountain Precipitation Experiment (IPEX). The analysis is based on high-density surface observations collected by the MesoWest cooperative networks, special radiosonde observations, wind profiler observations, Next-Generation Weather Radar (NEXRAD) data, and conventional data. A complex storm evolution was observed, beginning with frontal distortion and low-level frontolysis as a surface occluded front approached the Sierra Nevada. As the low-level occluded front weakened, the associated upper-level trough moved over the Sierra Nevada and overtook a lee trough. The upper-level trough, which was forward sloping and featured more dramatic moisture than temperature gradients, then moved across Nevada with a weak surface reflection as a pressure trough.Over northern Utah, detailed observations revealed the existence of a midlevel trough beneath the forward-sloping upper-level trough. This midlevel trough appeared to form along a high-potential-vorticity banner that developed over the southern Sierra Nevada and moved downstream over northern Utah. A surface trough moved over northern Utah 3 h after the midlevel trough and delineated two storm periods. Ahead of the surface trough, orographic precipitation processes dominated and produced enhanced mountain precipitation. This period also featured lowland precipitation enhancement upstream of the northern Wasatch Mountains where a windward convergence zone was present. Precipitation behind the surface trough was initially dominated by orographic processes, but soon thereafter featured convective precipitation that was not fixed to the terrain. Processes responsible for the complex vertical trough structure and precipitation distribution over northern Utah are discussed.