478 MONTHLY WEATHER REVIEW VOLUME 134 Terrain Influences on Synoptic Storm Structure and Mesoscale Precipitation Distribution during IPEX IOP3 JASON C. SHAFER,W.JAMES STEENBURGH, AND JUSTIN A. W. COX NOAA Cooperative Institute for Regional Prediction, and Department of Meteorology, University of Utah, Salt Lake City, Utah JOHN P. MONTEVERDI San Francisco State University, San Francisco, California (Manuscript received 3 December 2004, in final form 28 April 2005) ABSTRACT 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 conven- tional 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 moun- tain 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 precipita- tion that was not fixed to the terrain. Processes responsible for the complex vertical trough structure and precipitation distribution over northern Utah are discussed. 1. Introduction ern California (NCDC 2000, 26–27). Over northern Utah, the storm was responsible for 60–90 cm of moun- A major winter storm affected much of the western tain snow, and up to 2.1 cm of snow water equivalent United States on 12–13 February 2000. Across the low- (SWE) at lowland locations. Significant precipitation lands of north-central California, heavy rains produced gradients were observed over northern Utah. For ex- flooding, the collapse of a Home Depot roof in Colma, ample, the mountain site Snowbasin (SNI, 2256 m) re- and more than 2 million dollars of damage (NCDC corded 4.9 cm of SWE, over twice the 2.1 cm that fell at 2000, p. 29). Mountain snows of 60–120 cm (2–4 ft) fell Ogden (OGD, 1360 m), a lowland site only 15 km away. over the Sierra Nevada, and prefrontal wind gusts In addition, significant precipitation contrasts were ob- reaching 26 m sϪ1 uprooted trees across parts of south- served between sites at comparable elevations. For ex- ample, the 2.1 cm of SWE at OGD was nearly 4 times greater than the 0.6 cm observed 45 km away at Salt Corresponding author address: Jason C. Shafer, Dept. of Me- teorology, Lyndon State College, 1001 College Rd., Lyndonville, Lake City (SLC, 1280 m). The storm system also fea- VT 05851. tured a complex evolution from landfall along the Cali- E-mail: [email protected] fornia coast to the interior Intermountain region, in- © 2006 American Meteorological Society Unauthenticated | Downloaded 10/07/21 02:51 PM UTC MWR3051 FEBRUARY 2006 S HAFER ET AL. 479 FIG. 1. Topography and major geographic features of (a) the western United States and (b) northern Utah. Elevation (m) based on scale at lower left in (a). Boxed area in (a) identifies location of IPEX target area in (b). cluding the development of a complex vertical trough (2005) for a detailed analysis of the kinematic structure structure over the Sierra Nevada and Great Basin (Sha- of this event near the Wasatch Mountains and to Colle fer 2002; Schultz et al. 2002). et al. (2005) for a microphysical analysis and numerical This event occurred during the third intensive ob- simulation of this event. serving period (IOP3) of the Intermountain Precipita- tion Experiment (IPEX; Schultz et al. 2002), a research 2. Data and methods program designed to improve the understanding and prediction of precipitation over the Intermountain re- The analyses presented in this paper were performed gion of the western United States. IOP3 was character- using a variety of field program and conventional istic of many Intermountain cool-season storms where a datasets. Surface observations were provided by the landfalling Pacific storm system weakened and under- MesoWest cooperative networks (Horel et al. 2002), went a complex evolution across the Intermountain re- including data from approximately 2500 stations in the gion. As will be discussed further within this paper, the western United States and 250 stations across northern storm’s evolution and precipitation-producing mecha- Utah. MesoWest observations were quality controlled nisms were intrinsically related to the complex topog- by comparing the observed values to an estimate based raphy of the Intermountain region, which is character- on multivariate linear regression (Splitt and Horel ized by basin-and-range terrain where narrow, steeply 1998), and by spatial and temporal consistency checks sloped mountain ranges are separated by broad low- performed while preparing manual analyses. Precipita- land valleys and basins (Fig. 1a). For example, northern tion observations were quality controlled by Cheng Utah features the meridionally oriented Stansbury, (2001), and generally featured 0.01-in. resolution Oquirrh, and Wasatch Mountains, which rise 1500–2000 (amounts were converted to millimeters for publica- m above the surrounding lowlands (Fig. 1b). tion). For a description of precipitation gauge charac- This paper intends to advance the understanding of teristics see Cheng (2001). Special 1-s Automated Sur- cool-season Intermountain storm systems through two face Observing System (ASOS) data were also down- objectives. The first is to describe the large-scale storm loaded from the National Climatic Data Center 1 evolution from the Pacific coast to northern Utah, with (NCDC) and used for the surface time series. Since emphasis on the surface and vertical structure. The sec- MesoWest observations are irregularly spaced horizon- ond objective is to examine the precipitation distribu- tally and vertically, the Advanced Regional Prediction tion over northern Utah and its relationship to the in- System (ARPS) Data Assimilation System (ADAS; teraction of the kinematic and thermodynamic struc- Lazarus et al. 2002) was used to generate 1-km gridded ture of the storm with local topography. The vertical analyses of surface convergence and winds over north- structure of this system underwent dramatic change ern Utah. from California to Utah, and the precipitation distribu- tion across northern Utah featured both similarities and 1 Although available at high temporal resolution, the special departures from climatological precipitation versus al- ASOS temperature observations are only to the nearest degree titude relationships. Readers are referred to Cox et al. Celsius. Unauthenticated | Downloaded 10/07/21 02:51 PM UTC 480 MONTHLY WEATHER REVIEW VOLUME 134 Gridded data used for most upper-air analyses were elevation of Intermountain observing sites. Surface provided by the National Centers for Environmental frontal analysis over the Intermountain region is com- Prediction Rapid Update Cycle (RUC2; Benjamin et al. plicated by numerous issues, including but not limited 2004), an hourly assimilation forecast cycle run at 40- to the effects of thermally and terrain-driven circula- km horizontal grid spacing, but available at 60-km hori- tions (Stewart et al. 2002), diabatic processes (Schultz zontal and 25-hPa vertical grid spacing. Comparison and Trapp 2003), and limitations of the observing net- with observations and other gridded data suggested work (Horel et al. 2002). Nonetheless, the traditional that these RUC2 analyses provided the best overall de- definition of a frontal zone as an elongated area of piction of the storm evolution. However, Eta Data As- strong horizontal temperature gradient (e.g., Bluestein similation System (EDAS) gridded data were also used 1986; Keyser 1986), with the front as the boundary on because they featured a qualitatively more realistic rep- the warm side of the zone, was implemented with con- resentation of the potential vorticity banners generated sideration of the aforementioned difficulties. Owing to downstream of the Sierra Nevada. MesoWest observa- the higher mean elevation of the Intermountain region, tions, RUC2, and EDAS gridded data were plotted us- the remainder of this paper will refer to vertical levels ing General Meteorological Package (GEMPAK) soft- as follows: low level from the surface to 700 hPa, ware. midlevel 700–500 hPa, and upper-level above 500 hPa. Upper-air data included radiosonde observations at the