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W. James Steenburgh

Abstract

Synoptic, orographic, and lake-effect precipitation processes during a major winter storm cycle over the Wasatch Mountains of northern Utah are examined using radar imagery, high-density surface data, and precipitation observations from Alta Ski Area [2600–3200 m above mean sea level (MSL)] and nearby Salt Lake City International Airport (1288 m MSL). The storm cycle, which occurred from 22 to 27 November 2001, included two distinct storm systems that produced 108 in. (274 cm) of snow at Alta Ski Area, including 100 in. (254 cm) during a 100-h period. Each storm system featured an intrusion of low equivalent potential temperature (θ e) air aloft, well in advance of a surface-based cold front. Prefrontal precipitation became increasingly convective as low-θ e air aloft moved over northern Utah, while cold frontal passage was accompanied by a convective line and a stratiform precipitation region. Postfrontal destabilization led to orographic and lake-effect snowshowers that produced two-thirds of the observed snow water equivalent at Alta.

Storm stages were defined based on the passage of the above features and their accompanying changes in stability and precipitation processes. Contrasts between mountain and lowland precipitation varied dramatically from stage to stage and storm to storm, and frequently deviated from climatology, which features a nearly fourfold increase in precipitation between Salt Lake City and Alta. Based on the two storms, as well as other studies, a schematic diagram is presented that summarizes the evolution of Intermountain West snowstorms featuring an intrusion of low-θ e air aloft ahead of a surface cold front. Implications for short-range quantitative precipitation forecasting and seasonal-to-annual hydrometeorological prediction are discussed.

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W. James Steenburgh
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Marcel Caron and W. James Steenburgh

Abstract

In August 2018 and June 2019, NCEP upgraded the operational versions of the High-Resolution Rapid Refresh (HRRR) and Global Forecast System (GFS), respectively. To inform forecasters and model developers about changes in the capabilities and biases of these modeling systems over the western conterminous United States (CONUS), we validate and compare precipitation forecasts produced by the experimental, preoperational HRRRv3 and GFSv15.0 with the then operational HRRRv2 and GFSv14 during the 2017/18 October–March cool season. We also compare the GFSv14 and GFSv15.0 with the operational, high-resolution configuration of the ECMWF Integrated Forecasting System (HRES). We validate using observations from Automated Surface and Weather Observing System (ASOS/AWOS) stations, which are located primarily in the lowlands, and observations from Snowpack Telemetry (SNOTEL) stations, which are located primarily in the uplands. Changes in bias and skill from HRRRv2 to HRRRv3 are small, with HRRRv3 exhibiting slightly higher (but statistically indistinguishable at a 95% confidence level) equitable threat scores. The GFSv14, GFSv15.0, and HRES all exhibit a wet bias at lower elevations and neutral or dry bias at upper elevations, reflecting insufficient terrain representation. GFSv15.0 performance is comparable to GFSv14 at day 1 and superior at day 3, but lags HRES. These results establish a baseline for current operational HRRR and GFS precipitation capabilities over the western CONUS and are consistent with steady or improving NCEP model performance.

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W. James Steenburgh and Sento Nakai

Abstract

A remarkable snow climate exists on the Japanese islands of Honshu and Hokkaido near the Sea of Japan. Mean annual snowfall in this “gosetsu chitai” (heavy snow area) exceeds 600 cm (235 in.) in some near-sea-level cities and 1,300 cm (512 in.) in some mountain areas. Much of this snow falls from December to February during the East Asian winter monsoon when frequent cold-air outbreaks occur over the Sea of Japan. The resulting sea-effect precipitation systems share similarities with lake-effect precipitation systems of the Laurentian Great Lakes of North America, but are deeper, are modulated by the regional coastal geometry and topography, and can sometimes feature transversal mode snowbands. Snowfall can maximize in the lowlands or the adjoining mountains depending on the direction and strength of the boundary layer flow. Remarkable infrastructure exists in Japan for public safety, road and sidewalk maintenance, and avalanche mitigation, yet snow-related hazards claim more than 100 lives annually. For winter recreationists, there is no surer bet for deep powder than the mountains of Honshu and Hokkaido near the Sea of Japan in January, but the regional snow climate is vulnerable to global warming, especially in coastal areas. Historically, collaborative studies of sea- and lake-effect precipitation systems involving North American and Japanese scientists have been limited. Significant potential exists to advance our understanding and prediction of sea- and lake-effect precipitation based on studies from the Sea of Japan region and efforts involving meteorologists in North America, Japan, and other sea- and lake-effect regions.

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W. James Steenburgh and James R. Holton

Abstract

The conceptual model for height tendency presented by Hirschberg and Fritsch directly links upper-level virtual temperature tendency with low-level height tendency, overlooking the essential dynamics of mass divergence. An analysis of the complete height tendency equation shows that upper-level virtual temperature change car only indirectly induce low-level height change by driving ageostrophic circulations. To avoid misconceptions about middle- and lower-tropospheric height tendency, the dynamics of height tendency are reviewed.

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David M. Schultz and W. James Steenburgh

Abstract

A cold-frontal passage through northern Utah was studied using observations collected during intensive observing period 4 of the Intermountain Precipitation Experiment (IPEX) on 14–15 February 2000. To illustrate some of its nonclassic characteristics, its origins are considered. The front developed following the landfall of two surface features on the Pacific coast (hereafter, the cold-frontal system). The first feature was a surface pressure trough and wind shift associated with a band of precipitation and rope cloud with little, if any, surface baroclinicity. The second, which made landfall 4 h later, was a wind shift associated with weaker precipitation that possessed a weak temperature drop at landfall (1°C in 9 h), but developed a stronger temperature drop as it moved inland over central California (4°–6°C in 9 h). As the first feature moved into the Great Basin, surface temperatures ahead of the trough increased due to downslope flow and daytime heating, whereas temperatures behind the trough decreased as precipitation cooled the near-surface air. Coupled with confluence in the lee of the Sierra Nevada, this trough developed into the principal baroclinic zone of the cold-frontal system (8°C in less than an hour), whereas the temperature drop with the second feature weakened further. The motion of the surface pressure trough was faster than the posttrough surface winds and was tied to the motion of the short-wave trough aloft. This case, along with previously published cases in the Intermountain West, challenges the traditional conceptual model of cold-frontal terminology, structure, and evolution.

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Clifford F. Mass and W. James Steenburgh

Abstract

Observational analyses and a high-resolution simulation using The Pennsylvania State University–NCAR Mesoscale Model Version 5 (MM5) were used to describe the coastally trapped wind reversal of 19–21 July 1994. Major findings include the following.

  • The event was initiated and controlled by changes in the synoptic-scale flow, and particularly by the development of offshore flow over the coastal terrain of Oregon. Although synoptic control was dominant, blocking of the inversion-capped marine layer and mesoscale coastal pressure ridging were important components of the event.

  • The synoptic evolution associated with the trapped reversal was characterized by a shift from climatological near-zonal 500-mb flow to high-amplitude upper-level ridging over the eastern Pacific. As this upper ridge moved northeastward, the 850-mb flow over Oregon became northeasterly and then southeasterly, and 850-mb heights fell over western Oregon.

  • The combination of falling heights aloft and increasing low-level offshore flow, with associated downslope subsidence warming and offshore advection of warm continental air, resulted in sea level pressure falling along the Oregon coast and the northward extension of the California thermal trough into northern Oregon. The extension of the thermal trough caused a reversal of the alongshore pressure gradient along the Oregon coast, so that pressure increased to the south, as well as the attenuation or reversal of the normal cross-shore pressure gradient over the coastal waters.

  • As the coastal trough intensified, there was an increase in nearly geostrophic, onshore-directed coastal flow, with appreciable blocking and deflection of the inversion-capped marine layer by the coastal terrain. With an increase in the northward-directed pressure gradient force due to the troughing to the north, and a lesser contribution from damming of the marine air on the coastal terrain, the blocked low-level winds developed a coastally trapped southerly component over a considerable expanse of the southern and central Oregon coast.

  • Before wind reversal, the northerly flow over the coast was nearly geostrophic. After the coastal winds turned southerly, near-geostrophic balance in the cross-shore direction was established, with the offshore-directed pressure gradient force associated with the coastal pressure ridge balanced by the eastward-directed Coriolis force associated with the southerly flow. In contrast, the momentum balance in the alongshore direction was highly ageostrophic after wind reversal, with near-antitriptic balance close to shore between the northward-directed pressure gradient force and friction, while farther offshore the pressure gradient and Lagrangian accelerations were roughly in balance.

  • The offshore scale of the blocking within the marine layer was less than in the stable layer immediately above, a result consistent with previous theoretical and modeling studies. This difference in offshore blocking scale resulted in the offshore wind reversal occurring in the stable layer aloft before it occurred at the surface.

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Jason C. Shafer and W. James Steenburgh

Abstract

Motivated by the intensity and severity of winds and temperature falls that frequently accompany rapidly developing cold fronts in northern Utah, this paper presents a 25-yr climatology of strong cold frontal passages over the Intermountain West and adjoining western United States. Using conventional surface observations and the North American Regional Reanalysis, strong cold frontal passages are identified based on a temperature fall of 7°C or greater in a 2–3-h period, a concurrent pressure rise of 3 hPa or greater, and the presence of a large-scale 700-hPa temperature gradient of at least 6°C (500 km)−1. The number of strong cold frontal passages exhibits a strong continental signature with very few events (<10) along the Pacific coast and more than 200 events east of the Continental Divide. The number of events increases dramatically from the Cascade Mountains and Sierra Nevada to northern Utah, indicating that the Intermountain West is a frequent cold front breeding ground.

A composite of the 25 strongest events at Salt Lake City (based on the magnitude of the temperature fall) reveals that confluent deformation acting on a broad baroclinic zone over central Nevada commonly initiates Intermountain frontogenesis. The confluent deformation develops in southwesterly large-scale flow and appears to be enhanced by flow deflection around the Sierra Nevada. Quasi-stationary development and intensification of the southwest–northeast-oriented cold front then occurs as a mobile upper-level trough approaches from the west. The front becomes mobile as cold advection and ascent associated with the upper-level trough overtake the low-level front. Cloud and precipitation observations suggest that differential diabatic heating contributes to the rapid frontal intensification in many events.

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W. James Steenburgh and Clifford F. Mass

Abstract

Observational analyses and numerical simulations are used to investigate the interaction of an intense extra-tropical cyclone with the coastal orography of the Pacific Northwest. Known as the “Inauguration Day cyclone,” the system made landfall upon the Washington State coast on 20 January 1993, producing one of the most damaging wind storms in Pacific Northwest history. The strongest winds accompanying the storm were associated with an intense low-level pressure gradient that was concentrated along a bent-back front. Mesoscale pressure perturbations produced by the time-dependent interaction of the cyclone and bent-back front with the coastal orography were isolated using numerical simulations. The simulations showed that during the period of peak winds over Puget Sound, there was only a minor enhancement of the local pressure gradient by troughing to the lee (east) of the Olympic Mountains. Gradual amplification of this Olympic Mountain lee trough over a period of 2–3 h extended the period of strong winds by enhancing the pressure gradient over Puget Sound as the bent-back front moved out of the region.

The influence of orographically induced coastal ridging and pressure surges was also investigated. It was found that the evolution of coastal ridging was closely connected to the progressive northward development of onshore flow behind the bent-back front. There was no evidence that a self-propagating feature, such as a Kelvin wave or gravity current, was triggered during the landfall of the cyclone and its attendant fronts. The momentum budget in the coastal zone following passage of the bent-back ftont is also discussed.

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Lucas Bohne, Courtenay Strong, and W. James Steenburgh

Abstract

Orographic precipitation gradients (OPG) relating to the increase or decrease in precipitation amount with elevation are not well studied or analyzed except for case examples. A quality controlled daily OPG dataset for the western United States that is based on a linear regression framework of gauge precipitation observations and elevation for a 39-yr time period was created and analyzed to identify spatial and temporal patterns and variability in OPG and some of the drivers of variability on seasonal, annual, interannual, and climatological time scales. Most locations in the western United States experience positive OPG during most of the year, exhibiting an annual cycle with the highest magnitude of OPG in the winter season and lowest magnitude of OPG in the summer season. Coastal locations tend to have OPG with higher magnitude and larger variability in OPG than do interior locations during cool seasons. Empirical orthogonal function analysis identifies two principal components that account for 33% of the variability in a subset of the OPG dataset, and these modes of variability are related to precipitation amount and atmospheric circulation over the Pacific Ocean. Comparison of daily OPG with similarly calculated 3-day and monthly OPG identifies that OPG magnitudes are sensitive to the choice of length of the precipitation accumulation period.

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