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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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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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William Y. Y. Cheng and W. James Steenburgh

Abstract

Despite improvements in numerical weather prediction, model errors, particularly near the surface, are unavoidable due to imperfect model physics, initial conditions, and boundary conditions. Here, three techniques for improving the accuracy of 2-m temperature, 2-m dewpoint, and 10-m wind forecasts by the Eta/North American Meso (NAM) Model are evaluated: (i) traditional model output statistics (ETAMOS), requiring a relatively long training period; (ii) the Kalman filter (ETAKF), requiring a relatively short initial training period (∼4–5 days); and (iii) 7-day running mean bias removal (ETA7DBR), requiring a 7-day training period. Forecasts based on the ETAKF and ETA7DBR methods were produced for more than 2000 MesoWest observing sites in the western United States. However, the evaluation presented in this study was based on subjective forecaster assessments and objective verification at 145 ETAMOS stations during summer 2004 and winter 2004/05. For the 145-site sample, ETAMOS produces the most accurate cumulative temperature, dewpoint, and wind speed and direction forecasts, followed by ETAKF and ETA7DBR, which have similar accuracy. Selected case studies illustrate that ETAMOS produces superior forecasts when model biases change dramatically, such as during large-scale pattern changes, but that ETAKF and ETA7DBR produce superior forecasts during quiescent cool season patterns when persistent valley and basin cold pools exist. During quiescent warm season patterns, the accuracy of all three methods is similar. Although the improved ETAKF cold pool forecasts are noteworthy, particularly since the Kalman filter can help better define cold pool structure by producing forecasts for locations without long-term records, alternative approaches are needed to improve forecasts during periods when model biases change dramatically.

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Peter G. Veals and W. James Steenburgh

Abstract

Lake-effect snowstorms east of Lake Ontario are frequently intense and contribute to substantial seasonal accumulations, especially over the Tug Hill Plateau (hereafter Tug Hill), which rises at a gentle 1.25% slope to ~500 m above lake level. Using a variety of datasets including radar imagery from the KTYX (Fort Drum, New York) WSR-88D, this paper examines the characteristics of lake-effect precipitation east of Lake Ontario over 13 cool seasons (16 September 2001–15 May 2014). During this period, days with at least 2 h of lake effect account for 61%–76% of the mean cool-season snowfall and 24%–37% of the mean cool-season liquid precipitation. Mean monthly lake-effect frequency and snowfall peak in December and January. The highest lake-effect frequency and snowfall occur over the western and upper Tug Hill, with an arm of relatively high lake-effect frequency and snowfall extending to the southeast shore of Lake Ontario. To the east (lee), lake-effect frequency and snowfall decrease abruptly over the Black River valley, although relatively high frequency and snowfall extend downstream into the western Adirondack Mountains. Broad coverage and long-lake-axis-parallel (LLAP) bands dominate the lake-effect morphology throughout the region. There is no diurnal modulation of lake-effect frequency during winter, but weak modulation in fall and spring, especially of LLAP bands.

Collectively, these results quantify the role that lake effect plays in the cool-season hydroclimate east of Lake Ontario. The increase in lake-effect frequency and snowfall over Tug Hill suggest an inland/orographic intensification of many lake-effect systems, with evidence for shadowing in the lee.

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Gregory L. West and W. James Steenburgh

Abstract

Recent studies indicate that strong cold fronts develop frequently downstream of the Sierra Nevada over the Intermountain West. To help ascertain why, this paper examines the influence of the Sierra Nevada on the rapidly developing Intermountain cold front of 25 March 2006. Comparison of a Weather Research and Forecasting (WRF) model control simulation with a simulation in which the height of the Sierra Nevada is restricted to 1500 m (roughly the elevation of the valleys and basins of the Intermountain West) shows that the interaction of southwesterly prefrontal flow with the formidable southern High Sierra produces a leeward orographic warm anomaly that enhances the cross-front temperature contrast. Several processes generate this orographic warm anomaly, including flow modification by the Sierra Nevada (i.e., windward blocking of low-level Pacific air, leeward subsidence, and increased southerly flow from the Mojave Desert and lower Colorado River basin into the Intermountain West), diabatic heating and water vapor loss associated with orographic precipitation, and increased sensible heating and reduced subcloud diabatic cooling in the downstream cloud and precipitation shadow. In contrast, the postfrontal air mass experiences comparatively little orographic modification as it moves across the relatively low northern Sierra Nevada. These results show that the Sierra Nevada can enhance frontal development, which may contribute to the high frequency of strong cold-frontal passages over the Intermountain West.

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W. James Steenburgh and Clifford F. Mass

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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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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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Leah S. Campbell and W. James Steenburgh

Abstract

Finescale variations in orographic precipitation pose a major challenge for weather prediction, winter road maintenance, and avalanche forecasting and mitigation in mountainous regions. In this investigation, ground-based X-band radar observations collected during intensive observing period 6 (IOP6) of the Storm Chasing Utah Style Study (SCHUSS) are used to provide an example of these variations during a winter storm in the Wasatch Mountains of northern Utah. Emphasis is placed on precipitation features in and around Little Cottonwood Canyon (LCC), which cuts orthogonally eastward into the central Wasatch Mountains. Precipitation during the weakly stratified prefrontal storm stage featured a wavelike barrier-scale reflectivity maximum over the Wasatch Crest and upper LCC that extended weakly westward along the transverse ridges flanking LCC. This precipitation pattern appeared to reflect a veering wind profile, with southwesterly flow over the transverse ridges but cross-barrier westerly flow farther aloft. Sublimation within dry subcloud air further diminished low-level radar reflectivities over lower LCC. In contrast, the cold-frontal stage was associated with stronger reflectivities over lower LCC and the adjoining north- to northwest-facing canyon wall, consistent with shallow, northwesterly upslope flow. These results highlight the finescale precipitation variations that can occur during winter storms in complex terrain and demonstrate the potential for improved analysis and forecasting of precipitation in LCC using a gap-filling radar.

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