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Multiscale Analysis of the 7 December 1998 Great Salt Lake–Effect

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Multiscale Analysis of the 7 December 1998 Great Salt Lake–Effect 1296 MONTHLY WEATHER REVIEW VOLUME 129 Multiscale Analysis of the 7 December 1998 Great Salt Lake±Effect Snowstorm W. J AMES STEENBURGH AND DARYL J. ONTON NOAA Cooperative Institute for Regional Prediction, and Department of Meteorology, University of Utah, Salt Lake City, Utah (Manuscript received 3 May 2000, in ®nal form 16 October 2000) ABSTRACT The large-scale and mesoscale structure of the Great Salt Lake±effect snowstorm of 7 December 1998 is examined using radar analyses, high-density surface observations, conventional meteorological data, and a simulation by the Pennsylvania State University±National Center for Atmospheric Research ®fth generation Mesoscale Model (MM5). Environmental conditions during the event were characterized by a lake±700-hPa temperature difference of up to 22.58C, a lake±land temperature difference as large as 108C, and conditionally unstable low-level lapse rates. The primary snowband of the event formed along a land-breeze front near the west shoreline of the Great Salt Lake. The snowband then migrated eastward and merged with a weaker snowband as the land-breeze front moved eastward, offshore ¯ow developed from the eastern shoreline, and low-level convergence developed near the midlake axis. Snowfall accumulations reached 36 cm and were heaviest in a narrow, 10-km-wide band that extended downstream from the southern shore of the Great Salt Lake. Thus, although the Great Salt Lake is relatively small in scale compared to the Great Lakes, it is capable of inducing thermally driven circulations and banded precipitation structures similar to those observed in lake-effect regions of the eastern United States and Canada. 1. Introduction cold-frontal northwesterly ¯ow at 700 hPa, a lake±700- hPa temperature difference of at least 178C (which ap- The prediction of lake-effect snowstorms that develop proximately represents a dry adiabatic lapse rate), and over and downwind of the Great Salt Lake (GSL) is one an absence of stable layers or inversions near or below of the major forecast challenges facing meteorologists 700 hPa.1 Steenburgh et al. (2000) used observations in northern Utah. Occurring several times each year, from a recently installed National Weather Service Great Salt Lake±effect (GSLE) snowstorms last from a Weather Surveillance Radar-1988 Doppler (WSR-88D) few hours to more than a day, frequently produce snow- to identify GSLE events between September 1994 and falls of 20±50 cm, and have contributed to the state May 1998. During this period, 16 well-de®ned GSLE record 129-cm lowland storm-total snowfall that was events were observed, with the synoptic, mesoscale, and observed near Salt Lake City (SLC) from 24 to 28 Feb- convective characteristics of these events examined us- ruary 1998 (Carpenter 1993; Slemmer 1998; Steenburgh ing National Centers for Environmental Prediction et al. 2000). Despite signi®cant improvement in obser- (NCEP) Rapid Update Cycle version 2 analyses (RUC2; vational technologies and numerical forecast systems, Benjamin et al. 1991, 1994), SLC radiosonde obser- GSLE snowstorms remain dif®cult to predict with lead vations, and local WSR-88D radar observations. In ad- times of more than a few hours. dition to supporting the ®ndings of Carpenter (1993), Previous studies have identi®ed the climatological Steenburgh et al. (2000) also found that GSLE events characteristics, large-scale conditions, and mesoscale tend to occur during periods of positive lake±land tem- precipitation structures associated with GSLE snow- perature differences, usually exceeding 68C, and are storms. Based on lake-effect events identi®ed by visual most active during the overnight and early morning observations and spotter reports, Carpenter (1993) hours. It was hypothesized that the positive lake±land found that GSLE snowstorms were associated with post- temperature difference results in the development of Corresponding author address: Dr. W. James Steenburgh, De- 1 Due to the elevation of the GSL (;1280 m above mean sea level), partment of Meteorology, University of Utah, 135 South 1460 East surface and 700-hPa observations are used instead of surface and Room 819, Salt Lake City, UT 84112-0110. 850-hPa observations as is commonly done in studies of lake-effect E-mail: [email protected] snowstorms over the Great Lakes (e.g., Niziol et al. 1995). q 2001 American Meteorological Society Unauthenticated | Downloaded 09/28/21 02:24 PM UTC JUNE 2001 STEENBURGH AND ONTON 1297 United States (Carpenter 1993; Steenburgh et al. 2000). Wiggin (1950) described the general characteristics of lake-effect snowstorms in the Great Lakes region, in- cluding their potential for large accumulations and sig- ni®cant variations in snowfall over short spatial scales. Additionally, Wiggin (1950) noted that such storms were favored in polar continental air masses during pe- riods of large lake±air temperature differences, near- adiabatic lapse rates, and long overwater fetches. Peace and Sykes (1966) studied a lake-effect snowband using a mesoscale surface observing network over the eastern end of Lake Ontario. It was found that a narrow con- vergence line accompanied the snowband and it was hypothesized that surface sensible heating caused the formation of the snowband, with winds aloft controlling the location and movement of the band. Subsequent studies over the Great Lakes have identi®ed a variety of lake-effect precipitation structures including (i) broad area coverage, which may include multiple wind-par- allel bands or open cells (Kelly 1982, 1984); (ii) shore- line bands that form roughly parallel to the lee shore due to the convergence of a land breeze with the large- FIG. 1. Geographic features of northern Utah. Surface elevation in scale wind ®eld (Ballentine 1982; Braham 1983; Hjelm- meters shaded according to scale at bottom left. Station locations discussed in text are Salt Lake City (SLC), Tooele (TOO), Hat Island felt and Braham 1983; Hjelmfelt 1990); (iii) midlake (HAT), Gunnison Island (GNI), Great Salt Lake Desert (S17), and bands that form when the large-scale ¯ow is parallel to the Salt Lake City NEXRAD radar site (KMTX). Railroad causeway the long axis of a lake and a lake±land temperature identi®ed by a dashed line. contrast exists (Peace and Sykes 1966; Passarelli and Braham 1981; Braham 1983; Hjelmfelt 1990; Niziol et land breezes and low-level convergence that focus the al. 1995); and (iv) mesoscale vortices that form in a development of convection over the GSL. The greater polar air mass under conditions of a weak surface pres- frequency of lake-effect precipitation during the over- sure gradient and large lake±air temperature differential night and early morning hours may be related to the (Forbes and Merritt 1984). diurnal modulation of the lake±land temperature dif- Precipitation during GSLE events is most frequently ference and associated land-breeze convergence, similar characterized by the irregular development of radar ech- to that suggested by Passarelli and Braham (1981) over oes over and downstream of the GSL (Steenburgh et al. Lake Michigan. 2000). The most commonly observed organized precip- GSLE snowstorms share many similarities with lake- itation structures are solitary wind-parallel bands resem- effect snowstorms over the Great Lakes region of the bling midlake bands found over the Great Lakes, and FIG. 2. Daily mean lake-surface temperature at HAT (solid), air temperature at HAT (dashed), and air temperature at SLC (dotted) from 2 Sep 1998 to 31 Jan 1999. Large dots demarcate period of missing lake-surface temperature data from HAT. Unauthenticated | Downloaded 09/28/21 02:24 PM UTC 1298 MONTHLY WEATHER REVIEW VOLUME 129 FIG. 3. Regional RUC2 analyses and observed SLC upper-air sounding at 1200 UTC 6 Dec 1998. (a) Sea level pressure (every 2 hPa) and 10-m winds (full and half barbs denote 5 and 2.5 m s21, respectively). (b) 700-hPa temperature (every 28C), wind [as in (a)], and relative humidity (%, shaded following scale at upper right). Geopotential height trough axis denoted by dashed line. (c) 500-hPa geopotential height (every 60 m) and absolute vorticity (31025 s21, shaded following scale at upper right). Geopotential height trough axis denoted by dashed line. (d) SLC skew T±logp diagram with temperature and dewpoint (8C) denoted by heavy solid lines. Short-dashed line represents surface parcel ascent. Filled circle represents lake temperature. Wind as in (a). broad-area coverage precipitation shields that form near model, Lavoie (1972) found that frictional convergence the lee shoreline. In addition, GSLE precipitation some- due to land±water roughness contrasts, and surface sen- times occurs in concert with orographic precipitation, sible heating due to lake±air temperature differences, or within a broader-scale precipitation shield associated produce upward vertical motion and elevated inversion with synoptic-scale lifting. Signi®cant enhancement of heights near the lee shoreline of Lake Erie. The lake± GSLE events can occur when lake-induced precipitation air temperature difference was found to be dominant. features, such as solitary wind-parallel bands, extend Hjelmfelt (1990, 1992) examined the importance of low- over the downstream orography. level instability, lake±land temperature difference, sen- Several studies have used numerical models to ex- sible and latent heat ¯uxes, topography, capping inver- amine lake-effect snowstorm dynamics (e.g., Lavoie sions, and upstream moisture in producing lake-effect 1972; Ballentine 1982; Hjelmfelt and Braham 1983; snowstorms over Lake Michigan. He found that both Hjelmfelt 1990). Using a three-layer primitive equation shoreline-parallel and midlake snowbands were favored Unauthenticated | Downloaded 09/28/21 02:24 PM UTC JUNE 2001 STEENBURGH AND ONTON 1299 FIG. 4. Same as Fig. 3 except for 0000 UTC 7 Dec 1998. by strong lake±land temperature differences, weak sta- though changes in snowband location in response to the bility, and the absence of capping inversions at low evolving synoptic-scale ¯ow had timing errors of a few elevations.
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