Lake Effect of the Great Salt Lake: Scientific Overview and Forecast Diagnostics Jim Steenburgh NOAA Cooperative Institute for Regional Prediction Department of Meteorology University of Utah

1. Overview of Event Characteristics
2. Forecasting Methodology
3. Predictability Limitations
4. Potential Forecast Diagnostics
5. Observations of lake-effect events
6. References
7. Acknowledgements

Lake-effect snowstorms produced by the Great Salt Lake represent a major winter-season forecast challenge for meteorologists in northern Utah. Such storms are notoriously difficult to predict and have major socio-economic impacts. Lowland snowfall of 10-30 cm is common, and in extreme events, lake-effect precipitation has contributed to storm total accumulations of as much as 130 cm.

Progress in the understanding and prediction of these events has been hampered by several factors, including a lack of radar and surface observations over and around the Great Salt Lake. Installation of a new Doppler radar by the National Weather Service on Promontory Point in 1994, and subsequent expansion of the Mesowest Cooperative Networks of the Western United States in the late 1990s, has greatly improved our ability to monitor lake-effect precipitation events.

Several studies have examined lake-effect snowstorms produced by the Great Salt Lake (e.g., Carpenter 1993; Halvorson 1998; Slemmer 1998; Onton 2000; Steenburgh et al. 2000; Steenburgh and Onton 2001; Onton and Steenburgh 2001). Building on previous investigations of similar events over the Great Lakes, these studies have identified several characteristics of the Great Salt Lake and the lake-effect snowstorms that it produces:

• Climatological lake temperatures are similar to climatological mean air temperatures at SLC, but the shallow nature of the GSL allows for rapid changes in lake-surface temperature over short time periods.

• The salinity of the GSL prevents freezing over most of the lake surface and results in a significant reduction of saturation vapor pressure and latent heat flux over the GSL.

• Because the lake never freezes and can warm rapidly, lake-effect precipitation has been observed from September through May.

• Lake-effect precipitation is most commonly characterized by the irregular development of precipitation echoes over and/or downstream of the Great Salt Lake. Occasionally, solitary wind-parallel bands develop. Sometimes, wide-areal coverage is observed to the lee. It is not uncommon for lake-effect precipitation to occur in concert with precipitation produced by large-scale or orographic processes.

• A difference in temperature between the lake-surface temperature and 700 hPa air temperature of at least 16 C is needed for lake-effect precipitation to develop. This is a necessary but not sufficient condition. Some cases of very large (>25 C) lake-700 hPa temperature differences have not produced lake-effect precipitation.

• Prior events have never featured an inversion or stable layer base below 700 hPa.

• Large lake-land temperature differences favor the development of thermally-driven land breezes, over-lake convergence, and lake-effect precipitation. Solitary wind-parallel bands may be initiated by convergence between a land-breeze and the large-scale flow, or between land-breezes from the opposing shorelines.

• Lake-effect precipitation exhibits considerable diurnal modulation and is most commonly initiated during the overnight hours when land-breeze convergence is favored. During periods of strong solar heating, lake-effect precipitation frequently disipates in the afternoon as scattered convection develops over the surrounding land.

• The steering-layer (700 hPa) flow influences the climatological structure and geographic position of lake-effect precipitation.

• Most events occur with limited directional shear, although statistics calculated to date have not fully examined the relationship of vertical wind shear to lake-effect precipitation. If winds are light, lake-effect precipitation may develop even with significant directional shear.

• Moisture fluxes from the GSL are not required for lake-effect precipitation to occur, but can enhance events. Because of the limited overwater fetch, upstream moisture is likely an important variable in many events.

The characteristics described above suggest that an ingredients based approach (e.g., Doswell et al. 1996) for predicting lake-effect events may be justified. We believe that lake-effect precipitation is produced by moist convection, and therefore all the following ingredients are required for an event to occur:

• Instability: The environmental lapse rate, either in the ambient upstream environment or the lake-modified environment, must be conditionally unstable.

• Moisture: There must be sufficient moisture for a parcel, lifted using traditional parcel methods, to reach a level of free convection. Moist convective updrafts must be sufficiently strong and deep to produce precipitation.

• Lift: A mechanism to lift parcels to their level of free convection must be present. This may be accomplished/aided in some cases by thermally-driven land-breeze convergence.

• 1. The post-frontal environment can support moist convection even without boundary-layer modification by the Great Salt Lake. CAPE is relatively low and is consumed by moist convection almost as quickly as it is generated. Convection is preferentially initiated over the Great Salt Lake in the nighttime and early morning hours when localized heating over the lake produces land-breeze convergence. During the afternoon, solar insolation initiates scattered open-cellular convection over land, with convection less common over the lake where low-level divergence and mesoscale subsidence may develop.

• 2. The post-frontal environment will not support moist convection. Boundary-layer modification by surface sensible and latent heat fluxes is necessary for moist convection, as well as a mechanism to lift parcels to their level of free convection. Careful diagnosis of the boundary-layer modification by the Great Salt Lake and the development of land-breeze circulations is needed to predict if such an event will develop.

It is unclear how rapidly predictability decreases for lake-effect events as lead-time increases. While some have hypothesized that fixed surface forcing in regions of complex orography may extend predictability, it is also possible that such forcing will merely accenuate model errors and biases, resulting in more false alarms and decreased skill.

Considering the errors found in present day modeling systems, it is not surprising that lake-effect precipitation associated with the Great Salt Lake is difficult to predict. At just 12 h, the average Eta-model winter-season root-mean-squared errors over SLC are 1.6 C, 15 m, 18%, and 5.5 m/s for 700-mb temperature, height, relative humidity, and wind speed, respectively (White et al. 1999). These errors are very significant given the sensitivity of moist convection to small changes in the large-scale environment.

In addition, lake-surface temperature, and its impact on boundary-layer modification, are also not well known. Observations are taken at only two points (HAT and GNI), and most meteorologists simply use the latest reports from these points as an estimate of lake-surface temperature. In a typical day, however, the lake-surface temperature fluctuates approximately 2C, and when cold air moves over northern Utah, the lake-surface temperature can fall several degrees. Therefore, an additional source of uncertainty is the estimate of lake-surface temperature during the forecast period.

To illustrate the sensitivity of lake-effect precipitation to some of the uncertainties described above, we ran simulations of the 6-7 December 1998 lake-effect event that incorporated changes in the relative humidity and lake-surface temperature. Significant differences in the QPF were found for minor changes in the ambient relative humidity (+/- 10%) and lake temperature (+/- 2 C). These results illustrate that uncertainties in model forecasts and the observation of lake temperature should be considered by forecasters, particularly when applying parcel methods to model soundings.

The recently developed real-time MM5 at the University of Utah is the only modeling system available to SLC forecasters that resolves, to some degree, the Great Salt Lake. Run with an inner grid resolution of 12-km, twenty-nine grid points are treated as water, with the lake temperature for the entire simulation set to the most recent observation at HAT. The model lake will have an impact on wind, temperature, and precipitation forecasts, although the skill of such forecasts has yet to be determined. Subjective experience from last year suggests that the 12-km MM5 may overpredict the number of heavy precipitation events, although it did provide accurate guidance for the two potential lake-effect events that were observed in October 1999. Forecasters should note that errors in the large-scale forecast significantly degrade the utility of high resolution models. If the large-scale is not forecast well, then the mesoscale forecast will likely have problems. Careful evaluation of the MM5 is needed this winter to determine its operational utility.

A variety of diagnostics are currently being tested to determine their utility for nowcasting applications. A long laundry list of potential products is provided below. These products utilize experimental products like the Utah ADAS and diagnostics derived from the Utah Mesonet. Eventually, we hope to reduce the product list and also extend our efforts to longer-range prediction using NWP guidance such as the University of Utah real-time MM5. We encourage you to evaluate the utility of the products below for nowcasting and diagnosis of lake-effect events. Please note that these products are usually current, but may at times be old depending on product availability at the University of Utah.

Objective	Product(s)	Comments/Caveat
Regional-scale overview	RUC Dynamic tropopause, 500 mb, 700 mb, and surface analyses	Obtained from NCEP. Usual caveats about regional data assimilation system analyses apply. Link included here simply for student use at University of Utah.
Mesoscale precipitation structure	KMTX 0.5 degree radar imagery	Available only to University of Utah. Link included here for student use at the University of Utah.
Mesoscale wind/temperature structure	Mesonet loop	Wasatch Front mesonet observations. Locamotive Springs not on plot yet.
Thermodynamic structure, instability, and boundary-layer modification by the GSL	Observed SLC sounding ADAS sounding for HAT ADAS sounding for TOO ADAS sounding for SLC ADAS Lake Diagnostic ADAS CAPE analysis Lake-700 mb temperature difference time series	For assessing convective environment. Forecasters should first use observed SLC sounding to assess convective conditions.
Low-level moisture distribution and boundary-layer modification by the GSL	Mesonet dew point analysis LMS time series HAT time series TOO time series SLC time series.	Can also be used for interactive sounding analysis.
Lift/land-breeze convergence	Mesonet surface analysis GSL Circulation Diagnostics	Calculated using mesonet observations over and surrounding the Great Salt Lake; Developed by Mike Splitt and Bryan White
Steering-layer flow	ADAS sounding for HAT ADAS 700 mb analysis Promontory Point time series	PRP is probably the best observation above lake-level for approximating steering-layer flow

• Events Investigated by Carpenter (1993)
  
• Events since the installation of the KMTX WSR-88D
  

• Carpenter, D. M., 1993: The lake effect of the Great Salt Lake: Overview and forecast problems. Wea. Forecasting, 8, 181-193.
• Doswell, C. A., III, H. E. Brooks, and R. A. Maddox, 1996: Flash flood forecasting: An ingredients-based methodology. Wea. Forecasting, 11, 560-581.
• Halvorson, S. F., 1999: Climatology of lake-effect snowstorms of the Great Salt Lake. M. S. thesis, Dept. of Meteorology, University of Utah, 78 pp. PDF File.
• Onton, D. J., 2000: An observational and numerical modeling investigation of Great Salt Lake effect snow. Ph. D. dissertation, Dept. of Meteorology, University of Utah, 131 pp. PDF File.
• Onton, D. J., and W. J. Steenburgh, 2001: Diagnostic and sensitivity studies of the 7 December 1998 Great Salt Lake-effect snowstorm. Mon. Wea. Rev. , in press. PDF File.
• Slemmer, J. W., 1998: Characteristics of winter snowstorms near Salt Lake City as deduced from surface and radar observations. M. S. thesis, Dept. of Meteorology, University of Utah, 138 pp.
• Steenburgh, W. J., S. F. Halvorson, and D. J. Onton, 2000: Climatology of lake-effect snowstorms of the Great Salt Lake. Mon. Wea. Rev. , 128, 709-727. Online abstract/manuscript.
• Steenburgh, W. J., and D. J. Onton, 2001: Multiscale analysis of the 7 December 1998 Great Salt Lake-effect snowstorm. Mon. Wea. Rev., in press. PDF File.
• White, B. G., J. Paegle, W. J. Steenburgh, J. D. Horel, R. T. Swanson, L. K. Cook, D. J. Onton, and J. G. Miles, 1999: Short-term forecast validation of six models.Wea. Forecasting., 14, 84-107.
  

This research was supported by grants provided by the National Science Foundation and National Oceanic and Atmospheric Administration.