NOAA Technical Memorandum NWS WR-190 GREAT SALT LAKE EFFECT SNOWFALL: SOME NOTES AND AN EXAMPLE \ David M. Carpenter l ) \~ National Weather Service Forecast Office Salt Lake City, Utah October 1985 UNITED STATES I National Oceanic and I National Weather DEPARTMENT OF COMMERCE Atmospheric Administration Service Malcolm Baldrige, Secretary John V. Byrne. Administrator Richard E. Hallgren. Director This pub1ication has been reviewed and is approved for pub1ication by Scientific Services Division, Western Region. ~scfhi~Scientific Services Division Western Region Headquarters Sa1t Lake City, Utah i i TABLE OF CONTENTS PAGE I. Introduction 1 II. Elements of a Great Salt Lake Effect Snowstorm 2 III. An Example 11 IV. Summary 14 v. Acknowledgments 15 VI. References 16 i i i Great Salt Lake Effect Snowfall: Some Notes and an Example David M. Carpenter National Weather Service Forecast Office Salt Lake City, Utah I. Introduction Following are some ideas result­ ing from the study of the various A lake effect snowfall along the elements which should contribute to Wasatch Front is typically a post cold Great Salt Lake effect snowfalls. frontal event in which the duration of Twenty one lake effect cases dating snowfall downwind of the lee shore is from March 1971 to October 1984 were extended in cold, northwest flow. used. The last case October 18, Where one would normally expect 1984, is used as an example of a lake snowfall to end with the passage of effect storm. This storm resulted in the upper level trough, lake effect the heaviest 24 hour snowfall on snows usually continue for several record at the Salt Lake Interna­ hours after that event. tional airport. All of the cases This paper is based on data taken produced four or more inches of snow from lake effect cases that occurred at some point in the valley and seven in the northwest flow following the of them produced over a foot. This passage of an upper level trough. set of data is not intended to Occasionally it appears that a lake represent a complete listing of all effect may have occurred in southwest Great Salt Lake effect cases which flow, which enhances or produces occurred during this time period. snowfall close to the northeast Instead, it is more likely to be a shoreline. These cases are relatively representation of the most dramatic infrequent and are not included in or most noticeable cases. this study. The necessity of a rapid The percentage of annual infusion of cold air, usually in the snowfall in the Salt Lake Valley form of a cold pocket, and the which may be attributable to lake fact that this air would have to come effect is not known. However, the from a southerly direction makes a average number of lake effect cases conventional lake effect hard to come per year has been estimated to be by. What probably occurs in most between 6 and 8 (Alder,l977). The cases when snowfall is heavier along average number of days per year in the northeast shoreline is a warm which 0.1 inch of snow or more is advection pattern associated with a measured at the Salt Lake Interna­ cut-off low over Nevada as described tional Airport is 34 (Figgins,l984). by Williams (1962). Terrain produced Other studies indicate that lake upslope and differential friction from effect from the Great Salt Lake the lake surface to the shoreline probably occurs a rather small would enhance snowfall in that percentage of the time (Dunn, 1983). vicinity, but a contribution of Considering these facts and the instability and moisture from the restrictive nature of the required Great Salt Lake to the air mass is temperatures and wind flows necessary probably not a factor in most of these to produce a lake effect, it is felt cases. Even after limiting the by most of the forecasting staff at study to storms that occur in north­ the Salt Lake International Airport west flow, the effect of the Great Salt Lake on snowfall is not one that can be easily measured. 2 that the total contribution df lake effect snowfall to the annual snowfall of the Salt Lake Valley1 and nearby areas is probably rather small. Nevertheless, the lake effect can produce dramatic results, and . is the object of much interest and specu­ Brigham lation among those who watch the 0 weather along the Wasatch Front. "" 423~ < II. Elements of a Great Salt Lake 1 OgderJr-L?: effect Snowstorm 4300"' 0 ( :::> LL o A. Topographical Influences ~~:0:: The Lake is literally surrounded by mountain ranges which at one time formed islands and the shoreline (Fig. 1). The most prominent of these north-south mountain ranges is the Wasatch Range. The Great Salt Lake is oriented approximately along the 320 degree radial from the Salt Lake International Airport. The longest fetch, or over water trajectory, is approximately 75 miles long along that 320 degree radial. This fetch is realized in northwest flow for the Salt Lake Valley and in northerly flow for the Tooele Valley. Both Figure 1 -Map of the Great' Salt Lake valleys slope upward away from the and surrounding topographical · fea­ Lake shore and form a rough half bowl tures. shape. In addition to upslope, the the Lake (Hess,l959). In reality the topography makes two other modifi­ dynamics and kinematics of this cations to the surface wind that are process are much more complicated. worthy of note. The first of these At all four points,. the pressure is illustrated in Figure 2. An gradient force (PG) is the same, but exaggeration of a ·s.imple balance of the frictional drag force (F) varies forces is used to explain frictional with the roughness of the surface. convergence · along the lee shore of Over the relatively smooth lake surface the frictional force is less than that over land. Therefore, the 1 The term Salt Lake Va'lley will be wind velocity (V) is greater over the used to refer to the S~l t Lake City lake than over land. This difference metropolitan area including all in speed is enough to cause an area municipalities near Salt Lake City and of low level convergence along the located west of the Wasatch Mountains lee shoreline (Lavoie,l972). The and east of the Oquirrh Mountains, second modification to the low level south of the Great Salt Lake and north flow is the channeling that occurs as of the Transverse Mountains. The air is pushed against the sides of Transverse Mountains are shown in the mountain ranges. Figure 1 as the point of the Mountain Figure 3 shows the resulting just north of Alpine. streamlines of a hypothetical case of 3 N N I . >' i - ' w--,-~ w--~-E \/ i s' sI Figure 2 - A simple balance of forces Figure 3 - An idealized illustration illustration of the effect of differ­ of the surface stre811Jline analysis of ential friction on the surface wind northwest flow when the effects of flow in the vicinity of the Great Salt differential friction and channelling Lake. by the Wasatch Front are taken into account. northwest, low level flow. Note that the channeling effect and the In a Great Salt Lake local front, backing of the wind as it moves cold air at the surface becomes onshore, combine to produce low level dammed up against the mountains in convergence in the upper portions of the upper portion of the valley and the Tooele Valley and along the east forms a small cold dome (Figure 4). bench area of the Salt Lake Valley. Then, the low level flow coming off During some lake effect storms, the Great Salt Lake rides up and over the s~rface wind at the Salt Lake this cold dome, producing vertical International Airport has been known motion. The colder air in the valley to shift to the southwest and then to is maintained as being colder than the southeast, while the wind at the air coming off the Lake by nearby surrounding stations remains convective down drafts and precipi­ northwesterly. This is possibly a tation. Eventually with the help of a reflection of something called a local frictional disturbance along the lee front or boundary layer front by shoreline, the flow in the valley Garner (Garner,l983). This is becomes detached from the larger something similar to a miniature scale flow and can switch to a New England Coastal Front (Bosart, southerly direction. Without meso­ Vaudo, and Helsdon, 1972). scale data to confirm the above 4 cannot be overlooked. This will be dealt with in another section. For the purpose of this study, it would be best to have a record of Great Salt Lake temperature measure·· ments for several locations at the ~---~ same time of the day over many years. For the purpose of real time forecasting, it would be best to have the same temperature data available in real time. However, neither of these situations is the case and we will have to settle for data taken at one point (the southern shore near the Salt Air Resort), twice a month by the United States Geological Survey. These data were then compared to the previous seven days actual mean air temperature at the Salt Lake International Airport (Figure 5) and to the normal mean air temperature of the airport (Figure 6). Two least squares curves were fitted to the data to predict the temperature of the Great Salt Lake. Figures 5 and 6 show the resulting least squares curves (solid lines with dots) and the distribution Figure 4 - Illustration of Great Salt of data points about the curves Lake local front and cold air damming.