700 mb Winds & Temperature	12 Z, 15 Z, 18 Z, 21 Z	500 mb Heights & Vorticity	12 Z, 15 Z, 18 Z, 21 Z
850-400mb Wind Shear	12 Z, 15 Z, 18 Z, 21 Z	500 mb Winds & Temp	12 Z, 15 Z, 18 Z, 21 Z
250 mb Winds & Temperature	12 Z, 15 Z, 18 Z, 21 Z	250 mb Height & Winds	12 Z, 15 Z, 18 Z, 21 Z
Other Model Fields
Salt Lake City, UT LAPS Sounding	15 Z, 16 Z, 17 Z, 18 Z, 19 Z, 20 Z, 21 Z
LAPS CAPE	15 Z, 16 Z, 17 Z, 18 Z, 19 Z, 20 Z, 21 Z
University of Utah ADAS CAPE	16 Z, 17 Z, 18 Z, 19 Z, 20 Z
Other Observational Data
GOES-10 IR Satellite Data	1645 Z, 1700 Z, 1715 Z, 1730 Z, 1745 Z, 1830 Z, 1845 Z
GOES-10 Visible Satellite Data	1645 Z, 1700 Z, 1715 Z, 1730 Z, 1745 Z, 1830 Z, 1845 Z
GOES-10 Water Vapor Satellite Data	1645 Z, 1715 Z, 1745 Z, 1815 Z, 1845 Z
Surface Observations (w/RUC MSLP contours)	12 Z, 15 Z, 18 Z, 21 Z
Utah Mesonetwork Surface Observations	15 Z, 16 Z, 17 Z, 18 Z, 19 Z
SLC Observed Sounding	12Z
Hi-Res Topographic Map	SW United States

Environmental Discussion

The environment over the Salt Lake City (SLC) area became destablized by shortwave/jet streak dynamical forcing, and midlevel cold advection during the morning hours of 11 August 1999. An upper-level trough was moving over the Great Basin area at 250 mb (250 mb Height & Winds: 1200 UTC). An 80-kt jet streak was located downstream of the trough axis with its left front quadrant over the Salt Lake City (SLC) area by 1800 UTC. The 500 mb trough axis (500 mb wind & vorticity: 1200 UTC, 1800 UTC) was displaced to the west of the 250 mb trough axis. There were two vorticity maxima associated with this trough. The first vorticity maximum, which brought precipitation to northwest Utah the previous night, moved from the Great Salt Lake (GSL) toward western Wyoming (500 mb wind & vorticity: 1200 UTC, 1800 UTC). Extensive clouds over NW Wyoming/E Idaho associated with this shortwave trough were still visible the following morning (vis: 1615 UTC; water vapor: 1645 UTC). The second, stronger vorticity maximum moved from southern Nevada (1200 UTC) southeast into Utah (1800 UTC). At 500 mb, cold advection occured over SLC during the morning hours. The cold advection aloft, resulted in a greater lapse rate which, in turn, increased the CAPE over the SLC area from approximately 500-1000 Jkg^-1 in the morning (Observed sounding: 1200 UTC) to 1300-2000 Jkg^-1 by midday (LAPS sounding: 1800 UTC; CAPE: 1800 UTC). The Lifted Index for the Salt Lake valley decreased below -8 by 1800 UTC, also. In addition to the destablizing atmosphere, weak-to-moderate vertical wind shear was present in the 850-400 mb layer(850-400 mb shear: 1800 UTC). This product, which is used operationally in the Great Basin in the same manner as 0-6 km shear is used at lower elevations, showed 30-35 kts of shear were present in this layer over SLC at 1800 UTC. The 1800 UTC sounding showed the 0-3 km Storm Relative Helicity was only 3 m²s^-2 and the Bulk Richardson Number was 65. The weak-to-moderate shear and moderate CAPE suggests multicell storms are the primary threat.

At the surface, a weak trough extended southwestward from a surface low over northeastern Wyoming to the GSL (Surface map:1200 UTC, 1800 UTC). Data from the Utah Mesonetwork show a lake-breeze boundary located south of the lake by 1800 UTC. The 50°F dewpoints around the region are higher than normal, perhaps due to the precipitation the night before. Temperatures vary little near the lake-breeze boundary, but there is a slight change of low-to-middle 70s °F south of the boundary to upper 60s °F over the GSL.

The high-resolution topography map illustrates the important topographic features of the the region. SLC is located in a flat valley approximately 35 miles across and 4200 feet above sea level (ASL). The shallow GSL lies to the northwest of SLC and is nearly always cooler than the surrounding land surface during summer days. The valley rises quickly into the Wasatch Mountains to the east. A series of smaller mountain ranges are located along the west side of the valley. Of particular interest is the Oquirrh Mountains, located southwest of SLC, with peaks reaching over 10,000 feet ASL. Visible and infared satellite imagery (visible: 1715 UTC; IR: 1715 UTC) show convection initiating over the Oquirrh Mountains and moving towards SLC (visible: 1730 UTC, 1745 UTC IR: 1730 UTC, 1745 UTC).

SLC was located in a region of forecasted general thunderstorm activity made by the Storm Prediction Center (SPC), but no specific threat of severe weather was mentioned in the morning outlooks (Convective Outlook: text, graphic). No watch was issued for the storms that occurred during the event in this scenario.

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