Case Study 25"Salt
Lake City Tornado and Long Island Flash Floods"
11 August 1999
Salt Lake City Tornado
Case Summary
One of the most devastating weather events in Utah's history occurred on 11 August 1999 as a tornado moved through downtown Salt Lake City between 18:45 and 18:55 UTC. The tornado damage was rated as F2 on the Fujita scale, and was responsible for 1 death (the first recorded tornado death in the state's history) and dozens of injuries. Hail up to 1.5 inches in diameter was also reported from this storm.
Synoptic Discussion
The morning of 11 August 1999, an upper-level trof was approaching Utah from the west, with an associated weak vorticity max, shown here at 15Z. There was also a jet streak aloft, just to the southeast, putting Salt Lake City in the area of cyclonic vorticity.
The 12Z SLC sounding shows no great instability or directional shear, but decent speed shear with height. Aloft, the 15Z RUC analysis shows cold air advecting in, destabilizing the atmosphere, providing an environment for rapid thunderstorm development. The 500-700mb lapse rate at 15Z rose to near 20C, due to the cold air moving in aloft. The 15Z RUC also depicted an area of CAPE approaching from the west, with values as high as 1000 J/Kg.
At the surface, dew points were in the low 50's through the morning, while surface temps rose into the mid 70's in the area, due to solar heating.
Long Island Flash Floods
Case Summary
Between 13Z and 17Z on August 11, clusters of thunderstorms formed along a warm front. As they moved very slowly east, they produced heavy rain that caused flash flooding from Deer Park to Manorville, NY. Rainfall rates estimated at 1 to 2 inches per hour for up to 3 hours caused rainfall totals from 3 to 5 inches. Water, rapidly accumulating in low-lying and poor drainage areas, swept cars away and trapped people in their vehicles.
Synoptic Discussion
On the morning of 11 August, Long Island sat just to the east of a shortwave aloft. Slightly cooler air, in addition to some divergence due to weak PVA, was advecting in from the west at 700mb. Analysis of vertical velocity fields at 12Z showed a bullseye of upward motion from sfc-500mb centered over and just south of Long Island.
Additionally, Long Island sat just to the north of a warm front, with warm, moist flow overrunning from the south. A pronounced theta-e ridge can be seen at 950mb into central Long Island. Here you can see a radar image of Long Island as the storms began to develop.