The June 4, 1999 Oglala Tornado Event Brief Synoptic Analysis (page 2)

The 4 June tornadic supercell developed within an environment which demonstrated the classic features for a tornadic event, including great instability, high low-level moisture, strong vertical wind shear, and a mid-level short wave moving through the area. This brief discussion discusses these features as they were exhibited during the 4 June Oglala tornado event. Data from 00Z on June 5th will be used for most of the analyses.

Beginning with the higher levels of the atmosphere, the 500 mb height / vorticity at 00Z (below, from the 00Z analysis of the ETA model) exhibited an unseasonably strong and deep trough over the southwest United States, with nearly meridional flow through the panhandle of Nebraska and western South Dakota. A relatively strong shortwave is evident in this flow moving to the north through the Panhandle of Nebraska and southwest South Dakota. These features are also well-represented in the DIFAX charts at 300mb, 500mb, 700mb, and 850mb.

500 mb GIF

The upper-level flow above engendered a strong surface low pressure center over northeast Colorado, which wrapped copious moisture westward into southwest and central South Dakota. This is evident in the image below (RUC surface analysis) by the high dewpoints (shaded) which were in the upper 60s and low 70s in this area.

RUC Surface Analysis

The copious surface moisture, combined with the relatively cool atmosphere aloft, created a locally very unstable atmosphere (below).  Lifted Indices of less than -10 C were present over the area, with CAPES of 2500 to 3000 J/Kg. The most severe storms formed and moved along the  instability axis which extended from southwest South Dakota to south-central South Dakota.

00Z RUC Lifted Index and Surface Wind

The satellite image and surface observation composite at 23Z (5 PM MDT) shows the relationship between the position of the convection which was forming in northeast Nebraska to the surface features (below). As can be inferred from the surface analyses above, and the observations in the image below, an area of convergence was present along the moisture gradient north of the axis of highest dewpoints in western South Dakota. A well-defined line of cumulus (as well as the thunderstorms which later became the Oglala storm) are oriented along this region of convergence (moisture gradient) which extended from south of Douglas, WY to north of Aberdeen, SD. Note also the position of the surface low pressure (contoured in white) south of the tornadic storms. This is a common orientation for Northern Plains tornadic events. You may also want to check out the DIFAX surface chart at 00Z.

23Z 1km Visible Satellite with Station Plot and MSL Pressure

The aspect of the vertical profile of the wind (wind shear) is critical in determining what types of storms will form on a given day. During the evening of June 4th, there was strong wind shear between the surface and 6 km AGL, with values of 44 to 50 ms-1 (as demonstrated by the hodographs from the Rapid City profile, and the Merriman NE profiler). In addition to the strong shear, the helicity was high in both wind profiles, ranging from 200 to nearly 500 m2/s2.

More analyses are being performed on this interesting case, and will be posted when available.

Return to June 4, 1999 Oglala, SD Tornado Event


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