6/23/02 tornadic supercell environment near Aberdeen, South Dakota

On the evening of 6/23/02, a tornadic supercell produced several tornadoes north of Aberdeen SD, including a couple of strong or possibly violent intensity.

This satellite photo showed storms developing over South Dakota and North Dakota before 00 UTC, including an area northwest of Aberdeen, and another area near and north of Pierre:
< visible satellite photo 2345 UTC

The 23 UTC surface map indicated an east-west warm front near the SD/ND border north of Aberdeen with backed winds north of the front, and hotter temperatures south of the front.  The Eta 12 hr 500 mb forecast also showed a short wave moving across the Dakotas and a wind max (> 40 kts) along the SD/ND border near the warm front:
< surface map 23 UTC  < Eta 12 hr forecast 500 mb winds and height contours valid 00 UTC (from UCAR/RAP web site)

Estimated parameter fields in the 23-00 UTC time frame from the SPC mesoscale analysis page suggested that 0-1 km EHI was maximized with increased 0-1 km SRH values (> 100 m2/s2) in backed winds near the warm front north of Aberdeen.   Another shear-CAPE parameter, 0-3 km VGP (vorticity generation parameter, Rasmussen and Blanchard 1998) was also maximized in the same area, and deep layer shear (BL-6 km shear) was around 50 knots.  Low-level thermodynamic parameters also appeared favorable for tornadoes north of Aberdeen, with lower LCL heights north of the front, decreased CIN (around 50 J/kg or less) suggesting that the storms would be surface-based, and acceptable LFC heights of 2000-2400 m given the large CAPE (near 4000 J/kg) just south of the SD/ND border.  These factors all suggested a favorable environment for supercell tornadoes near and north of Aberdeen with storms developing in that area:
< 23 UTC 0-1 km EHI   < 00 UTC 0-1 SRH 
< 00 UTC 0-3 km VGP  < 00 UTC BL-6 km shear
< 23 UTC LCL heights   < 00 UTC CIN & CAPE  < 23 UTC LFC heights

The Aberdeen (ABR) environment from both the observed sounding and RUC-2 analysis profile at 00 UTC confirmed that factors appeared favorable for support of supercell tornadoes as suggested by the SPC graphics.  Both profiles showed good shear-CAPE combinations, and even less CIN with lower LFC heights than suggested by the SPC depictions.  One would also need to take into account that LCL heights would be lower and low-level humidity would be larger north of Aberdeen with the surface warm front in place, and that low-level shear (SRH) might be larger further north as well:
< observed sounding at Aberdeen SD 00 UTC  < Aberdeen SD RUC-2 analysis sounding 00 UTC

By comparison, the environment further south and west near Pierre (PIR) appeared to have unfavorable low-level shear for supercell tornadoes (negative SRH values).  LCL heights were also excessively high (near 2000 m), with LFC heights high as well (> 2500 m), suggesting an unfavorable low-level thermodynamic environment for tornadoes, in spite of strong deep layer shear:
< Pierre SD RUC-2 analysis sounding 00 UTC

The storm cluster NW of Aberdeen developed into a large supercell on the south end just north of the warm frontal boundary (seen on radar below), and began producing large tornadoes after 0030 UTC.  In contrast, the storm cluster near Pierre, while taking on supercell characteristics and prompting a tornado warning, did not produce any tornadoes:
< ABR radar base refl. 0004 UTC, 0044 UTC, 0114 UTC
< surface map 00 UTC  < visible satellite image 0045 UTC

< tornado tracks north of Aberdeen 00 UTC to 02 UTC (courtesy NWS Aberdeen)

Using relative parameter value strengths:  , the table below summarizes the different environments near Aberdeen and Pierre for comparison:

The supercell north of Aberdeen moved eastward in a favorable location just north of and parallel to the warm front, which may help account in part for the storm's ability to produce several tornadoes over a nearly 2 hour period.

- Jon Davies 7/1/02

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