Wednesday, June 30, 2010

After-dark EF4 tornado in NW Iowa 6/25/10 - a look at the environment

Violent (EF4-EF5) tornadoes are rather rare after dark in the Plains. Last Friday's tornadoes during 03-05 UTC 6/26/10 after dark in northwest Iowa (see video image above) included an EF4 intensity tornado near Sibley, Iowa, which certainly makes the environment worth examining.

The tornadoes occurred just ahead of a cool front moving slowly southeastward (not shown) and an approaching upper trough (not shown), associated with the west end cell of a line sagging southward over northern Iowa (see SPC mosaic radar overlays above). The 03 UTC SPC mesoanalysis of 0-1 km energy-helicity index (EHI; see 2nd image above), which combines MLCAPE and 0-1 km SRH, showed very large values (near 10!) ahead of this west-end supercell over northwest Iowa, suggesting strong potential for low-level mesocyclones and possible tornadoes with discrete or tail-end supercells. The SPC mesoanalysis of low-level CAPE (0-3 km MLCAPE, see 3rd graphic above) at the same time also showed a notably surface-based setting over northwest Iowa, with low-level MLCAPE values near 100 J/kg. From a 2009 NWA electronic journal paper by Davies and Fischer, environments with large CAPE and SRH that are also relatively surface-based offer important support for tornadoes after dark, but aren't that common in the Plains due to nighttime cooling beneath typical warm layers advected eastward from the desert southwest. This latter issue did not appear to be a problem last Friday evening over northwest Iowa.

The 03 UTC RUC analysis sounding at Sheldon, Iowa (just south of the tornadic tail-end supercell; see 4th graphic above) confirmed large MLCAPE and SRH (near 4000 J/kg and 500 m2s2) and sizable deep layer shear (> 40 kts), along with a relatively surface-based setting (0-3 km MLCAPE > 100 J/kg, and MLCIN around -50 J/kg), all excellent supporting ingredients for tornadoes with supercells in that environment. In contrast, 110 miles to the east-southeast with another tornado-warned embedded supercell near Clarion, Iowa, the RUC sounding (see last graphic above) showed _no_ low-level CAPE, and MLCIN was quite large (-250 to -300 J/kg) with not nearly as much total MLCAPE above the CIN layer (only around 1600 J/kg),. This suggested somewhat "elevated" nighttime storms with considerably less tornado potential, and no tornadoes were reported in this area farther east.

NWS Sioux Falls has a page detailing their survey of the EF4 tornado near Sibley, Iowa, north of Sheldon.

- Jon Davies 6/30/10

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