Tuesday, October 26, 2010

Late October EF2 tornado at Rice TX

There's been no shortage of interesting tornado events the past couple months, with a tropical system producing tornadoes in the Dallas TX area and a bow echo type tornado in the Queens/Brooklyn area of New York City in September, and damaging tornadoes in the high country of central Arizona in early October. I've been busy, so no case study posts on those events. Last Sunday saw a photogenic October EF2 tornado at Rice TX, southeast of Dallas and just north of Corsicana TX (see storm chaser Bridget Geaughan's photo above). Video shown on CNN from a range of about 100 yds from the tornado was quite impressive. Although there's nothing terribly unusual about an October tornado in Texas, I thought I'd at least post some brief material about the setting to help make up for lack of posts in recent weeks.

Probably the most interesting aspect of Sunday's afternoon setting is how quickly the low-level shear increased during the afternoon in response to the strong mid-level trough (see 2nd image above at 500 mb) moving west to east across Texas that helped fire up the storms ahead of a pre-frontal surface trough and wind shift (see surface map, 3rd graphic above). The SPC mesoanalysis showed 0-1 km storm-relative helicity (SRH, a measure of low-level wind shear, see 4th graphic above) at 2100 UTC / 4 pm CDT to be in the 100-150 m2/s2 range in the Rice TX area, values somewhat marginal for support of supercell tornadoes. However, by 2300 UTC / 6 pm CDT, about the time of the tornado, SRH values had doubled (> 250 m2/s2, same graphic) in response to the approaching mid-level trough, increasing the combination of SRH and MLCAPE (around 3000 J/kg) that could help generate low-level mesocyclones with stronger supercells (energy-helicity index near 3.0 and above by 2300 UTC, same graphic).

The RUC analysis profile at Corsicana TX, a few miles to the south of Rice at 2300 UTC during the tornado (see last graphic above), suggested strong combinations of parameters for supporting supercell tornadoes, even with a surface wind slightly west of due south. So a strong tornado occurring with a discrete supercell in this environment was not a surprise; SPC had a tornado watch issued well in advance, and a tornado warning was issued by NWS Ft. Worth based on radar roughly 15 minutes prior to the tornado.

- Jon Davies 10/26/10