Wednesday, April 7, 2010
Surprise tornadoes near Wakeeney KS 4/6/10?
Although not in yesterday's NWS storm reports for 4/6/10, storm chaser Dick McGowan (on his way to Oklahoma from Denver) happened to be in the right place at the right time, apparently photographing a couple tornadoes near Wakeeney, Kansas, just north of I-70. I haven't seen any pictures posted yet. but this was well northwest of the true warm sector, and in the vicinity of a secondary front moving across northwest Kansas. This wasn't a forecastable event, but it is worth looking at because it was certainly unusual.
The first graphic above is a SPC mesoanalysis estimation of 0-3 km MLCAPE and surface vorticity at 2100 UTC (4 pm CDT), with radar reflectivity overlaid. Notice the pronounced low-level CAPE maximum in the Wakeeney/Hays KS area. By itself, this doesn't mean much, but given its location far away from the warm sector over eastern Kansas, it suggests an area of cold air aloft combined with surface heating. The surface analysis just before 2100 UTC (2nd graphic above) shows an east-west frontal boundary in the area, with surface temperatures in the mid 60s F along this front (70s F further south) and dew points in the mid-upper 40s F. The RUC midlevel map analyses at 2100 UTC (3rd graphic above) definitely show cold air coming in aloft at 700 mb (0 deg C over northwest Kansas) and a closed 500 mb low over northern Colorado. So there was warmth at the surface, cold air aloft at 700 mb, and a frontal zone with just enough moisture in the local area. One probably wouldn't call this a real "cold-core" setting given the odd surface pattern and the distance from the midlevel low, but the cold air aloft with this system certainly played a role.
Visible satellitie and radar (4th graphic above) show the location of a small cell (possibly a mini-supercell) near Wakeeney, moving east, with plenty of clear skies (heating) to its south and east. This is near the time the tornadoes were reported. A RUC analysis sounding at Hays Kansas at 2100 UTC (east of Wakeeney, last graphic above), modified for more heating than the raw RUC file indicated, yields 200-300 J/kg of CAPE, all bunched below 500 mb, with decent wind shear, fairly steep low-level lapse rates, and small CIN. This is another one of those small CAPE soundings I've discussed recently, and the type of environment that would likely generate rapid low-level stretching within local updrafts. Given all the above ingredients, it's not too much of a "stretch" to see how the setting might support some weak tornadoes with the isolated cell over northwest Kansas. But it was certainly not an event that could really be anticipated.
What an oddball case! And as I write this, more cold air aloft from the same system is now generating a small late pm severe cell over north Kansas City, just behind a secondary surface cold front.
- Jon Davies 4/7/10