I haven't done a blog post in a long long time. But several people asked me why last Tuesday (April 26, 2016) wasn't a big tornado day in north & northeast Kansas as had been forecast. So... it seemed a useful topic to start up posting again.
With our many computer models, it's not hard to see potentially significant systems coming several days in advance, but whether they produce tornadoes or not often has much to do with features such as outflow boundaries and their orientation, which models certainly can't forecast very well.
Tuesday morning April 26 saw a strong area of thunderstorms move east-southeast across northeast Kansas and over into Missouri, producing a strong trailing boundary of cool outflow air (see satellite photo and surface map below).
Strong surface-based instability (large CAPE) was present south of this boundary, but southeast winds producing wind shear supportive of potential tornadic supercells were largely limited to the area north of this boundary, separate from the warm sector instability (see SPC analyses of sfc-based CAPE & storm-relative helicity/SRH below at 20 UTC / 3 pm CDT).
By 20 UTC, storms were forming along this outflow boundary, but because of the south-southwest to north-northeast mid-level flow aloft (see SPC 500 mb chart below), these storms moved immediately into the cooler surface air north of the outflow boundary.
Indeed, several supercells did develop north of the outflow boundary, although none could produce tornadoes in the cool surface air. The radar panels below show one of these supercells (white arrow) that developed southwest of Topeka and moved north-northeast, prompting a radar-based tornado warning just northwest of Topeka shortly after 22 UTC (5 pm CDT).
The photos below show the cool air clouds north of the stalled outflow boundary, and the non-tornadic supercell as it passed west and northwest of Topeka within the cool surface air.
A somewhat similar situation (not shown) involving outflow and a stationary front over northeast Texas three days later on April 29 did generate a significant tornado at Lindale Texas, northwest of Tyler. In that case, mid-level flow aloft was more west to east so that supercell storm motion was mostly parallel to the boundary, instead of perpendicular and up into the cool air, as in the Topeka April 26 case.
Outflow boundaries can help produce tornadoes, or they can hinder tornado production... it all depends on their orientation and strength relative to how storms move near them.
- Jon Davies 5/1/16