Monday, April 30, 2012

April 27 and April 21, 2012: Examples of boundaries hurting & helping tornado potential

Last Friday's (4/27) SPC outlook with a moderate risk and strong tornadoes expected in central and eastern KS did not really verify (there were only 2 or 3 very brief EF0 tornado reports with no damage). Looking back, this case was a good example of a narrow warm sector where, although ingredients (CAPE and wind shear) looked very supportive of supercell tornadoes, storms persistently formed too close to a surface stationary frontal boundary.  They moved across it and into a cooler surface environment where they rapidly became elevated supercells (not surface-based), with the only lifted parcels that could produce CAPE/instability coming from well above ground, which worked against tornadoes.

To illustrate this, the first graphic above shows surface-based (SB)CAPE and storm-relative helicity (SRH, low-level wind shear) from the SPC mesoanalysis at 7 pm CDT when a tornado-warned storm was moving into Topeka KS from the southwest.  Notice how far north of the surface warm sector (indicated by SBCAPE) this cell was located.  As seen from the Topeka Rapid Refresh sounding at the same time (2nd graphic above; no observed sounding was done due to ongoing severe weather), this environment was clearly elevated, with no CAPE from lifted parcels near the ground.  Even though SRH was quite large, it is difficult for tornadoes to develop in such an elevated setting north of a stationary surface boundary, and any tornadoes that could manage to develop here would be weak and very brief.   Because supercell storms could not spend much time in the surface-based warm sector before crossing the stationary boundary into much cooler surface air, the supercell tornado threat as originally anticipated could not materialize.

The Saturday before (4/21) in the northern plains saw another type of boundary generate a surprise round of weak (EF0-EF1, see storm report map and photo above) tornadoes in western Minnesota that were not expected.  These were non-supercell tornadoes that developed from enhanced stretching processes along a sharp north-south surface wind-shift boundary that provided vorticity for spin.  With very cold air aloft, a vertically compact area of CAPE (see the sounding above) developed from surface heating accompanied by very steep low-level lapse rates.  Both of these factors increased the potential for strong low-level stretching as storms formed along the surface boundary, which resulted in several tornadoes that did some damage to farmsteads.  Notice on the SPC graphics above that the overlapping of steep low-level lapse rates with low-level CAPE suggested very well the enhanced stretching environment along and just west of the boundary (see my paper with Jim Caruso here).  This was also indicated by the SPC Non-supercell Tornado parameter (also shown above), which incorporates these same ingredients.

So, 4/21 was an example of a different type of boundary that helped rather than hurt tornado potential.  Fortunately, difficult-to-forecast events like these usually produce generaly weak tornadoes and don't happen that often.  One certainly would not expect tornadoes in April with northwest flow aloft and surface dew points only in the 40's F (quite unusual!!!).  But a look back shows that some required ingredients for non-supercell tornadoes were there.

- Jon Davies  4/30/12

No comments: