Sunday, April 27, 2008

Nighttime tornado setting in north-central Kansas on 4/24/08-4/25/08

Nighttime tornadoes, particularly after midnight, are somewhat unusual in Kansas and the central plains. This is largely because nighttime cooling at the surface below warmer temperatures aloft in many plains severe weather episodes prevents storms from being surface-based where “parcels” of air near the ground can rise rapidly to aid in tornado development. An exception sometimes occurs when strong warmth and moisture moves north in the plains at night in response to the approach of a strong springtime storm system. The deadly nighttime Greensburg area tornadoes last year were an extreme example of this type of situation.

On 4/25/08 after midnight (around 12:26 am CDT), a damaging tornado (rated EF-2, 15 mile path) developed north and northeast of Beloit in north-central Kansas (see this link from NWS Hastings). The parent supercell had been traveling east for nearly 5 hours before producing this tornado (see composite radar images above). Why did this storm suddenly produce a moderately long-track tornado in the middle of the night in Kansas? There were several factors present over north-central Kansas to suggest why.

First, the storm environment was becoming more surface-based as the supercell moved east. Above, compare estimated 0-3 km CAPE (this low-level instability is one indicator of how surface-based a setting may be) on maps from the SPC mesoanalysis at 02z (9 pm CDT) and 05z (midnight CDT). Notice that earlier, when the tornado-warned supercell (circled “S”) had been near Hill City in northwest Kansas, there was no low-level CAPE (red), suggesting an environment that wasn’t really surface-based. However, by 05z, an area of low-level CAPE (red) was showing up over north-central Kansas, with the supercell moving into it. This was a more surface-based setting, a result of strong warm moist advection, with surface weather maps indicating a significant temperature and dew point rise at Salina (not shown) from 68/63oF to 70/64oF during 03z (10 pm CDT) to 05z (midnight CDT), at night.

A similar setup is seen with SPC maps of estimated total CAPE and CIN, also above. In red, CAPE (instability) was present across northwest and north-central Kansas throughout the evening. But large CIN in dark blue over northwest Kansas at 02z (indicating inhibition that would slow near-surface air parcels from rising rapidly beneath storm updrafts) was not a good environment for significant tornado development with the supercell located near Hill City. But later, at 05z, notice that the supercell was approaching an area with much less CIN (in light blue or white) over north-central Kansas, indicating a more surface-based environment, increasingly favorable for tornadoes if low-level wind shear was also present.

Also above, SPC maps of the 0-1 km energy-helicity index (EHI) at both 02z and 05z showed large combinations of low-level wind shear and instability supportive of supercell tornadoes (suggested by large EHI values) pointing northward into north-central Kansas. This is where the more surface-based environment at 05z was also located as discussed above, with the long-lived supercell moving eastward into this “more favorable” axis. These factors likely contributed to the unusual middle-of-night tornado near Beloit, when such events are relatively rare. As clusters of storms increased over southeast Nebraska and northeast Kansas after 06z, widespread organized outflow brought an end to the tornado threat.

Computer forecasts from the NAM/WRF model valid at 06z (1 am CDT) on 4/25/08 did a reasonable job hinting at potential for a nighttime supercell tornado environment as far out as 18 hours in advance. Note above the model forecast of strong EHI values (wind/instability combinations supportive of tornadoes, bright colors) at 06z, as well as the more surface-based environment suggested by forecast 0-3 km CAPE values at 06z (orange and yellow colors). These overlapping parameters from north-central and northeast Kansas into southeast Nebraska (where an EF-1 tornado occurred from another storm) suggested a model forecast “heads-up” to forecasters for nighttime tornado potential well in advance of this event.

Jon Davies 4/27/08


CheekyGeek said...

I really appreciate your "post mortem" of this event. Very educational! I hope this will continue.

Sean Mullins, Age 22 said...

Very informative! Its really great to see detailed insight only a few days after the event. I targeted way too far south on the nose of that dry punch despite the obvious cap. Your analysis is not only helpfull to me, but could lead to an overall increase in tornado documentation by chasers gaining knowledge and bettering their selves thanks to you. By the way, my nephew loves your new book. He will be chasing with me before long.