Tuesday, March 16, 2021

March 13, 14, and 15 bring big tornadoes and cold-core action to the Plains


Friday, March 12 through Monday, March 15, 2021 saw tornado activity really ramp up in the Central Plains.  There were no deaths or injuries reported, as the biggest day (Saturday, March 13) was well-forecast and warned.

Over 10 tornadoes occured on March 13 in the central and southern Texas (TX) panhandle, including the twin large tornadoes pictured above from the same supercell near the town of Happy, along with a storm-relative velocity image at roughly the same time showing the two mesocyclones side by side.  The tornado and mesocyclone labeled "1" above generated a tornado rated EF2 on the ground for 17 miles according to NWS Amarillo (had the tornado hit a town, the EF rating would likely be higher).

Farther east, according to radar, it appeared there were also two tornadoes on the ground simultaneously at times with the Happy, TX supercell according to radar (not shown).  But these were not photographed as the storm became increasingly wrapped in rain and the radar circulations occurred in difficult spotter/chaser terrain northeast of the Palo Duro Canyon State Park.  It doesn't appear that there were any tornado mergers, but that's not entirely clear, and there were also times when multiple mesocyclones appeared to be interacting with each other (not shown).

The following two days, other localized tornadoes occurred in northwest Kansas (KS) and southwest Nebraska (NE), including the "white" landspout tornado pictured below (1st photo) west of Trenton, NE on Sunday March 14, and a tornado near Stilwell, KS, south of Kansas City (2nd photo) where some minor damage occurred (rated EF0):

I'll briefly touch on the meteorological setting for March 13, March 14, and March 15, as these days involved a closed 500 mb low within a large mid-level trough moving slowly northeastward from the southwestern U.S.  On March 13, the 500 mb low was too far to the west to be directly involved in the Texas tornado outbreak, but on the subsequent two days, the cold-core low aloft had a direct impact on tornadoes occurring in rather marginal settings.

Below is a visible satellite image at mid-afternoon on March 13, with surface weather map features drawn in, and the location of the 500 mb low well to the west:

The tornadic supercells occurred in the warm sector ahead of a Pacific cold front and south of a warm front over the TX panhandle, including the Happy, TX supercell (labeled with arrow).   Below are SPC mesoanalysis images of two composite tornado parameters, suggesting how favorable for supercell tornadoes the environment was over the panhandle at 2200 UTC (4:00 pm CST) while tornadoes were ongoing near and east of Happy, TX:

And here's an analysis sounding from the RAP model at Plainview TX (south of the Happy, TX supercell) at 2100 (3:00 pm CST) at about the time the outbreak started:

Notice that parameters such as mlCAPE (> 2000 J/kg), 0-1 km storm-relative helicity (SRH > 250 m2/s2), surface to 6 km shear (> 70 kt), and 0-3 km mlCAPE (> 150 J/kg) were all highly favorable in this environment for supercells capable of generating strong tornadoes.

The next day (Sunday, March 14), satellite with surface features (below) showed that the 500 mb low had moved to eastern Colorado (CO), where a blizzard dumping over 2 feet of snow was in progress over northern CO and southeast Wyoming.  The surface low had also evolved northward to eastern CO, with a massive dry slot/clear slot punching northward across most of Kansas with the 500 mb low, while the Pacific cold front had advanced eastward:

Along the occluded front extending northeastward from the CO surface low, the wind shift along the boundary combined with surface heating and CAPE (from the cold air aloft with the mid-level low) to help produce a couple of brief landspout tornadoes at early afternoon, including the Nebraska EF0 tornado pictured earlier.

An SPC image of 0-3 km mlCAPE and surface vorticity at 1900 UTC (2:00 pm CDT) showed very well the low-level CAPE (in red) overlapping the west-east boundary (blue surface vorticity lines) over northwest KS and southwest NE:

These are ingredients that can generate non-supercell tornadoes such as landspouts when low-topped storm updrafts from CAPE and heating phase properly with the boundary so that vertical vorticity along the wind shift can be stretched upward.

Here's a RAP model 1-hr forecast sounding near Trenton, NE  shortly before the brief NE tornado pictured earlier -- notice how the CAPE is relatively small but bunched low in the  atmosphere, typical of tornado settings near 500 mb lows:

And here's an image showing the crisp low-topped storms looking north from KS into NE associated with this occluded front and cold-core setting:

Last, here's the satellite/surface setting on Monday, March 15 between 2000 UTC and 2100 UTC (3:00-4:00 pm CDT) over the KS-Missouri(MO) area:

The 500 mb low had "split" or evolved into two centers, with one drifting east across Kansas and generating a surface low over northeast KS near the stalled occluded front.  With all the cold air aloft associated with the 500 mb low and the west-east boundary near Kansas City, the stage was set for a couple more brief tornadoes, including the one pictured earlier near Stilwell, KS.

The SPC depiction of low-level CAPE and surface vorticity at 2100 UTC showed a setting similar to the day before (low-level CAPE overlapping the boundary ), except this time near Kansas City:

And here's a RAP model 1-hr forecast sounding at Harrisonville, Missouri, shortly before the tornado at Stilwell, KS just west across the KS-MO border:

With a somewhat more clockwise-curving hodograph, the Stilwell storm appeared more supercellular on radar (not shown), but the components of low-level CAPE and the boundary were also likely significant contributors to the tornado in this occluded front and cold-core setting.

In the March 13, 2021 Texas case, parameters and ingredients were strong within the warm sector away from pre-existing fronts and boundaries to help generate several classic-type supercell tornadoes.  But on March 14 and 15 as the 500 mb low moved slowly out into the Plains, the tornadoes were more a result of low-topped storms interacting with an occluded front, where the boundary was a key factor.

It is interesting to note that the Texas twin tornadoes pictured at the top of this article occurred 31 years to the day after the twin Hesston-Goessel KS tornadoes that went on to merge together into an EF5 tornado.

And I must note that St Patrick's Day 2021 (tomorrow) will likely see a significant outbreak of severe weather and tornadoes in the South over the Arkansas-Louisiana-Mississippi-Alabama area.  I hope everyone in those areas stays informed and up-to-date on the weather.. stay safe!

- Jon Davies  3/16/21

No comments: