Thursday, July 16, 2020

EF4 tornado in Minnesota on July 8, 2020 - a rather subtle environment for a violent tornado

After four EF4 tornadoes in April (3 in Mississippi, 1 in South Carolina), there were none in the U.S. in May and June.  But on July 8 in west-central Minnesota (MN), an EF4 tornado (images above) near Dalton, MN shortly after 5:00 pm CDT (2200 UTC) killed one person.  This was the first violent tornado in July in the U.S. since 2004!

The environment at first glance did not appear likely to generate a violent tornado, as low-level wind shear (storm-relative helicity, or SRH) wasn't notably large.  But there was a boundary in the area and large instability (CAPE), along with enough deep-layer wind shear to support tornadic supercells.  Here's a look at the setting...

SPC mesoanalysis graphics at 2200 UTC (5:00 pm CDT) showed large total CAPE (mlCAPE > 4000 J/kg, 1st panel below) near the soon-to-be tornadic storm that had developed during the previous hour in west-central MN:

Deep-layer shear (2nd panel above) was enough (30-40 kt) to help organize the isolated cell into a supercell.  Just as important, the cell developed near an east-west boundary (warm front /stationary front, also shown on the graphics above) that extended eastward from a surface low, a favorable setting for tornadoes when the surrounding environment offers additional support.

Also at 2200 UTC, low-level instability (0-3 km mlCAPE) was large over west-central MN (> 125 J/kg, 1st panel below), although 0-1 km SRH was nothing that would raise eyebrows much (0-1 km SRH 75-100 m2/s2, 2nd panel below):

But together, the combination of just enough low-level shear/SRH with strong low-level and total CAPE in the vicinity of the east-west boundary east of the surface low was more favorable for tornado potential than a look at wind shear parameters alone might suggest.

In fact, supercell and tornado development was rather fast and 'explosive', as seen in the two satellite images below:

In just less than an hour from initiation, the storm became a supercell and planted a strong/violent tornado on the ground.  That's quite fast compared to many supercells that take two hours or more to produce tornadoes.

A contributing factor to this rapid development may have been low-level lapse rates (below), not shown in the SPC graphics above:
The axis of red dots in the graphic above shows strong surface heating extending from the southwest into the supercell's environment near the boundary over west-central MN.  Such steep lapse rates (> 7.0 degrees C per km, a rapid change in temperature above ground) can enhance the stretching of low-level air entering storm updrafts, adding a 'kick' to an already unstable situation and helping to make the most of available low-level wind shear (SRH) as it tilts and stretches into an updraft.

Back around 2005, I came up with a parameter called 'enhanced stretching potential' (ESP) that combined low-level lapse rates with low-level CAPE to help locate areas that might have this extra 'kick'.  It was designed mainly for diagnosing short-term potential for landspout tornadoes along sharp boundaries (see this paper).  But on 7/8/20, ESP overlapped the area we discussed above that already had potential for tornadoes suggested by more typical supercell tornado parameters:
It may have added some 'gasoline' to the fire in this case, speeding up supercell and tornado development.  But that's only speculative.  I will mention that the prolific tornadic supercell in southern Saskatchewan (Canada) on July 4 that I touched on in my previous post had some of the same ingredients (SRH < 100 m2/s2, large CAPE and large ESP, as well as an east-west boundary; not shown).

As ESP was originally intended for highlighting short-term possibility for landspouts, it is worth noting on 7/8/20 that the ESP graphic above showed an ESP maximum over north-central Nebraska, where landspouts did occur later that evening along a boundary southwest of a storm near Thedford, Nebraska:

And other landspouts occurred on 7/8/20 along a boundary and boundary intersection over northeast Colorado where ESP was also large (not shown).

Finally, here's the large-scale NAM model 500 mb forecast for mid-afternoon on 7/8/20 showing the strong shortwave disturbance (red dashed line) in mid-levels moving through the northern plains that helped intiate the tornadic supercell in MN:

It is interesing that the early half of July 2020 has been more active for tornadoes in the central and northern Plains of the U.S. than in the last half of June.

- Jon Davies  7/16/20

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