Review of July 17th, 2011 LaMoure County, North Dakota, Tornado: An EXTRAORDINARY Mesoscale Environment

Two-panel image of °0.5 storm-relative mean radial velocity (L) and 0.5° base reflectivity from Aberdeen, South Dakota (KABR) radar at 2313z (6:13 PM CDT) Sunday, June 17th, 2011. Of note is that the TVS (tornado vortex signature) maxes out at 171 knots gate-to-gate and the reflectivity features a debris ball of 65dBz intensity. This is likely near the time that a farmhouse was reportedly completely destroyed west of Berlin, North Dakota. Level II image created on GRLevel2 Analyst Edition.

The year of 2011 has been no stranger to strong-violent tornadoes. From the three-day outbreak that ravaged areas from Tushka, Oklahoma, to Raleigh, North Carolina, on April 14th-16th, to what's being dubbed the second Super Outbreak of April 25th-28th, to the horrific Joplin, Missouri, EF5 killer on May 22nd, the United States has been no stranger to incredible tornado events this year. And though July 17th will likely go down as a fairly minor footnote in the 2011 record book, it was extraordinary in its own right.

Climatologically, strong-violent tornado activity peaks in April in the United States, with May and June the next closest contenders. July typically sees a tailoff in strong-violent tornadoes, but when they do occur, they most often occur in the northern plains and upper midwest. In this regard, the events of July 17th were far from extraordinary. In fact, they were exactly what would be expected this time of year.

What sets July 17th apart is the environment in which the supercell that produced the LaMoure County tornado developed. The combinations of exceptional instability, rather strong shear, a lift mechanism, and lack of thermonuclear cap created one of the most impressive mesoscale environments I have witnessed or studied.

Let's start with instability and forcing. With temperatures in the lower 90s and dewpoints in near 80 (no, that's not a misprint as you can see below), the stage was set for an incredibly unstable airmass to take shape. With the jet stream overhead helping to bring in cooler temperatures aloft, that was exactly what took place. Additionally, a boundary was in place across North Dakota, as evidenced by the wind shift at the sfc and the maximum in moisture convergence. This boundary would provide the focus for convection.

23z sfc map from SPC mesoanalysis.

Sfc moisture convergence from SPC mesoanalysis.

By 23z in the afternoon, an extraordinary amount of CAPE had built in across southern North Dakota. SBCAPE values of 8000J/kg or greater and MLCAPE values of an incredible 7000J/kg with MLCINH completely eroded were in place across southern North Dakota. Incredibly, as can be seen on the MLCAPE graphic, the supercell that produced the strong tornado had these incredible instability values directly within its near-storm environment, providing an incredible amount of fuel and updraft velocity to the storm.

SBCAPE from SPC mesoanalysis.

MLCAPE from SPC mesoanalysis.


RUC 1 hr fcst sounding from the 22z run for 23z on 7/17/11 near supercell location. Note lack of major capping inversion, low LCLs, and extreme lapse rates. Sounding from www.twisterdata.com

Also of note were high values of 0-3km CAPE and sfc vorticity in the near-storm environment. These high values, attributed to the 750-1000m LCLs and lack of low-level inversion to eliminate low-level instability, acted to rapidly accelerate near-sfc based parcels and to stretch the low-level vorticity into the updraft, aiding in tornadogenesis.

0-3km CAPE and sfc vorticity from SPC mesoanalysis. Note the higher regions of both values near the North Dakota supercell.

LCL heights from SPC mesoanalysis.

It is most typically the case that, when such tremendous instability is in place without a capped atmosphere, that vertical shear is extremely weak. This is because vertical shear is indicative of warm air advection in a layer, a process that typically, in hot environments, promotes capping. In North Dakota, however, significant amounts of shear were in place in the near-storm environment, with 40-50 knots of 0-6km shear and 150-200m**2/s**2 of 0-1km SRH. Additionally, the storm moved quite a bit to the right of the storm motion vector used to calculate the 0-1km SRH, thus increasing the actual amount of helicity available to the storm, as described below.

0-6km shear from SPC mesoanalysis.

0-1km SRH from SPC mesoanalysis.


22z RUC fcst hodo for 23z on the 17th. The shaded area represents a (very) rough approximation of 0-1km SRH. The green is for a storm moving along the given vector. The red is for the supercell as it turned to the right of the given vector. Note the increase in shaded area, indicating an increase in 0-1km SRH. Hodo from www.twisterdata.com

This combination of extreme instability and strong shear led to off-the-chart severe indicies, with effective-layer significant tornado parameter of 8 and 0-1km EHI of 8 in the near-storm environment.

Effective layer STP from SPC mesoanalysis.

0-1km EHI from SPC mesoanalysis.

All-in-all, with the boundary in place, minimal capping, strong shear, and incredible instability, the environment across southern North Dakota on July 17th was extraordinary. It is with great fortune that this storm did not strike a major population center, like many of the other major 2011 tornadoes.