2020-07-08 EVENT: MN

[Dean Baron]

Starting a discussion thread for the setup in MN on July 8th. The main tornado has officially been rated an EF4 by the NWS- Grand Forks. This is the first EF3 or stronger tornado in MN since 2010. The last EF4 tornado in MN was 8-7-2010, which was in the same general area as Wednesday's tornado. The 8-7-10 tornado was also a slow moving and highly photogenic drill bit tornado, eerily similar to the one on Wednesday.

Wednesday's tornado is particularly interesting because it occurred outside (or perhaps on the very edge) of the 2% tornado risk from the SPC. Most of the day the CAMs (especially the HRRR) was firing supercells farther east into central MN. Around the 19z run of the HRRR, a quick shift to the west happened. The SPC introduced an upgrade to a 5% tor probability with their 1630 outlook for the area the CAMs initially showed potential tornadic sups across central MN but these probabilities were not extended west for the 20z outlook, even though most recent model output indicated convective initiation farther west than originally thought. Convective initiation farther west meant storms would be developing in a more favorable environment for tornadoes, as indicated by mesoanalysis and the vorticity shown on the vorticity/0-3km CAPE overlay. This area was also closer to the strongest instability, with SBCAPE AOA 6000 and MLCAPE over 4000. The only question was whether storms would fire in this environment, especially with the 12C H7 temp line nearby, but the CAMs were bullish on CI. An increasingly large and agitated area of cumulus was noted on satellite prior to the storm's development. For those of us chasing, it was not coincidence that most of us ended up on a tornadic storm well away from the SPC's 5% tornado contour. When looking back at the forecast, it shouldn't be surprising that such a tornado occurred when looking at the mesoscale factors that led up to the event. Any other input is appreciated. I have attached a screenshot of the archived 0-3km CAPE/vorticity overlay from mesoanalysis to show how vorticity rich the environment was. Now we know what large amounts of vorticity and extreme instability can do, even in an otherwise seemingly marginal environment.

EDIT: I should note that the tornadic storm is developing on the attached image. This storm also formed at the intersection of the warm front and cold front. This intersection is another reason why convective initiation in this environment should not have been a surprise.

 

[Jeff Duda]

I was very much not paying attention to this day, so when I heard the news it really took me back. As a result, I am not familiar with the setup for the case. But I can provide resources.

Archived HRRRv3 and HRRRX imagery remains on the GSL website for quite some time following events. You can look through the images at this link (HRRR Model Fields - Experimental). Just change the boxes at the top to select between the model types, initialization times, and domain regions.

GSL also runs the 3DRTMA, which is better than the SPC mesoanalysis software IMO, minus the lack of composite indices. Unfortunately there was a gap in the runs during this event, so it is impossible to know what this higher-resolution version of the atmospheric scenario looked like at the time of the event. HRRR Model Fields - Experimental

Archived satellite imagery is available going back 7-10 days at CSU-CIRA...their SLIDER product. Seems to show that storm absolutely exploded out of nothing (no precursor CU seemed present). That is very uncharacteristic of typical dryline-forced CI, which is more burbling/roiling cumulus that gradually builds up into a robust CB. If there was a boundary that moved into that region, that could possibly explain the explosive development.

The presence of extreme buoyancy likely compensated for the seemingly marginal deep layer shear. This seems common, but is probably worth deeper discussion. Likely the vorticity provided by the surface boundary also contributed to this tornado being so particularly strong.

 

[Dean Baron]

If there was a boundary that moved into that region, that could possibly explain the explosive development.

Morning storms laid down an OFB. I couldn't pay much attention to it much while I was at work but it was the general consensus among local chasers that this would help reinforce the warm front, which could help explain the explosive nature of the storm. I wish I had easy access to old radar data but IIRC the echo tops went from nothing to 40k ft in about 2 scans. The extreme instability likely helped the explosive nature of it too.

Everything seemed to come together at just the right time. Had the warm front stayed too far south it would have been too far removed from the better shear and vorticity. Farther east it was also removed from the better vorticity, and even if it did end up farther east, the terrain turns to lakes and trees quickly which would have made chasing much more difficult. Had the wf ended up farther north it would have been in lower instability. Where the storm formed was about the exact perfect place for it to happen. There was a kink in the wf (just east of the cold front) in this area too where it was laid out SW-NE, and with the storm motion to the NE, it likely was able to latch onto the wf as opposed to simply crossing it. That might help explain the long lived nature of the tornado.

EDIT: I attached the surface analysis from FGF's tornado summary page. The intersection of the warm front and cold front on that analysis is almost exactly where the storm fired. You can also see the bulge in the warm front I mentioned above, with the storm's motion roughly paralleling the orientation of this part of the warm front.