07/27/10 DISC: MN, WI, SD, ND, IA, MI

[Jeff Duda]

Tornado watch fail? Especially when viewing the reports.

I know instability was very high, but deep layer shear was never awesome, and low level shear was quite poor. Plus the surface winds ahead of the boundary were SSWly, thus really reducing convergence at the surface. Why did SPC issue a watch with such high probabilities? I can't see the reasoning for it.

 

[Dean Baron]

I dont know for sure, and the 10% hatched TOR was issued well before these became defined, but by early afternoon OFB's started becoming well defined, particularily across central Minnesota. I was busy from 2:00 pm onward, but prior to that I was looking at data and one OFB had setup from western WI westward over the Twin Cities metro and westward still in to central MN with southeasterly surface just south of the boundary. Never had a chance to look at meso-analysis so I'm not sure what other parameters could have influenced any of those decisions.

 

[Jeff Snyder]

The SPC mesoanalysis loops of various low-level shear parameters doesn't support that low-level shear was "quite poor". True, the surface winds veered to WSW or SW southwest of I94 in Minnesota, but I still saw quite a few S and occasionally SSE winds N of I94 between Minnesota and Duluth (and E/W from there). There was some juxtaposition of decent low-level shear and instability (both looking at individual parameters and various composite indices like SCP and STP). Even with the veered sfc flow, however, forecast hodos still supported moderate low-level shear and decent hodograph structures. Just look at the 00z MPX sounding -- ~4000 j/kg MLCAPE, 40 kt 0-6km shear, 340 m2/s2 0-1km SRH (that's with a southwest wind!), SCP of 40.2, and an effective STP of 5.7. Those aren't shabby numbers, and the 0-1 km SRH is actually rather impressive for this time of year... Winds on that MPX sounding were 35-40 kts from the W or WSW 750 mb to 250 mb, so there was essentially no shear over a relatively large depth of the troposphere. Flow aloft may have been a bit stronger farther north.


I think the biggest problem was storm mode. There WERE at least a few supercells today, but most were quickly munched by expanding convection nearby. Deep-layer shear wasn't particularly impressive, but should have been "good enough" given the instability. However, when there's strong linear forcing, it isn't terribly surprising to see a rapid QLCS evolution.