What happened on 5/8/14

[JeremyS]

I've seen a few of these types of posts over time on this board, so I thought I would throw the question out there to get some insight from some of the other experts here. Yesterday was a moderate risk in northern Iowa and Minnesota. There were morning and very early afternoon showers across most of the area that quickly lifted out and dissipated across the region. In fact the sun came out for most places across the risk area by 2pm. The dryline and cold front didn't really move into Iowa until 6ish. The mesoanalysis across northwest Iowa into very southern Minnesota showed 2000 J/kg CAPE and no cap. Shear increased throughout the afternoon to 40-60 kts. I chased and saw barely a shower. There was the storm that dropped a tornado up in St James Minnesota which was quite a surprise to me because it seemed like it was quite a bit north of the warm front where the winds were almost northerly and temps were in the 50s and low 60s. Down in the warm sector blips would show up on the radar but disappear usually within 20-30 minutes. Watching the clouds all day, it seemed like they wanted to go big but just when you would get something decent looking it would fall apart. What do you think was missing? It's almost like there was still a cap remaining and the little storms that did form were very pulse like as if there wasn't enough shear to keep them going either. Just curious if anyone had any thoughts....

 

[Will Campbell]

My best guess would be based on my observations in central IA. I started on the storm that fired just east of Omaha and followed it all the way to NE of Carroll IA. Storms would initially form and look really good, fairly low bases considering the somewhat meager moisture. The storms would intensify and shortly after become much higher based. While I haven't chased many CF storms as I prefer the DL/WF when available, it appears as though the CF was undercutting the updrafts causing the storms to uproot and lose their good inflow. I was on both of the tornado warned cells in Iowa at a point before and during the warning, they both looked exponentially better before they were even severe warned. When both storms were T Warned they were very high based with no signs of anything more than an extremely broad mesocyclonic rotation.