Mar 212011

Monday, 21 March 2011

The Storm Prediction Center has a high-end slight risk (30%) for much of Iowa tomorrow. They mention the possibility for supercells and isolated tornadoes. However, they also outline quite clearly their concern for dry air impeding both storm development and substantial instability to support supercells and/or tornadoes.

While I admit i haven’t spent a great deal of time looking at this setup over the last couple days, I did look over the 12z March 21 WRF and GFS models. Both show an intense shortwave ejecting through the central plains tomorrow, bringing with it a surface low through eastern Nebraska/western Iowa, and strong southwest upper air winds. But the wave also brings with it quite veered 850mb winds. Throughout much of the day, very warm and dry air from western Canada will be advecting northeastward into eastern Nebraska and Iowa. This will not only scour out any deep moisture, but it will also bring a pronounced elevated mixed later to Iowa. In other words, there is going to be an enormous CAP to break in the afternoon!

Luckily, the intensity of the vort max associated with the strong shortwave will cool the mid levels in the afternoon/evening and weaken the CAP. So it’s safe to assume at least isolated thunderstorms will fire along the dryline over eastern Nebraska/western Iowa very late in the afternoon/evening. But the next big problem will be moisture. With dry air advecting all day at 850mb, you have to assume some of that will mix down to the surface. So surface dewpoints will pay the toll. And the current forecast for dewpoints will probably be very generous, since they rarely handle mixing well in these situations.

Finally, instability will be a problem since moisture is going to be an issue — especially closer to the dryline or the further west you go in the warm sector. I’m just not a big fan of this setup. But if I lived in Iowa and had nothing to do tomorrow, I might sit along the warm front in central or eastern Iowa. *IF* enough instability can indeed develop there (better 850mb moisture should be present) then I think the tornado potential would be pretty good.

I also want to say that conditional setups sometimes produce nice tornadoes. I hope my post here isn’t completely discouraging to any chasers out there in the central plains. I’m just expressing my opinion as I see things at the moment.

Good luck to anyone chasing tomorrow and be safe.

- Jim