While I’m writing this I’m keeping tabs on what looks like is a pretty decent severe weather day in Minnesota. Chasers have reported at least two tornado touchdowns in the past hour and the cells are looking great. It’s a shame we were not able to make it up there today as it looks like today may now better then what tomorrow is offering.

Unfortunately after stellar 06Z model runs came in this morning and we got excited the 12Z and 18Z runs came in and pretty much gave us the finger. The NAM along with the GFS are now indicating a much weaker low level wind field, as shown in the graphic below.

The problem with the graphic above is first off the winds are veered to the southwest. This type of setup favors linear lines instead of tornadic supercells. Also the winds have come down from previous model runs, only showing 20 to 25 knots. This number is fairly insignificant and thus not favorable for much tornadic activity, especially with the veered direction.

These two graphics are from the 18Z NAM. The first graphic is the Surface Based CAPE of Convective Available Potential Energy graphic. It’s still showing CAPE values upwards of 6000 to 7000 J/Kg which seems reasonable as the 22Z RUC is showing 7000 J/KG of SBCAPE over Iowa this afternoon. That type of instability is off the charts and just creates an explosive atmosphere. Graphic #2 is the 1 KM Helicity. Any number over 150 is generally favorable for low level mesocyclones. There is no question there will be a substantial severe weather threat tomorrow in parts of Iowa, but the question is will that be in the form of a high wind event or will it also include a tornado threat? Right now unfortunately it is appearing the low level wind fields will be too weak for a sustained tornado threat so whether or not we chase is in question, thus the chase indicator has been lowered to moderate.

The models have been having convective feedback issues that could be having a biased affect thus Connor and I will be taking a very close look at several models off the 0Z suite. Just to show an example of a biased model run, I’ve posted a temperature graphic from 7 PM CDT Friday for Iowa. As you can see, the NAM is indicating parts of Western Iowa in the low 70′s due to convection it is showing, thus anything at this time is unreliable in terms of wind data and the overall environment because of convective feedback. We’re keeping an eye on everything and I’ll have another discussion out later tonight. If we do end up chasing Friday we will be departing Dallas around 3:30 AM for the ten hour drive to Iowa.

- David Reimer