A potential fall tornado outbreak may be setting up across portions of the Midwest and upper Mississippi River Valley this Thursday into Friday.
An deep trough an area of surface low pressure is forecast to develop across the Midwest and move northeast toward Minnesota by this weekend. As the low deepens, it is forecast to occlude somewhere near the North Dakota/Minnesota/Iowa border early Saturday. A warm front extending northeast into the Great Lakes will be the focus for some potential severe weather on Thursday, but Friday may be the biggest day with the greatest tornado potential.
The computer models are coming into better agreement with timing, although there are some details to resolve. A surface low is forecast to be located over Nebraska on Friday. Ahead of the low, an unstable air-mass with dew-points well into the 60’s is forecast to set the stage for severe weather. The focus points will be close to the low pressure center and its associated warm front. Near the Minnesota/Iowa border, the tornado “ingredients” appear to be most impressive, especially with the NAM model. Here, backing low level winds combine with good speed and directional shear aloft to strongly support rotating updrafts. Instability may be on the high side, especially if there is enough sunshine, as the NAM brings CAPE values to over 3000 J/kg over much of the area. Based on the latest data, I would think that northwest Iowa into southern Minnesota would stand the greatest risk to see tornadoes, and some of them could be strong, on the order of EF-2 or stronger.
There is still time for the setup to evolve and it’s especially the case this late in the year that daytime heating is critical. If any low-level moisture and/or clouds limit instability, then the severe threat may be mitigated. However, if the models are correct and we see strong wind shear coupling with higher levels of instability, then watch out.
I am tentatively planning on heading into Iowa on Friday for storm chasing, but I will obviously be watching the computer models very closely over the next 24 hours. For updates on my storm chasing adventures and thoughts for areas outside of Connecticut, follow my special Twitter account @stormchaserq.