Yesterday, there were a few tornadoes, including one sited near Trinidad, CO, a place I have stopped frequently in my travels.
The Storm Prediction Center (SPC) has issued an Enhanced Risk for most of Iowa this afternoon.
Associated with the Enhanced Risk is a 2%Tornado Threat Ring.
The SPC is basing this threat on a surface low pressure system and a shortwave moving around the trough.
Synoptically speaking, the 300 mb NAM chart (from Unisys) shows an approaching trough, as well as a very obvious shortwave over eastern Nebraska that corresponds to a surface low pressure system.
The 500 mb NAM chart shows some Positive Vorticity Advection (PVA) into the western parts of Minnesota and Iowa by 0Z this evening. The vorticity isn’t very strong, but the winds blow at a sharp angle to the vorticity gradient.
The 700 mb NAM chart shows that the largest Upward Vertical Velocities (UVV) are over eastern North Dakota by 0Z, and that there is actually sinking air over western Iowa this evening.
The 850 mb NAM chart shows that there is some moderate Warm Air Advection (WAA) as winds flow into the surface low pressure system. The warm sector for this storm includes all of Iowa and the southern-half of Minnesota.
TwisterData.com shows that the CAPE is not super-impressive with today’s threat. It is actually much lower than I expect, given the Enhanced Risk.
TwisterData.com shows that the shear is favoring western Minnesota over Iowa, though some marginal bulk shear is available by 0Z in western Iowa as well.
Currently, the surface observations (from the SPC Mesoscale Analysis Map) show overcast skies for much of Minnesota and Iowa. The wind profile and some precipitation on radar mark the location of the surface low.
The surface pressure chart shows that there is a low pressure system over eastern Nebraska this morning. Notice the winds spinning cyclonically around the system.
The RAP shows the surface low will deepen over the next six hours. It is forecasted to move towards the northeast.
The RAP also shows that, over the next six hours, the supercell parameter is expected to increase in western Minnesota. This is consistent with the forecasted position of hte surface low.
The visible satellite imagery is unavailable at this time.
The enhanced infrared satellite imagery shows some morning convection at the western edge of the Enhanced Risk area. These showers and thunderstorms are expected to clear out throughout the morning, leaving the skies open for diurnal heating throughout the day.
The water vapor satellite imagery shows both the trough and embedded shortwave, with convection to the east of both of these features.
Overall, I expect a few storms, but it wouldn’t surprise me to see the risk area shift of extend farther north, based on the projected path of the low pressure system. The CAPE is low in all places, and the shear is only marginal, but that shortwave is very distinct. I could see today being a busted chase day for folks as well, based on these parameters. Or maybe I’m biased, as I always cap bust in Iowa…
Thank you for reading my forecast.
*UPDATE* The latest SPC charts (I’m not capturing them, as I am about to get off the train),, show that the Enhanced Risk and a new 5% Tornado Threat ring have shifted north into Minnesota. I love it when a plan comes together!
The upper air soundings and mesoscale analysis plots are from the Storm Prediction Center website.
The forecasted upper air soundings are from TwisterData.com.
The surface observation and upper level charts are from Unisys Weather.
The satellite data is from NASA – MSFC