The Storm Prediction Center (SPC) has issued a Slight Risk today for parts of the Northern Great Plains.
Associated with the Slight Risk is a 5% Tornado Threat ring.
The 300 mb NAM chart shows a weak speed max nosing into the Dakotas by this evening. This will act to increase shear. The shortwave that is behind this speed max will be a large influence in severe weather over the next few days.
At the 500 mb level, the NAM chart shows some weak Positive Vorticity Advection (PVA) moving into the south central South Dakota by 0Z Wednesday (this evening). The gradient is not strong, but vorticity will increase in this region, and with it, the likelihood of rotating storms.
The 700 mb level NAM chart shows that rising air is lagging just behind (west) the PVA and the speed max. Notice the rapidly rising air over Wyoming and the western part of the South Dakota and Nebraska.
The 850 mb NAM chart shows no strong thermal advection, but notice how the winds are rotating around a lee-side low pressure system over southeastern Wyoming. Air is in motion from the Gulf of Mexico to this point, and perhaps this will be shown as some moisture advection.
The 12Z upper air sounding from Rapid City, SD, does not look much at all like a severe weather sounding. We have very little Convective Available Potential Energy (CAPE) and a ton of Convective Inhibition (CIN), instead of the other way around. Bismark, ND, and Aberdeen, SD, show similar stories, with Aberdeen having more CAPE than anywhere else.
In all three places, low-level shear is good, but deep-layer shear is marginal. This will increase with the speed max that is approaching at 300 mb, though it will only help to fire storms if diurnal heating or advection can increase the CAPE and decrease the CIN. The 12Z hodograph for Rapid City, SD, is shown below.
The visible satellite imagery shows mostly sunny skies over the entire Slight Risk area. This region will have time to sit and bake in the hot sun, making the most of the diurnal heating.
The SPC Mesoscale Analysis CAPE graphic shows that CAPE has increased significantly throughout the morning and early afternoon. There is a bull’s eye of 3000 J/kg CAPE that is not capped at this time. There are several pockets of 3500 J/kg, with CIN still in place.
The RAP shows the CAPE intensifying in the south (over Nebraska), and the northern pocket of CAPE decreasing by 1Z Wednesday (this evening). This predicted decrease could be because storms have already formed at this point. The CIN over Nebraska is still present, though light.
The Lifted Condensation Level (LCL) heights are marginal throughout the entire Slight Risk area. Storms will be high based, and tornadoes will have a long path from the wall cloud to the ground under this environment. There is a tongue of 2000 m LCLs, which are high for tornadoes. It doesn’t mean they won’t happen; in fact, these are some of the more photogenic tornadoes, as they stretch so far from cloud base to ground; but they are less likely than if the LCL heights are lower.
The surface-based Lifted Index (LI) is best around the North and South Dakota border. However, I am still watching the pocket in southwestern Nebraska, as that area shows -7, and could be a threat with so much CAPE available. If a few strong updrafts can break the capping inversion in this area, they will have a strong pocket of energy all to themselves.
The effective bulk shear is not great in southwestern Nebraska. In fact, it’s currently not great anywhere in the Slight Risk area.
However, by 1Z, the effective bulk shear is expected to increase throughout the entire region, with a few rings of 40 kt shear over southwestern Nebraska, and northeastern Wyoming, as well as a shear max in north central North Dakota.
Overall, if I were chasing today (I almost was, but then balked to get ready for tomorrow instead), I am intrigued by the high CAPE and adequate shear possible today in southwestern Nebraska. I definitely risk the “cap bust” in this region, but I could evaluate on the road whether to travel farther north. For now, I would probably target Imperial, NE, or McCook, NE, and just watch for a few hours.
Thank you for reading my prediction.
The Severe Weather Outlook is from the Storm Prediction Center website.
The satellite imagery is from the NOAA Satellite and Information Service.
The surface observation and upper level charts are from Unisys Weather.