This is a severe weather discussion concerning the expected severe weather outbreak on June 14th.
The Storm Prediction Center has issued a Moderate Risk of severe weather for Saturday, June 14th, affecting eastern Nebraska, western Iowa, far north central Kansas, and even extreme southeast South Dakota. Moderate Risks aren't issued too often by the SPC, and it's even more uncommon to see them issued a full day in advance. Typically, a Moderate Risk will be posted on the day of the severe weather event. Posting the risk a day in advance of the event means the severe weather threat is likely more threatening, or confidence in the event is higher.
Model guidance forecasts of 500mb wind speeds on Saturday evening show an upper level low located over northeastern Montana into southern Canada producing an extended trough over the northern Plains. This is defined well by the mid-level jet maximum developing in the overnight hours of Saturday into Sunday, when the above image is valid. Wind speeds at the base of this trough look to exceed 75 knots, a beneficial environment for creating convection. The trough pressing eastward into the dominating ridge, pulling up moist and unstable air in the process, is likely to create opportunities for severe weather on Saturday. This mid-level jet and supportive lower-level wind fields confirm this possibility.
The forecast for instability on Saturday evening shows multiple opportunities for severe weather, as well as a few caveats. This projection of convective available potential energy, or CAPE, shows values exceeding 3000 j/kg, indicating the presence of a very unstable environment. These high CAPE values, combined with the supportive wind fields, will likely play a significant role in producing and sustaining the expected convection. Also of note is how the uncapped instability is located in the Moderate Risk, while areas with a capped atmosphere (a thunderstorm suppressant) are not in the Moderate Risk area. This tells me that the capping inversion that prevents thunderstorms may be strong, but areas that manage to 'break the cap' will experience some intense weather.
A zoomed-in forecast of the Supercell Composite on Saturday night also shows the potential for supercell thunderstorms, considered to be some of the most violent thunderstorms. We see the elevated Supercell Composite values from Oklahoma and Texas to Iowa, but again, due to the capping inversion, we'll be focusing our attention mainly on southeast Nebraska and Iowa, where potential seems the greatest for this event.