This is a severe weather discussion concerning the potential multi-day severe weather threat from June 17 to 20.
The severe weather potential kicks off on the evening of June 17th, when this instability image above is valid for. In this forecast, we see a reservoir of extremely unstable air extending from the Gulf Coast to southern Canada. At the heart of all of this instability, we see convective available potential energy, or CAPE values exceeding 6000 j/kg, an absolutely massive amount of fuel for thunderstorms. This kind of extreme instability finds itself from Nebraska to Minnesota, down through Illinois and Missouri. All of these areas may be in line for the first of potentially multiple rounds of severe weather on the back end of this upcoming workweek.
Fast-forwarding to Thursday evening, we see lower-level wind fields are suddenly much more supportive for severe weather than they had been on Tuesday and Wednesday. This comes as a low pressure system slowly meanders across the Plains gradually attaining a negative tilt in the process. A negative tilt means the contour lines appear to be digging to the southeast direction, which then enhances the risk of severe weather. In this case, we see low-level wind speeds exceeding 40 knots in the Midwest, and this is where I believe we may see some organized severe weather on Thursday evening, as continued extreme instability and now-supportive wind fields combine to create for what could be an impressive environment.
We can also use analog dates to predict the future. This method of forecasting takes decades of weather observations and matches a handful of select dates up that are closest to the forecasted conditions. This image shows compiled severe weather reports from the top 15 analog dates, basically the 15 days that were the closest to projected conditions on Wednesday evening. On this image, we can see that the majority of severe weather events that happened under similar conditions as this coming Wednesday occurred in the Plains regions. We see an anomalous swath of hail, wind and tornado reports throughout the Dakotas, Nebraska, Kansas and Colorado. We then see an area where strong tornadoes hit under similar conditions in central Oklahoma into Texas. While this coming Wednesday won't exactly follow this set-up and put tornadoes/hail/damaging winds down in these exact areas, it does give us a good glimpse at some regions that might be affected by potentially tornadic weather. While I don't have images for Thursday's threat and onward, it can be anticipated that the threat would shift east.