Thunderstorms will roll through tomorrow, then much cooler air arrives. It won't be "cool" necessarily, but it will feel really nice. Highs will only warm into the mid 80s for the Upstate by Wednesday and upper 70s in the mountains with low humidity. Therefore, you may be thinking spring and not summer when you step out the door!
All of this is made possible by a dip in the jet stream. The jet exists because of temperature contrast. Colder air resides north of the jet, and warmer air to the south. The jet will head toward the Carolinas as a piece of polar air makes it way into the U.S. We won't get a big chill, as I described above, but it will bring an airmass that just won't feel like summer!
So, calling this a summer-Polar Vortex isn't totally incorrect! If it was wintertime, this pattern would equal some bitter cold.