This upper air map shows (1) there is no strong heatwave ridge over the Midwest or Northeast any more and (2) a broad trough populated with smaller disturbances will move from the Mississippi Valley to the Appalachians by Sunday.
Thunderstorms will continue to erupt near the northern edge of the heatwave, enhanced by a series of disturbances rippling along in the upper air flow. This is the NWS Storm Prediction Center's severe thunderstorm outlook for today
... the main upper air steering current moves eastward across the northern Plains, then dives southeastward toward the Middle Atlantic states. The core of this current defines the rim of the hottest weather and serves as a conduit for clusters of thunderstorms.