This video outlines the forecast for the Great Lakes and Northeast from now through early next week.
Meteorologists use a variety of weather maps to study patterns that affect day-to-day or minute-to-minute changes. One map is the 500mb chart. Below are two versions of the same map. One shows a contour interval of 60 meters, which is standard for this kind of map. However, what if you wanted to see more details. You could use a far smaller contour interval, in this case 5 meters (second map). When looked at this way, you can see two distinct flows in the East: one from the south with moisture and one from the west that is dry. There is a problem, however: the model solutions evolve over time, and as we get closer to next Monday afternoon (the time the forecast maps are using), the lines and orientations will probably change.
In this picture, the sun had set as seen from the ground. However, sunlight was able to light up the clouds from below to create this look outside my home:
Now, drier air has arrived, and most of the Northeast will have abundant sunshine today and tomorrow.
A front that will usher in slightly less humid air for the Northeast tomorrow will trigger locally strong thunderstorms today.
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.
3. Hot air will be moving east from the Plains, reaching the major East Coast cities Friday and Saturday. This map shows the upper-air flow that will make this happen.