Monday 9 a.m.
A strong trough swinging through the Northeast is responsible for the unseasonably chilly air mass now in place. As the trough departs, the flow aloft will change from northwest to westerly. That may not seem like much, but consider this: Northwest of Pennsylvania and New York, temperatures were generally in the 30s this morning. However, across the Dakotas, daybreak temperatures were as high as the 50s. The coming westerly flow will bring that warm air east. This video has more.
On Wednesday, it will feel like summer from Missouri to Virginia, but the warmup will be a bit muted farther north, and the flow on this map shows why. The isobars show a pretty straight show of warm air from Texas to Virginia. However, north of there, the flow bends to become more south to north. The curvature zone is the location of a warm front. North of the front, it is in the process of getting warmer, but it is not as warm there as it is south of the front. Also, north of the front, the action of warm air rising up and over retreating cool air can lead to clouds and showers. The forecast map is for 2 p.m. EDT Wednesday.
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.