A storm system will trudge towards our region tomorrow and slowly cross through the Delaware Valley through Saturday morning before bringing lower humidity in its wake. The combination of instability in the atmosphere, high amounts of humidity, and some dynamics with the storm system itself lead to a threat for severe weather for parts of the region on Thursday and for the whole of the region on Friday.
From a thunderstorm standpoint, Thursday's threat is not a widespread squall line type of day...it is more scattered afternoon and evening storms that will fire up, some of which becoming severe. The main front will still be back over the Lakes, with the best pocket of severe potential in the Corn Belt with a weak area of low pressure centered over Illinois on Thursday. This low will ride northeast along the front as the front itself pushes east. All that thrown in, we should have a few stronger thunderstorms that pop in the heat and humidity and some of them will flirt with or exceed severe limits...but it doesn't look like a widespread thunderstorm day.
Friday, however, will have that threat for widespread thunderstorms over the region as the low and front get closer. Thunderstorms will be in the forecast for the region, with some of the storms not only producing severe weather but also a threat for heavy rainfall within the thunderstorms. This is one of those days that could feature storms at all times through the day but more numerous and heavier thunderstorms in the afternoon and evening hours.
With the threat for heavier rainfall looming for Friday's storms, some localized flooding and ponding of water on roads can't be ruled out. The front will push through early Saturday, ending the heavy thunderstorm threat and bringing lower humidity levels through most of the weekend.